Preparation of Stimuli-Responsive Functionalized Latex Nanoparticles

Article pubs.acs.org/Langmuir

Preparation of Stimuli-Responsive Functionalized Latex Nanoparticles: The Effect of Spiropyran Concentration on Size and Photochromic Properties Amin Abdollahi, Ali Reza Mahdavian,* and Hamid Salehi-Mobarakeh Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran 1497713115, Iran

ABSTRACT: Incorporation of photochromic compounds to polymer matrix through chemical bonding results in an enhancement of photoactivity and stabilization of optical properties. Here, spiropyran ethyl acrylate monomer (SPEA) was synthesized, and then photochromic particles bearing epoxy functional groups were prepared through semicontinuous emulsion copolymerization. Dynamic light scattering (DLS) and scanning electron microscopy (SEM) results depicted an increase in particle size and particle size distribution with the increase in SPEA monomer−surfactants ratio. Studies on photochromic properties by UV−vis analysis demonstrated a decrease in the absorption intensity despite the increase in SPEA content due to the enhancement in particle size. The prepared acrylic copolymer particles showed reasonable photostability, photoreversibility, and fast photoresponsivness according to the convenient test methods under UV/vis irradiation. DSC and DMTA analyses indicate an increase in Tg of the obtained copolymers with the increase in SPEA content. Finally, stimuli-responsive cellulosic papers were prepared by impregnation, and their photochromic behavior was investigated in dry and wet forms in various media under UV radiation. Morphology studies, due to stabilization of the photochromic copolymer on cellulose fibers, were conducted by SEM micrographs and showed good adhesion and compatibility between the two phases. siveness,15 these compounds could be introduced inside hydrophobic cavities via doping11,16 or a copolymerization15,17,18 process. On the other hand, inclusion of these dyes through copolymerization is more attractive for preventing

1. INTRODUCTION Photochromic compounds have particular importance and vast applications in the fields of medicine,1−3 biotechnology,4,5 optical data storage,6−9 security documents,10,11 chemical sensors,12,13 and ophthalmic lenses4,9 due to their reversible optical-switching and changes in physical and chemical properties. In order to protect a photochromic dyes from environmental degradation14 and keeping their photorespon© 2015 American Chemical Society

Received: July 15, 2015 Revised: August 29, 2015 Published: September 16, 2015 10672

DOI: 10.1021/acs.langmuir.5b02612 Langmuir 2015, 31, 10672−10682

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Langmuir

Figure 1. Schematic representation for preparation of the functional photochromic latexes containing SPEA groups.

glass matrix, cross-linking was carried out through UV irradiation. Measuring optical properties of these coatings under dry and wet (swelled with water) conditions reveals an increase in their optical response in the swollen state. This increase comes from the reduction of steric hindrance around spiropyran groups, which facilitates their isomerization. Sun et al.11 prepared photochromic papers through impregnation of cellulosic paper with a polystyrene-acrylic latex containing spiroxazine groups. This method suffers from inefficient bonding between the above two phases because of physical interaction between spiroxazine and polystyrene-acrylic latex as the carrier. In fact, the existence of chemical bonding is a key point to protect the photochromic dye from environmental degradation and induces appropriate stability.25,28 They also prepared similar photochromic papers using cellulose nanocrystals as a carrier of spiroxazine dye, and the same results were obtained.16 In contrast to previous studies on this subject,11,16 Tian et al.29 prepared fluorescent papers by introducing carboxylated spiropyran derivative without any polymeric carrier into the cellulose via esterification reaction. This study illustrates an increase in fluorescence efficiency with sacrificing smart photochromic behavior and irreversible color changes under UV radiation. This phenomenon is called negative photochromism and is attributed to the establishment of strong hydrogen bonding between resulting merocyanine intermediate and cellulosic hydroxyl groups.6,29−32 Therefore, the important role of polymeric carriers and preserving reversible photochromic properties of the consequent materials can be realized here. In this study, an acrylic-modified spiropyran derivative, piropyran ethyl acrylate (SPEA), was synthesized, and novel corresponding photochromic polymeric nanoparticles, bearing epoxy functional groups, were prepared through semicontinuous emulsion polymerization. Also, the effect of SPEA content on particle size distribution, photochromic, and thermomechanical properties were investigated. The presence of epoxy functional groups provides the reactivity of such latex particles with cellulosic matrix. Then, cellulosic papers were

their undesired aggregation and reduced photochromic properties.8,11,16,17 Copolymerizations of these dyes with other monomers by emulsion or miniemulsion polymerization result in producing smart polymeric (nano) particles, which can be used for several applications. The size of such polymeric particles17 and the concentration of photochromic dye19 affect their optical properties significantly, and it has to be less than the wavelength of irradiated light essentially. Spectral properties of the light responsive polymer particles can be affected by particle size, because scattering phenomenon will dominate and suppress other photoactivities. The phase structures and sizes of micelles can be tuned by changing chemical structure of the surfactants and the surfactant-monomer ratio, respectively.20−22 Zhu et al.8 copolymerized a spiropyran-based monomer with styrene, N-isopropylacrylamide and divinylbenzene as the crosslinking agent through emulsion polymerization and studied the photochromic and luminescence optical switchability, while attached to the living cells as nanodetector. In 2007, this group23 prepared similar nanoparticles containing a fluorescent polymerizable dye beside similar photochromic dye. Such compounds are known as dual-color systems owing to the combination of photochromic and fluorescence properties. In some particular ratios, they show synergic effect through the fluorescence resonance energy transfer (FRET) phenomenon.23,24 In 2013, the effect of electron withdrawing and donating substituents on optical and dual-color properties of the polymeric nanoparticles were studied.25 In recent years, much attention has been paid to smart photochromic polymers with specific functionalities. Florea et al.26 successfully polymerized a spiropyran derivative with norbornene monomeric moieties on the inner wall of microcapillary tubes using ring-opening metathesis polymerization. Then, the prepared capillary tubes were placed under UV irradiation to determine the polarity of some solvents passing through them. In another attempt, hydrophilic coatings were prepared using solution polymerization of spiropyran derivatives, photo-cross-linkable benzophenon and dimethyl acrylamide.27 After coating of this layer onto the silicon and 10673


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Langmuir Table 1. Procedure for Preparation of Functional Photochromic Nanoparticles with Layered Structure first stage sample

water (mL)

NaHCO3 (g)

SDS (g)

Triton X-100 (g)

KPS (g)

CSPEA × 10‑3 (mol/L)

SPEA (g)

MMA (g)

SPEA monomer−surfactants ratio

particle size (nm)

PL-10-A PL-10-B PL-10-C PL-10-D PL-10-E PL-10-F PL-10-G PL-10-H PL-10-J

27 27 27 27 27 27 27 27 27

0.030 0.030 0.030 0.030 0.030 0.035 0.040 0.040 0.040

0.040 0.040 0.040 0.045 0.045 0.052 0.057 0.067 0.082

0.010 0.010 0.010 0.012 0.012 0.015 0.015 0.020 0.025

0.030 0.030 0.030 0.030 0.030 0.035 0.040 0.040 0.040

0.82 4.10 8.20 12.60 16.90 24.60 32.80 41.00

0.01 0.05 0.10 0.15 0.20 0.30 0.40 0.50

2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3

0 0.20 1.00 1.75 2.63 3.00 4.16 4.60 4.70

62 70 79 88 290 320 405 426 685

required time for completion of polymerization reaction was obtained by monitoring monomer conversion through the gravimetric method. Final conversions for all the samples were above 95%, and the amount of coagulation at the end of polymerization for each sample was below 1 wt %. The total amount of incorporated SPEA in the nanoparticles was determined by coagulation of the latexes and absorption measurement of the remaining serum by UV−vis analysis with respect to a standard solution. It was found that more than 95% of the added SPEA were incorporated in these samples. 2-4. Modification of Cellulosic Paper with Photochromic Polymer. In order to prepare a stimuli-responsive paper, filter paper (MUNKTELL-Grade: 391, Lot No: 09-158) was treated with PL-10D latex through an impregnation method at ambient temperature for 5 h by using a shaker. Then the above impregnated paper was dried at 80 °C for 12 h. The prepared stimuli-responsive paper was immersed in water, ethanol, and methanol and then exposed to UV irradiation (λ = 365 nm) for 5 min. In addition, photochromic properties were detected through different color changes in each media, and the observations were recorded.

impregnated with the prepared photochromic latex, and their optical behavior under UV irradiation was studied in detail. The produced photochromic papers showed outstanding features such as chemical bonding between spiropyran groups and the polymeric carrier, absence of undesired negative photochromism, and their potential toward alternate and reversible color changes under UV irradiation in different polar media. To the best of our knowledge, this is the first report on the preparation of such photochromic papers with the above-mentioned specifications.

2. EXPERIMENTAL SECTION 2-1. Materials. 2,3,3-Trimethylindolenin, glycidyl methacrylate (GMA) and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. All of the solvents, 2-hydroxy-5-nitrobenzaldehyde, methyl methacrylate (MMA), potassium persulfate (KPS), sodium hydrogen carbonate (NaHCO3), triethylamine, Triton X-100, 2bromoethanol, and acryloyl chloride (AC) were supplied by Merck Chemical Co. All chemicals were used without further purification. Deionized (DI) water was used in all recipes. 2-2. Synthesize of 1′-(2-Acryloxyethyl)-3′,3′-dimethyl-6nitrospiro-(2H-1-benzopyran-2,2′-indoline) (SPEA). (R/S)-2(3′,3′-Dimethyl-6-nitro-3′H-spiro[chromene-2,2′-indol]-1′-yl)ethanol (Spiropyran, SP) was synthesized based on the modified method described previously,33 and SPEA was prepared through modification of SP with acryloyl chloride (AC) according to a known procedure34 with some improvement. Modification reaction was carried out through reaction of SP (6 mmol, 2 g) with AC (18 mmol, 4.2 mL) in dry THF (20 mL) as the solvent by using of triethylamine (8 mmol, 0.8 mL) at 0 °C. Dropwise addition of AC into the solution was completed in 30 min. Then temperature rose to ambient temperature, and the reaction continued for 8 h. Finally, the mixture was filtered and the obtained yellow solid was washed with cold acetone to remove unreacted compounds and gave yellow precipitate of SPEA (1.78 g, 4.4 mmol). 2-3. Emulsion Polymerization and Preparation of the Photochromic Latex Nanoparticles. Functionalized photochromic layered nanoparticles were prepared using semicontinuous emulsion polymerization, in which SPEA was introduced inside the hydrophobic core (Figure 1). The external layer containing epoxide groups was prepared by a second feeding based on MMA and GMA comonomers. The amount of reactants in each recipe has been given in Table 1. In general, sodium hydrogen carbonate as the buffer, ionic surfactant (SDS), and nonionic surfactant (Triton X-100) were dissolved in DI water. The obtained mixture was then transferred into the reactor under continuous flow of nitrogen gas. After addition of KPS as the initiator at 75 °C, a solution of SPEA in 2 mL DI water and MMA were added separately and simultaneously into the reactor within 10 min dropwise. The polymerization was continued after completion of addition for 45 min. Afterward, and to form the external layer in the second stage, a mixture of GMA (0.4 mL) and MMA (0.75 mL) was added to the above latex within 10 min and continued for 20 min. The

3. CHARACTERIZATION Identification of epoxy functional groups in polymeric particles was carried out by using an FT-IR BRUKER-IFS48 spectrophotometer (Germany). Sample preparation was performed after coagulation of the latex with concentrated sulfuric acid, drying at 50 °C, and preparation of KBr pellets. Diluted latexes (up to 100 folds) were used for measurement of particles size and size distribution by using SEMATECH laser light scattering (LLS) (Nice, France) at 633 nm wavelength, which the laser angle was set at 90°. A scanning electron (SEM) micrograph was taken by a Tescan Vega II (Czech Republic). A drop of diluted latex was placed on the sample holder and dried under vacuum at 25 °C. Then, they were put under vacuum, evacuated, and a layer of gold was deposited under flushing with argon by using EMITECH K450x sputter-coating (England). Morphology studies of cellulose fibers were performed after washing of the photochromic paper with DI water and drying at 50 °C. Photochromic properties of the samples were investigated by UV−vis analysis and by using Shimadzu-UV2550 UV−vis spectrophotometer (Japan). For this reason, the initial latex was diluted to about 0.2 wt %. To evaluate photochromic properties, the excitation was done by a UV lamp (365 nm), CAMAG 12VDC/VAC (50/60 Hz, 14VA, Switzerland). Also the source for visible light was a common LED lamp with white light. UV and Vis irradiation time was set 5 min for all the samples. Dynamic-mechanical thermal analysis (DMTA) of the coagulated latex powder was performed on TRITON analyzer (UK). The samples were heated in the range of 25−200 °C by heating rate 5 °C/min under N2 atmosphere, in dual cantilever bending mode at 1 Hz. Differential scanning 10674


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Figure 2. FTIR spectrum of the PL-10-D sample.

Figure 3. DLS analysis for prepared latexes with different SPEA monomer−surfactants ratios.

beam.8,17,23 Polymeric particles larger than 100 nm do not usually show remarkable photochromic properties,8,17 because the effective surface area is decreased with the increase in particle size. For those particles containing spiropyran moieties, a few concerns should be taken into consideration such as their large steric hindrance and zwitterionic nature after UV irradiation. This will result in an increase in particle size while using these compounds at high concentrations. Here, the strategy for preparation of photochromic latexes including SPEA is illustrated in Figure 1. The obtained particles were modified with epoxy functional groups to induce susceptibility toward incorporation into the reaction with cellulosic paper. Polymeric nanoparticles were prepared using semicontinuous emulsion polymerization. In the second stage of the polymerization process, MMA and GMA comonomers were added to the above latex. In this stage, the functionalized outer layer with

calorimetry (DSC) and thermogravimetric analysis (TGA) were carried out under N2 atmosphere at 10 °C/min heating rate by NETZSCH Instruments Co. (DSC 200, F3Maia, Germany; 30−150 °C) and TGA-STA: PL-1500 (UK) (30− 600 °C), respectively. Dried latexes were used for these analyses directly.

4. RESULTS AND DISCUSSION One of the most important methods for incorporation of the photochromic materials into the polymer matrix and preparation of light sensitive smart fabrics is to copolymerize them with other acrylic monomers via emulsion8,23 or miniemulsion15,17,25 polymerization. Because of the spherical morphology of polymeric particles in the latex form, particle size should essentially be less than the wavelength of incident 10675


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Figure 4. SEM images of some latexes with different concentrations of SPEA.

notable that SPEA was added to the reactor as a red aqueous solution and in fact, its merocyanine form was copolymerized with MMA at the first stage. Increase in the merocyanine concentration leads to an abrupt elevation of ionic strength of the dispersion. Based on the DLS results, the increase in ionic strength (with SPEA conc.) enhanced particle size, although the role of emulsifiers’ concentrations was in the opposite direction. As a consequence, SPEA conc. is a critical parameter for controlling particles’ size and their distribution in this emulsion polymerization. On the other hand, PL-10-F was selected randomly to investigate the effect of SPEA addition speed on the particle size distribution. It was found that particle size distribution was reduced significantly by precise control of the addition speed of SPEA in the polymerization process. This could be attributed to the restriction in secondary nucleation due to the high water solubility of SPEA and its concentration in the feeding. Morphology and particle size distribution of photochromic latex particles were studied by scanning electron microscopy, and some typical micrographs have been shown in Figure 4 with good agreement with DLS analysis. It was found that the increase in SPEA content resulted in some instability of the obtained latex. Hence, higher amounts of the surfactants were required to maintain the stability (Table 1). It is observable that the increase in SPEA content has resulted in formation of secondary particles beside the growth of previous ones. This owes to the solubility of SPEA according to the existence of merocyanine isomeric form. Therefore, higher concentration of SPEA will increase the probability of secondary nucleation and broadening of particle size distribution, although all of the obtained latexes are completely stable. On the other hand, the

epoxy groups is formed on a hydrophobic core containing SPEA. Typically, PL-10-D latex was coagulated and dried for FTIR analysis (Figure 2). The characteristic peaks of the carbonyl groups (CO) and C−H stretching vibrations of the aliphatic moieties appeared at 1724 cm−1 and 2952−2992 cm−1, respectively. In addition, stretching vibrations of epoxy groups were found at 754 and 844 cm−1. These reveal that the prepared latex has been functionalized by GMA, which could be exploited in further reactions. 4-1. Size and Morphological Studies. Emulsion polymerization was chosen for incorporation of SPEA within the hydrophobic core of the latex particles. Size of polymer particles can be controlled by effective parameters such as surfactant concentration,35−37 stirring speed and initiator concentration,38,39 as well as ionic strength.40,41 Conversion of the colorless spiropyran to the colored merocyanine zwitterion through ring opening and C−O (spiro) cleavage can be observed under UV irradiation or dissolving spiropyran in a polar media such as water.42−45 The reverse reaction will be conducted by heating or visible light irradiation.6,8 Hence, particle size and size distribution can be affected by the presence of merocyanine isomer with ionic character during polymerization, which results in particle growth because of the increase in ionic strength.40,41 In order to investigate the influencing parameters on the photochromic properties, polymer particles containing various amounts of SPEA were prepared via semicontinuous emulsion polymerization (Table 1). Particles size and size distribution for the prepared samples were determined by dynamic light scattering (DLS) analysis and shown in Figure 3. The obtained sizes were between 60 to 700 nm, and their size distributions became broader by the increase in SPEA monomer-surfactants ratio in the feed. It is 10676


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Figure 5. DLS analysis of the seed and final nanoparticles (a) and SEM image (b) of PL-10-D photochromic latex.

Figure 6. UV−vis analysis of the diluted prepared photochromic latexes up to 0.2 wt % under UV (---) and visible () irradiation.

of polymerization, DLS analyses of the seed particles and final PL-10-D latex have been given in Figure 5 beside the SEM micrograph. It could be recognized that no secondary particle is formed during addition of GMA and MMA in the second feed (Figure 5a). Also, a slight increase in particle size relating to the particle growth is observed. The uniform size and morphology of the PL-10-D latex particles is evident from the corresponding SEM image (Figure 5b).

growth of primary particles can be attributed to the enhancement of ionic strength in the polymerization course. By considering the role of SPEA in controlling particle size, it is necessary to optimize the situation to obtain particles less than 100 nm with narrow size distribution for appropriate photochromic properties. In this regard, PL-10-D with particle size of about 90 nm (PSD: 1.24) was nominated as the proper sample. To verify the particle growth through the second stage 10677


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Figure 7. Color changes of the prepared latexes before and after UV irradiation at 365 nm.

Figure 8. DSC (a) and TGA−DTG (b) thermograms of PL-10-A and PL-10-D samples.

4-2. Studies on Photochromic Properties. Dimensions of polymeric nanoparticles have a significant impact on the intensity and magnitude of the photochromic properties. Two concerns exist in this manner:6,8 (i) particle size should be less (< λ/2) than the wavelength (λ) of irradiated light, and (ii) scattering will be dominated with the increase in particle size undesirably. Here, photochromic properties of all the samples were investigated using UV−vis analysis (Figure 6). These studies revealed that the absorption intensity reduced significantly with the increase in particle size. In addition, it is expected that SPEA may tend to migrate to the surface of polymer particles due to its hydrophilic nature when converted to merocyanine colored form in the aqueous media. As a result, the photochromic efficiency could be improved by reducing particle size and enhancement in surface area. DLS and SEM analyses depict that PL-10-B to PL-10-D have particles below 100 nm, and it is expected to show remarkable absorption intensities in UV−vis analysis. Low absorption for PL-10-B was anticipated to the low concentration of SPEA in

the diluted latex. On the other hand, PL-10-C and PL-10-D have significant absorption intensities, which are attributed to the appropriate concentration of spiropyran as well as appropriate latex particle size. No reasonable absorption was observed for PL-10-E to PL-10-J samples because of their large particle sizes. A comparison between UV absorption of PL-10C and PL-10-J, with the increase in SPEA concentration up to 10-fold, reveals that particle size plays a more important role than the SPEA amount. Although SPEA locates on the particles surface desirably, the quantity of scattered light plus the reduced overall surface area (with the increase in particle size) are responsible for the dramatic decrease in absorption intensity. It is notable that, to have comparable results, all the latexes were diluted similarly to 0.2 wt % prior to UV−vis analysis. Afterward, the prepared photochromic latexes were exposed to UV irradiation (365 nm), and the color changes were examined (Figure 7). Latexes with particles greater than 100 nm display a slight color change upon UV irradiation. PL-10-B 10678


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Figure 9. Variation of storage and loss modulus (a) and Tan δ (b) with temperature for PL-10-A and PL-10-D samples from DMTA analysis.

achievement of copolymerization reaction, and slight changes in thermal stability are related to the low content of SPEA. Dynamic-mechanical thermal analysis (DMTA) was used to study thermo-mechanical properties of the prepared dried latexes (Figure 9). Figure 9a reveals an increment in storage modulus (E′) and loss modulus (E″) curve from PL-10-A to PL-10-D. These little differences could be explained by the difference in their rigidity through inclusion of SPEA of about 3 wt % in the copolymer structure. However, the storage modulus diagrams reveal that the length of glass transition region has been shortened for PL-10-D. This reduction can be ascribed to the brittle structure of PL-10-D, which is in conformation with DSC results. These parameters are also affected by the restrictions in the mobility of the polymeric chains that is caused by the improved dipolar interactions due to the presence of SPEA in the copolymer. The changes in brittleness and rigidity of the samples have resulted in a decrease in Tan δ peak for PL-10-D with respect to PL-10-A. Although a small amount of SPEA had been used in the polymerization feed, it influenced the thermomechanical properties. Totally, these observations illustrate the progress of the copolymerization reaction between SPEA and MMA. 4-5. Cellulosic Paper with Photochromic Properties. Stimuli-responsive photochromic papers have received much attention in recent years with respect to their potential applications in photosensitive paper coatings, security, and authentication documents.11,16,29,32 In all studies, the photochromic dye has been introduced into a polymeric carrier by doping and subsequent mixing with a cellulosic paper. The lack of chemical bonding between photochromic compounds and polymeric carriers results in negative photochromism,29,32 aggregation,11 and reduction of photostability and lifetime16 of these compounds, which reduces their photochromic properties dramatically. In order to induce suitable photochromic properties in cellulosic papers, stimuli-responsive papers were prepared by impregnation of the filter paper (MUNKTELL-Grade: 391-Lot No: 09-158) in PL-10-D photochromic latex and then dried at 80 °C. The effect of polarity of the media on color changes was studied by immersing the paper in water, ethanol and methanol (Figure 10). Different color changes were seen when these papers were exposed to UV irradiation (365 nm). These observations can be attributed to the interaction of merocyanine colored form in various polar media. Ionic merocyanine structure can be

displays a negligible color change as a result of a low concentration of SPEA, which was also observed from UV− vis analysis. In addition, similar photochromic properties with insignificant color changes could be seen for PL-10-E to PL-10J samples. The most intense macroscopic color changes are related to PL-10-C and PL-10-D ones. Consequently, PL-10-D with optimum photochromic properties, minimum particle size and highest absorption intensity was selected for our further studies. It is worth mentioning that PL-10-D latex showed reasonable photostability, photoreversibility, and fast photoresponsivness according to the convenient test methods for similar systems.8,17 4-3. Studies on Thermomechanical Properties. Investigation of glass transition temperature (Tg) for PL-10-A and PL-10-D latexes was performed by using differential scanning calorimetry (DSC). Comparative thermograms of PL-10-A and PL-10-D samples (Figure 8a) show an increase in Tg as a result of incorporation of SPEA into the polymer backbone. PL-10-A shows a transition at 109 °C, indicating the amorphous structure of PMMA. However, PL-10-D sample (containing about 3 wt % of SPEA) reveals a single transition temperature at 123 °C. Considering the aromatic structure of SPEA and also its melting point (98−101 °C), the slight increase in Tg (about 14 °C) could be attributed to the incorporation of SPEA as a comonomer into the polymer chain. Due to the large volume of spiropyran moiety, its introduction to the polymer structure can lead to an increase in steric hindrance and limitation of mobility of polymer chains.46 Also, the presence of dipolar interactions in SPEA would be another reason for restriction in chain mobility and increase in Tg. These results depict that the copolymerization of SPEA with MMA has been conducted, and no phase separation was observed in the final copolymer according to the appreanence of a merely single transition temperature. These were approved by other thermal analysis too and are discussed in the following. The effect of incorporation of SPEA on thermal resistance of the prepared copolymer was studied by TGA (Figure 8b). The results reveal a slight increase in thermal stability of PL-10-D in comparison with PL-10-A. It is worth mentioning that both samples have similar degradation profiles, and are performed in a single step. If SPEA generates its own domains during polymerization, it will be expected that the degradation occurs in different patterns and that multistep decomposition will be observed. The uniformity of thermal degradation confirms the 10679


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solvated easily (either by hydrogen bonding or dipolar interactions) in polar solvents and thus absorbs light with different wavelengths and exhibits various colors under UV irradiation. Morphology changes of cellulose fibers after being impregnated with photochromic latex has been recorded by SEM images taken from the surface of photochromic papers (Figure 11). Comparison between the morphology of cellulose fibers before (Figure 11a,b) and after (Figure 11c,d,e) impregnation reveals the diffusion and stabilization of PL-10D polymer between cellulose fibers. The uniformity of the modified paper demonstrates a good interaction and compatibility between PL-10-D polymer and cellulose fibers. This returns to the possible reaction between hydroxyl groups (on cellulose) with epoxy groups (PL-10-D) during impregnation and thermal treatment, and forms such a compatiblized structure. The existence of some cracks (Figure 11e) relates to the brittleness of PL-10-D polymer with high Tg (123−125

Figure 10. Color changes of the prepared stimuli-responsive photochromic papers modified with PL-10-D latex.

Figure 11. SEM images of the primary (a and b) and impregnated (c, d, and e) cellulose fibers with PL-10-D latex in different magnifications. 10680


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Langmuir °C). In some reports, the photochromic compound is doped into the polymer latex without any chemical bonding.11,16 This will cause a lack of sufficient stability of the optical properties. On the other hand, if the photochromic compound is added to the cellulose paper directly,29 its smart behavior regarding to reversible isomerization is sacrificed. This is attributed to the establishment of strong hydrogen bonding between merocyanine isomer and hydroxyl groups in cellulose and is called as negative photochromism. The main advantage of this work is to cover both of the above-mentioned deficiencies by designing an appropriate copolymer matrix. The presence of epoxy groups in the photochromic latexes provides the possibility of chemical modification of the cellulose matrix. This matter is under investigation by our group and will be reported soon. This will result in the stability of photochromic properties of corresponding stimuli-responsive cellulose coatings.

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5. CONCLUSION In this work, functionalized photochromic latex nanoparticles with epoxy groups were prepared using semicontinuous emulsion polymerization. The effects of SPEA content, as the optically active monomer, on particle size and photochromic properties were investigated in detail. The increase in particle size in the range of 60 to 700 nm was observed by increasing SPEA monomer-surfactants ratio, owing to the elevation in ionic strength of the medium because of the existence of merocyanine isomeric form. UV−vis analysis reveals reduced absorption intensity with the increase in particle size. Maximum photochromic properties were found for the latex containing 3 wt % of SPEA with particles size of about 90 nm. Investigations of thermal and thermo-mechanical properties illustrated the incorporation of SPEA into the polymer chains. Photochromic papers were prepared by impregnation of cellulosic paper in the photochromic latex and resulted in stimuli-responsive papers with the ability to respond to UV radiation. Different and reversible color changes for the paper were observed in dry and wet form (in liquids with different polarities). Morphological studies showed complete stabilization of the photochromic polymer within the cellulose fibers, and their compatibility is related to the susceptibility of chemical reaction between these two phases. This will result in enhanced photostability of such smart and stimuli-responsive photochromic papers.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +9821 4478 7000; Fax: +9821 4478 7023; E-mail: a. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to express our gratitude to Iran Polymer and Petrochemical Institute (IPPI) for financial support of this work (Grant# 24761167).

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REFERENCES

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Preparation of Stimuli-Responsive Functionalized Latex Nanoparticles

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