Preparation of Fast Photoresponsive Cellulose and Kinetic Study of

Apr 26, 2016 - Jaber Keyvan Rad and Ali Reza Mahdavian. Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran...
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Preparation of Fast Photoresponsive Cellulose and Kinetic Study of Photoisomerization Jaber Keyvan Rad and Ali Reza Mahdavian*

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Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran, 14967 Iran ABSTRACT: Exploitation of a polymer carrier for introducing photochromic properties to cellulose matrix has several advantages such as photostability, photoreversibility, elimination of dye aggregation, and elimination of undesirable negative photochromism. The switching rate of a photochromic compound in the polymer matrix depends on steric restrictions, polarity, protic characteristics, and flexibility of the surrounding media. Here, the copolymerization of a spiropyran-based monomer with butyl acrylate and methyl methacrylate comonomers is reported through semicontinuous emulsion polymerization and the kinetics of isomerization and switching rate are investigated. The obtained latex was incorporated into cellulosic paper through chemical modification. The analysis confirmed a ring-opening reaction between hydroxyl groups in cellulose and epoxy functional groups in the low Tg latex. Morphological studies and contact angle measurements demonstrated an improved uniformity and enhanced hydrophobicity in the modified cellulose. Solid state UV−vis spectroscopy was employed to determinate the switching rate, kinetics analyses, maximum reflection wavelengths, and removal of undesired negative photochromism. Analyses revealed that flexible epoxy-functionalized photochromic modified cellulose exhibited reasonable fatigue resistance, photoresponsivity, and photoreversibility upon alternative UV and visible irradiation.

1. INTRODUCTION Photochromism is a reversible transformation of chemical species between two isomers promoted in one or both directions by light that have vastly different properties.1 The main attention in photochromic compounds has been paid to their applications such as fluorescence modulation,2−4 rewritable papers,5 ophthalmic lens,6 real time holographic materials,7 self-erasing papers or self-healing coatings,8,9 optical data storage,10,11 three-dimensional printing,12 color-changeable textiles,13 and controlled release systems.14,15 Spiropyran derivatives are known as important members of the photochromic materials family which can switch from a stable colorless state (spiropyran, SP) to a metastable colored state (merocyanine, MC) upon exposure to external stimuli such as optical irradiation,16−18 solvents,19,20 ions,21 mechanical force,22 temperature,23 and pH.24 Negative photochromism is an undesired phenomenon where the colored state becomes more stable than the colorless one and converts to a colorless isomer upon exposure to visible irradiation in a reduced rate.25 Highly polar environment,19 hydrogen bonding,26 polymer matrix,27 conjunction with macromolecules,28 and complexation with metal ions29 are some factors that may lead to stabilization of the MC form and observation of negative photochromism. The isomerization rate of a photochromic compound is closely dependent on the free volume, flexibility, and polarity30−33 of the surrounding medium, because photochromic transformation is accompanied by changes in physical and chemical properties. Hence, photochromism © 2016 American Chemical Society

efficiency in a rigid polymer matrix decreases due to the restriction in essential free volume changes for isomerization.33,34 Exploitation of a polymeric matrix may result in a substantial decrease in negative photochromism of the photochromic dyes. However, flexible polymer matrixes have remarkable improvement in the coloration efficiency and isomerization rate of such chromophores.30,31,35 Incorporation of photochromic spiropyran into a polymer matrix by covalent bonding1 has many advantages over simple doping36 such as prevention of dye leaching, dye aggregation, and negative photochromism; enhanced stability; improved fatigue resistance; solvent compatibility; improved fluorescence; and biocompatibility. Evans and co-workers37 studied the effect of integrating low glass transition temperature (Tg) materials on the switching speed of a photochromic dye in a rigid polymer matrix (such as in ophthalmic lens). They reported that the attachment of flexible oligomers such as poly(dimethylsiloxane) to the photochromic dye make a significant enhancement in its switching rate. Ishii et al.7 reported the controlled coloration and thermal fade speed of the photochromic dye in rigid poly(methyl methacrylate) matrix by means of adding plasticizers. Their results show that the plasticized polymers change the viscosity of the surrounding medium around the Received: March 12, 2016 Revised: April 19, 2016 Published: April 26, 2016 9985

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The Journal of Physical Chemistry C Table 1. Preparation of Epoxy-Functionalized Photochromic Latexes with Different Tg’sa,b inner layer

a

outer layer

latex

SDS

Triton X-100

KPS

NaHCO3

SPEA

MMA

BA

MMA

GMA

RFP FFP

0.045 0.06

0.02 0.03

0.03 0.03

0.03 0.03

0.1 0.1

2.1 0.5

0.0 1.6

0.4 0.4

0.4 0.4

The total amount of water in each recipe was 27 mL. bAll amounts are in grams, and the total solid content was set to 10 wt %

Figure 1. Preparation of various photoresponsive papers and their color changes under UV (365 nm) and visible (520 nm) irradiation.

photochromic dye and enhance the free volume and intramolecular rotations associated with the photochromic isomerization between the two states. These introduce potential applications in real-time dynamic holograms. Hatano et al.38 synthesized a novel photochromic compound in which the colored state is normally stable and photochemically converts to its metastable colorless state upon exposure to visible light. Then the photogenerated species can return thermally to their initial state. Sun and co-workers39 studied the effect of entrapped spirooxazine dye in polystyrene latex and its photochromic efficiency relative to simply dissolved spirooxazine in acetone and its deposition on cellulose paper. Their results demonstrated that the use of polystyrene matrix as a carrier for the photochromic dye induced higher photochromism efficiency, color stability, and fatigue resistance, and avoided dye aggregation in comparison with just-dissolved spirooxazine in acetone in the absence of any carrier.

In this study, a functionalized photochromic spiropyran dye was employed and covalently linked to the backbone of both flexible and rigid epoxy-modified polymeric latexes. The prepared rigid epoxy-functionalized photochromic (RFP) particles and flexible epoxy-functionalized photochromic (FFP) particles were reacted with the dispersed paper pulp as a cellulosic matrix in water. Direct inclusion of spiropyran into the highly polar medium of cellulose paper caused strong hydrogen bonding between the hydroxyl groups of cellulose and the resulting MC form of spiropyran (after isomerization of SP to MC); that is a reason for observing undesired negative photochromism. It was found that application of the polymer carrier for introducing spiropyran to cellulose matrix obviated the above negative photochromism. The prepared photoresponsive papers from FFP showed a uniform and uncracked surface with high coloration efficiency and isomerization speed. These were attributed to the ease of conformational trans9986

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The Journal of Physical Chemistry C formations in the flexible polymer matrix with appropriate free volume for the isomerization of spiropyran. The results confirmed the effect of Tg of the polymeric matrix on the photochromic phenomenon, beside improvements in the uniformity and quality of the obtained cellulosic paper, optical fatigue resistance, hydrophobicity, and prevention of negative photochromism. To the best of our knowledge, this is the first report on the kinetics and studies on the switching rate of photoisomerization of spiropyran in solid phases, and typically cellulosic substrate.

nm) irradiation. To compare, 10 mL of the paper pulp mixture was treated with a solution of (R/S)-2-(3′,3′-dimethyl-6-nitro3′H-spiro-[chromene-2,2′-indole]-1′-yl)ethanol [(R/S)(SPOH)] in water (1.7 mL, 8.2 × 10−3 M). They were mixed by mechanical stirring (1200 rpm) at ambient temperature for 5 h. Then, the impregnated pulp paper was cast in a Petri dish and air-dried at room temperature. 2.4. Characterization. To prepare paper pulp, a SONOPULS ultrasonic homogenizer (20 kHz, HF-GM 2200, BANDELIN electronic GmbH & Co., Germany) was used with a titanium microtip KE-76 probe (D 6 mm). Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra were recorded for solid films on a BRUKER-IFS48 (Germany) spectrometer to endorse the reaction between functional groups of the photochromic latex and the hydroxyl group of the cellulosic paper pulp. Scanning electron microscopy (SEM) was recorded by a Vega Tescan II (Czech Republic). Prior to scanning, a piece of the prepared photoresponsive paper was placed on the sample holder and a layer of gold was deposited by using EMITECH K450x sputter-coating (England), under vacuum and flushed with argon. The contact angle of water on the surface of stimuli-responsive papers was measured using a KRUSS G10 (Germany) at room temperature and 23% relative humidity. A double distilled water (DDW) droplet (5 μL) was placed on the surface of as-prepared papers. A minimum of three measurements were carried out for each sample and then averaged and reported. The isomerization speed and photochromic efficiency of the photoresponsive papers were investigated by solid phase UV−vis analysis and by using a high performance double beam scanning spectrophotometer T90+ (PG Instrument, England). The excitation was done with a UV lamp (365 nm), Model Camag 12VDC/VAC (50/60 Hz, 14VA, SER 1206, Switzerland). The source for visible light was a common LED lamp with white light.

2. EXPERIMENTAL SECTION 2.1. Materials. 2,3,3-Trimethylindolenine, glycidyl methacrylate (GMA), and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. All of the solvents and 2hydroxy-5-nitrobenzaldehyde, butyl acrylate (BA), methyl methacrylate (MMA), potassium persulfate (KPS), sodium hydrogen carbonate (NaHCO3), triethylamine, Triton X-100, 2-bromoethanol, and acryloyl chloride were supplied by Merck Chemical Co. All chemicals were used without further purification. Deionized (DI) water was used in all recipes. 2.2. Preparation of Epoxy-Functionalized Photochromic Latex Particles with Different Tg’s. The rigid epoxyfunctionalized photochromic (RFP) particles containing spiropyran were prepared according to our previously reported procedure.40 Flexible epoxy-functionalized photochromic (FFP) particles were produced via a semicontinuous emulsion polymerization in which 1′-(acryloxyethyl)-3′,3′-dimethyl-6nitrospiro-(2H-1-benzopyran-2,2′-indoline) (SPEA) was incorporated into the flexible hydrophobic core. The functionalized outer layer with epoxy groups was fabricated by the second feeding of comonomers (GMA and MMA). The amounts of components used in the formation of photoresponsive latex are listed in Table 1. In general, ionic and nonionic surfactants (SDS and Triton X-100), buffer (sodium hydrogen carbonate, NaHCO3), and KPS were dissolved in 24 mL of DI water and kept under continuous flow of nitrogen gas. Then the obtained mixture was placed in an oil bath at 70 °C. An aqueous solution of SPEA in 3 mL of DI water and a mixture of MMA and BA were added separately and dropwise into the reactor within 10 min. To build up the outer layer, the mixture of GMA and MMA was added dropwise within 10 min to the above mixture after progress of the previous step in about 45 min. The required time for completion of reaction was 30 min after the second addition and according to monitoring monomer conversion by gravimetric method. The amount of coagulation was less than 1 wt % and also SPEA content in nanoparticles was determined to above 95% due to the previous report.40 2.3. Preparation of the Stimuli-Responsive Cellulosic Paper. High quality filter paper (MUNKTELL-Grade 391, Lot No. 09-158) (10 g) was converted to pulp by mechanical stirring (at 1200 rpm) in DI water (190 mL) at room temperature and then sonicated for 20 min at a power of 75% to obtain excellent dispersion. In continuum, the prepared pulp paper was used for the preparation of different stimuliresponsive papers (Figure 1). To prepare the stimuli-responsive paper strips, 10 mL of the above pulp paper dispersion (5 wt %) and 1.7 mL of the prepared RFP or FFP latexes were mixed for 5 h at room temperature and corresponding photoresponsive paper strips were cast and dried at ambient conditions. The coloration efficiency and switching speed of the prepared photoresponsive papers were investigated upon UV (365 nm) and visible (520

3. RESULTS AND DISCUSSION Exploitation of polymer carriers for incorporation of photochromic compounds in different media will improve their photochromic efficiency, color stability, and fatigue resistance and prevents some undesired phenomena such as like negative photochromism, dye leakage, and also probable environmental degradative issues. However, these advantages will be highlighted more if the photochromic compound is covalently linked to the polymeric matrix.2,40 Flexibility of the polymer carrier is known as an important factor in the photochromism efficiency and switching rate between two isomeric forms. To prepare smart and quick responsive cellulosic papers, FFP latex was prepared by semicontinuous emulsion polymerization and consequent photochromic efficiency and coloration rate were investigated and compared with those of an RFP latex of similar composition. Flexible polymer carriers have additional advantages such as fast switching between colored states, obviation of surface cracks, and improvement in hydrophobicity of the obtained stimuli-responsive cellulose. 3.1. Preparation and Characterization of Photoresponsive Papers. The ring-opening reaction between epoxy groups of FFP and RFP latexes and hydroxyl groups in cellulose was studied by attenuated total reflectance (ATR) mode of FTIR analysis (Figure 2). The characteristic peaks of epoxides appear in 754 and 844 cm−1, relating to stretching and bending vibrations, respectively (Figure 2a). The stretching vibration of the hydroxyl groups in cellulose paper could be found clearly at 3331 cm−1 (Figure 2b). Photoresponsive 9987

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explained by the availability of epoxy groups. By incorporation of butyl acrylate (BA) in the copolymer composition in FFP latex, the particles become more hydrophobic, and this would result in repelling of polar chains containing GMA comonomer. Hence, epoxy groups will be localized in the outer layer (relative to RFP latex particles) and are more accessible to react with hydroxyl groups in cellulose. Surface morphologies of cellulose fibers were recorded before and after modification with RFP and FFP latexes by SEM analysis (Figure 3). These micrographs demonstrate effective wetting, diffusion, and deposition of both photochromic latexes in cellulose fibers, and they are considerably coated with both copolymers (Figure 3). This returns to the establishment of strong hydrogen bonding (cellulose with RFP and FFP latex) and reaction between epoxy groups on the FFP particle surface and hydroxyl groups of cellulose. However, images with higher magnifications reveal some cracks in the coated fibers with RFP latex and this could be attributed to the brittleness of the employed copolymer. In contrast, no crack or brittle behavior was observed for those modified with FFP latex. Coating of cellulosic fibers, which are primarily hydrophilic, with an organic polymer will make them hydrophobe. This can be followed by measurement of the contact angle (θ) of a DDW droplet. The observed contact angles for the coated papers with RFP and FFP latexes were 32.7 and 97.5, respectively (Figure 4). Although these data confirm the existence of applied copolymer on the paper surface, the difference in θ goes back to the difference in their compositions. FFP copolymer includes more butyl acrylate comonomer that lowers the corresponding Tg. Hence, it is expected to be more hydrophobic with a higher contact angle than RFP copolymer including just MMA monomer. 3.2. Photochromic Properties and Photoswitching Rate. Solid-state UV−vis spectroscopy was used to observe

Figure 2. ATR-FTIR analysis of primary functionalized photochromic latex (a), cellulosic paper (b), mixture of cellulosic paper with RFP latex (c), and FFP latex (d).

cellulose paper prepared by mixing of RFP latex and paper pulp did not show any specific change in the characteristic peaks of epoxy groups apparently (Figure 2c). The existence of characteristic epoxy peaks depicts the incompletion of reaction between cellulose hydroxyl groups and RFP latex. However, this reaction proceeded well for FFP latex and corresponding epoxy peaks disappeared (Figure 2d). This is a qualitative method to follow up the reaction progress between these functional groups. The correlation between the reactivity of such latexes and Tg’s of the corresponding copolymers could be

Figure 3. SEM images of the pure cellulose (a) and coated cellulose with RFP (b, d) and FFP (c, e) latexes with different magnifications. 9988

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papers modified with RFP and FFP latexes, no reflectance was observed in 500−600 nm before UV irradiation at 365 nm (Figure 5b,c). This illustrates that none of the SPEA chromophores has been transformed from SP to MC isomer upon inclusion into the cellulosic matrix initially. In other words, the employed acrylic copolymer, as the carrier for SPEA, has achieved preservation of the chromophores and prevention of negative photochromism. λmax for the reflectance index of MC usually changes by variation in polarity, flexibility, and protic or aprotic characteristics of the medium.25 These were measured and found to be 529, 583, and 565 nm for the modified cellulose with SP-OH, RFP, and FFP samples, respectively. The impregnated cellulosic with SP-OH has the lowest λmax because of the highly polar and protic medium of the cellulosic substrate. Among modified samples, the changes in λmax could be explained by the environmental effects and resulting dipolar interactions between newly formed MC and polymer chains. Inclusion of BA in the copolymer composition has two effects: (i) lowering Tg and (ii) hydrophobizing polymer medium. The later will result in a red shift for λmax in the modified paper with FFP relative to the impregnated one with SP-OH and a blue shift with respect to the RFP-modified cellulosic paper. Real-time analysis is a reliable and accurate method for investigating SP to MC isomerization in SPEA. UV analysis was recorded for the prepared RFP and FFP-modified photochromic papers in different irradiation times at 365 nm (Figure 5b,c). Obviously, reflective intensities are decreased with the increase in irradiation time and this returns to the increase in MC concentration at longer irradiation time. It could be noted that the reflectance intensity reached 13% for FFP-modified cellulose upon UV exposure for 30 s and this went to 18% for the RFP-modified one even for 510 s exposure time. These demonstrate that higher chain flexibility and more free volume between copolymer chains provide proper conditions for the fast and efficient switching rate in the prepared photochromic papers. The kinetics of isomerization and variation in reflectance intensity of spiropyran chromophore could be described in the obtained photosensitive papers by eq 1.

Figure 4. Contact angle measurement of a water droplet on the modified cellulosic papers with RFP (a) and FFP (b) latexes.

reflectance spectra of the prepared photoresponsive papers upon UV irradiation at 365 nm (Figure 5). Reflectance spectra of the prepared photochromic paper by impregnation of cellulosic fibers with SP-OH solution before UV irradiation (365 nm) demonstrates a reflective index of approximately 54% in the range 500−600 nm (λmax = 529 nm), which is ascribed to the existing negative photochromism (Figure 5a). For cellulosic

ln((R ∞ − R 0)/(R ∞ − R t )) = k isot

(1)

where R0, R∞, and Rt are the maximum reflectance intensity of spiropyran in the photoresponsive papers at 500−600 nm after UV irradiation (365 nm) at times 0, infinity, and t, respectively. These data could be extracted from Figure 5, and kiso represents the isomerization rate constant of the spiropyran moiety in the smart papers. The time profile response for the RFP-modified cellulose during UV irradiation demonstrates a deviation from the firstorder kinetics (Figure 6a). First, the reflectance intensity decreases with a constant rate (kiso = 0.0304) and it continues to decrease with a slower speed (kiso = 0.0053). Facile switching in the first step returns to the existence and availability of SPEA chromophores that are susceptible to receiving energy. In continuum, further isomerization to MC upon exposure to UV needs more free volume. This cannot be provided because of the stiff polymer chains with limited motions. Therefore, lower switch speeds were observed after 60 s of UV irradiation at 365 nm. This issue was quite different for the FFP-modified cellulose (Figure 6b), in which the switching speed followed a first-order kinetics with high isomerization rate constant (kiso = 0.1464).

Figure 5. Time-dependent reflective indices of the impregnated photoresponsive papers with SP-OH (a) and after UV irradiation at 365 nm for those modified with RFP (b) and FFP (c) latexes. The images in the right show the colors of the corresponding paper strips. 9989

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photoresponsivity, photoreversibility, and fatigue resistance which arise from three main reasons: (i) covalent bonding between cellulose and FFP latex, (ii) flexibility, and (iii) hydrophobic surrounding medium of spiropyran.

4. CONCLUSION In this study flexible and rigid epoxy-functionalized photochromic latexes were employed to prepare UV responsive cellulosic papers. The ring-opening reaction between cellulose and latex nanoparticles proceeded more with the FFP one. SEM and contact angle analyses revealed that utilization of FFP latex had remarkable enhancement in obviation of surface cracks and increase in hydrophobicity of the obtained stimulicellulose papers. Solid-state UV−vis spectroscopy and corresponding kinetic studies illustrate that the flexibility and mobility of the polymer chains, surrounding SPEA as the photochromic dye, have prominent effects on the switching rate, maximum reflectance intensity wavelength, and efficiency of isomerization between SP and MC forms. This study will open up new developments in the preparation of facile and efficient photoresponsive polymeric devices with their mechanistic features.

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AUTHOR INFORMATION

Corresponding Author

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

Figure 6. Isomerization kinetics for the modified cellulosic paper with RFP (a) and FFP (b) latexes after UV irradiation at 365 nm.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to express our gratitude to the Iran Polymer & Petrochemical Institute (IPPI) for financial support of this work (Grant 24761172).

A comparative study between switching rates of photochromic spiropyran in FFP- and RFP-modified celluloses emerges from the dramatic difference owing to the polymer features, such as flexibility and existing free volumes, which facilitate isomerization. In addition, the fatigue resistance, photoresponsivity, and photoreversibility of FFP-modified cellulose with desired optical properties were investigated. For this reason, FFPmodified cellulose was exposed to UV and visible alternative irradiation at 365 and 520 nm for 10 times at 30 and 60 s, respectively (Figure 7). The observed responsiveness under cycles of UV and visible irradiation demonstrated significant

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REFERENCES

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Figure 7. Variation in reflective index for FFP-modified cellulose upon alternative UV (365 nm) and visible (520 nm) irradiation for 30 and 60 s, respectively. 9990

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DOI: 10.1021/acs.jpcc.6b02594 J. Phys. Chem. C 2016, 120, 9985−9991