Thermochromic Polymers—Function by Design - Chemical Reviews

Review pubs.acs.org/CR

Thermochromic PolymersFunction by Design Arno Seeboth,* Detlef Lötzsch, Ralf Ruhmann, and Olaf Muehling Department of Chromogenic Polymers, Fraunhofer Institute for Applied Polymer Research, Volmerstraße 7b, 12489 Berlin, Germany example, transforms on heating, when exceeding sufficient temperature, into a shiny green and returns upon cooling. Known since antiquity,1 this phenomenon has been studied by scientists for centuries.2 In 1963 and 1968, reviews written by Day appeared reporting on reversible and irreversible thermochromism by single compounds as well as by organic or inorganic composites.3 In the last decades, the field of thermochromism has made tremendous progress. Thermochromic composites or pigments based on leuco dyes4 or cholesteric liquid crystals5 as well as their incorporation into polymers6 have become a state of the art technology and were described in detail.7 Within the group of polymers with inherent thermochromic properties conjugated polymers were studied extensively.8 The investigations on these thermochromic material CONTENTS classes seem to have elucidated their specific potential and limitations. Thus only marginal stimuli can be expected from 1. Introduction A them in the future. 1.1. Origin of Thermochromism: ThermoresponThe strategy to design thermochromic systems by physical or sive Effect on Light A chemical interaction of their nonthermochromic components 1.2. Thermochromic Polymers: Inherent and encourages the topic as never before. As a consequence the Doped Systems B following systems will be highlighted in the present review: 2. Temperature Tunable Photonic Crystals C 2.1. One Dimensional Photonic Crystals C • photonic crystals including Bragg stacks and crystalline 2.2. Three Dimensional Photonic Crystals: Cryscolloidal arrays talline Colloidal Arrays F • nanoparticle based effects: surface plasmon absorption 3. Thermochromism in Polymers Based on Nanoand quantum dots particles M • dye−dye or polymer−dye aggregation−disaggregation 3.1. Surface-Plasmon Resonance M mechanisms 3.1.1. Growth of Particle Size N In cases in which the same strategies were applied to develop 3.1.2. Particle Agglomeration N novel thermoresponsive fluorophoric polymers these are also 3.1.3. Change of Particle Shape N reported. 3.1.4. Change of Interparticle Distance O Pioneering works in the field of polymers with stimuli3.1.5. Refractive Index Modulation of the responsive colorimetric properties, especially with thermochroSurrounding Medium R mic and/or mechanochromic properties were reported by the 3.2. Quantum Dots R groups of Asher, Weder, and Pucci. The origin of the respective 4. Thermochromism by Self-Assembling of Dye− thermochromic effects, the specific design strategies, state of the Dye Aggregates S art, structure−property-relationships, and the strengths and 5. Thermochromism by Polymer−Dye Interaction X weaknesses of the different material classes will be evaluated 5.1. Proton Equilibrium in Hydrogels X critically. 5.2. Polymer−Dye Complex Formation AA To give an overview of the widespread field of thermochrom5.3. Charge-Transfer Complex AB ism, section 1.1 will present a classification of polymers with 6. Conclusion AB thermoresponsive optical properties according to their thermorAuthor Information AC esponsive effect on light while in section 1.2 thermochromic Corresponding Author AC polymers will be classified into inherent and doped systems. Notes AC Biographies Abbreviations References

1.1. Origin of Thermochromism: Thermoresponsive Effect on Light

AC AD AD

The interaction of matter with visible (vis) light determines how an object is perceived. Possible interactions can be classified into a small number of general phenomena: scattering, reflection,

1. INTRODUCTION Thermochromism denotes the phenomenon of a color change with dependence on temperature. The red color of the ruby, for © XXXX American Chemical Society

Received: August 22, 2013

A

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Table 1. Classification of Polymers with Thermoresponsive Optical Properties thermoresponsive effect on light scattering

origin

macroscopic behavior

phase separation processes

thermotropism

refractive index changes of one component of a heterophasic composite

thermotropism

reflection

changes of the optical path length in periodic structured materials

thermochromism

absorption

structural changes of chromophoric moieties

thermochromism

emission

structural changes of fluorophoric moieties

thermoresponsive shift of the emitted light color

polymer class hydrogels and polymer blends with lower or upper critical solution temperatures9 lyotropic liquid crystalline hydrogels10 polymer-domain systems9c,11 liquid crystalline polymers forming helical superstructures12 crystalline colloidal arrays embedded in a polymer matrix (section2.2) self-assembling of block copolymers (section2.1) or cross-linked polymeric colloids (section2.2) forming a photonic crystal leuco dye−developer−solvent systems embedded in a polymer matrix6a−f conjugated polymers8a−e inorganic thermochromic complexes dissolved in a polymer matrix7,9c,13 surface-plasmon absorption by nanoparticles embedded in a polymer matrix (section3.1) dye−dye aggregation−disaggregation in a polymer matrix (section4) polymer−dye aggregation−disaggregation (section5.2) hydrogel-indicator dye composites (section5.1) quantum dots coupled to chain ends of poly(N-isopropylacrylamide) brushes on gold (section3.2) dye−dye aggregation−disaggregation in a polymer matrix (section4) hydrogel-fluorescence indicator dye composites (section5.1)

Table 2. Classification of Thermochromic Polymers into Inherent and Doped Systems inherent

doped with thermochromic pigments

liquid crystalline polymers forming helical superstructures12 conjugated polymers8a,b,d−g

leuco dye-developer-solvent systems embedded in a polymer matrix6a−f conjugated polymers embedded in a polymer matrix8c inorganic thermochromic complexes dissolved in a polymer matrix7,9c,13

doped with nonthermochromic additives self-assembling of block-copolymers or cross-linked polymeric colloids forming a photonic crystal crystalline colloidal arrays embedded in a polymer matrix surface-plasmon absorption by nanoparticles embedded in a polymer matrix hydrogel-indicator dye composites dye−dye aggregation−disaggregation in a polymer matrix polymer−dye aggregation−disaggregation

thermochromic by the interaction between a polymer matrix and additives. In the present review, the term “function by design” denotes the creation of thermochromism in the third of these polymer classes. Examples for this type of classification are listed in Table 2. Liquid crystalline and conjugated polymers can possess inherent thermochromic properties. Several liquid crystalline phases can form helical superstructures, if a chiral dopant is added or the liquid crystalline compound itself has a chiral molecular structure.5c,7,15 Incident circular polarized light, whose handedness and wavelength fits with the periodic structure of the liquid crystalline phase is reflected according to the Bragg’s law. The pitch length of the helical structure can range from about 100 nm to infinite. Frequently the reflected light has a wavelength in the visible range and thus structural color occurs. Since the pitch length of these liquid crystalline materials changes generally with temperature, the appearance of structural color is accompanied generally by thermochromism. The most common of these liquid crystalline phases is the cholesteric phase. Thermochromic cholesteric liquid crystalline polymers12a−d and polymer gels12e,16 as well as thermochromic pigments consisting of microencapsulated low molecular weight cholesteric liquid crystals5a are described in numerous papers as well as in several reviews and books. Cholesteric liquid crystalline polymer materials are well-understood and commercially available in various forms. Conjugated polymers consist of a polymer

absorption, and emission. A classification of polymers with thermoresponsive optical properties sorted by their effect on light upon temperature change is given in Table 1. Changes of the light scattering properties on temperature change do primarily not affect the color but the transparency of the material, e.g., a transformation between a clear and a milkywhite state can occur. These so-called thermotropic effects form a subclass of thermochromism.5,7−9 In the present review we use the term thermochromism in its closer sense denoting color changes with temperature. These can have their origin in the phenomena reflection or absorption. They result from shifts of the selective reflection wavelength or due to changes of the absorption properties in the visible region. Light emission requires a previous transformation of the material into an excited state as in case of fluorescence dyes on ultraviolet (UV) radiation. Shifts of the emitted light wavelength in the visible region are denoted frequently as thermochromic effects too.14 However, in the present review we will term this class of materials as thermoresponsive fluorophoric for a differentiation from the group of thermochromic polymers. 1.2. Thermochromic Polymers: Inherent and Doped Systems

Another way to classify thermochromic polymers is to distinguish between polymers with inherent thermochromic properties, polymers which become thermochromic by embedding of thermochromic pigments, and polymers which become B

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backbone structure with a conjugated π-electron system. Polyacetylenes, polydiacetylenes, polythiophenes, and poly(phenylene vinylidenes) belong to this class of polymers. A large number of them absorb light in the visible range leading to the appearance of colored states. Any change of the effective conjugation length of the π-electron system in the polymer backbone with temperature results in a shift of the absorption wavelength and thus in thermochromism. Even small variations of the conformational structure can already cause significant color changes, especially if they affect the planarity of the polymer chain. These color changes can occur continuously within a phase or discontinuously at a phase transition. Thermochromic effects of conjugated polymers are in principle reversible but can be also irreversible due to kinetic effects. From the applicative point of view, the introduction of thermochromism into commercial paints, lacquers, inks, thermoplastics, and thermosetting plastics is of special interest. In the past decade a lot of efforts were made to develop thermochromic pigments applicable for a wide range of polymer matrices or to tune the properties of a thermochromic pigment for a specific application. Microencapsulated leuco dye−developer−solvent systems are the most common thermochromic pigments. A polymer shell separates the matrix from the thermochromic composite which therefore forms a separate phase in the matrix. Leuco dye−developer−solvent systems are generally colored in the solid state and transform into a colorless liquid on heating. However, a few examples of a color change in the opposite direction were also reported.4c Since a large number of suitable leuco dyes, developers, and solvents are known, color and transition temperature of this type of thermochromic composites can be tuned easily. In their microencapsulated form leuco dye− developer−solvent systems have become a state of the art technology and are commercially available. However, they could never prevail due to their limited lifetime with its fading thermochromism. Advantages in the nanotechnology and a better understanding of functional dyes and polymer−dye interaction have led to the development of several types of thermochromic polymers using nonthermochromic polymer matrices and nonthermochromic additives. Physical or chemical interactions between the nonthermochromic components lead to the occurrence of thermochromism. In other words, the thermochromic effect itself is created by the material design. This strategy opens the way for novel thermochromic materials and has tremendously stimulated this topic.

Figure 1. Schematic drawings of the structures of 1-D, 2-D, and 3-D photonic crystals.

sections 2.1 (1-D photonic crystals) and 2.2 (3-D photonic crystals). Photonic crystals formed by self-assembling of polymer colloids in fluid media are not part of this review.21 Both sections start with a brief introduction of the various design concepts. Organized in the same manner a detailed description of several examples follows. 2.1. One Dimensional Photonic Crystals

The simplest structure of a photonic crystal is a periodic onedimensional stack of alternating layers of high and low refractive index materials. This so-called Bragg mirror or Bragg stack reflects specific wavelengths of light. For incident light normal to the surface the Bragg’s law can be expressed by eq 1.

mλ = 2(∑ nidi) i

(1)

Here, m is an integer which stands for the order of diffraction, λ is the wavelength of the first order diffracted light, and ni and di are the refractive index and thickness of the respective layer i. According to eq 1, a thermoresponsive 1-D photonic crystal will be obtained, if at least one of the layer materials alters spacing (d) and/or refractive index (n) with temperature. Different design strategies ensue from this. 1-D photonic crystals, whose thermochromic effects are caused primarily by the alteration of spacing of one involved layer material, were obtained by: (1) spin-coating or self-assembling of alternating thermoresponsive and nonthermoresponsive polymer materials;22 (2) selfassembling of block copolymers in which the degree of segregation between the different blocks depends on temperature;23 and (3) addition of a low molecular weight component to a self-assembling block copolymer which causes a stretching of the layer thickness of one layer material by hydrogen bonding. A temperature stimulated break of the hydrogen bonds and a change of the distribution of the low molecular weight additive between the different layers modulates the layer thickness.24 A thermochromic effect by a significant alteration of the refractive index of one layer material was also achieved.25 Addition of a low molecular weight additive induced a smectic liquid crystalline phase in one of the layers of a 1-D photonic crystal. Its transition into the isotropic phase alters the refractive index of the specific layer due temperature variation. All these thermochromic 1-D photonic crystals will be described in detail in the following section. The preparation of a spin-coated thermochromic multilayer polymer film consisting of alternating layers of thermoresponsive and nonthermoresponsive polymers were reported by Hayward et al.22a and by Yang et al.22b Temperature changes vary the thickness of the thermoresponsive polymer layer (Figure 2).

2. TEMPERATURE TUNABLE PHOTONIC CRYSTALS Photonic crystals are regular structures formed by at least two dielectric materials.17 They possess a periodical modulation of the refractive index. Incident light is reflected partially at each boundary where the refractive index changes. The optical interference of these multiple light paths results in the reflection of light of specific wavelengths and can give rise to the appearance of structural color. According to the dimension of the regular structure, photonic crystals can be classified as one-dimensional (1-D), twodimensional (2-D), and three-dimensional (3-D; Figure 1). Intensive studies on photonic crystals have started with the pioneering works of Yablonovitch18 and John19 in 1987. Especially the creation of stimuli-responsive photonic crystals, which are able to visualize environmental changes by color changes, has come into the focus.20 This includes thermochromic polymers and gels, whose design concepts are presented in C

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Figure 2. Model of a 1-D photonic crystal built from alternating layers of thermoresponsive and nonthermoresponsive polymer materials. Figure 4. Molecular structures of the poly(p-methyl styrene) (PpMS) and the poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-coPAAc) copolymer used in ref 22a for the fabrication of a thermo- and pH-responsive 1-D photonic crystal.

Both authors used a cross-linked poly(N-isopropylacrylamide) (PNIPAM) copolymer as thermoresponsive layer material. Cross-linked PNIPAM (Figure 3) is well-known for its temperature responsive volume change in aqueous media.26

Figure 3. Molecular structure of poly(N-isopropylacrylamide) (red) cross-linked with N,N′-methylene-bis-acrylamide (blue).

As long as the temperature is below the lower critical solution temperature (LCST) of about 32 °C the polymer network is hydrated and swollen. When heated above 32 °C the hydrated state collapses into a dehydrated state accompanied by a distinct and discontinuous volume change. The sharp and reversible transition at an easily accessible transition temperature in combination with its simple synthesis from commercially available precursors has made this polymer gel attractive for both fundamental and applicative studies. Thus it has become a model system in the field of thermoresponsive gels. In detail, Hayward et al. employed a PNIPAM copolymer with 1 mol % of the cross-linker precursor acrylamidobenzophenone and 5 mol % of the ionizable monomer acrylic acid to build the thermoresponsive layers and a poly(p-methyl styrene) (PpMS) copolymer with the same cross-linker precursor to construct the corresponding nonthermoresponsive layers (Figure 4).22a Similar to cross-linked PNIPAM this cross-linked poly(Nisopropylacrylamide-co-acrylic acid) (PNIPAM-co-PAAc) copolymer swells extensively in water at room temperature and undergoes a pronounced collapse on heating. With increasing number of layers the reflectivity of the polymer stack increases. As a compromise of production time and reflectance an eleven layer sensor stack consisting of alternating layers of 90 nm thick cross-linked PpMS and 40 nm thick cross-linked PNIPAM-coPAAc copolymer was prepared. Layer-by-layer, each was spincoated, cross-linked by photopolymerization, developed in a suitable solvent mixture to remove noncross-linked polymer, and dried (Figure 5a). After immersion in room-temperature water, the polymer stack possesses a Bragg reflection peak at 710 nm with a reflectivity of about 0.49. Upon heating, the thermoresponsive

Figure 5. (a) Schematic representation of sensor fabrication [red layers, PpMS; blue layers, PNIPAM-co-PAAc], (b) peak reflectance wavelength, and (c) photographs of sensors swelled in water at different temperatures. Reprinted with permission from ref 22a. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

PNIPAM-co-PAAc copolymer deswells causing a blue shift of the wavelength of the Bragg reflection peak and a reduction of the reflectance (Figure 5b,c). At 50 °C Bragg reflection occurs at 431 nm with a reflectivity of about 0.15. Over at least five temperature cycles between 20 and 50 °C, the swelling/deswelling process was found to be fully reversible. A similar 1-D photonic crystal was reported by Yang et al.22b The structures of the used polymers are displayed in Figure 6.

Figure 6. Molecular structures of poly(N-isopropylacrylamide-coglycidylmethacrylate) (PNIPAM-co-PGMA) and poly(methylmethacrylate) (PMMA) used in ref 22b for the fabrication of a thermoresponsive 1-D photonic crystal. D

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The poly(N-isopropylacrylamide-co-glycidylmethacrylate) (PNIPAM-co-PGMA) copolymer was prepared by a radical polymerization of 2 g of N-isopropylacrylamide and 1 mL of glycidylmethacrylate with azobisisobutyronitrile (AIBN). Alternating 62 nm thick layers of PNIPAM-co-PGMA and 110 nm thick layers of poly(methylmethacrylate) (PMMA) were spincoated on a silicon substrate forming a Bragg stack film with 26 layers. This spin-coating process was carried out by using specific solvents for dissolving only the respective polymer but not the other. In particular, hexanol was used to dissolve PNIPAM-coPGMA and toluene to dissolve PMMA. After spin-coating the PNIPAM-co-PGMA layers were cross-linked by photopolymerization of the epoxy groups in the glycidylmethacrylate moieties. Notice that only one photopolymerization step and no layer-bylayer photopolymerization as in the case of the above-described cross-linked PpMS/cross-linked PNIPAM-co-PAAc copolymer layer stack is necessary.22a The Bragg stack film thereby prepared also exhibits thermochromic properties in water. However, its temperature sensitivity is significantly lower. As the temperature rises the Bragg reflection wavelength shifts from 600 nm at 0 °C to 577 nm at 10 °C, 557 nm at 20 °C, 542 nm at 30 °C, and 531 nm at 40 °C. This blue shift is accompanied by a decrease of the reflection intensity. The reason for the lower temperature sensitivity is the high cross-linking density of the PNIPAM layer. With increasing cross-linking density the swelling capability in water is reduced thus UV light exposed and unexposed regions develop different colors in water. Hence, this system can be used as photonic paper in the following way: prewritten by UV light a text becomes visible by exposing the polymer Bragg stack to water. While the layer structures of the 1-D photonic crystals described above were prepared by a layer-by-layer spin coating process, all following layer stacks were formed from selfassembling copolymers. A red-shifting 1-D photonic crystal consisting of alternating layers of a soft thermoresponsive and a hard nonthermoresponsive polymer material was reported by Gong et al.22c The nonthermoresponsive polymer layers consist of poly(dodecylglyceryl itaconate) (PDGI) bilayers with a thickness of 4.7 nm and the thermoresponsive polymer layers of poly(acrylamide) (PAAm) and poly(acrylic acid) (PAAc) with stimuli-dependent thicknesses in the range of hundreds of nanometers. The latter were obtained by a two-step synthesis starting with the preparation of self-assembled PDGI-PAAm layers. Subsequently PDGI-PAAm was immersed in an aqueous solution of acrylic acid followed by a second polymerization step resulting in the PAAm/PAAc network formed by H-bonds (Figure 7). In deionized water a thermochromic effect occurs within the temperature range of 5−50 °C. At low temperatures hydrogen bonds are formed between the two polymers of the network. Increasing the temperature results in a gradual dissociation of the hydrogen bonds and the involved layers swell. Thus an increase of the layer thickness on rising temperature stands in contrast to thermoresponsive PNIPAM layers. The gradually increase of the layer thickness leads to a continuous color change from blue to red on heating. In addition to thermoresponsive properties the photonic crystal shows also stress/strain and pH-responsive properties leading in this specific case to a response to overall three different external stimuli. A 1-D photonic crystal formed by a self-assembled block copolymer gel was reported by Thomas et al.23 The structures of the used block copolymer and solvent are shown in Figure 8.

Figure 7. Molecular structures of poly(dodecylglyceryl itaconate) (PDGI) and of poly(acrylamide) (PAAm) and poly(acrylic acid) (PAAc) network used in ref 22c for the preparation of a thermoresponsive 1-D photonic crystal.

Figure 8. Molecular structures of the block copolymer poly(styrene-bisoprene) (PS-b-PI) and the solvent cumene used in ref 23 for the fabrication of a thermoresponsive 1-D photonic crystal.

A 50% w/w solution of poly(styrene-b-isoprene) (PS-b-PI) in cumene was casted between two glass substrates and sealed with a fast-curing epoxy resin to preserve a constant solvent concentration. By this procedure a 1-D photonic crystal with the lamellae oriented parallel to the glass substrates was obtained as shown by the angle dependence of the Bragg reflected color. The sample appeared green under incidence normal to the glass plates. Tilting the sample in any direction provoked a blue shift of the Bragg reflected color. A temperature rise from 30 to 140 °C led to an overall blue shift of 60 nm measured at normal incidence (Figure 9).

Figure 9. Normal incidence reflectivity spectra of the block copolymer poly(styrene-b-isoprene) lamellar photonic gel at 12 different temperatures between 30 and 140 °C in 10 K increments. Reprinted with permission from ref 23. Copyright 2008 American Chemical Society.

The contribution of three factors to the thermochromic effect was discussed: (1) the temperature dependence of the refractive index of all components, (2) a thermal expansion of the layers, and (3) a decrease of the degree of segregation between the polystyrene and polyisoprene blocks with increasing temperature. Thermal expansion on temperature rise shifts the Bragg reflection peak to longer wavelengths while the other two effects cause a shift to shorter wavelengths. Computerized calculations E

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copolymer and the low molecular weight liquid crystal displayed in Figure 11.

taking these three effects into account were found to match the experimental data. The weaker segregation with increasing temperature is by far the dominating factor and leads to the overall observed blue shift of the Bragg peak on heating. Ikkala et al. designed a 1-D photonic crystal showing a sharp and reversible thermochromic effect in the solid state.24 No solvent contact is necessary for the thermochromic effect. The photonic crystal consists of a block-copolymer and a low molecular weight additive leading to the formation of a selfassembled polymeric solid film. Earlier investigations had shown that bonding of dodecylbenzenesulfonic acid to the poly(4vinylpyridine) block of a polystyrene-block-poly(4-vinylpyridine) causes large stretching of the chains thus leading to Bragg reflection in the visible range.27 Based on this finding the authors concluded that weakening of the interaction between the block copolymer and the low molecular weight additive might lead to a thermoresponsive behavior. To realize this concept, a comb-shaped supramolecular complex formed by hydrogen bonding instead of strong ionic interactions was created. The poly(4-vinylpyridine) block was transformed into a poly(4vinylpyridinium methanesulfonate) block by stoichiometric protonation with methanesulfonic acid (Figure 10). Then 3-npentadecylphenol was added in order to form a comb-shaped supramolecular complex by hydrogen bonding.

Figure 11. Molecular structures of poly[styrene-block-poly(methacrylic acid)] and of the liquid crystalline mesogen used in ref 25 for the preparation of a self-assembled hydrogen bonded liquid crystalline block copolymer.

Both components were dissolved in tetrahydrofuran and solvent casted. During this process the low molecular weight component is hydrogen-bonded to the methacrylic acid units forming the side groups of the resulting liquid crystalline block copolymer, which self-assembles into a 1-D photonic crystal of alternating polystyrene and poly(methacrylic acid)-liquid crystal layers. The poly(methacrylic acid)-liquid crystal layer displays a homeotropically oriented smectic structure. Alteration of the order parameter of the anisotropic structured mesogens is accompanied by changes of the refractive index. The order parameter drops to zero at the smectic to isotropic phase transition (Figure 12a). Accordingly a discontinuous change of the refractive index occurs (Figure 12b) resulting in a color change from green to orange (Figure 12c). However, this color change was found to be irreversible. On cooling no recovery of the solvent casted state was observed. The ways to produce 1-D photonic crystals, which include coating techniques or self-assembling systems, fulfill the requirements for large-scale applications. On the downside, PNIPAM copolymers dominate the field of 1-D photonic crystals allowing only small variations of the polymer.

Figure 10. Molecular structures of the block copolymer polystyreneblock-poly(4-vinylpyridinium methanesulfonate) and the low molecular weight additive 3-n-pentadecylphenol used in ref 24 for the fabrication of a thermoresponsive 1-D photonic crystal.

The obtained complex averages 1.5 equiv 3-n-pentadecylphenol per 4-vinylpyridinium methanesulfonate unit. At room temperature 3-n-pentadecylphenol is not soluble in polystyrene but dissolves well in poly(4-vinylpyridinium methanesulfonate), where it forms hydrogen bonds to the sulfonate groups. This architecture causes a stretching of the polymer chains. The periodicity of the lamellar structure adds up to 160 nm and the sample appears green showing a Bragg reflection at about 530 nm. On heating no structural change and no shift of the Bragg reflection wavelength occurs below 117 °C. Within the temperature range of 120−130 °C the hydrogen bonds gradually break. Additionally, 3-n-pentadecylphenol becomes soluble in polystyrene and migrates into these domains. Both effects contribute to a collapse of the periodicity of the lamellar structure leading to a shift of the Bragg reflection wavelength into the UV region. The described effects are reversible and on cooling the original structure and color is recovered. Preparation and thermo-optical properties of a thermochromic Bragg stack with a discontinuous refractive index change of one layer material through temperature alteration were reported by Thomas et al.25 The reported Bragg stack was formed by a selfassembled hydrogen bonded liquid crystalline block copolymer consisting of a poly[styrene-block-poly(methacrylic acid)] block

2.2. Three Dimensional Photonic Crystals: Crystalline Colloidal Arrays

3-D colloidal photonic crystals are formed by periodically arranged monodisperse colloidal spheres. Contrariwise, inverse 3-D photonic crystals consist of periodically arranged holes embedded in a matrix material.20,28 The spherical particles or holes can form close-packed or nonclose-packed crystalline colloidal arrays (CCA) as displayed in Figure 13. Their periodic structure leads to light reflection of specific wavelengths as determined by the Bragg’s law formulated in eq 2: mλ = 2ndhkl sin θ

(2)

where m is an integer, which stands for the order of diffraction, λ is the wavelength of the first order diffracted light, n is the mean refractive index, d is the interplanar spacing of the planes defined by the Miller indices (hkl), and θ is the angle between the incident light and the diffracting planes. The lattice constant of photonic crystals typically range from 50 to 500 nm giving rise to diffracted wavelength in the UV, visible, or infrared (IR) region. Taking the structure of 3-D colloidal photonic crystals into account and combining Bragg’s law with Snell’s law the reflected wavelength can be expressed by eq 3.29 F

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In contrast to the wavelength, the intensity of the Bragg reflection cannot be calculated exactly at the moment. However, as long as multiple scattering effects are small, computerized calculation methods can give accurate results. 30 These calculations demonstrate for example the influence of different sphere diameters or incident light angles on the diffraction efficiency of CCA. The intensity of the Bragg reflection of a photonic crystal can be understood as an interference of all scattering contributions from each individual colloidal particle of the system. Generally, single and multiple scattering contributions have to be considered which is numerically expensive to solve and thus restricted to small numbers of scattering particles. In case of photonic crystals with relatively low differences of the involved refractive indices multiple scattering effects can be neglected. This enables to carry out simulations for realistically sized macroscopic photonic crystals by calculating the total scattering properties as a sum of the scattering properties of each individual sphere obtained by applying Mie theory. The relationship between wavelength/intensity of the diffracted light and the material parameters of photonic crystals provides a guidance for a purposive tuning of their optical properties and thus for the creation of thermochromism. A photonic crystal with thermochromic properties will result, if any of the parameters appearing in the Bragg equation is designed to change its magnitude with temperature. The embedding of a CCA in a matrix possessing a temperature caused volume change, which modulates the center-to-center distance (D) of the colloidal spheres, was the first strategy to create thermochromic 3-D photonic crystals polymer materials.21a,31 Since the pioneering work in 199621a, further examples were reported whereupon isotropic31 and anisotropic32 matrix volume changes were realized. Embedding particles in either a nonthermoresponsive or in a thermoresponsive polymer matrix with a temperature dependent volume change is another concept.33 By this, changes of the refractive index contrast between particles and matrix were obtained. They result in changes of the Bragg reflection intensity. Matrix volume changes through elastic deformations resulting in shifts of the Bragg reflection wavelength were also achieved. A further design concept is to prepare covalently cross-linked thermoresponsive nanoparticles. They can either show an order−disorder transition resulting in a disappearance of the Bragg reflection in the ordered state during the transition into the disordered state34 or a temperature dependent volume change resulting in a modulation of the center-to-center distance.35 Moreover, the preparation of a 3-D photonic crystal by embedding a CCA in a polymer matrix having the same refractive index as the colloidal particles at a certain temperature but a different refractive index change with temperature was implemented successfully.36 All these thermochromic 3-D photonic crystals will be described in detail in the following section. The most common concept for the development of thermochromic photonic crystals is the enclosure of CCA into a hydrogel matrix exhibiting a temperature depending volume change. In a CCA-matrix system the lattice spacing of the periodic structure is coupled with the volume of the matrix. Swelling or shrinking of the hydrogel matrix in dependence on temperature leads to an alteration of the lattice spacing and thus to changes of the wavelength of the diffracted light (Figure 14). For this type of photonic crystals eq 4 formulates the dependence of the diffracted wavelength on the degree of swelling (d/ d0):20a,31a

Figure 12. (a) Structural change of the self-assembled hydrogen bonded liquid crystalline block copolymer at the smectic to isotropic phase transition (PS, polystyrene layer; PMAA-LC, poly(methacrylic acid)liquid crystal layer). (b) Illustration of the alteration of the refractive index of one layer material forming the 1-D photonic crystal. (c) Photographs of pieces of the reflective polymer (size: 1 mm × 2 mm, the cracks were accidentally introduced during sample removal.): Sample at 30 °C (left) and at 80 °C (right). Reprinted with permission from ref 25. Copyright 2002 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 13. Schematic drawings of the structures of 3-D photonic crystals.

mλ =

8 D(∑ ni 2Vi − sin 2 Φ) 3 i

(3)

Here, D is the center-to-center distance of the colloidal spheres, ni and Vi are respectively the refractive index and volume fraction of the component i, and Φ is the angle between the incident light beam and the sample normal. G

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Figure 14. Illustration of an alteration of the lattice spacing by a temperature dependent volume change of the matrix.

mλ =

8 ⎛d⎞ D⎜ ⎟(∑ ni 2Vi − sin 2 Φ) 3 ⎝ d0 ⎠ i

(4)

Here, d denotes the diameter of the gel in the actual state and d0 the diameter in the reference state. Color changes on temperature alteration occur as long as the Bragg reflection wavelength is in the visible range. The first thermochromic 3-D photonic crystal of this type was reported in 1996 by Asher et al.,21a who also created similar chromogenic 3-D photonic crystals sensitive to other stimuli.37 A cross-linked PNIPAM matrix was used to obtain a temperature dependent volume change.26 Preparation of a CCA embedded in a cross-linked PNIPAM hydrogel matrix was performed in two steps: (1) dispersion of 0.23 g of highly negatively charged and monodisperse polystyrene spheres (99 nm diameter, 19% solids) in an aqueous solution of 0.35 g of N-isopropylacrylamide (monomer), 0.02 g of N,N′-methylene-bis-acrylamide (crosslinker), and 0.004 g of diethoxyacetophenone (UV-photoinitiator). (2) Initiation of a photochemical polymerization of the dissolved acrylamide monomers to build-up the hydrogel matrix. During the first step the polystyrene spheres form a nonclosepacked CCA with a body-centered cubic lattice structure by selfassembling in the aqueous medium. Electrostatic repulsion of the charged polystyrene spheres is the driving force for the selfassembling. By varying the concentration of the polystyrene spheres it is in principle possible to adjust the lattice constant to any desired value. The formation of the cross-linked polymer network around the polystyrene spheres permanently locks the embedded CCA. The obtained photonic crystal swells and shrinks in water reversibly and continuously upon temperature alteration within the range of 10−35 °C. Spectra of a 125 μm thick sample were recorded in a temperature range of 11.7−34.9 °C with the incident light normal to the (110) plane of the bodycentered cubic lattice (Figure 15). At 11.7 °C the spectrum shows a diffracted wavelength of 704 nm. By increasing the temperature, the diffraction wavelength shifts to lower wavelengths ending with 460 nm at 34.9 °C and having crossed the entire visible range during the 23 K span. Correspondingly, the color of the photonic crystal changed from red to indigo via orange, yellow, green, and blue. In the last two decades several further examples proved this concept for the preparation of thermochromic 3-D colloidal photonic crystals.31 Watanabe and Takeoka et al. reported the use of a closepacked colloidal crystal formed by silica particles as a template to introduce interconnecting periodic porosity into thermoresponsive hydrogels.31a−d In the first step a close-packed silica colloidal crystal was prepared on a glass substrate.36b After that, a

Figure 15. Spectrum of a 125 μm thick polymerized CCA film of 99 nm PS spheres embedded in a PNIPAM gel illustrating the change of Bragg diffraction upon temperature alteration. The shift of the diffracted wavelength results from the temperature-induced volume change of the gel which alters the lattice spacing. The sample was placed normal to the incident light beam. The inset shows the temperature dependency of the diffracted wavelength for this polymerized CCA film when the incident light is normal to the (110) plane of the lattice. Reprinted with permission from ref 21a. Copyright 1996 Science.

solution of the gel forming monomers was infiltrated into the voids of the close-packed colloidal crystal and the monomers were polymerized. A 5% w/w hydrofluoric acid aqueous solution was then used to remove the silica particles from the gel. One gel matrix used was N-isopropylacrylamide cross-linked with N,N′methylenebisacrylamide (0.0133 g/g of N-isopropylacrylamide).31b In pure water the hydrogel matrix undergoes a temperature dependent matrix volume change at about 32 °C. The volume change is isotropic and does not depend on the shape of the hydrogel. This effect also appears in the prepared periodically porous gel leading to a thermoresponsive change of the Bragg reflected wavelength. A gel prepared from a template of 0.5 μm sized silica particles by the gravity sedimentation method was reported to exhibit heterochromatic color. Above 32 °C the color disappeared and reappeared on cooling below this temperature. The observation of heterochromatic color was explained by random orientation of diffracting planes. An advantage of porous gels is the strongly reduced response time compared to similar nonporous samples. The response time of swelling and shrinking processes is determined by the diffusion process of the solvent through the polymer network which depends on the characteristic length of the gel (R) and on the collective-diffusion coefficient D as expressed by eq 5. τ=

R2 π 2D

(5)

Since the interconnecting porous structure increases the surface area and thus reduces the characteristic length of the gel, a smooth diffusion of the solvent into and out of the gel occurs and an improved response speed for the matrix volume change is obtained. This effect becomes more and more evident with increasing size of the sample. For sudden heating from 25 to 40 H

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°C a porous cylindrical gel sample with a diameter of 3 mm exhibits a response time of 30 s to reach a completely collapsed state. Similar shaped nonporous samples require more than 1 day for the same response. The specific reported thermo- and pHresponsive gel matrix in ref 31c is a copolymer of methacrylic acid, N-isopropylacrylamide, and N,N′-methylenebisacrylamide. Spherical silica particles with a diameter of 210 nm were used to fabricate the template for the interconnecting periodically porous structure. Instead of the gravity sedimentation method used in reference31b a solvent evaporation method38 was applied. The solvent evaporation method produces large single crystal and not polycrystalline silica particle templates as obtained by the gravity sedimentation method. The N-isopropylacrylamide unit causes the temperature-sensitivity and the methacrylic acid unit the pHsensitivity of the swelling ratio of the cross-linked copolymer. As an example for the thermochromic properties of the prepared periodically porous gel, the temperature dependent change of the Bragg reflection wavelength of the gel in an HCl aqueous solution of pH 2.6 was reported. Within the temperature range of 16−31 °C a continuous decrease of the Bragg reflection wavelength from 650 to 330 nm occurred accompanied by the corresponding color changes from red via orange, green, blue, and violet to finally nearly colorless (λmax in the UV region). A thermochromic porous gel made from a 4-vinylbenzo-18crown-6, N-isopropylacrylamide, and N,N′-methylenebisacrylamide copolymer exhibits additional ionochromic capabilities provided by the 4-vinylbenzo-18-crown-6 unit.31a An advantage of porous gels in comparison with homogeneous gels is the strongly reduced elastic energy induced by the coexistence of different swollen states, thereby accomplishing an improved local resolution of color changes by temperature. A gel matrix with multiple chromogenic effects was fabricated from a copolymer of 4-acryloylaminoazobenzene, N-isopropylacrylamide, and N,N′-methylenebisacrylamide.31d The azobenzene group can be switched reversibly by light between the trans and the cis forms. The switch induced dipole moment change influences the degree of swelling of the gel leading to a photochromic response in water additionally to the thermochromic response of the PNIPAM moiety. The latter was investigated in the dark (Figure 16). Within the temperature range of 15 to 25 °C the Bragg reflection wavelength was shifted through the entire visible range. Kanai et al. yielded a temperature dependent linear shift of the refracted wavelength over a wide temperature range by modifying the thermoresponsive copolymer matrix system.31e The matrices consist of randomly linked thermoresponsive Nisopropylacrylamide and nonthermoresponsive N-methylolacrylamide with various molar ratios and were cross-linked by the addition of a small fraction of N,N′-methylenebisacrylamide. In these matrices flow-aligned loosely packed arrays of monodisperse silica particles with a diameter of 210 nm were embedded giving rise to selective reflection in the visible range. The performed flow-alignment technique enables to prepare large area single-crystal like domains of the colloidal photonic crystals. By varying the molar ratio of the two monomers the thermo-sensitivity of the 3-D colloidal photonic crystal changed. Due to a distinct volume change at the LCST of about 32 °C PNIPAM homopolymers exhibit a distinct shift of the Bragg reflection wavelength at this temperature. A rise of the mole fraction of N-methylolacrylamide above 0.4 causes a linear shift of the Bragg reflection on temperature alteration for the copolymers instead. In particular, at a molar fraction of 0.5 a shift of −2.1 nm/K was measured within the temperature range

Figure 16. Photographs and reflection spectra of the porous azobenzene/PNIPAM gel at various temperatures. The spectroscopic characterization was carried out in the dark. Reprinted with permission from ref 31d. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

of 15−55 °C. Further increase of the mole fraction of the nonthermoresponsive N-methylolacrylamide leads to a decrease of the slope of the wavelength shift with temperature. A polystyrene colloidal crystal template was used by Lee et al. to prepare a dually tunable inverse opal hydrogel colorimetric sensor.31f 2-Hydroxyethyl methylacrylate as the basic building block, N-isopropylacrylamide as thermoresponsive unit, and the pH sensitive acrylic acid were infiltrated into the template and photopolymerized to form the hydrogel matrix. A small amount of the cross-linker N,N′-methylene-bis-acrylamide was additionally incorporated to provide elastic restoration. The polystyrene template was removed afterward by immersing the hydrogel in chloroform for 24 h. The obtained sensor material was found to response much faster to temperature changes than to pH variations. On a temperature drop from 26 to 10 °C, a 25 μm thick sensor responds in merely 0.5 s. Weitz et al. presented a capillary microfluidics technique for the fabrication of monodisperse microcapsules with hydrogelimmobilized CCA shells.32 The fabrication method is illustrated schematically in Figure 17.

Figure 17. Procedures used for fabrication of microcapsules with hydrogel-immobilized CCA shells through microfluidic techniques and photopolymerization. Reprinted with permission from ref 32. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Oil−water−oil double emulsions were generated by the capillary microfluidics technique. Their water phase contains Nisopropylacrylamide (monomer), N,N′-methylene-bis-acrylamide (cross-linker), and Irgacure 2959 (photoinitiator) as well as charged polystyrene particles forming a CCA. After UVradiation the aqueous phase transforms into a hydrogelI

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occurs within the temperature range of 25−50 °C. This temperature induced volume change causes a shift of the Bragg reflection wavelength in the 111 direction of the fcc structure from 2200 nm at 25 °C to 1800 nm at 50 °C. Although the reported Bragg reflection wavelength is not in the visible range and thus the photonic crystal does not possess thermochromic properties its thermo-optical response is another proof of the described concept. Asher et al. described the preparation and thermoresponsive optical properties of PNIPAM nanogel colloidal particles embedded in a loose-knit acrylamide/bisacrylamide hydrogel matrix.33a The colloidal particle assembled to a fcc structure. Due to the particle concentration a Bragg reflection at 720 nm occurs from the fcc 111 planes. At 10 °C the used PNIPAM particles have a diameter of 350 nm, are highly swollen and consist mostly of water. Since the refractive index difference between matrix and particles is small, the Bragg reflection is weak. As the temperature is increased to 40 °C, fairly above the LCST of PNIPAM, the particles expel water and shrink in diameter to about 125 nm. This effect is displayed schematically in Figure 20.

immobilized CCA shell surrounding the inner oil phase. Typical diameters of the prepared microcapsules were 100−150 μm. The CCA forming polystyrene particles had a diameter of 120 or 198 nm. In the aqueous phase, they self-assemble into a facecentered-cubic (fcc) structure. As indicated by the detection of only 111 reflection peak, the colloidal particles were well oriented parallel to the spherical interface as the fcc (111) lattice planes. Whereas a conventional bulk or spherical gel shrinks or swells uniformly in all directions, an anisotropic volume change appears in the hydrogel shell. The nearly temperature independent volume of the inner core leads to an increase of the shrinking or swelling of the hydrogel shell in the radial direction. Thus the change of the Bragg reflection wavelength with temperature, displayed in Figure 18, is more distinct in the hydrogelimmobilized CCA shells than in a spherical photonic crystal of the shell material (bulk gel).

Figure 20. Illustration of size alteration of the colloidal particles by a temperature dependent volume change of the particles forming nanogel. The face-centered cubic structure of the photonic crystal remains unaffected by the particles size alteration. Figure 18. Temperature dependence of the Bragg reflection wavelength of the cross-linked PNIPAM shell immobilized CCA (●, measured; Δ, calculated) and of a similar bulk material (◊, calculated, assuming an isotropic shrinkage of the gel). Reprinted with permission from ref 32. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

By the shrinking of the gel particles the refractive index difference between matrix and particles rises and leads to a dramatic increase of the diffraction efficiency. A similar system was reported by Zhang et al.33b A CCA of poly(N-isopropylacrylamide-co-acrylic acid) particles was embedded into a acrylamide/bisacrylamide hydrogel matrix. As the PNIPAM homopolymer the poly(N-isopropylacrylamide-coacrylic acid) copolymer exhibits a temperature dependent volume phase transition. However, the transition is shifted slightly to lower temperatures. It appears within the temperature range of 30−32 °C. Two interactive effects were observed between the thermoresponsive gel particles and the nonthermoresponsive hydrogel matrix. First, the volume phase transition temperature of the gel particles becomes broader in the hydrogel matrix and shifts to the temperature range of 32−35 °C. In this range a strong increase of the Bragg reflection intensity occurs on heating. Second, the shrinking of the gel particles with increasing temperature causes a shrinking of the hydrogel matrix. This leads to a shift of the Bragg reflection wavelength to lower wavelength. The embedding of thermoresponsive, uncharged, and lightly cross-linked spherical colloidal particles forming a close-packed CCA in both thermoresponsive and nonthermoresponsive matrices was reported by Zhu et al.33c These colloidal particles consist of poly(N,N-diethylacrylamide-co-N-ethylacrylamide-co2-hydroxyethyl methacrylate) (Figure 21). They were cross-linked by the addition of small amounts of N,N′-methylenebisacrylamide to the mixture of the three

Yang et al. described the preparation of a three-dimensionally structured hydrogel with thermoresponsive optical behavior by three-dimensional holographic lithography.39 Interference pattern of four light beams generate a fcc array of spherical holes in a poly(hydroxyethyl methacrylate-co-methyl methacrylate) matrix (Figure 19). After swelling with water a structured hydrogel with thermoresponsive optical properties was obtained. On temperature alteration a viscoelastic deformation of the hydrogel matrix

Figure 19. Molecular structure of poly(hydroxyethyl methacrylate-comethyl methacrylate). In ref 39, a copolymer with a molar ratio of m = 7 and n = 3 was used. J

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covalent bonding between the nanoparticles contributes to the structural stability and prevents the system from redispersing as observed in similar noncross-linked systems.21c Upon cooling the nanoparticles reassemble into the ordered close-packed CCA (Figure 23). Figure 21. Molecular structure of poly(N,N-diethylacrylamide-co-Nethylacrylamide-co-2-hydroxyethyl methacrylate).

monomers previously to their polymerization. Within the concentration range of 3% w/w to 10% w/w the particles are compressed and form CCA at temperatures below their LCST of about 20 °C. Repeated temperature cycling between 10 and 45 °C leads to the formation of well-ordered fcc CCA. Embedding of the CCA in four different polymer matrices was carried out by photopolymerization of a 10:1 mixture of the respective matrix monomer with the cross-linker N,N′-methylenebisacrylamide at 5 °C. Two of the used matrix polymers, cross-linked polyacrylamide and cross-linked poly(N,N-dimethylacrylamide), are nonthermoresponsive matrices. However, in these matrices a temperature dependent shift of the Bragg reflection wavelength by about 50−100 nm was observed. A higher particle concentration results in larger wavelength shifts with temperature. Stress induced elastic deformations are the origin of this thermochromic effect. At low temperatures the colloidal particles are compressed and the volume of the photonic crystal is elastically expanded. Increasing the temperature leads to a reduction of the compression between the colloidal particles and thus to a reduction of the elastically expanded volume of the photonic crystal (Figure 22).

Figure 23. Scheme of the ordered and disordered states. A reversible phase transition occurs at the LCST of the covalently bond hydrogel nanoparticles. In the ordered state the nanoparticles form a close-packed CCA.

In detail, spherical nanoparticles of a poly(N-isopropylacrylamide-co-2-hydroxyethyl acrylate) copolymer self-assembled in a close-packed CCA and the nanoparticles were then cross-linked by the addition of divinylsulfone. At room temperature Bragg reflection of green light was observed. On heating the ordered state transforms at about 50 °C into a disordered light scattering and non-Bragg reflecting state. At this temperature the sample appears milky white. After cooling the sample back to 21 °C, the Bragg reflection reappears within 10 s. This is about 1000 times faster than the reassembling of the CCA of similar noncrosslinked systems. Further examples of thermochromic 3-D photonic crystals based on covalently cross-linked thermoresponsive colloidal particles were reported by Hu et al. in 2008 (Figure 24).35

Figure 22. Illustration of stress induced elastic deformations of closepacked colloidal particles embedded in an elastic polymer matrix.

Due to the shrinking of the photonic crystal the center-tocenter distance of the particles decreases and the Bragg reflection wavelength shifts to shorter wavelengths with increasing temperature. The other two used polymer matrices, cross-linked PNIPAM and cross-linked poly(N,N-diethylacrylamide), are thermoresponsive matrices possessing LCSTs of 32 and 28 °C, respectively. Again, a blue shift of the Bragg reflection wavelength with increasing temperature was observed. However, the correlation of the diffraction wavelength and temperature shows now two distinct regions at which a pronounced shift of the wavelength occurs. The first region is located at the LCST of the colloidal particles and the second at the LCST of the matrix. Both contribute to the overall shift of the Bragg reflection wavelength of about 200 nm on heating. Hu et al. presented a polymer network consisting of covalently bond thermoresponsive hydrogel nanoparticles.34 At low temperatures the swollen hydrogel nanoparticles self-assemble into a close-packed CCA that reflects light according to the Bragg’s law. On heating above the LCST of the hydrogel the particle volume collapses and an order−disorder transition takes place. No Bragg reflection occurs in the disordered state. The

Figure 24. Center-to-center distance (D) changes of covalently crosslinked thermoresponsive colloidal particles by temperature.

On heating particle shrinkage occurs accompanied by volume shrinkage of the investigated microgels. The particles take the nonthermoresponsive particle-connecting-polymer-chains to move with them. Without losing the order of the CCA a reduction of the interparticle distance occurs leading to a shift of the Bragg reflection. Microgels based on poly(ethylene glycol) (PEG) derivatives were used to build up 3-D photonic crystals. As PNIPAM, these microgels possess LCST. Moreover, spherical monodisperse particles of these microgels can self-assemble into CCA. In the first step of the preparation outlined in Figure 25 a copolymer of poly(ethylene glycol) ethyl ether methacrylate, poly(ethylene glycol) methyl ether methacrylate, and poly(ethylene glycol) acrylate was synthesized by using the free radical polymerization method. The ratio of the first two components enables to tune the LCST while the third component provides the functional groups which enable the K

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Figure 25. Outline of the preparation of covalently cross-linked poly(ethylene glycol) microgel particles forming CCA.

extrusion and pressed against a flat plate the shell material fuses into a matrix. It embeds the shear assembled spheres of the core material which form a fcc structure with the (111) plane parallel to the surface. The described manufacturing method is industrially scalable. Sheets of 1 m × 100 m were already produced.36b Refractive indices of colloidal particles and matrix can be designed to be equal at room temperature resulting in a transparent material showing no Bragg reflection. This type of thermochromic polymer opal is denoted by the term balanced. The used matrix material exhibits a glass transition temperature (Tg) of 0 °C and the colloidal particles forming material of 110 °C. On heating the increase of the refractive index of the matrix material is much stronger than that of the colloidal particles and thus a refractive index contrast develops. Additionally a small isotropic expansion occurs. Accordingly, Bragg reflection appears on heating. Its intensity increases quadratically with temperature but without significantly changing its maximum wavelength (Figure 26).

attachment of vinyl groups in a second step. After attaching the vinyl groups, thermoresponsive particles with an average diameter of about 142 nm in dry state were obtained. In the third step the vinyl group attached microgels were mixed with a UV photoinitiator and either a poly(ethylene glycol) acrylate or an acrylamide monomer in aqueous solution. After the vinyl group attached microgels had formed a CCA, they were connected covalently to the surrounding poly(ethylene glycol) acrylate or poly(acrylamide) chains by a free radical polymerization. Bragg reflection in the visible range was observed for both prepared gel films. A decrease of the Bragg reflection wavelength with increasing temperature occurs within a temperature range of 20−42 °C for poly(acrylamide) chains or 21−55 °C for poly(ethylene glycol) acrylate chains, respectively. A CCA embedded in a thermoplastic matrix was reported by Baumberg et al.36a The 3-D photonic crystal was manufactured from monodisperse core−shell particles comprised of a PMMA core and a composite shell consisting of 70% poly(ethylacrylate) and 30% poly(benzylmethacrylate) (PBMA). Sheared by L

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Table 3. Overview of Silver and Gold Nanoparticles with Various Morphologies and Their Typical Ranges for the Surface-Plasmon Resonance Peak Position in the Visible Region40d metal silver silver silver silver gold/silver gold gold gold

Figure 26. (a) Photographs of a balanced thermochromic polymer opal at 25 °C and after heating to 100 °C. (b) Schematic drawing of the structural change of a balanced thermochromic polymer opal showing both refractive index changes and isotropic expansion on heating. Reprinted with permission from ref 36a. Copyright 2009 AIP Publishing LLC.

morphology spheres rods cubes plates alloyed spheres spheres rods shells with hollow interiors

typical ranges of absorption band position (λmax/nm) 400−430 435−750 405−530 570−730 405−515 485−515 515−750 515−750

Supposing a decrease of the interparticle distance of the nanoparticles, a coupling of their individual surface plasmon resonance effects occurs and a band splitting effect is observed. In case of longitudinal oscillations of the waves charge attraction forces lead to a shift of the plasmon resonance peak to longer wavelengths and in case of transverse oscillations repulsion forces lead to a shift to shorter wavelengths (Figure 27).

Varying the size of the colloidal particles enables to tune the wavelength of the Bragg reflection across the visible and near-IR region. The reported sample contains spheres with a diameter of 220 nm giving rise to Bragg reflection at 580 nm corresponding to a green color. Additionally, a nanoparticle-tuned polymer opal was reported.36 By incorporating only 0.05% w/w of carbon black nanoparticles into the balanced thermochromic polymer opal a dramatic enhancement of the perceived color was obtained. The incorporation of the nanoparticles was carried out by adding them to the precursor mixture previously to the extrusion process. During the extrusion the nanoparticles are distributed homogeneously within the interstitial voids of the colloidal particles without affecting the lattice quality. In addition to the enhancement of the perceived color intensity a widening of the viewing angle of this color beyond 40° was observed which is in contrast to the pronounced dependence of the perceived color on the viewing angle by Bragg reflection. Carbon black nanoparticles absorb and scatter light. The described optical effects were explained by color generation through spectrally resonant scattering inside the fcc lattice of the low-refractiveindex-contrast 3-D photonic crystal. An advantage over the 1-D photonic crystals is the use of a wider range of polymer matrices. Apart from PNIPAM, acrylate or glycol based polymers come into use facilitating a solvent-free thermoplastic processing.

Figure 27. Band splitting effect due to coupling of the surface plasmon resonance effects of the colloids.

In large nanoparticle aggregates multiple band splitting results in broad absorption peaks, which can cover the entire visible range. Temperature dependent changes of the surface plasmon resonance effect can be both, reversible or irreversible. They will occur, if shape, size, or the interparticle distance of the nanoparticles is influenced by temperature. Since the surface plasmon resonance wave is located at the boundary between the metal nanoparticles and the surrounding medium, it is sensitive to the refractive index of the surrounding medium, whose modulation by temperature can also result in thermochromism. An illustration of the designed mechanisms causing thermochromic effects of the surface plasmon resonance band of nanoparticle−polymer composite materials is displayed in Figure 28. Irreversible thermochromic effects will occur, if nanoparticles grow by disintegration of the smaller particles42 (mechanism I in Figure 28), if a particle agglomeration takes place43 (mechanism II in Figure 28), or the particle shape changes44 (mechanism III in Figure 28). The driving force for the first and the third effect is the reduction of the specific surface free energy of the nanoparticles by reducing their surface area. Irreversible agglomeration of nanoparticles is caused by the attractive forces

3. THERMOCHROMISM IN POLYMERS BASED ON NANOPARTICLES 3.1. Surface-Plasmon Resonance

Plasmons are quantized waves of mobile electrons oscillating around their equilibrium positions. They occur, e.g., in metals, where the electrons are collectively oscillating against the restoring forces of the nuclei. Plasmons at the surface of a metal are significantly lower in frequency than bulk plasmons. These so-called surface plasmons can interact with light when the frequency of the photons matches the surface plasmon resonance frequency. For certain metals, as gold and silver, the surfaceplasmon resonance takes place in the visible range. Individual nanoparticles possess a Gaussian shaped surface plasmon resonance band with a narrow bandwidth. Its position depends on size and shape of the particles40 as well as on the surrounding medium.41 As shown in Table 3 the surface-plasmon resonance peak can be tuned within the entire visible range by control of composition, morphology, and structure of the nanoparticles. M

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nanoparticles occurs. After a period of 15 h, the average diameter increases from 4.5 to 10 nm accompanied by a shift of the absorption maximum from 435 to 463 nm. Transmission electron microscopy (TEM) micrographs of the polymer composites revealed the partial aggregation of the silver nanoparticles. A uniaxial stretching of the sample with a draw ratio of 15 leads to a pearl-necklace type of arrangement for the spherical nanoparticles with high aspect ratios and by that to anisotropic optical properties. Under incident linearly polarized light the color of the anisotropic polymer composite changes depending on the polarization direction of the light with respect to the stretching direction of the polymer. In case of parallel polarized light, the film appeared red while a perpendicularly oriented polarization led to a yellow appearance. Likewise, an irreversible thermochromic effect occurs for the nonstretched polymer composite when the sample is annealed for 15 h at 180 °C. Despite the growth of the nanoparticles the polymer composite remains anisotropic. After annealing the polymer composite appears violet to purple depending on the polarization direction of the incident linear polarized light. 3.1.2. Particle Agglomeration. An irreversible agglomeration of previously well dispersed gold nanoparticles in polysiloxane elastomeric nanocomposites was reported by Caseri et al.43 These nanocomposites were manufactured by dispersion of gold nanoparticles in toluene containing poly(dimethylsiloxane). Prior to this, the gold was coated with 1dodecanethiol. The poly(dimethylsiloxane) is cross-linked via hydrosilylation. Evaporation of the toluene induces a color change. The red dispersion transforms into blue-grayish polymer films. In detail, 0.3−0.8 mm thick polysiloxane elastomeric films containing 0.5% w/w surface modified gold particles with an average diameter of 2−3 nm were prepared by the described procedure. These initially obtained blue-grayish films could be transformed into red films by swelling them with dichloromethane or toluene and subsequent fast evaporation of the solvent. If this is performed to slowly, a blue-grayish film will be obtained again. The red color indicates that the gold nanoparticles are well dispersed in the polymer matrix, while the bluegrayish color of the initial polymer films indicates the formation of gold nanoparticle agglomerates. As long as the 1dodecanethiol chains are attached to the gold nanoparticles, deagglomeration by swelling is possible. Once the polysiloxane elastomeric nanocomposite is dried, the gold nanoparticles are immobile and the film keeps its color until it is heated above 200 °C. At this temperature an irreversible thermochromic effect occurs. Desorption of the 1-dodecanethiol leads to a marked increase of the attractive forces between the gold nanoparticles and thus to the formation of gold nanoparticle agglomerates. Agglomeration is accompanied by a color change from red to blue. With rising temperature the desorption rate increases and the color change accelerates. It requires 10 min at 220 °C or 5 min at 240 °C. 3.1.3. Change of Particle Shape. An irreversible thermochromic effect based on thermal reshaping of nanoparticles embedded in polymer matrices was achieved by Composto et al.44a and Mecerreyes et al.44b The surface free energy of a nanoparticle depends on its morphology. For example, the specific surface free energy of rod-like particles decreases with a decreasing aspect ratio and reaches a minimum for spherical particles. Thus rod-like particles can reshape into spherical particles upon heating. A polymer composite film consisting of 5% v/v uniformly distributed surface modified gold nanorods in PMMA revealed

Figure 28. Illustration of the structural changes of nanoparticles leading to reversible or irreversible thermochromic effects. (I) Irreversible growth of particle size, (II) irreversible particle agglomeration, (III) irreversible change of particle shape, (IVa) reversible change of the interparticle distance through matrix expansion on heating or through an aggregation−disaggregation effect, (IVb) reversible change of the interparticle distance through matrix volume collapse on heating at its LCST, and (V) reversible increase of the refractive index of the surrounding matrix on heating.

between individual metal nanoparticles. Disintegration of smaller particles and particle agglomeration require a certain mobility of the nanoparticles within the matrix. Thermochromism occurs through triggering of the particle mobility in the polymer matrix by temperature. At low temperatures the particles are immobilized by the matrix while at high temperatures the required mobility is achieved. These thermochromic effects require generally high temperatures or long switching times. Reversible thermochromic effects are created by variation of the interparticle distance of the nanoparticles (mechanisms IVa and IVb in Figure 28) or by a temperature induced modulation of the refractive index of the surrounding matrix (mechanism V in Figure 28). Alteration of the metal nanoparticles interparticle distance is achieved by the following methods: (1) using a hydrogel with a matrix volume collapse at its LCST,45 (2) using polymer matrices exhibiting a glass transition with a distinct volume change,46 or (3) creating a reversible aggregation− disaggregation effect of the nanoparticles through the chain interaction of substituted metal nanoparticles.47 The volume phase change of a thermoresponsive hydrogel is accompanied by a distinct change of the refractive index. This refractive index modulation can be used to tune the surface plasmon resonance band of metal nanoparticles with temperature.48 Detailed descriptions of the thermochromic effects based on the plasmon resonance effect of metal nanoparticle−polymer-composite materials are given in the sections 3.1.1−3.1.5, which are classdivided according to the underlying mechanism (I−V in Figure 28). 3.1.1. Growth of Particle Size. An irreversible thermochromic effect based on the growth of primary nanoparticles embedded in a polymer matrix by disintegration of the smaller particles (Oswald ripening) was reported by Caseri et al.42 Silver nanoparticles coated with a dodecanethiol layer were dispersed in high density polyethylene. These coated silver nanoparticles had an average particle diameter of 4.5 nm. During annealing the polymer composite at 180 °C, irreversible growths of silver N

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After 5 min at a temperature of 160 °C the polymer film with gold nanorods changes its color from violet to blue and at an annealing temperature of 200 °C a pink color appears. Similar experiments with silver nanoparticles show an alteration of the purple color at room temperature to a red color at 130 °C, an orange color at 155 °C, and a yellow color at 190 °C. Investigations on the kinetics revealed that the silver nanorods are less stable in the prepared polymer film than the gold nanorods. At an annealing temperature of 100 °C spherical silver nanoparticles form within 5 h while gold nanorods still have an aspect ratio of 2 after 24 h. 3.1.4. Change of Interparticle Distance. The reversible volume change of thermoresponsive hydrogels was used by several authors to create a reversible thermo-optical effect based on changes of the interparticle distance of metal nanoparticles.45 Willner et al. prepared a gold nanoparticle PNIPAM composite film with a layer thickness of 0.4 mm.45a The spherical gold nanoparticles were introduced into the hydrogel matrix by a repeated “breathing-in” process. During the swelling in a gold nanoparticle suspension at 20 °C the polymer was charged with gold. A subsequent deswelling in water at 40 °C allowed several repetitions for further loading with nanoparticles. The prepared composite film is red in its swollen state at 20 °C and turns blue upon heating to 40 °C (shrunken state). A reduced interparticle distance transforms the plasmon resonance of individual particles to a coupled plasmon resonance and causes the reversible thermochromic effect. However, no spectroscopic measurements or photographs of the different colored states were reported. Kawaguchi et al. presented thermoresponsive microgel particles in which spherical metal nanoparticles of various structures were dispersed homogeneously.45b Preparation scheme and the thermoresponsive structural change of the investigated microgels are outlined in Figure 30. N-Isopropylacrylamide, glycidyl methacrylate, and N,N′methylene-bis-acrylamide copolymerized microgels were treated with 2-aminoethanethiol resulting in an amino functionalized thermoresponsive hydrogel. Gold nanoparticles were in situ synthesized within the microgel, followed by a one or two step seed mediated growth with gold or silver in order to produce (a) larger gold nanoparticles, (b) gold nanoparticles with a silver shell, or (c) gold nanoparticles with an intermediate silver shell and an outer gold shell. A dispersion of the prepared microgels in water shows temperature dependent reversible swelling/ deswelling within the temperature range of 25−40 °C. A temperature increase reduces the diameters of the microgels and shortens the interparticle distances of the specific embedded

several irreversible color changes above 100 °C.44a The nanorods averaged a length of 42.1 ± 3.7 nm and a diameter of 12.6 ± 1.0 nm giving it an aspect ratio of 3.3. They are modified with poly(ethylene glycol) brushes via terminal thiol groups. After a period of 8 days, the color changes reach different metastable states. A brown-red polymer film is obtained at an annealing temperature of 50 °C, a green film at 100 °C, a blue film at an temperature of 150 °C, and a pink-red film at an annealing temperature of 200 °C. Applying a temperature gradient produces a gradient of the aspect ratio of the nanoparticles as displayed by a color gradient of the film (Figure 29).

Figure 29. (a and b) Schematics showing how a polymer composite film with gradient optical properties can be fabricated. (c) Photograph of the fabricated polymer composite film. Reproduced from ref 44a by permission of The Royal Society of Chemistry. Copyright 2009.

Spectroscopic measurements revealed the longitudinal plasmon resonance peak of the metastable state blue-shifts linearly with increasing annealing temperature. Shortly above 200 °C the shift reached an equilibrium state. The origin of this irreversible thermochromic effect is a reduction of the aspect ratio of the gold nanorods until finally spherical nanoparticles are obtained. The same effect using not only gold but also silver nanorods was observed in poly(1-vinyl-3-ethylimidazolium)bis(trifluoromethanesulfonimide) films.44b On heating, these films show irreversible color changes due to the reduction of the aspect ratios of the incorporated metal nanorods. The morphological changes of the aspect ratio of 3.5 and the accompanying color changes occur continuously. With increasing temperature and proceeding time the aspect ratio of the nanoparticles decreases.

Figure 30. Preparation route and thermoresponsive swelling/deswelling mechanism of the investigated microgels containing gold, gold/silver or gold/ silver/gold nanoparticles. Reprinted with permission from ref 45b. Copyright 2006 American Chemical Society. O

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increasing their refractive index and lowering the interparticle distance of the nanorods at their surface (Figure 32). This effect is fully reversible. The original structure recovers on cooling.

metal nanoparticles (gold, gold−silver, or gold−silver-gold). Consequently, the band splitting effects of the plasmon resonance peaks strengthen leading to a broadening of the respective absorption peaks accompanied by slight shifts of the peak maxima to larger wavelength. UV/vis spectra measured at 25 and 40 °C as well as photographs of the investigated microgels at these temperatures are displayed in Figure 31.

Figure 32. Illustration of the structural change of thermoresponsive microgels bearing gold nanorods at their surface.

UV/vis spectroscopic measurements revealed two optical effects: (a) an increase of Rayleigh light scattering caused by the rising refractive index of the microgel particles during collapse and (b) changes of the surface plasmon resonance of the gold nanorods. The surface plasmon resonance effect of the gold nanorods consists of two well-separated bands. One band, located in the near IR range, belongs to the longitudinal and the other, located in the visible range, to the transversal electron oscillation. Both, the increasing refractive index of the microgel particles and the changes of the interparticle distance of the nanorods influence their surface plasmon resonance effect. The rising refractive index increases the absorbance which shifts slightly to longer wavelengths while the decreasing interparticle distance leads to less intensive but broader peaks, which are also shifted to longer wavelengths. At higher gold nanorod surface coverages the change of the interparticle distance on the surface plasmon resonance effect dominates. Especially for the longitudinal surface plasmon resonance peaks, wavelength shifts of about 30 nm were observed. A silver nanoparticle monolayer with a temperature tunable interparticle distance was presented by Lee et al.45d The temperature tunability of the interparticle distance was achieved by depositing the silver nanoparticles on the surface of a crosslinked PNIPAM hydrogel film. In detail 20 nm sized silver nanoparticles capped with oleylamine and oleic acid were spread on a water surface, where they formed a Langmuir−Blodgett film. The silver nanoparticle film was then “picked up” by an about 100 μm thick cross-linked PNIPAM hydrogel film on a glass substrate. On heating above 35 °C a distinct and discontinuous volume change of the cross-linked hydrogel film occurs. The hydrogel shrinks in all directions and thus reduces the distance between the silver nanoparticles on its surface. This effect is illustrated schematically in Figure 33. Absorption spectra recorded at various temperatures revealed a shift of the surface plasmon resonance peak from about 480 to 590 nm as the temperature increased from 20 to 40 °C. Thermoresponsive variations of the interparticle distance of metal nanoparticles by a glass transition of the used polymer matrices were described by Carotenuto et al.46 One of the reported systems consists of spherical silver nanoparticles embedded in polystyrene. In the first step of the preparation silver dodecyl mercaptide polymer films were prepared via casting a solution of the components in chloroform on a glass substrate at room temperature. These silver dodecyl mercaptide polymer blends were annealed to generate silver by a thermolysis of the mercaptide. Moderate thermolysis temperatures of about

Figure 31. Absorption spectra and photographs of the investigated microgels containing (a) gold, (b) gold/silver, and (c) gold/silver/gold nanoparticles. The spectra were measured at 25 °C (bold diamond), at 40 °C (gray square) and again at 25 °C after ten heating−cooling cycles (open diamond). Reprinted with permission from ref 45b. Copyright 2006 American Chemical Society.

The optical properties of the microgels depend strongly on the incorporated nanoparticles. On heating from 20 to 40 °C, the microgel with gold nanoparticles exhibits a shift of the absorption peak maximum from 537 to 545 nm which was accompanied by a color change from red to purple (Figure 31a). The absorption peak of the second microgel shifts from 424 to 442 nm changing its color from yellow to orange (Figure 31b). In the case of embedded gold/silver/gold nanoparticles, heating causes a shift from 712 to 784 nm accompanied by only very little changes of the perceived color (Figure 31c). In contrast to the other two investigated microgels, the thermoresponsive change of the absorption properties in the latter microgel is not fully reversible over ten heating−cooling cycles. This indicates a partially irreversible aggregation of the nanoparticles. If the size of the respective nanoparticles is increased over 35 nm, the same effect will also occur for the other microgels, Thermoresponsive microgels bearing gold nanorods at their surface were created by Hellweg et al.45c The used gold nanorods averaged a length of 57 nm and a diameter of 15 nm; thus, their average aspect ratio amounts to 3.8. They were surface modified by wrapping them with a poly(styrene sulfonate) and a poly(allylamine hydrochloride) layer. By this procedure a uniform positive surface charge is obtained enabling a fast and quantitative adsorption of the nanorods on the surface of the used PNIPAM microgel particles. Thermo-optical investigations were carried out on aqueous dispersions of five types of microgel particles with different gold nanorod surface coverages. The PNIPAM microgel particles collapse on heating from 15 to 50 °C P

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polycarbonate blend, and at 200 °C in the case of the poly(phenylene oxide) blend. A reversible thermochromic effect with silver nanoparticles not occurring at Tg of the polymer matrix was created by Carotenuto et al.47 The effect is based on a reversible aggregation of thiolatecapped spherical silver nanoparticles through interdigitation of their thiolate chains. Thiolate-capped spherical silver nanoparticles dispersed in amorphous polystyrene were prepared by dissolving the respective silver mercaptide and polystyrene in chloroform, followed by fast evaporation of the solvent and annealing of the silver thiolate/polymer blend at 200 °C. The fast evaporation of the solvent leads to strong concentration gradients within the silver thiolate/polymer blend. During annealing the silver thiolates decompose generating thiolatecapped silver nanoparticles. These show reversible temperature controlled aggregation−disaggregation behavior in the polymer matrix attended by a distinct color change. The appearing structural changes are shown schematically in Figure 35.

Figure 33. Illustration of the structural change of the silver nanoparticle monolayer due to thermoresponsive volume change of the polymer substrate layer.

150−250 °C enable the use of a wide range of commodity polymers. Quantity and size of the silver nanoparticles vary on the concentration of the added silver dodecyl mercaptide and on the conditions of the annealing process. Longer annealing times result in larger particles and a broader particle size distribution. Annealing of a 5−15% w/w silver dodecyl mercaptide polystyrene blend for 30 s at 200 °C yields in a thermochromic polymer material. Independently from the mercaptide concentration silver nanoparticle spheres of 2−3 nm are obtained. On heating, the polymer blends exhibit a color change from deepbrown to yellow. It occurs at Tg of polystyrene at about 80 °C. During the glass transition the polymer volume undergoes a distinct change. This matrix volume change triggers the interparticle distance of the silver nanoparticles and by this the intensity of band splitting of the surface plasmon resonance effect. Below Tg smaller interparticle distances cause a broadening of the absorption peak and the composite appears brown. Above Tg the absorption peak has the characteristic small bandwidth of isolated silver nanoparticles and the polymer blend appears yellow. This effect is also observed in the UV/vis spectra measured above and below Tg (Figure 34).

Figure 35. Illustration of the temperature controlled aggregation− disaggregation effect of thiolate-capped silver nanoparticles in amorphous polystyrene.

Below a certain temperature the silver nanoparticles form aggregates through interdigitation of their thiolate chains. In these aggregates strong band splitting effects of the plasmon resonance occur leading to a broad absorption band. The polymer composite appears dark-brown in this state. On heating disaggregation occurs and the thiolate-capped silver nanoparticles are isolated. Their higher interparticle distance results in a weak band splitting effect. The plasmon resonance peak has a narrow bandwidth and the polymer composite appears yellow. Alterations of the thiolate chains influence the stability of the aggregates. Therefore the thermochromic switching temperature can be tuned by variation of the chain length or by introducing polar groups into the alkyl chains to strengthen the chain−chain interaction. A certain chain length of the alkyl thiolate is necessary to obtain the thermochromic effect (Table 4). No aggregation is observed for a hexyl thiolate-capped silver nanoparticle/ polystyrene blend and thus no thermochromic effect either.

Figure 34. UV/vis absorption spectra of a silver/polystyrene nanocomposite film (obtained by annealing a 5% w/w silver dodecyl mercaptide containing polystyrene film at 200 °C for 30 s) at 25 °C (brown), and at 110 °C (yellow). Reprinted with permission from ref 46. Copyright 2005 Elsevier.

Table 4. Variation of the Thermochromic Switching Temperatures of Thiolate-Capped Silver Nanoparticle/ Polystyrene Blends by Alteration of the Used Silver Precursors thermochromic switching temperature (°C)

silver precursor

A wide range of polymer matrices as poly(vinylacetate), poly(methyl methacrylate), polycarbonate, and poly(phenylene oxide) gives similar results. Any time the nanoparticles polymer blends reach their Tg, a brown to yellow color change on heating occurs. Thus the color change appears at 50 °C in case of the poly(vinylacetate) blend, at 120 °C in the case of the poly(methyl methacrylate) blend, at 160 °C in the case of the

silver hexyl mercaptide silver dodecyl mercaptide silver hexadecyl mercaptide silver octadecyl mercaptide silver 11-hydroxyundecyl mercaptide Q

− 133 129 123 162

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3.2. Quantum Dots

Above the required minimum alkyl chain length only small temperature shifts are obtained by length variation. An increase of the chain length from 12 to 18 carbon atoms reduces the switching temperature by about 10 K. However, a strong increase of the switching temperature by about 30 K is achieved with the introduction of a hydroxyl group at the end of the alkyl chain opposing to the thiolate group. These results demonstrate that structural variations of the thiolates provide a suitable tool to tune the thermochromic switching temperature of thiolatecapped silver nanoparticle/polystyrene blends within wide ranges. 3.1.5. Refractive Index Modulation of the Surrounding Medium. A reversible shift of the surface plasmon resonance band in the visible range by modulating the refractive index of the surrounding medium was created by Mitsuishi and Miyashita et al.48 Negatively charged spherical gold nanoparticles with an average size of 30 nm were immobilized on a glass substrate coated with a positively charged polymer nanosheet consisting of two layers of a poly[N-(n-dodecyl)acrylamide-co-N-(2-(2-(2aminoethoxy)ethoxy)ethyl)acrylamide] copolymer [p(DDA/ DADOO)]. After removal of the polymer coating from the noncovered areas, PNIPAM brushes were grown on these areas. On heating above the LCST of PNIPAM, the length of the brushes decreases drastically attended by an increase of the local effective refractive index (Figure 36).

An optical transduction of the temperature modulated volume phase transition of PNIPAM at the LCST can also be realized by using quantum dots.49 Embedding CdTe quantum dots in PNIPAM leads to their immobilization and to a modulation of their interparticle distance by temperature.49a With increasing temperature a strong decrease of the photoluminescence intensity accompanied by a red-shift of the peak position was observed. Scattering of the hydrogel matrix above the LCST and a reduction of the photoluminescence efficiency upon heating were proposed as the origin of the obtained quenching effect. The red shift of 13 nm, caused by a temperature rise from 25 to 41 °C was explained by a transformation from a nonclose to a close-packed state of the quantum dots on heating. A quenching effect of the quantum dot luminescence intensity also occurs in the proximity of a gold surface. This is due to a nonradiative energy transfer from the quantum dots to the gold surface. The reported concept envisages a temperature induced variation of the distance between the gold surface and the quantum dots. This is accomplished with the help of PNIPAM brushes as temperature responsive spacer molecules (Figure 37).49b

Figure 37. Proposed structure and temperature dependent behavior of the Au/PNIPAM/quantum dots assembly. Reprinted with permission from ref 49b. Copyright 2009 IOP Publishing.

Figure 36. Illustration of the temperature dependent length change of the polymer brushes.

Heating and cooling of a prepared gold substrate, PNIPAM brushes, core/shell cadmium selenide/zinc sulfide quantum dots hybrid multilayer assembly in an aqueous medium proves the concept. Below the LCST the polymer brushes of PNIPAM are in the extended configuration (Figure 37) and an intensive green luminescence occurs (Figure 38). Above the LCST the PNIPAM

The modulation of the refractive index of the brushes is revealed by a shift and an intensity change of the surface plasmon resonance band of the gold nanoparticles. A brushes length of 40 nm in the collapsed state shows the largest sensitivity. Below the LCST the brushes stretch from 40 nm to about 130 nm. This leads to a shift of the surface plasmon resonance band from 546 nm at T > 32.5 °C to 540 nm at T < 31 °C accompanied by an intensity change from 0.291 ± 0.005 to 0.267 ± 0.005. In contrast to the systems discussed for photonic crystals, the matrices used for the surface-plasmon resonance effect show a wider range of structural different polymers. As long as one aims for an effect in the visible region, the limitation for these kinds of systems is the choice of the metal component since it is limited to gold and silver. Moreover, the thermochromic effect of these systems is often very small, shifting typically only a few nanometers.

Figure 38. Left: luminescence images of Au/PNIPAM/quantum dots assembly at 25 °C (1), after heating to 50 °C (2), and after cooling back to 25 °C (3). Right: spectra measured at below the LCST (1, 3) and above the LCST (2), showing the quenching and recovery behavior. Reprinted with permission from ref 49b. Copyright 2009 IOP Publishing. R

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chains collapse leading to a strongly reduced distance between quantum dots and gold surface and to a practically complete quenching of the luminescence. However, an exact explanation for the quenching effect cannot be given until now. Quantum dots offer the advantage of a small absorption bandwidth paired with large contrast as Figure 38 shows perfectly.

Figure 40. Dye aggregation−disaggregation of an aggregachromic dye triggered by temperature.

4. THERMOCHROMISM BY SELF-ASSEMBLING OF DYE−DYE AGGREGATES Cyano substituted oligo(p-phenylene vinylene) dyes (cyano OPV) and other so-called aggregachromic dyes can form H- or Jaggregates by π−π stacking interactions among their planar aromatic backbones. The dye aggregation is accompanied by pronounced changes of the absorption properties which have their origin either in charge transfer interactions within the dye aggregates or in conformational changes of the dye structure. The coupling of the transition dipole moments results for Haggregates (plane to plane parallel stacking) in hypsochromic and for J-aggregates (head to tail stacking) in bathochromic shifts of the absorption bands (Figure 39).

polymer53 or phase separation processes54 for morphologically controlled dye aggregation−disaggregation. Another design concept based on crystallization/melting processes of semicrystalline polymers was only applied to thermoresponsive fluorophoric polymer blends.55 These mechanisms will be described in detail in the following section. For the sake of completeness, it should be mentioned that thermochromic56 and thermoresponsive fluorophoric57 properties are also reported for polymers functionalized with a covalently bound aggregachromic dye. Those exhibit a LCST in aqueous medium. Below the LCST the polymers are soluble in water and their dye moieties are separated. Above the LCST the polymers form a coacervate phase and the dyes aggregate as a result of the reduced microenvironmental polarities. This solvent based aggregachromic effect will not be discussed in more detail. Polymer blends possessing kinetically controlled thermochromic effects can be fabricated by dissolving the respective dye in the melt of the polymer followed by rapid cooling into the glassy state. Low temperatures reduce the solubility of the dye thus if the concentration is high enough, phase separation and crystallization of the dye will be favored thermodynamically. However, the dye molecules are kinetically immobilized by the high viscosity of the glassy polymer matrix in their thermodynamically unstable monomeric form. Heating the polymer blends above the respective Tg reduces dramatically the viscosity and thus increases the mobility of the dye, which starts to build irreversibly a separate phase and to form nanometer sized aggregates accompanied by a change of the perceived color. In this temperature region the polymer blends act as time−temperature indicators.58 A higher temperature reduces the switching time of the thermochromic effect following Arrhenius behavior (Figure 41), until above an upper temperature limit the thermochromic effect disappears. This temperature limit results from the increasing solubility of the dye in the polymer matrix on heating. Accordingly, the aggregation− disaggregation equilibrium of the dye shifts toward the disaggregated monomeric form. Finally, far above Tg the dye is completely solved in the polymer matrix and no dye aggregates are formed anymore. Weder et al. designed a series of thermochromic cyano OPV polymer blends with various switching temperatures.52 Many cyano OPV dyes tend to form aggregates and possess a good thermal stability. The molecular structures of the investigated cyano OPV and perylene dyes as well as the Tg of the used polymer materials are displayed in Figure 42 and Table 5, respectively. One of the detailed described systems is a 1.1% w/w Cyano OPV I/poly(ethylene terephthalate glycol) blend.52a Polymer dye blends were manufactured by melt-mixing using a corotating twin-screw extruder and then formed to films by compression molding. Freshly quenched films are yellow colored indicating the monomeric dye form. Annealing at temperatures above Tg (78 °C) leads to an irreversible color change from yellow to orange (Figure 43).

Figure 39. Transition dipole moment interactions in H- and Jaggregates and the resulting shifts of the absorption bands. Adapted with permission from ref 50. Copyright 2008 American Chemical Society.

Three criteria are pointed out to guide the design of aggregachromic dyes:51 (1) a rigid and rod-shaped core consisting of highly conjugated and aromatic nuclei, which promote π−π stacking interaction between dye molecules; (2) electron-withdrawing and/or electron-donating substituents conjugated to the aromatic core and thus part of the chromophoric system and capable to modulate the optical properties of the dye; and (3) absence of bulky substituents, which would cause a steric hindrance of dyes self-assembly. Thermochromism will occur if the dye and/or the dye aggregates absorb in the visible range and a temperature dependent dye aggregation−disaggregation behavior arises (Figure 40). Creative use of viscosity or morphological changes of polymers for triggering the aggregation or disaggregation of dyes have led to different concepts for thermochromic polymer blends based on aggregachromic dyes. The fundamental idea is to utilize viscosity changes at Tg for a kinetically controlled dye aggregation−disaggregation52 and structural changes of the S

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Figure 44a, b. The ratio between the absorption of the dispersed dye (AD) and the absorption of the aggregated dye (AA) was found to follow a single exponential function (eq 6).

AD A = D ∞ + Ce−t / τ AA AA ∞

(6)

AD∞ and AA∞ denote the absorbance of dispersed and aggregated dye form after annealing to equilibrium. τ is the aggregation rate constant and C is a constant representing the magnitude of change in AD/AA. These measurements were iterated at various annealing temperatures for both blends. Plotting the aggregation rate constants τ logarithmically against the reciprocal temperature (1/T) reveals a linear Arrhenius type behavior for both dye polymer blends (Figure 44c). A comparison of the two investigated dyes shows, that Cyano OPV II aggregates more slowly than Cyano OPV I under the same conditions. This is in agreement with the expectation that an expansion of the rigid core of the dye reduces its mobility in the polymer matrix. While Cyano OPV I has a rigid core length of 16 Å, the core length of Cyano OPV II is increased to 29 Å. Studying the aggregation kinetics in dependence of the dye concentration reveals a logarithmic relationship between the dye concentration and the aggregation rate constants τ (Figure 44d). Thus tailoring of the dye structure and its concentration in the polymer provide tools to control the time−temperature relation of the thermochromic effect. Although the aggregation rate constants of both Cyano OPV/poly(ethylene terephthalate glycol) blends differ under the same conditions, they have the same aggregation rates but at different temperatures (Figure 44e). The Cyano OPV I blend shows at 95 °C the same relative color change as the Cyano OPV II blend at 110 °C and at 100 °C the same as the Cyano OPV II blend at 130 °C. Another parameter characterizing the thermochromic behavior of these types of systems is the threshold temperature at which the dye aggregation starts. This temperature should depend on Tg of the polymer dye blend. To prove this concept 2% w/w Cyano OPV I were blended with poly(methylmethacrylate), poly(butylmethacrylate), and two poly(methylmethacrylate-co-butylmethacrylate) copolymers. Tg of these polymers spread between 13 and 108 °C (Table 5). Blending these polymer matrices with 2% w/w Cyano OPV I leads to small shifts of Tg, which appear at 23, 42, 56, and 104 °C, respectively. The findings of the kinetic investigations are in consistence with the expected relationship between Tg and the threshold temperature for the described thermochromic effect. Selecting the polymer matrix thus enables a target-oriented tuning of the threshold temperature. A further extension of the threshold temperature range was obtained by using poly(ethylene-co-norbornene) copolymers with Tg of 131−149 °C (Table 5).52d Cyano OPV II blends of these two copolymers are suitable for operating as time− temperature indicators within the temperature range of 130−200 °C. Another design concept for thermochromism by aggregachromic dyes embedded in polymer matrices was created by Pucci et al.53 The extent of dye aggregation in the polymer matrix was modulated by temperature, either through changes of the polar and protic properties of the matrix polymer (Figure 45 top), or through changes of the dye solubility (Figure 45 bottom). In detail, the water-soluble perylene diimide PZPER (Figure 42) was embedded via solvent casting into poly(vinyl alcohol) (PVA) and poly(ethylene-co-vinyl alcohol) copolymers with

Figure 41. Mechanism of the irreversible thermochromic effect on heating above Tg of the polymer dye blend.

The switching times of the color change depend strongly on temperature. At an annealing temperature of 90 °C it takes hours to produce the orange color while annealing at 120 °C generates it in less than 5 min. Similar results were obtained for a 0.9% w/w Cyano OPV I/poly(ethylene terephthalate) blend.52a,b Poly(ethylene terephthalate) is a semicrystalline polymer with a Tg of likewise 78 °C. The thermochromic effect is the same than outlined above for the Cyano OPV I/poly(ethylene terephthalate glycol) system. Systematic studies of the aggregation kinetics of 1% w/w Cyano OPV I/poly(ethylene terephthalate glycol) and Cyano OPV II/poly(ethylene terephthalate glycol) blends were carried out to elucidate the time−temperature relation of the thermochromic effect (Figure 44).52c For both blends the aggregation rate constants were determined by measuring absorption spectra at a constant annealing temperature in correlation with the annealing time. The obtained data for the Cyano OPV I blend are shown in T

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Figure 42. Molecular structures of 1,4-bis(α-cyano-4-octadecyloxystyryl)-2,5-dimethoxybenzene (Cyano OPV I),52a−c,59 2-(4-[2-(4-ethoxy-phenyl)vinyl]-phenyl)-3-(4-(2-(4-[2-(4-ethoxy-phenyl)-vinyl]-phenyl)-2-isocyano-vinyl)-2,5-bis-octyloxy-phenyl]-acrylonitrile (Cyano OPV II),52a,c,d N,N′bis(2-(1-piperazino)ethyl]-3,4,9,10-perylenetetracarboxylic acid diimide dichloride (PZPER),53 and N,N′-bis-(R)-(1-phenylethyl)-perylene-3,4,9,10tetracarboxyldiimide (R-Pery)54 used for the preparation of thermochromic polymer blends.

Table 5. List of Tg for Various Polymer Materials Used in the Preparation of Thermochromic Cyano OPV Polymer Blends polymer material poly(ethylene terephthalate glycol) poly(ethylene terephthalate) poly(methyl methacrylate) poly[(methylmethacrylate)0.417-co(butylmethacrylate)0.583] MW = 100 000 poly(methylmethacrylate-cobutylmethacrylate) MW = 150 000 poly(butyl methacrylate) poly(ethylene-co-norbornene) TOPAS 5013 poly(ethylene-co-norbornene) TOPAS 6015

glass transition temperature (°C)

ref

78 78 108 58

52a 52a,b 52c 52c

41

52c

13 131 149

52c 52d 52d

environment. In the absorption spectra of the investigated PZPER polymer blends two peaks occur. One is located at 540 nm corresponding to the monomeric dye and one at 500 nm corresponding to the aggregated dye. Even in the copolymer with the lowest vinyl alcohol content dye aggregates were observed. Increasing the temperature leads to an increased solubility of the dye in the matrix polymers. A continuous change of the polar and protic properties of the matrix polymers is assumed by the authors as the origin of the increased dye solubility, by which the dye aggregation−disaggregation equilibrium is shifted toward the disaggregated monomeric form. Thus the ratio between the absorbance at 540 nm and that at 500 nm increased with temperature. A similar behavior was observed for R-Pery (Figure 42) in linear low-density polyethylene.54 The blends were prepared by melt-mixing. R-Pery exhibits two absorption peaks located at 480 and 520 nm from its monomeric form and an unstructured band centered at 549 nm from its aggregated form. At a concentration of 0.1% w/w dye in linear low-density polyethylene all three absorption peaks were detected clearly by spectroscopic measurements at room temperature. With increasing temperature the solubility of the dye in the polymer matrix increases. Hence, the absorption changes by temperature displayed a continuous shift toward the disaggregated dye form on heating. This effect is fully reversible. On cooling to room temperature the original state recovers. The same concept was proved to be applicable for the manufacturing of thermoresponsive fluorophoric polymer blends.52−54,60 These blend systems consist of a fluorescence dye which can form aggregates concomitant with changes of the emitted light color under UV light exposure. One of the investigated dye classes was cyano OPV́ s. These dyes can exhibit wavelength shifts of the emitted light up to 140 nm. Indeed, examples of this type of polymer dye blends were the first systems investigated to prove the underlying idea of kinetically immobilizing the dyes in their thermodynamically unstable monomeric form by quenching the blends below Tg.60 1,4-Bis(αcyano-4-methoxystyryl)benzene (Cyano OPV III, Figure 46)

Figure 43. Picture of 1.1% w/w Cyano OPV I/poly(ethylene terephthalate glycol) blend after annealing as indicated. Reprinted with permission from ref 52a. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA.

molar ethylene fractions of 0.27 and 0.44, respectively. PZPER forms H-type aggregates via π−π stacking interactions between perylene nuclei and via hydrogen bonding between the protonated piperazine sites. The strength of the hydrogen bonds depends on the polar and protic properties of the dye U

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Figure 44. (a) Absorption spectra of an initially quenched 1% w/w Cyano OPV I/poly(ethylene terephthalate glycol) blend as a function of annealing time at 100 °C. (b) Color change (expressed as AD/AA) extracted from the absorption spectra shown in (a) as a function of time (○). Also shown are data obtained in a similar manner at 95 °C (■), 105 °C (▲), 110 °C (◇), and 115 °C (●). Lines represent least-squares fits according to eq 6. (c) Plot of the natural logarithm of the aggregation rate constants τ (min) of 1% w/w Cyano OPV II/poly(ethylene terephthalate glycol) (■) and Cyano OPV I/ poly(ethylene terephthalate glycol) (○) blends against 1/T. (d) Plot of the natural logarithm of the aggregation rate constants τ (min) of Cyano OPV II/poly(ethylene terephthalate glycol) (■) and Cyano OPV I/poly(ethylene terephthalate glycol) (○) blends annealed at 100 °C against the natural logarithm of concentration (% w/w). (e) Relative color change extracted from absorption spectra as a function of time for initially quenched 1% w/w Cyano OPV I/poly(ethylene terephthalate glycol) blends (○) annealed at 95 °C (dashed) and 100 °C (solid) or 1% w/w Cyano OPV II/poly(ethylene terephthalate glycol) blends (■) annealed at 110 °C (dashed) and 130 °C (solid). Reprinted with permission from ref 52c. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

ID I = D ∞ + Ce−t / τ IA IA ∞

was melt-blended with poly(methyl methacrylate) or poly(bisphenol A carbonate) and afterward compression molded to films of the polymer dye blends. For example, a 10% w/w Cyano OPV III/poly(bisphenol A carbonate) blend was fabricated. During annealing at 150 °C over a period of 42 h, the emitted color changed from blue to green upon excitation with UV light. Spectroscopic investigations of the thermoresponsive fluorophoric effect can also be used to determine the aggregation rate constant τ of the dye (eq 7).

(7)

In eq 7 ID and IA denote the fluorescence intensity of the dispersed and aggregated dye form and ID∞ and IA∞ the respective fluorescence intensities after annealing to equilibrium. Using Cyano OPV I or Cyano OPV II, both, thermochromic and thermoresponsive fluorophoric effects were observed in the polymer blends.52a−c Under UV light exposure Cyano OPV I emits green in its molecular form and orange-red in its aggregated form. Cyano OPV II emits yellow-green in its molecular form and orange in its aggregated form. Monitoring V

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Figure 45. Schematic representation of the thermochromic effects triggered by either a structural change of the matrix or by a change of the dye solubility.

w BBS (highest concentration allowing dispersion of the dye in its monomeric form) in this polymer blend by melt-mixing and subsequent rapid cooling to room temperature result in a composite exhibiting a blue fluorescence under excitation by UV light (366 nm). In comparison to the BBS-PBS blend system the optical stability at room temperature is increased strongly. When annealed at temperatures above Tg of 61 °C, a blue to green color change of the emitted light occurs. Independently from the temperature the dye aggregation reaches a plateau after 8 min within the investigated range of 60−100 °C. These aggregates keep stable until further warming to 140 °C. At this temperature disaggregation and the color change back to blue emission occurs. Rapid cooling of the sample to room temperature preserved the dye in its monomeric form and regenerates the threshold temperature indicator. Differential scanning calorimetric investigations on the BBS-PLA and BBS-PLA-PBS blend systems revealed morphological changes of the polymer matrix as the origin of the observed thermochromic effects (Figure 47). Cold crystallization of PBS forms crystallization nuclei, which induce crystallization of PLA above its Tg. This results in an increase of the crystallinity degree by 23%, starting from 5% in pure PLA to 28% in the PLA-PBS blend leading to higher BBS dye concentrations in the amorphous regions of the semicrystalline polymer matrix and triggers the dye aggregation. At about 140 °C the crystalline PLA regions melt and the dye disaggregates. The thermoresponsive fluorophoric properties of BBS poly(ethylene-co-norbornene) copolymers with a norbornene molar fraction of 15.3 mol % were also investigated.61b Polymer blends with a BBS concentration up to 0.1% w/w were prepared by melt-mixing and rapid quenching of the melt at 0 °C. By this procedure the dye was trapped kinetically in its monomeric form. Even after 7 months no dye aggregation occurred at room temperature. Tg of the copolymer matrix is 64 °C. Heating above this threshold temperature leads to the irreversible formation of H-type dye aggregates as illustrated by fluorescence emission spectra (Figure 48a). The emission color changed upon UV light excitation from blue to green displaying the dye aggregation effect (Figure 48b). The aggregation rate

Figure 46. Molecular structures of 1,4-bis(α-cyano-4-methoxystyryl)benzene (Cyano OPV III) and 4,4′-bis(2-benzoxazolyl)stilbene (BBS) used for the preparation of thermoresponsive fluorophoric polymer blends.

the kinetics of thermochromic and thermoresponsive fluorophoric color changes proved that the underlying mechanism of both effects is identical. Examples of thermoresponsive fluorophoric polymer blends based on the excimer forming stilbene dye BBS (Figure 46) were reported by Pucci et al.55,61 Contrary to the cyano OPV dyes, BBS complies with the FDA regulations. It is highly temperature stable (degradation temperature 380 °C) and thus processable by extrusion technology to melt-mix the dye with commodity polymers. Incorporating 0.05% w/w BBS in poly(1,4-butylene succinate) (PBS) at 200 °C followed by rapid quenching at 0 °C avoids the formation of excimers in the polymer blend.61a At room temperature, roughly 60 °C above the Tg of the polymer matrix (Tg = −34 °C), no excimer formation was detected until 1 month after preparation. Annealing the polymer blend at 65 °C leads to a color change of the fluorescence emission from blue (molecular state) to green (aggregated state) within 16 h. Similar investigations of BBS-poly(lactic acid) (PLA) blends revealed no thermoresponsive fluorophoric behavior.55 However, by using a polymer blend containing 85% w/w PLA and 15% w/w PBS clear changes of the emission properties with temperature were found.55 Both used polymers are biodegradable. PBS has plasticizing activity onto PLA and improves its ductility. The resulting matrix system possesses two Tg located at −33 and +61 °C indicating thereby that the used polymer components are immiscible for thermodynamic reasons. Incorporating 0.07% w/ W

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Figure 47. (a) Differential scanning calorimetric scan and photographs of the sample under irradiation at 366 nm before (left) and after (right) thermal annealing at 70 °C. Adapted from ref 55 by permission of The Royal Society of Chemistry. Copyright 2010. (b) Schematic representation of the thermochromic effects triggered by morphological changes of the matrix polymer blend consisting of 85% w/w PLA and 15% w/w PBS.

increases with temperature (64 °C → 74 °C) and dye concentration (0.03 → 0.1% w/w). The perylene diimide dyes PZPER and R-Pery change not only their absorption behavior but also their fluorophoric properties upon aggregation.53,54 Thus the temperature modulated aggregation−disaggregation of PZPER in PVA and poly(ethylene-co-vinyl alcohol) copolymers as well as of R-Pery in linear low-density polyethylene described above cause also thermoresponsive shifts of the fluorescence spectra. With rising temperature a continuous increase of the ratio between the light emission intensity of the monomeric form (560 nm for PZPER, 525 and 565 nm for R-Pery) and that of the corresponding aggregated form (600 nm for PZPER and 620 nm for R-Pery) occurs. In addition to their thermochromic and thermoresponsive fluorophoric properties, this class of materials has found considerable interest due to its ability to create mechanochromic dye polymer blends.51,62 A shear induced disaggregation of dye aggregates takes place in mechanochromic dye polymer blends concomitant with a change of the absorption and/or fluorescence properties. Uniaxial drawing unfolds the macro-

molecular chains, destroys the dye aggregates, and orients the single dye molecules along the drawing direction. Incorporating cyano OPV dyes in shape memory polymers enables to fabricate polymer blends which shape memory effects are accompanied by color changes.59 Only considering the immense range of usable dyes the dye−dye aggregate approach holds great potential for new thermochromic systems.63

5. THERMOCHROMISM BY POLYMER−DYE INTERACTION 5.1. Proton Equilibrium in Hydrogels

pH-Indicator dyes change their color in aqueous solution in response to alteration of the pH-value which triggers the equilibrium between the protonated and the deprotonated form of the dye. Certain polymer matrices such as hydrogels are known to vary their protic properties on temperature change. Thus the incorporation of pH-indicator dyes in these polymer matrices should result in the occurrence of thermochromism. Based on this concept thermochromic hydrogels were created by Seeboth et al.6d,64 A physically cross-linked hydrogel network X

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propanesulfonate for cresol red. In both hydrogel/indicator dye systems a continuous shift of the phenol−phenolate-equilibrium toward the phenolate form was observed on heating. The hydrogel with incorporated DTPP is colorless at 10 °C and turns more and more violet on heating to 80 °C. Similarly, the hydrogel containing the cresol red changes gradually its color from yellow to wine-red on heating from 16 to 80 °C. In both cases the thermochromic effects were found to be fully reversible. Notice that the formation of a gel network is essential for the observed thermochromism. Neither aqueous solutions of the indicator dye, nor solutions with aqueous surfactants or solutions in aqueous PVA showed any temperature dependent color change. Thus clearly synergetic interactions between the indicator dye and the PVA-borax hydrogel matrix are the origin of the appearing thermochromic effect. To quantify the thermochromic effect temperature dependent spectroscopic measurements were carried out in the visible range. As an example the results for the cresol red containing hydrogel are displayed in Figure 50.

Figure 48. (a) UV/vis emission (λexc = 270 nm) spectra of a BBS poly(ethylene-co-norbornene) copolymer (0.05% w/w BBS, 15.3 mol % norbornene) after different annealing durations at 67 °C. (b) Picture of the same film after different annealing durations at 74 °C taken under irradiation at 366 nm. Reprinted with permission from ref 61b. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

consisting of PVA and borax produces a clear and colorless hydrogel. The temperature dependency of the phenol− phenolate equilibrium of the Reichardt betaine dye65 2,6diphenyl-4-2,4,6-(triphenyl-1-pyridinio)-phenolate (DTPP) and of cresol red were investigated in the described hydrogel (Figure 49).64a The acidic DTPP is colorless and deprotonation results in the violet phenolate form while basification of the acidic cresol red turns the color from yellow to wine-red. For an increase of the dye solubility in the hydrogel matrix a dye specific surfactant was added. N,N-dimethyl-N-tetradecylammonioacetic acid bromide was used for DTPP and 3-(N,N-dimethyl-N-dodecylammonio)

Figure 50. Photographs and absorption spectra of a PVA-borax-cresol red-3-(N,N-dimethyl-N-dodecylammonio) propanesulfonate hydrogel network at various temperatures.

The spectrum revealed two absorption peaks: one located at 419 nm which corresponds to the yellow phenol form and a second one at 581 nm which corresponds to the wine-red phenolate form of cresol red. A rise in temperature lowers the absorption intensity at 419 nm and increases the absorption intensity at 581 nm simultaneously. All spectra meet at an isosbestic point located at 486 nm. The observation of an isosbestic point indicates that both peaks belong to the same dye and thus support the proposed mechanism. Further investigations on the PVA-borax-indicator dye system showed that thermochromism occurs only for incorporated indicator dyes with a pKa value between 7.0−9.4 which dissolve in the hydrogel matrix. Phenol red,64b thymol blue,6d phenolphthalein, and bromothymol blue66 all fulfill these criteria. With these indicator dyes yellow to purple (phenol red), yellow to green (thymol blue), colorless to red (phenolphthalein), and green to blue (bromothymol blue) color changes were realized on heating. In case of the last two indicator dyes no surfactant but n-butanol was used as further additive for the preparation of the thermochromic PVA-borax hydrogels. n-Butanol is known to act in combination with PVA as a cosurfactant while PVA itself already exhibits surfactant properties in aqueous solution. As shown by these examples the PVA/n-butanol (surfactant/cosurfactant) mixture can make a further surfactant unnecessary. The wide pKa working

Figure 49. Temperature dependent phenol−phenolate equilibria of DTPP and of cresol red. Y

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glycol units on average (TX-100)].69 While the anionic surfactant SDS decreases the absorbance with increasing concentration, the addition of the cationic surfactant CTAB was found to increase the absorbance. The nonionic surfactant TX-100 makes the absorbance increase at lower concentrations and decrease strongly on higher concentrations. A polyether cross-linked by LiCl70 and Nafion71 were likewise used successfully for the creation of thermochromism by incorporating pH-indicator dyes. The used polyether with a molecular weight of 4800 consists of 87% w/w propylene oxide and 13% w/w ethylene oxide.70 Adding 0.8 g of a dye specific buffer solution, 0.25 g of LiCl, and 0.12 g of a 2.2% w/w aqueous solution of an indicator dye (bromothymol blue, chlorophenol red, or nitrazine yellow) to 3.95 g of the polyether result in the formation of thermochromic hydrogels. A green to yellow (bromothymol blue), a red to yellow (chlorophenol red), and a blue to green (nitrazine yellow) color change on heating was obtained for these thermochromic hydrogels. In contrast to the PVA-borax-indicator dye hydrogels the phenol−phenolate equilibrium is shifted toward the phenol form on heating. In addition to the thermochromic effect, all three investigated polyether based hydrogels exhibit also thermotropic properties. Above the LCST a separation of a water phase occurs. For the bromothymol blue containing hydrogel a LCST of about 33 °C was reported. On heating from 33 to 47 °C the normal−normal transmittance was found to decrease from about 88% to near zero. Nafion, whose structure is shown in Figure 51 is a perfluorosulfonate ion exchange polymer.

range of the PVA-borax hydrogel matrix enables to produce a more complex color behavior by a combined use of two indicator dyes. For example the thermochromic behavior of a bromothymol blue and cresol red containing PVA-borax hydrogel was reported.67 Within the temperature range of 0− 80 °C three different colors occur. Starting from yellow (0−5 °C) the color of the hydrogel turns to green (15−25 °C) and on further heating to violet (60−80 °C). The influences of blending with other polymers64b,68 and of various surfactants and their concentration64b,68,69 were investigated in further studies. Inducing thermotropism into a thermochromic PVA-boraxphenol red hydrogel was achieved by adding a polyether.64b Already at polyether concentrations of 0.8% w/w a LCST of about 35 °C was observed, above which separation of a water phase occurred. With increasing polyether content a decrease of the LCST and an increase of the slope of the transparency change with temperature took place. At the highest investigated concentration of 1.5% w/w polyether the LCST appeared at 18 °C and the normal−normal transmittance dropped down from about 90% to about 20% within 10 K. Moreover, the influence of the zwitterionic sulfobetain surfactant 3-(N,Ndimethyl-N-dodecylammonio)propanesulfonate on the thermotropic and thermochromic properties was investigated. Above the critical micelle concentration (CMC) the surfactant was expected to dissolve both, the indicator dye and the polyether. This should influence the thermochromic and also the thermotropic properties of the PVA-polyether-borax-phenol red hydrogels. Addition of the sulfobetaine to a PVA-boraxphenol red hydrogel containing 1.1% w/w polyether reduces the absorbance of both dye forms (phenol and phenolate) and increases the LCST. Both effects intensify with increasing sulfobetaine concentration. Unexpectedly, these effects were already observed at surfactant concentrations below the CMC. Blending of a thermochromic PVA-borax-indicator dye hydrogel with agar, poly(ethylene oxide), gelatin type A (acid processed), or poly(vinyl pyrrolidone) as well as the effect of the anionic surfactant sodiumdodecyl sulfate (SDS) on the thermochromic properties were investigated recently.68 In this study phenolphthalein was used as indicator dye. The prepared thermochromic hydrogel consists of 2.5% w/w PVA, 0.075% w/ w borax and 0.008% w/w phenolphthalein. On heating from 20 to 85 °C the colorless hydrogel becomes more and more pink until maximum color intensity is reached at 70 °C. Further heating results in a color fading and above 80 °C the sample appears colorless again. Blending this hydrogel with agar (0.05% w/w) or poly(ethylene oxide) (0.5% w/w) was found to enhance the gel strength of the PVA-borax hydrogel and to increase the absorbance of the colored state while addition of gelatin type A (0.5% w/w) or poly(vinyl pyrrolidone) (0.5% w/w) had the opposite effect. However, no thermochromism is observed in any case the PVA concentration in the blends is less than 60% respective to the overall polymer mass. To study the effect of SDS on the thermochromic properties 2 mM and 12 mM SDS were incorporated into the thermochromic hydrogel. With increasing SDS concentration an increase of the absorbance was observed starting from about 0.26 with no SDS to about 0.44 at a SDS concentration of 12 mM. A complex influence of the surfactant nature on the thermochromic properties of a PVA-borax-phenolphthalein hydrogel was found by adding an anionic (SDS), a cationic [cetyl trimethyl ammonium bromide (CTAB)], or a nonionic surfactant polyethylene [a glycol tert-octylphenyl ether with 9.5

Figure 51. Idealized molecular structure of Nafion.

Preparation and thermochromic properties of thin films of Nafion−indicator dye composites were reported by Baron et al.71 They were casted from a solution of low alcohols and water. Thermochromic properties were obtained for Nafion composites with safranin-O, phenolphthalein or methylene blue. In case of the phenolphthalein composite, which shows the most pronounced effect, a colorless to intense pink color change occurs on rising the temperature from 20 to 70 °C. Similar to the PVA-borax hydrogel matrix, Nafion shifts the phenol−phenolate equilibrium of phenolphthalein toward its deprotonated form during heating. Absorption measurements visualize this behavior. At about 25 °C an absorption band appears at a wavelength of about 502 nm and increases on further heating to 70 °C. The same idea was also used to prepare a thermoresponsive fluorophoric hydrogel.72 2-Naphthol, which is a well-known pHsensitive fluorescence indicator, was incorporated into a PVAborax hydrogel. Upon excitation with UV light (λ = 365 nm) the phenolate form of 2-naphthol emits blue light while the phenol form does not emit any visible light. With increasing temperature the phenol−phenolate equilibrium of 2-naphthol is shifted toward the phenolate form by the interaction with the PVA-borax hydrogel matrix. Thus heating from 30 to 80 °C leads to more Z

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than a doubling of the intensity of the emitted blue light (λ = 426 nm). The proton-equilibrium concept in hydrogels is currently receiving increased attention again. According to the variety of indicator dyes and the now broader knowledge of water-based polymer networks, much prospective potential inheres in these systems. Still, the reproducible production of hydrogels and their characterization remains challenging. But new biocompatible or self-healing biological systems give plenty of room for applications.

Figure 53. Schematic representation of the polymorphy and color changes of a thermochromic PLA/cyanidin chloride/dodecylgallate/ hexadecanoic acid composite.

5.2. Polymer−Dye Complex Formation

Anthocyanidin dyes are known to change the structure of their chromophoric moiety and thus their color in response to variations of the pH value. Formation of chelate complexes, selfassociation, copigmentation, or intramolecular sandwich-type stacking can also initiate a color change in anthocyanidin dyes. This wide range of color influencing mechanisms makes anthocyanidin dyes promising for the creation of thermochromic effects. A further motivation to use an anthocyanidin dye (E163) is that they are practically nontoxic and are approved food additives in Europe. Very recently the design of a nontoxic thermochromic polymer material representing the first example of thermochromism by anthocyanidin dyes was reported by Seeboth et al.73 This thermochromic polymer material consists of poly(lactic acid) (PLA), the anthocyanidin dye cyanidin chloride (E163), dodecylgallate (E312), and the fatty acid hexadecanoic acid (E570). All used additives are approved food additives in Europe. The cyanidin chloride dye is found to be either present in its red colored neutral or in its violet colored anionic anhydrobase form (Figure 52). A change between both is accomplished by alteration of the temperature.

thermochromism in the investigated polymer-composite material. At concentrations below the solubility limit the hexadecanoic acid is completely dissolved. No rigid amorphous phase is formed and no thermochromism appears. Above the solubility limit separate hexadecanoic acid rich domains are present as proved by scanning electron microscopy. This domains function as nucleating agent. They induce a transformation from the mobile into the rigid amorphous phase. Accompanied with this change of the polymorphy thermochromism appears. A reversible polymer−dye complex formation triggered by structural changes of the matrix polymer during the transformation from the mobile into the rigid amorphous phase was proposed as origin of the thermochromic effect. In the polymer melt and in the mobile amorphous phase polymer−dye complexes are expected to be formed via multiple H-bonds which stabilize the anionic anhydrobase form of cyanidin chloride. Conformational changes of the polymer backbone during the transformation into the rigid amorphous phase destabilize the polymer−dye complexes leading to disaggregation and to a change of the dye structure into the neutral anhydrobase form (Figure 54).

Figure 54. Scheme of the thermochromic effect and the proposed mechanism. Reproduced from ref 74. Copyright 2013 Scientific Research Publishing Inc.

In a further study variations of the anthocyanidin dye structure were carried out by Lötzsch et al.74 Cyanidin chloride was substituted by either pelargonidin chloride or delphinidin chloride to provide a homologous series with one to three hydroxyl groups on the B-ring of the anthocyanidin dyes (Figure 55). Since the delphinidin chloride also exhibits thermochromism in the PLA/dodecylgallate/hexadecanoic acid composite but the pelargonidin chloride shows none, one might speculate that two adjacent hydroxyl groups on the B-ring are mandatory for the formation of PLA−dye complexes. The pelargonidin dye was independently from the polymer structure always observed in its neutral anhydrobase form. The findings are in agreement with the mechanism in Figure 54.

Figure 52. Absorption spectra of a thermochromic PLA-composite at 20 °C (red) and 70 °C (violet) and the underlying molecular structures. Reproduced from ref 73 by permission of The Royal Society of Chemistry. Copyright 2013.

Detailed investigations on the polymorphy and the temperature dependency of the UV/vis absorption spectra indicated that the color change on heating occurs at the Tg and on cooling during the transformation from the mobile into the rigid amorphous phase of the glassy state (Figure 53). The concentration of hexadecanoic acid was found to play an important role on the polymorphy and the appearance of AA

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Figure 55. Molecular structures of the investigated anthocyanidin dyes.

6. CONCLUSION Improved knowledge about the preparation of nanoparticles, their assembly, and about the interaction of functional dyes with their environment have delivered novel tools for the creation of color effects in the last two decades. In combination with creative ideas for thermal control of these effects by structural changes of the surrounding medium a new generation of thermochromic polymers has been designed. These polymers comprise photonic crystals including Bragg stacks and CCA, metal nanoparticle assemblies showing surface plasmon resonance effects, as well as dye−dye and polymer−dye aggregation−disaggregation phenomena. Thermoresponsive color effects originate from the coaction of different nonthermochromic components. As eqs 1−3 reveal, the parameters d or D (layer thickness or particle distance) and n (refractive indices of the components) are the instruments affecting the color appearance. Their temperature dependent behavior results in the thermochromic color changes of Bragg stacks or CCA. Of course there is no linear correlation since both parameters change generally with temperature and are usually affected by the thermoresponsive effect itself. The size, shape, and distance of nanoparticles as well as the refractive index of the surrounding medium have an effect on the surface-plasmon resonance. The plenty options offer multifarious mechanisms of color adjustment by temperature. An optical transduction of temperature dependent distance changes as described for photonic crystals and surface plasmon resonance effects of metal nanoparticles can also be designed by using quantum dots. In aggregated systems morphological changes of the polymer matrix trigger the color change. These changes tune the dye−dye or polymer−dye interaction including the hydrogel driven proton-equilibrium of indicator dyes. From an applicative point of view some specific new generation thermochromic polymer materials exhibit promising properties. The structural colors of photonic crystals are highly light stable. No fading on irradiation with sunlight occurs as it is typical for leuco dye systems enabling the use of photonic crystals for long-term outdoor applications. A limiting factor is the leak of a mass market compatible production technology especially in view of large area applications. Even well-established thin film technologies, such as the layer-by-layer technique, do nothing to change this fact. The development of an extrusion based fabrication method for CCA embedded in thermoplastic polymer matrices is a first step to overcome this limitation. On the other hand, polymer materials exhibiting thermochromism by dye−dye or polymer−dye aggregation−disaggregation phenomena are already compatible with common polymer processing technologies. The wide range of potentially applicable polymers including biopolymers gives grounds to expect a strongly expanded range of thermochromic polymer materials in future. The same applies for dyes and other additives. Some of the novel thermochromic polymers are nontoxic and consist only

The development of nontoxic thermochromic systems by polymer−dye interaction and their processing are still at the beginning of research, but the group of anthocyanidins alone offers much potential. Other biopolymers as poly(hydroxybutyrate) or cellulose acetate including their copolymers will enter the field soon. 5.3. Charge-Transfer Complex

A further concept of thermochromism is to create a reversible thermal tuned charge-transfer-complex formation.75 This is well established in solid state75a−c and in solution.75d However, only a unique example of a thermochromic polymer film has been reported so far.76 The charge-transfer complex formation occurs in a hybrid organic−inorganic multilayer polymer film consisting of altering Preyssler-type heteropolytungstate K12.5Na1.5[NaP5W30O110] (NaP5W30) and poly(ethylene imine) (PEI) layers. The manufacturing of the investigated (NaP5W30/ PEI)80 multilayer film was carried out by a layer-by-layer technique based on electrostatic interactions. This self-assembly method is a powerful tool for the design of ultrathin films.77 Annealing the polymer film at 120 °C for 90 min or at 180 °C for 30 min results in a color change from yellow to blue. This process is reversible. After cooling the blue film to room temperature a bleaching process occurs as long as the film is in contact to an air or oxygen atmosphere. Within 180 min the film recovers its initial state. The origin of this color change is a ligand to metal chargetransfer bridge between NaP5W30 and PEI formed by hydrogen bonding (Figure 56).

Figure 56. Charge-transfer complex equilibrium of an alternating Preyssler-type heteropolytungstate K 12.5 Na 1.5 [NaP 5 W 30 O 110 ] (NaP5W30) and PEI multilayer film.

The back reaction which requires the attendance of oxygen is triggered by an electron transfer from the tungsten atom to an oxygen molecule. Results of electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) support the proposed mechanism. In agreement with the charge-transfer-complex formation the typical ESR signal of WV+ is not detected at room temperature but appears during annealing at 180 °C. XPS reveals binding energy values of W4f doublet of 35.6 and 37.7 eV for WVI+ and of 34.5 and 36.5 eV for WV+. This implies that WVI+ is reduced to WV+ during the thermochromic coloration process. The creation of hybrid organic−inorganic multilayered films exhibiting thermochromism by charge-transfer-complex formation is a hitherto little-studied topic. Nevertheless, the described system enriches the pool of strategies for the development of chromogenic polymers. AB

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of approved food additives. They offer excellent prospects for use in medical and biorelated applications. Process-controlled generation of concentration gradients and anisotropic structures might further contribute to enlarge the property profile. All these activities will stimulate a broad market launch of thermochromic polymers. Characterization of the color properties of thermochromic polymers including metameric effects and surface topography is complex. Only few thermochromic systems came up with color measurements by standardized methods so far.78 For the development of applicative materials, the extension of accurately described systems would be a push. From the scientific point of view improved knowledge can be expected from a better understanding of the mechanisms behind chromogenic effects63 of smart optical and biorelated polymers.79 These chromogenic effects include thermochromism,7 photochromism,80 and mechano-/piezochromism.81 Some of the photonic crystals described above are even responsive to multiple stimuli.22a,c,31a,d,f The mutual interaction of the different chromogenic effects was only rarely investigated so far. The prospective design of various stimuli-responsive effects will boost the spirit for these real chromogenic polymers and opens the way for significantly more complex sensor materials.

Detlef Lötzsch received his diploma degree in physical chemistry in 1986 and his doctoral degree in 1992 from the Technical University Berlin. He was guest scientist at the Raman Research Institute (Bangalore, India), the Naval Research Laboratory (Washington, D.C., USA), and the University of Hull (Hull, U.K.). Since 2001 he has worked as a senior research scientist in the department Chromogenic Polymers at the Fraunhofer Institute for Applied Polymer Research. He is an excellent

AUTHOR INFORMATION

expert in liquid crystals including ferro-, ferri-, and antiferroelectric,

Corresponding Author

dielectric, and electro-optical properties. Additional topics involve phase

*E-mail: [email protected]. Web site: www. thermochromic-polymers.com.

separation processes, dye complexes, and compounds and supra-

Notes

molecular structures. He has published 41 papers and numerous patents

The authors declare no competing financial interest.

and coauthored two books about thermochromism.

Biographies

Ralf Ruhmann (born in 1955 in Berlin/Germany) received his diploma degree in Rostock in 1981 and his doctoral degree at the Academy of

Arno Seeboth received his diploma degree in 1979. After working in industrial research for 8 years, he moved in 1987 to the German Academy of Science. During this time he received his doctoral degree by working in physical chemistry in 1985. He joined the Max PlanckInstitute of Colloids and Interfaces in 1992 and was guest scientist at the Institute of Food Research, Norwich/U.K. for one year and then had a leading position in the Institute of Applied Chemistry, Berlin. Since 2001 he has been head of the department Chromogenic Polymers at the Fraunhofer Institute for Applied Polymer Research. He has authored over 200 scientific publications including two books about thermochromism and 52 patents. The Editor in Chief for the Open Journal of Polymer Chemistry is also a member of advisory boards of different international conferences and a referee for respected journals over the years. His main research topics are chromogenic materials, selfassembling systems including liquid crystals, and display technology.

Sciences in Berlin with a work about thermosetting polymers in 1988. Ruhmann was employed in nonuniversity institutes and industrial research in responsible positions in several European countries. Since 2005 he has worked as deputy head in the department of Chromogenic Polymers of the Fraunhofer Institute for Applied Polymer Research. His scientific work has been published in numerous papers in respected journals and led to several patents. As an outstanding expert on material science especially in polymer synthesis and processing, functional materials, liquid crystals, and dyes, he works with different advisory committees. AC

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PMAA PVA PZPER R-Pery SDS TEM Tg UV vis XPS Olaf Muehling was born in 1973 in Berlin, Germany. After graduation from school (1992) and several years of self-employment in the music business (1992−1996) he studied chemistry at the Humboldt University of Berlin (1996−2001). In 2006 he completed his doctoral degree in organic chemistry under the supervision of Pablo Wessig. For his excellent doctoral thesis he received the Fischer−Nernst Award. Since 2006 he works as a research scientist and project manager at the Fraunhofer Institute for Applied Polymer Research, department Chromogenic Polymers. He is an expert in dispersed-phase polymerizations and its application to the design of nano- and submicrometersized particles of well-defined architecture and function. His current research interests are focused on novel smart materials and structures with temperature-responsive properties.

poly(methacrylic acid) poly(vinyl alcohol) N,N′-bis(2-(1-piperazino)ethyl]-3,4,9,10perylenetetracarboxylic acid diimide dichloride N,N′-bis-(R)-(1-phenylethyl)-perylene3,4,9,10-tetracarboxyldiimide sodium dodecyl sulfate transmission electron microscopy glass transition temperature ultraviolet visible X-ray photoelectron spectra

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ABBREVIATIONS 1-D 2-D 3-D AIBN BBS CCA CMC CTAB cyano OPV

one-dimensional two-dimensional three-dimensional azobisisobutyronitril 4,4′-bis(2-benzoxazolyl)stilbene crystalline colloidal arrays critical micelle concentration cetyl trimethyl ammonium bromide cyano substituted oligo(p-phenylene vinylene) dye DTPP 2,6-diphenyl-4−2,4,6-(triphenyl-1-pyridinio)-phenolate ESR electron spin resonance fcc face-centered cubic IR infrared LC liquid crystal LCST lower critical solution temperature monodisperse refers to a narrow size distribution PAAc poly(acrylic acid) PAAm poly(acrylamide) PBMA poly(benzylmethacrylate) PBS poly(1,4-butylene succinate) PDGI poly(dodecylglyceryl itaconate) PEA poly(ethylacrylate) PEG poly(ethylene glycol) PEI poly(ethylene imine) PLA poly(lactic acid) PNIPAM poly(N-isopropylacrylamide) PNIPAM-co-PGMA poly(N-isopropylacrylamide-co-glycidylmethacrylate) PS polystyrene PMMA poly(methylmethacrylate) PpMS poly(p-methyl styrene) AD

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AF

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