Shape Memory Cellulose-Based Photonic Reflectors - ACS Applied

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Shape Memory Cellulose-based Photonic Reflectors André Espinha, Giulia Guidetti, Maria Concepcion Serrano, Bruno Frka-Petesic, Ahu Gümrah Dumanli, Wadood Y. Hamad, Alvaro Blanco, Cefe Lopez, and Silvia Vignolini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10611 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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ACS Applied Materials & Interfaces

Shape Memory Cellulose-based Photonic Reflectors André Espinha,†,1 Giulia Guidetti,†,2 María C. Serrano,3 Bruno Frka-Petesic,2 Ahu Gümrah Dumanli,2 Wadood Y. Hamad,4 Álvaro Blanco,1 Cefe López,*,1 Silvia Vignolini*,2 1

Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Calle Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, Spain 2

Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW UK

3

Hospital Nacional de Parapléjicos, Servicio de Salud de Castilla La Mancha, Finca La Peraleda s/n, 45071 Toledo, Spain 4FP

Innovation, Departments of Chemistry and Chemical & Biological Engineering, University of British Columbia, 2665 East Mall, Vancouver, BC V6T 1Z4 Canada †

AE and GG contributed equally to this work.

*Email: C.L.: [email protected]; S.V.: [email protected] KEYWORDS: cellulose nanocrystals, cholesteric, polydiolcitrates, shape memory, biomimetic.

ABSTRACT Biopolymer-based composites enable to combine different functionalities using renewable materials and cost-effective routes. Here we fabricate novel thermoresponsive photonic films combining cellulose nanocrystals (CNCs) with a polydiolcitrate elastomer exhibiting shape memory properties, known as hydroxyl-dominant poly(dodecanediol-co-citrate) (PDDC-HD).

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Iridescent films of CNCs are first made by evaporation induced self-assembly, then embedded in the PDDC-HD prepolymer, and finally cured to obtain a crosslinked composite with shapememory properties. The fabricated samples are characterized by polarized optical microscopy, scanning electron microscopy and thermomechanical cycling. The obtained hybrid material combines both intense structural coloration and shape memory effect. The association of stiff cellulose nanocrystals and soft polydiolcitrate elastomer enhances the overall mechanical properties (increased modulus and reduced brittleness). This hybrid nanocomposite takes advantage of two promising materials and expands their possibilities to cover a wide range of potential applications as multiresponsive devices and sensors. As they perform from room to body temperatures, they could be also good candidates for biomedical applications. INTRODUCTION Plastics are ubiquitous materials in many aspects of daily life, ranging from packaging, to construction, to medical applications. The needs to replace conventional plastics by biocompatible and biodegradable ones motivate the exploitation of polymeric materials versatility using sustainable fabrication approaches. Within this context, low-cost photonic materials produced by self-assembly of biopolymers are receiving growing interest in the materials community.1-3 Many photonic structures with diverse optical responses have been produced using a large variety of biomaterials and fabrication methods.4-6 However, there are limitations on the different functionalities that can be achieved using only sustainable and biocompatible polymers. As an example, cellulose-based photonic structures can provide strong, intense colorations but they are generally very brittle.7 The addition of organic and inorganic matrices to such cellulose nanostructures is challenging as it can improve their mechanical properties but often compromises either the photonic effect or the full biocompatibility of the final composite.8


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Shape memory polymers9 have been implemented in the design of a large variety of thermomechanical programmable components with interest in photonics.10-13 As an emerging class of biocompatible polyesters, polydiolcitrate elastomers present a great potential in biomedical applications.14,15 They offer possibilities of programming structural features at the nanoscale16 or achieving shape programmable gain media.17 However, to our knowledge, the combination of shape memory polymers with different types of cellulose sources has produced only amorphous, colorless composites, in which the only objective was to enhance the mechanical response. For instance, Chiu and co-workers reported microfibrillated cellulose-poly(propylene carbonate) composites presenting shape memory and self-healing properties.18 Sonseca et al. investigated the feasibility of customizing the stiffness and toughness of poly(mannitol sebacate) by introducing cellulose nanocrystals (CNCs).19 Similarly, composites in which CNCs improved the fixity ratio in poly(ester-urethane) and polycaprolactone and polyethylene glycol have been described.20,21 Interestingly, recent developments enabled a water responsive shape memory effect in similar composite systems.22, 23 In this work, we present a hybrid cellulose-based photonic structure exhibiting shape memory functionality, fabricated by combining cellulose nanocrystals photonic films with a polydiolcitrate elastomer.24 In particular, we directly impregnate and embed colored CNC films with hydroxyldominant poly(dodecanediol-co-citrate) (PDDC-HD).25 The CNCs contribute both to the structural coloration of the film and to its overall mechanical cohesion, while the PDDC-HD provides both flexibility and reduced brittleness, due to its elastomeric properties, and adds functionality by introducing the shape memory effect. Our approach consists in the fabrication of thermoresponsive optically active composites, as summarized in the following steps: (i) CNC photonic film fabrication, (ii) impregnation in the PDDC-HD prepolymer, and (iii) thermal curing treatment of


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the composite to provide the prepolymer with shape memory properties. At each step, the evolution of the optical properties of the samples is monitored by polarized optical microscopy and spectroscopy, while the final structures are observed in cross-section by scanning electron microscopy (SEM). Finally, thermomechanical cycling is performed to assess the programmed shape-memory response.

EXPERIMENTAL SECTION Chemicals: Citric acid and 1,12-dodecanediol were purchased from Sigma-Aldrich and ethanol 99.5% from Panreac. All chemicals were used as received. Fabrication of cellulose nanocrystals films: The cellulose nanocrystal suspension was provided by CelluForce with the following specification: concentration 4 wt%, pH 2, conductivity 2100 µS/cm. Films of cellulose nanocrystals were fabricated by evaporation induced self-assembly.26 First, the pH of the suspension was adjusted to pH 5.5 by addition of NaOH. The initial suspension was then subjected to heat treatment in a water bath at 60 °C for either 12 h, 21 h and 30 h respectively. Then, a volume of 2 mL of each CNC suspension at 1 wt% was poured into polystyrene Corning® Petri dishes of 3.5 cm diameter and allowed to dry at ambient conditions. The dry films were then detached from the substrates and cut into individual flakes of similar shape, discarding the very edge of the films that were affected by a coffee-ring strain.27 Prepolymer synthesis: A mixture composed of citric acid (CA) and 1,12-dodecanediol (DD) (mol ratio of hydroxyl to carboxyl groups, 4:3) was prepared following a previously reported protocol.25 CA and DD were initially melted at 165 ºC for 15-20 minutes. Subsequently, the temperature was decreased to 140 ºC and the condensation reaction was allowed to continue for 1 h so that a viscous


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liquid was obtained. The so-formed prepolymer was then diluted in ethanol at a concentration of 0.5 g mL-1, with the help of mild heating and bath sonication. Preparation of shape memory composites: Free-standing CNC flakes were immersed overnight in the prepolymer solution diluted in ethanol, drained to remove the excess of prepolymer and finally left drying to allow ethanol evaporation. The samples were then thermally annealed at 80 ºC in an oven for periods from 16 h to 20 h to crosslink the polymer to form hydroxyl-dominant poly(dodecanediol-co-citrate), referred to as PDDC-HD. During this curing step, the samples were held by three contact points in order to minimize adhesion to the substrate. A schematic of the fabrication procedure is presented in the Supporting Information, Figure S1. Optical characterization: Optical characterization was performed in reflection mode on a customized Zeiss Axio microscope using a halogen lamp (Zeiss HAL100) as a light source using Koehler illumination. The light reflected off the sample passes through a quarter wave plate and a polarizing filter, specifically oriented to select either left-circularly-polarized (LCP) or rightcircularly-polarized (RCP) light before being split between a CCD camera (UI-3580LE-C-HQ, IDS) and an optical fiber mounted in confocal configuration and connected to a spectrometer (AvaSpec-HS2048, Avantes). This setup allowed for the spectra acquisition from specific areas in the sample, in both polarization channels; all the spectra were normalized to the reflection of a silver mirror in one polarization channel. With this normalization convention, a perfectly left (right) chiral nematic sample would have a 100% reflectivity in the left (right) channel. Scanning Electron Microscopy: SEM images were acquired using a Leo Gemini 1530VP system, Zeiss, working in cross section at an angle of 90° with respect to the electron beam. The samples were placed on aluminum stubs using conductive carbon tape and sputter-coated with a few


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nanometer thick layer to Au/Pd (Emitech K550) to minimize the charging effect. The acceleration voltage used was 4 kV and the working distance was 3-4 mm.

RESULTS AND DISCUSSION The first step of the preparation of the hybrid nanocomposite corresponds to the preparation of the CNC suspension and the fabrication of the photonic CNC films (see schematic in the Supporting Information, Figure S1). The heat treatment applied to the suspension for either 12 h, 21 h or 30 h prior to casting leads to a gradual desulfation of the CNC surface.28 This was performed in order to reduce the electrostatic interaction between rods by lowering their charge and increasing the ionic strength of the medium, as reported in previous work.29 After this, the three suspensions were left drying in a petri dish, from which dried films with structural colors were obtained, displaying blue, green and red color, respectively. The clear blue-shift is obtained as the heat treatment duration is increased, and gave us the possibility to test the next steps of the preparation starting for these three initial colors. In the second step, PDDC-HD prepolymer was added to the CNC films. The PDDC-HD was first diluted into ethanol to favor its diffusion into the swelling CNC film. After soaking and drying, the hybrid films presented a slightly opaque appearance. This can be attributed to the scattering of non-crosslinked prepolymer microdomains.16 For all the fabrication stages, the obtained films are characterized by polarized optical microscopy and spectroscopy.30 The corresponding data after the impregnation step are shown for initially blue, green and red CNC films in Figure 1a and 1b, respectively. As expected for CNC chiral nematic films, left circularly polarized light is reflected in a specific spectral region, due to the stacking periodicity induced by their chiral nematic structure.31 From the presence of a reflection


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peak in the LCP channel only, we can infer that the prepolymer impregnation does not affect the chiral nematic ordering of the CNCs. We observe a systematic red-shift for all the samples, consistent for all the investigated spectral regions (Figure 1). Such red-shift indicates the penetration of the prepolymer into the CNC films, either by affecting the average optical index of the infiltrated CNCs film (nPDDC-HD = 1.522) or by the macroscopic swelling of the CNC films. The latter hypothesis is also supported by a broadening of the reflection peaks as well as by SEM analysis of the films after curing (see below).

Figure 1. (a) Polarized optical microscopy images, for left (LCP) and right (RCP) circular polarization, for three CNC films initially reflecting in the blue, green and red (top line), and then redshifted respectively to the green, red and infra-red after prepolymer infiltration (bottom line).


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Scale bar corresponds to 50 µm. (b) Corresponding reflection spectra, for both polarizations, before (top line) and after (bottom line) the infiltration with the prepolymer, prior to any curing. Table 1. Average spectral position of reflectance maxima (av) along with average peak width (absolute FWHM - av and relative - avav) of the left-circularly polarized Bragg peak, for the blue, green and red CNC films, before and after the prepolymer infiltration, before any curing.

Initial film color

Blue

Green

Red

av (nm)

av (nm)

av /av

av (nm)

av (nm)

av /av

av (nm)

av (nm)

av av

initial

470 ± 20

61

13%

570 ± 20

39

7%

630 ± 70

80

13%

impregnated

560 ± 70

102

18%

660 ± 80

73

11%

700 ± 100

106

15%

Representative spectra of the three samples were collected before and after the infiltration and are shown in Figure 1b. As previously mentioned, the reflection peak was observed only using a circularly-left polarizing filter, assuring that the axes of the chiral nematic structure are perpendicular to the film surface.30 To take into account the intrinsic multi-domain nature of the CNC films, we measured several spectra in different positions; Table 1 reports the average spectral position of the reflectance maximum (in the left circularly polarized channel) (av) before and after the impregnation. After the impregnation process, av red-shifted about ~20% for all the samples. Interestingly, the standard deviation of the measured av increased after the infiltration and can result from an inhomogeneous infiltration throughout the CNC film thickness. Moreover, the swelling of the CNC films by the prepolymer appears to be smaller in the samples that were more exposed to heat treatment, which are also those expected to have stronger van der Waals attractive interactions.29, 32


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In the third and last step of the fabrication process, the impregnated films are subjected to a partial thermal treatment to crosslink the PDDC-HD polymeric chains and provide the shape memory functionality to the final samples. The samples were kept flat during this step in order to memorize it as the permanent shape of the films. The samples were then left to cool down in that same position and were stored at room temperature prior to observation. The optical characterization of a representative cured film is reported in Figure 2, which shows that the films maintain the color appearance. A strong reflection peak is observed in the left circularly polarized channel (LCP) only (Figure 2b), which confirms that the final hybrid material retains its chiral nematic structure. However, the presence of an increased signal in the right polarization channel (RCP), limited to localized defects of the cholesteric structure, suggests that the infiltration process could slightly affect the orientation of the axes of the chiral nematic domains. A comparison of the average positions of the reflection peak at the three different steps of the fabrication process is reported in Figure 2c, showing no noticeable color shift before and after curing.


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Figure 2. a) Representative left (resp. right) circularly polarized (LCP, resp. RCP) optical micrographs of a hybrid CNC/PDDC-HD film obtained from an initially blue CNC film after impregnation with PDDC-HD prepolymer and curing treatment. Scale bar corresponds to 100 µm. b) Reflectance spectra after curing. c) Evolution of the spectral position of av throughout the three different fabrication steps: initial, impregnation with PDDC-HD prepolymer and after PDDC-HD curing (error bars correspond to the statistical standard deviation). In order to characterize the morphology of the CNC/PDDC-HD hybrid materials after curing, the cross-section of the films was investigated using scanning electron microscopy (SEM), as reported on Figure 3 and Figure S2 in the Supporting Information. From the SEM we observe that the PDDC-HD forms a thick layer on both sides of the CNC film, increasing the total thickness of the CNC film from ~4.5µm to ~35µm (CNC film sandwiched between PDDC-HD layers), while the CNC layer itself seems on average thicker (~6.5µm), indicating that the PDDC-HD is infiltrated


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in the CNC layer. High magnification SEM imaging of the cross-section enables to recognize the Bouligand arches, characteristic of the cholesteric architecture in the cellulose film.33 A systematic and statistical analysis of the cholesteric pitch, measured as twice the pattern periodicity, shows a pitch increase of ~16% after the PDDC-HD impregnation, as reported in Table 2. Interestingly, no processes such as delamination were observed at the interface. A successive penetration and polymerization of the PDDC-HD inside the CNC film can explain the remarkable adhesion at the interface. Moreover, we hypothesize that the hybrid composite could be further stabilized by the formation of additional ester bonds between non-reacted carboxyl groups in the prepolymer and surface hydroxyl groups on the cellulose nanocrystal film. An additional indication of the swelling of the CNC film by the PDDC-HD can be obtained from the analysis of the Fabry-Perot oscillations observed in the reflection spectra. These oscillations are visible on Figure 1b and allow estimating the product navtav of the average optical index and the CNC cholesteric thickness) using the Fabry-Perot oscillations observed in the reflection spectra. The observed increase of navtav reported in Table 2 confirms the PDDC-HD incorporation in the CNC film. However, these oscillations interfere in the presence of the additional PDDC-HD layers and become difficult to detect on the samples after impregnation and curing.


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Figure 3. Representative SEM cross-section images of a CNC/PDDC-HD hybrid material after impregnation of the prepolymer and curing. Left: the elastomer layers surrounding both sides of the CNC film (the scale bar is 2 µm, interfaces are indicated by yellow arrows). Right: the arrangement of the cellulose nanocrystals into a chiral nematic structure (the scale bar is 200 nm).

Table 2. Detailed comparison of the physical properties of a sample before and after impregnation and curing, derived from SEM and optical analysis for an initially green sample.

initial

cured

variation

Thickness tlayer by SEM

PDDC-HD top layer

--

21 ± 0.5

N.D.

(µm)

CNC middle layer

4.5 ± 0.3

6.5 ± 0.5

+45%

PDDC-HD bottom layer

--

6 ± 0.3

N.D.

Pitch by SEM

Pitch average

355

412

+16%

(nm)

Pitch std dev

55

82

+50%

Spectral analysis

av (nm)

570

660

+16%

Fabry-Perot oscillations

navtav (µm)

5.27

5.73

+8.7%

The shape memory effect of the final, cured hybrid nanocomposites was then tested. Shape memory effect is predictable in the system due to the presence of PDDC-HD. Its mechanism is


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derived from the effect of prepolymer and DD rich microdomains that undergo a melting transition and therefore act as a physical switch.25 The temporary shape was conferred using the following sequence of heating/ mechanic deformation/ cooling. First, the samples were heated above their melting transition temperature (Ttrans ~ 30 °C) using a heat gun, then rolled up while maintaining the temperature above Ttrans, and finally left to cool down at room temperature, mechanically holding them in that shape to allow the material to solidify in a rolled configuration, until complete solidification of the polymer was observed (~10min). At that stage, the material was rigid again and had adopted the new, temporary shape. We tested the recovery of the permanent shape (memorized during the initial curing) by applying a mild heat stress. Such tests are reported in Figure 4 (and by the corresponding videos in SI), where the hybrid thin films exhibited shape recovery. Moreover, the films were able to restore their permanent shape regardless of the gravity bias. The optical response of the cellulose films was maintained during the thermal cycle, while only a transparency increase at high temperature was observed above Ttrans.16 Finally, the overall mechanical performances of the hybrid CNC/PDDC-HD composite can be tuned on demand. This is possible as the effective Young modulus depends on the relative thickness of the middle (CNC-rich) and outer (PDDC-HD based) layers. Using a simplified model of high Young modulus CNC film (usually around 2-6 GPa)34 surrounded by lower Young modulus PDDC-HD elastomer (2-20 MPa),16 we estimated for the example of the hybrid material in Figure S2 an effective Young modulus around ~40 MPa (calculated in a bending geometry, details in SI). This value is much lower than for a brittle, pure CNC film and about 2.7 times larger than pure PDDC-HD for the involved layer thicknesses reported here.


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Figure 4. Examples of thermomechanical cycles for the CNC/PDDC-HD composite, at four different times of the shape recovery process. The images at t = 0 s exhibits the temporary programmed shape. The top row corresponds to shape recovery in favor of gravity and the bottom one to a folding against it. Most importantly, the high mechanical cohesion of the hybrid CNC/PDDC-HD material is assured by the infiltration of the PDDC-HD inside the CNC layer and, in this way, a functional and responsive material, bridging structural coloration and shape memory properties, is achieved. Indeed, other investigated routes, such as direct co-assembly of the CNCs in the shape memory prepolymer and direct casting of the CNCs onto a shape memory polymer substrate either led to no structural coloration, or delamination. CONCLUSIONS In summary, we presented a proof of concept for the fabrication of novel thermoresponsive chiral nematic photonic materials. The fabricated hybrid materials are based on self-assembled cholesteric cellulose nanocrystal (CNC) films with photonic properties, impregnated with and


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embedded in a shape memory polymer of hydroxyl-dominant poly(dodecanediol-co-citrate), known as PDDC-HD. The evolution of the optical and structural properties at each of the fabrication steps indicates that the CNC film is partially swollen by the surrounding PDDC-HD without disrupting the CNC cholesteric order. This conformation appears responsible for the mechanical cohesion of the CNC-rich film providing the photonic properties of the composite, and the PDDC-HD surrounding layers responsible for the shape-memory properties. Furthermore, this hybrid nanocomposite takes advantage of two promising materials and leading to the fabrication of a wide range of possible applications beyond the functionalities of their individual constituents.

ASSOCIATED CONTENT Supporting Information. Additional experimental details; scanning electron microscopy images; videos showing the shape restoration; estimation of the Young modulus of the hybrid composites. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors: *Email: C.L.: [email protected]; S.V.: [email protected] Notes The authors declare no competing financial interest. Additional data related to this publication is available at the University of Cambridge data repository (http://dx.doi.org/10.17863/CAM.6086). Present Address


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Institut de Ciència de Materials de Barcelona, Consejo Superior de Investigaciones Científicas,

Carrer dels Til·lers S/N, Campus de la UAB, 08193 Bellaterra, Barcelona, Spain. Author Contributions †

AE and GG contributed equally to this work.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was partially funded by EU FP7 NoE Nanophotonics4Energy grant No. 248855, the Spanish MINECO project MAT2015-68075 (SIFE2), Comunidad de Madrid S2013/MIT-2740 (PHAMA_2.0) program. All the authors acknowledge the Royal Society (2014/R2-IE140719). AE was supported by the FPI PhD program from the MICINN. SV, BFP and AGP are funded by the BBSRC David Phillips fellowship [BB/K014617/1], and the ERC-2014-STG H2020 639088. GG acknowledges the EPSRC [1525292]. MCS acknowledges the Instituto de Salud Carlos III of Spain for a Miguel Servet I contract (MS13/00060). REFERENCES


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