Stimuli-responsive cellulose modified by epoxy-functionalized polymer nanoparticles with photochromic and solvatochromic properties

Carbohydrate Polymers 150 (2016) 131–138

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Stimuli-responsive cellulose modified by epoxy-functionalized polymer nanoparticles with photochromic and solvatochromic properties Amin Abdollahi, Jaber Keyvan Rad, Ali Reza Mahdavian ∗ Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran

a r t i c l e

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Article history: Received 25 January 2016 Received in revised form 30 April 2016 Accepted 4 May 2016 Available online 6 May 2016 Keywords: Stimuli-responsive Photochromic Solvatochromic Cellulose Spiropyran Nanoparticle

a b s t r a c t Photoresponsive papers are among the fast and simple tools for detection of polarity by solvatochromic and photochromic behaviors upon UV irradiation. Here, a new, green and facile modification strategy was employed to prepare novel stimuli-responsive cellulose materials containing spiropyran by mixing microcrystalline cellulose (MCC), as a model compound, with epoxy-functionalized photochromic latex. FTIR analysis, thermal and thermo-mechanical properties were used to confirm the microstructral properties. Crystallographic analysis revealed a decrease in crystallinity of cellulose matrix and approved the incorporation of photochromic copolymer. Then stimuli-responsive papers were prepared by using pulp paper as the cellulosic matrix and their smart characteristics were studied under UV irradiation while dried or immersed into some polar and non-polar solvents. Different color changes were observed and investigated by solid-state UV–vis spectroscopy. These significant results were attributed to the efficient chemical modification and confirmed by SEM, EDX and nitrogen mapping analyses. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Spiropyran as a classic photochromic compound has received much attention because of its superior physical, chemical, photochromic and reversible properties in recent decade (Crano & Guglielmetti, 1999; Crano, 2002; Klajn, 2014). The photochromism phenomenon and some of the related properties of spiropyrans were firstly investigated by Fischer and Hirschberg in 1952 (Crano & Guglielmetti, 1999; Crano, 2002). They observed the color changes of diluted solution of spiropyran while exposed to UV irradiation (less than 450 nm) and its return to colorless form under visible light (above 500 nm). Physical and chemical properties of spiropyran such as photochromism and solvatochromism characteristic can be reversibly switched “on” and “off” through isomerization reaction between spiro-form (SP, closed ring) and merocyanine-form (MC, open ring) under UV irradiation and visible light, respectively (Xia et al., 2014; Zhu et al., 2007; Zhu et al., 2006). However, SP-form have some characteristics such as three dimensional structure (Shiraishi et al., 2014; Wojtyk, Kazmaier, & Buncel, 2001), inert and colorless nature (Chen et al., 2015; Darwish et al., 2012), while the colored MC-form have the

∗ Corresponding author. E-mail address: [email protected] (A.R. Mahdavian). http://dx.doi.org/10.1016/j.carbpol.2016.05.009 0144-8617/© 2016 Elsevier Ltd. All rights reserved.

planar and zwitterionic structure with a large dipole moment. Recent studies have revealed that the response of spiropyran photochromic molecules to external stimuli such as heat (thermochromic) (Shiraishi, Miyamoto, & Hirai, 2009), UV/Vis light (Shiraishi et al., 2014; Wojtyk et al., 2001), polarity (causing solvatochromism) (Florea, McKeon, Diamond, & Benito-Lopez, 2013; Schenderlein, Voss, Stark, & Biesalski, 2013; Tian & Tian, 2014b), moisture (hydrochromic) (Sheng et al., 2014) and mechanical force (mechanochromic) (Gossweiler et al., 2015; Peterson, Larsen, Ganter, Storti, & Boydston, 2014) strongly depends on the interactions of spiropyran with the adjacent environment, as well as other factors such as molecular packing, charge, and orientation (Achilleos & Vamvakaki, 2010; Wan, Zheng, Shen, Yang, & Yin, 2014). Incorporation of photochromic compounds into the polymer matrix would occur through doping (Sun et al., 2013; Sun, Hou, He, Liu, & Ni, 2014) or chemical bonding (Cayre, Chagneux, & Biggs, 2011; Keyvan Rad, Mahdavian, Salehi-Mobarakeh, & Abdollahi, 2016; Liao et al., 2015; Wu, Zou, Hu, & Liu, 2009). Physical linkages lead to a decrease in photostability, photochromic properties, coloration efficiency and reversibility. On the other hand, fabrication of stimuli-responsive polymer particles via copolymerization of the photochromic monomer derivatives through emulsion (Keyvan Rad et al., 2016; Zhu et al., 2007) and miniemulsion (Chen, Zeng, Wu, Su, & Tong, 2009; Tian, Wu, & Li, 2009) polymerization

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Table 1 Preparation of the photochromic cellulose samples.a Photochromic copolymer contentb (wt%)

MCC (g)

Water (mL)

PL (mL)

Sample

20 25 30 40

4 3 2.35 1.5

10 10 10 10

10 10 10 10

PC-20 PC-25 PC-30 PC-40

a b

Amount of photochromic copolymer in each sample is 1 g. Relative to the total amount of photochromic copolymer and MCC.

are among the most promising techniques. The presence of a photochromic dye beside the cellulose matrix can induce the preparation of stimuli-responsive papers with potential application in different fields such as stimuli-responsive papers (Sun et al., 2013, 2014), smart inkjet printing (Tian et al., 2009), photo-sensitive textile (Feczkó, Samu, Wenzel, Neral, & Voncina, 2013), security documents (W. Tian & Tian, 2014b), sensors and authentication systems (Sheng et al., 2014). Exploitation of polymeric carriers for transformation of photochromic dyes inside the cellulose matrix is an effective factor for improving life-time of the photochromic dye (Abdollahi, Mahdavian, & Salehi-Mobarakeh, 2015; Sun et al., 2013, 2014), omitting negative-photochromism phenomenon (Tian & Tian, 2014a,b), and increase in fatigue resistance of the dye (Crano, 2002; Klajn, 2014). To increase photostability, lifetime, fatigue resistance and photochromic efficiency of chromophores in polar media (like cellulose), polymer particles could be preferably linked covalently to the matrix through specific processes (Sun et al., 2013, 2014). Functionalization of photochromic copolymers is a successful strategy to immobilize them onto other substrates through chemical bonding (Abdollahi et al., 2015). Glycidyl methacrylate (GMA), which bears an epoxy functional group, is a good candidate for preparation of such reactive polymeric particles and their involvement in stimuli-responsive cellulosic papers through a simple substitutionnucleation or ring-opening reaction. Sun et al. (2013, 2014) has recently reported the preparation of photochromic papers by using latex particles and cellulose nanocrystals (CNC) as the carrier for spiroxazine dye. In both studies, spiroxazine was incorporated into the carrier by doping and subsequent impregnation of the paper with these carriers. Hence, they showed a developed photostability, fatigue resistance, photochromic properties and lifetime of the spiroxazine in the cellulosic matrix in comparison with simple impregnation with spiroxazine solutions. Introducing a carboxylic derivative of spiropyran to cellulose through esterification reaction between hydroxyl groups of cellulose and carboxylic moieties of spiropyran was investigated by Tian et al. (W. Tian & Tian, 2014b). It could be concluded from several reports that utilization of appropriate polymer carriers to immobilize spiropyran into cellulose matrix would prevent negative photochromism phenomenon (Crano & Guglielmetti, 1999; Tian & Tian, 2014a,b), irreversible color changes (Abdollahi et al., 2015; Sun et al., 2013, 2014) and also decrease in photochromic behavior in addition to the increase in fluorescence efficiency. The irreversible photochromic responses are attributed to the stabilization of colored form of spiropyran (MC intermediate) through formation of hydrogen bonding between MC zwitterion and highly polar medium of cellulose (Tian & Tian, 2014a). Therefore, polymer carriers can play an influential role in the final properties of stimuli-responsive materials. In this paper and in continuum to our recent study (Abdollahi et al., 2015), we report a simple and green strategy for preparation of new stimuli-responsive cellulosic papers through chemical modification of cellulose. Photochromic epoxy-functionalized latex (PL) particles were prepared through semi-continuous emulsion polymerization with narrow size distribution and particle

size below 100 nm. The obtained PL copolymer was treated with microcrystalline cellulose (MCC) via ring-opening reaction. MCC was chosen primarily as a model compound to follow the reaction and performing optimizations before employing generalpurpose cellulose fibers. The result of affecting parameters for conducting this reaction were investigated by FTIR spectroscopy, crystallography, thermal and thermo-mechanical analyses. Then, stimuli-responsive paper pulps were fabricated. After full characterization, photochromic and solvatochromic properties were studied in dry and wet conditions in some different polar and nonpolar solvents. To the best of our knowledge, this is the first report on preparation of such novel stimuli-responsive cellulosic paper, which specifically points out the chemical modification of cellulose by PL particles without observing any negative photochromism. This may open up a research area and introduce new cellulosic materials for chemical detectors, optical switches and authentication systems.

2. Experimental 2.1. Materials 2,3,3-trimethylindolenin, glycidyl methacrylate (GMA), microcrystalline cellulose (MCC; 20 ␮m) and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. High quality filter paper (MUNKTELL-Grade: 391, Lot No: 09-158, made in Bärenstein, Germany) was used here. All of the solvents and 2-hydroxy5-nitrobenzaldehyde, methyl methacrylate (MMA), potassium persulfate (KPS), sodium hydrogencarbonate (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. Synthesis of epoxy- functionalized photochromic particles The photochromic latex (PL here and PL-10-D in reference #32) containing spiropyran dye (3 wt% relative to the total acrylic monomers) and epoxy functional groups from GMA (10% wt relative to the total amount of monomers) were prepared by semi-continuous emulsion polymerization according to our recent published work (Abdollahi et al., 2015). NaHCO3 , as the buffer, set pH of the obtained latex to 8–9. Size of the latex particles were in the range of 80–90 nm and had a narrow size distribution (PDI: 1.25) (Abdollahi et al., 2015). In addition, the solid content was about 10% wt and this was confirmed by the gravimetric method (supporting information). 2.3. Preparation of the photochromic cellulose samples In order to prepare photochromic cellulose based on spiropyran (PC-series), the aforementioned PL copolymer and microcrystalline cellulose were mixed with magnetic stirrer according to the given values in Table 1 at room temperature (25 ◦ C) for 5 h. The obtained dispersions were dried either at room temperature or at 80 ◦ C. The

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Fig. 1. Preparation of photochromic cellulose samples and its color changes under UV (365 nm) and visible irradiations or heat.

final prepared samples were finely powdered and used for further analyses.

2.4. Preparation of the stimuli-responsive paper First, the paper pulp precursor was prepared by mechanical stirring (1000 rpm) of the scraps of a high-quality filter paper in water at room temperature to yield a 10% wt paper-water dispersion. To improve the dispersion and uniformity of pulp paper, ultrasound waves were also applied to the above mixture for 20 min at 75% amplitude by probe-sonication with a titanium microtip probe. In order to prepare the stimuli-responsive paper, 70 mL of the pulp paper (7 g cellulose) and 30 mL of the photochromic latex (3 g neat photochromic copolymer) were mixed by mechanical stirring (1200 rpm) at 25 ◦ C for 5 h. Then, the obtained mixture was casted into a petri-dish and dried at 80 ◦ C. Their photochromic properties and color changes were investigated either in dried form or when immersed in polar and nonpolar solvents under UV irradiation (365 nm).

2.5. Characterization To prepare paper pulp, SONOPULS ultrasonic homogenizer (20 kHz, HF-GM 2200, GmbH & Co. Germany) was used with a titanium microtip KE-76 probe (D: 6 mm). Fourier transform infrared (FTIR) spectra were recorded on KBr pellets of the samples by using a BRUKER-IFS48 (Germany) spectrometer with a frequency range from 4000 to 400 cm−1 and resolution of 2 cm−1 at 25 ◦ C to endorse the reaction between functional groups of the photochromic latex and MCC. Thermal behavior of the prepared samples was analyzed by differential scanning calorimetry (DSC) on DSC 200, NETZSCH (Germany). All the samples were heated at 0 ◦ C to 200 ◦ C under N2 atmosphere at 10 ◦ C/min heating rate. Thermogravimetric analysis (TGA) was carried out on a TGA-STA-PL-1500 (UK). Samples were heated in the range of 25–700 ◦ C, and 10 ◦ C/min heating rate under N2 atmosphere. Dynamic-mechanical thermal analysis (DMTA) of the samples were studied by using TRITON Instrument (UK) and heating range was set from 30 ◦ C to 200 ◦ C(5 ◦ C/min heating rate) under N2 atmosphere (at frequency of 1 Hz and 50 mm

displacement). The powder form of MCC and PL copolymer were employed for this analysis. X-ray powder diffraction (XRD) was used to observe crystal type changes of the cellulosic samples, using a Siemens D5000 X-Ray diffractometer (Germany). The Cu-K␣ anode was used for radiation under the following conditions: voltage 40 kV, current 40 mA, scanning speed rate 1.2◦ min−1 , scanning step 0.02◦ and scanning scale (2␪) 5–60◦ . Crystalline index (CI) for each sample was calculated from XRD spectrum through equation CI = (Ac /At ) × 100, where Ac is the area of the crystalline peak (002) in the range of 18.5–26.3◦ and At is the total area (5–60◦ ) in each spectrum. Scanning electron microscopy (SEM) micrographs were obtained by a Vega Tescan II (Czeck Republic). Prior to scanning, a piece of the prepared stimuli-responsive paper was placed on the sample holder after washing with DI water and drying at room temperature. Then, a layer of gold was deposited by using EMITECH K450x sputter-coating (England), under vacuum and flushed with argon. To observe the distribution of nitrogen atoms, relating to spiropyran moieties in the photochromic paper, energy dispersive X-ray (EDX) analysis (INCA Model, Oxford Instron, England) was carried out. Photochromic properties of the papers were investigated by solid-phase UV–vis analysis and by using high performance double beam scanning spectrophotometer T90+ (PG Instrument, England). The excitation was done by 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. In all investigations, UV and Vis irradiation time were set 5 min for the samples. 3. Results and discussion Here, a new approach is used to prepare novel stimuliresponsive cellulose through chemical modification of cellulosic matrix with a photochromic epoxy-functionalized latex (PL) containing spiropyran moieties. For this reason, PL nanoparticles containing of about 3 wt% spiropyran, particle size of 88 nm with a narrow size distribution (PDI: 1.25) were used. According to our recent study (Abdollahi et al., 2015), these latex nanoparticles show photochromic characteristic with intensive color change upon UV irradiation, excellent reversibility and photostability in polar

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matrix and corresponding photochromic, solvatochromic properties and morphologies were investigated comprehensively. 3.1. FTIR analysis of the photochromic cellulose

Fig. 2. FTIR analysis of (a) MCC, (b) PL copolymer, (c) PC mixture at the beginning, (d) after 1 h of mixing (dried at room temperature), (e) after 1 h of mixing (dried at 80 ◦ C), (f) after 5 h of mixing, (dried at room temperature) and (g) after 5 h of mixing (dried at 80 ◦ C).

substrates. MCC was selected to check the chemical modification of cellulose with PL copolymer because of its simple and facile processing and easy characterization relative to usual cellulose fibers, while paper pulp was used for preparation of stimuli-responsive papers. In addition, the required mixing time for progress of ringopening reaction between epoxy functionalized latex particles and cellulose was determined by FTIR analysis. Afterward, the effect of heat and polymer content on the thermal, mechanical and physical properties were investigated by DSC, TGA, DMTA and XRD analyses. As can be seen in Fig. 1, the prepared photochromic cellulose (PC) samples display considerable reversible color changes from colorless to purple upon UV irradiation at 365 nm. Finally, photoresponsive papers were prepared by using paper pulp as the cellulose

Ring-opening reaction between hydroxyl groups in MCC and epoxy substituents in the prepared PL copolymer was studied by FTIR analysis and the effect of mixing time and heat on this reaction was investigated (Fig. 2). PC-30 sample was chosen for this reason arbitrarily. Epoxy functional groups of the PL copolymer were identified by two distinct peaks at 754 and 844 cm−1 (Fig. 2b), which remained almost unchanged at early stage of mixing process (Fig. 2c and d). The other considerable result was the role of thermal treatment (drying at 80 ◦ C) that led to the disappearance of epoxy characteristic peaks (Fig. 2e). Therefore, it was found that thermal treatment of the samples was a driving force for the progress of this reaction. On the other hand, the second effective parameter was the reaction time. This reaction reached to completion after 5 h of mixing at room temperature (Fig. 2f). In addition, thermal treatment showed no specific change in the characteristic peaks for this sample (Fig. 2g). Hence, these results could give an insight toward the progress of desired ring-opening reaction at room temperature and upon heating up to 80 ◦ C. Accordingly, this reaction is carried out through mixing of MCC with PL copolymer at room temperature for 5 h and thermal treatment of the samples improves its progress. 3.2. Thermal properties of the prepared samples DSC thermograms (Fig. 3) reveal various PC samples after 5 h of mixing with different amounts of the PL copolymer. To study the effect of thermal treatment and copolymer content on the efficiency of ring-opening reaction and corresponding thermal properties, two series of PC samples were prepared and investigated separately. Obviously, the reaction of epoxy groups in PL copolymer with hydroxyl groups on MCC molecules will result in reduction of

Fig. 3. DSC thermograms for the prepared photochromic cellulosic samples after 5 h of mixing and (a) dried at room temperature, (b) dried at 80 ◦ C and the comparative ones for (c) PC-30 and (d) PC-40.

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Fig. 4. Storage modulus and Tan␦ curves versus temperature for the dried samples at room temperature (a and b) and 80 ◦ C (c and d).

hydrogen bondings in MCC matrix and increase in its hydrophobicity. According to the obtained thermograms for MCC, the observed endothermic peaks below 100 ◦ C (for both series of PC samples) are attributed to the removal of entrapped moisture in the cellulose structure (Tian et al., 2009). The intensity of this peak gradually decreases by the increase in PL copolymer content in PC samples (Fig. 3a) and also after drying at 80 ◦ C (Fig. 3b). The effect of this modification and thermal treatment on hydrophobic character and moisture uptake for PC-30 and PC-40 have been show in Fig. 3c and d, respectively. The appeared endothermic peak in the range of 150–200 ◦ C confirms the reaction of residual epoxy functional groups of PL copolymer with hydroxyl groups of MCC in the samples. In addition, the area of this peak increased by the increase in PL copolymer content and maximum peak area was found for PC40 sample with the highest amount of PL copolymer. It is evident that thermal treatment of the samples at 80 ◦ C displays a significant role for the increase in the hydrophobicity of PC samples (based on the entrapped moisture), while it has a slight effect on progress of the ring-opening reaction after 5 h of mixing (Fig. 3c and d). This is in conformation with the FTIR observations. TGA and DTG thermograms were recorded to study the effect of thermal treatment and copolymer content on the efficiency of ringopening reaction and corresponding thermal properties (Fig. 1 in supporting information). All the results from DSC and TGA analyses endorsed the chemical reaction between photochromic latex and MCC and its enhancement by drying at 80 ◦ C and with approving FTIR observations. 3.3. Dynamic-mechanical thermal properties The effects of PL copolymer content and thermal treatment on the storage modulus and Tan␦ have been studied by DMTA analysis (Fig. 4). As shown in Fig. 4a, the overall storage modulus decreased significantly for the samples dried at room temperature with respect to MCC. This could be attributed to the diffusion

of PL copolymer chains into the cellulose structure and influencing its integrity and intramolecular interactions. The enhancement in the storage modulus from PC-20 to PC-40 indicates the ability of PL copolymer chains to restrict local motions of the MCC chains through bondings on cellulose molecules (Babaee, Jonoobi, Hamzeh, & Ashori, 2015). The observed Tan␦ peaks (Fig. 4b) display a shift to higher temperatures for PC-series samples with the increase in PL copolymer content. This owes to the bonding of copolymer chains and their effect on the segmental motions of MCC chains. On the other hand, the broad Tan␦ peaks for all PC-series samples (Fig. 4b) are the result of brittleness and higher Tg of PL copolymer chains that need a wide range of temperature to trigger chain mobility. However, bonding of higher amounts of the amorphous PL copolymer chains on the highly crystalline MCC structure acts as a softener for the cellulose matrix and leads to broadening of Tan␦ peaks. Fig. 4c reveals a slight change in storage modulus when the samples are dried at 80 ◦ C relative to room temperature, regarding to the effect of thermal treatment on the progress of ring-opening reactions. This will cause in restricted mobility of the polymeric chains. Similar trends to those dried at room temperature were observed for storage modulus and Tan␦ (Fig. 4d). It could be stated that the cross-linking has not progressed completely in Fig. 4a and this is the reason that an increase in storage modulus was observed in Fig. 4c after heating up to 80 ◦ C. However, the polymeric chains act as plasticizer for the cellulosic matrix when they are partially bonded. After increasing the cross-link density upon heating, higher storage modulus will be expecting (as observed). Totally, DMTA analyses confirm chemical modification of MCC with PL nanoparticles. 3.4. X-ray diffraction (XRD) analysis MCC structure is identified by its characteristic peaks at 2␪ of 15◦ , 22.4◦ and 34◦ (Liu et al., 2015; Sokker, Badawy, Zayed, Eldien, & Farag, 2009). The 002 peak (22.4◦ ) is the main characteristic peak

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Fig. 6. Photochromic response of the prepared stimuli-responsive papers under UV irradiation (365 nm) in different conditions.

Fig. 5. XRD pattern of MCC, water-swollen MCC and PC-series samples.

for measurement of crystallinity in MCC (Fig. 5a). Hence, it was chosen for monitoring the changes in crystallinity index (CI) of MCC matrix by inclusion of PL copolymer (results have been summarized in Table 2 of supporting information). The crystallinity decreased with the increase in PL copolymer content through the reaction of cellulose with the functionalized amorphous copolymer, while the minimum CI value was observed in PC-40 sample (33%). Interestingly, the dried water-swollen MCC showed similar crystallinity to MCC. This implies that water has not penetrated into the crystalline regions of cellulose and swelling has been occurred in the amorphous zones. These results depict that the crystalline structure of cellulose could be influenced by its physical interactions with amorphous PMMA chains due to the formation of new linkages and inability to re-establish previous hydrogen bondings in the MCC structure. So, the decrease in CI by the increase in PMMA content is expected and was observed in XRD patterns. The obtained results from several performed analyses reveal that the incorporation of spiropyran moieties into cellulose matrix has been carried out successfully. Here, PC-30 sample with optimum thermal and thermo-mechanical properties was selected for preparation of corresponding stimuli-responsive papers in the next steps. 3.5. Photoresponsive papers In this study, the incorporation of PL copolymer into the paper matrix is performed through covalent bond formation between the PL copolymer particles and cellulose fibers. Here, novel stimuliresponsive papers were prepared by replacing MCC with paper pulp in the mixing process. Photochromic and solvatochromic behaviors of the prepared smart papers were investigated upon UV irradiation in the dried form and after dipping into different solvents. Selection of the solvents was based on their dielectric constant (DC) in which the protic solvents like water and methanol with high DC (78.5 and 32.6, respectively) and aprotic solvents such as n-hexane and toluene with low DC (1.9 and 2.38 respectively) were nominated (Reichardt, 1994). On the other hand, toluene is a good solvent for the prepared PL copolymer and other three solvents are non-solvents for these copolymers. The prepared stimuli-responsive papers showed reversible color changes under UV irradiation at 365 nm. This is an indication of their photochromic characteristics and capability of switching “on” and “off” under UV and visible lights. MC form of the spiropyran moieties display wide range of color changes and different absorption spectra upon exposure to polar and non-polar media due to zwitterionic character of MC (Jonsson, Beke-Somfai,

Andre´ıasson, & Norde´ın, 2013; Tannouri et al., 2014). Distinct color changes of the chromophores because of their stability levels and interaction of these two isomers with several media is called solvatochromism (Goldburt, Shvartsman, & Fishman, 1984; Lee et al., 2014). Here, scraps of the prepared stimuli-responsive papers were immersed in the aforementioned solvents (water, methanol, toluene and n-hexane). Then exposed to UV irradiation (365 nm) and the color changes were recorded macroscopically (Fig. 6). Intense color changes were observed in water and methanol, while poor response was seen in n-hexane under UV irradiation. Remarkably, these photo-solvatochromic (photoresponsive) papers were able to respond in the wet form due to the presence of amorphous photochromic polymer phase to provide isomerization between SP and MC forms. However and in contrast to another report (Tian & Tian, 2014b), photochromic efficiency of the prepared photosolvatochromic papers has been developed due to the preservation of spiropyran moiety from degradation in the highly polar cellulose matrix owing to the presence of polymethyl methacrylate carrier. Also, no undesirable negative photochromism was found with improved photoreversibility. This capability introduces a potential application of such smart papers in chemical sensors for facile detection of the media polarities in polarity-detector devices. In addition, the colored papers are able to convert to the initial colorless form by applying heat or visible light repeatedly. The effect of solvent polarity on photochromic responses was studied for the prepared photoresponsive papers by solid-phase UV–vis spectroscopy too. Scarps of the prepared papers were immersed in different solvents and after wetting, they were exposed to UV irradiation (365 nm) for 5 min. Then the absorption wavelengths and corresponding intensities were examined by UV–vis analysis. Fig. 7a shows the solvatochromic and photochromic behaviors of these photoresponsive papers. The changes in UV–vis absorption bands refer to the occurrence of solvatochromic characteristic and it reveals a strong dependence of the photochromic behavior to the dipolar interactions between the conjugated zwitterionic merocyanine form and the medium. Obviously, the maximum absorption wavelength (max ) shifts to shorter ones by the increase in solvent polarity due to the hypsochromic or blue shift phenomenone (Florea et al., 2013). On the other hand, the higher intensities of the absorption bands and deep color changes were observed in polar solvents (water and methanol). Minimum absorption intensity and weak color change was found for the wet paper in n-hexane, depicting that the merocyanine form as an ionic molecule is not stabilized in such nonpolar medium. A comparison between UV–vis spectra for the dry and wet papers in n-hexane implies almost similar  max and absorption intensities. This points out that n-hexane has not achieved to penetrate inside

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Fig. 7. Solid-phase UV–vis analysis in dry and wet (in different solvents) conditions after UV irradiation at 365 nm (a); and variation in reflective index under UV (365 nm) and visible (520 nm) cycles (b) for the prepared photoresponsive papers.

copolymer chains to induce facile isomerization of SP to MC upon UV irradiation. The establishment of hydrogen bondings and dipolar interactions in protic solvents like water and methanol play an important role in exhibiting solvatochromic properties, and the merocyanine population is increased under UV irradiation subsequently. In addition, high absorption intensity and strong color change were observed in toluene, which is related to the diffusion of toluene in the polymeric phase and its consequent swelling to facilitate SP to MC isomerization. This owes to the higher existing free volumes between PL copolymer chains in the wet state in toluene due to the swollen PL copolymer phase. Anyway, the soaked papers demonstrated intense color changes and different colors relative to the dried ones. Hence, the obtained results from Figs. 6 and 7 confirm the photoresponsive characteristics of such prepared photochromic papers based on photochromism and solvatochromism phenomena. In addition, a scrap of dried photochromic paper was exposed to UV and visible alternative irradiations at 5 and 8 min, respectively. The observed responsiveness demonstrated photoreversibility and fatigue resistance because of efficient covalent bonding between the PL copolymer particles and cellulose paper (Fig. 7b). SEM analysis reveals effective wetting of cellulosic fibers with the added PL copolymer. The comparison between these

morphologies with our previous report (Abdollahi et al., 2015) illustrates that more uniform networks have been built without any crack in the polymeric layer because of the chemical modification and considered improvements (Fig. 2 in supporting information). In addition, EDX analysis (Fig. 3 in supporting information) confirms the presence of N, C and O atoms in this stimuli-responsive paper. Nitrogen-mapping of the surface of prepared photochromic paper shows the presence of PL copolymer containing spiropyran moieties in the cellulose and a well-distribution of nitrogen atoms in the cellulose matrix. 4. Conclusion Novel stimuli-responsive cellulose was prepared by mixing of MCC and paper pulp with the prepared epoxy-functionalized photochromic latex at room temperature and subsequent drying at 80 ◦ C. FTIR analysis revealed that hydroxyl groups of cellulose reacted with epoxy functional groups of the photochromic latex via ring-opening reaction. The obtained results from DSC, TGA and DMTA thermograms illustrated that drying of the samples at 80 ◦ C could improve their physical and chemical properties and increase their hydrophobicity. DMTA analysis showed developments in storage modulus and Tan␦ peak intensity either through the increase in

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latex content or upon drying the photochromic cellulose samples at 80 ◦ C. According to the XRD patterns, crystallinity index of MCC matrix decreased significantly by the increase in amount of amorphous PL copolymer and these resulted in reduction of physical interactions between cellulose molecules. The prepared stimuliresponsive papers demonstrated different colors by immersing in polar and nonpolar solvents under UV irradiation. Solid-phase UV–vis spectroscopy displayed a blue shift in maximum absorption band as a result of changes in the solvent polarity. SEM micrographs of the modified cellulose fibers showed efficient deposition of the PL copolymer because of the chemical reactions. This was approved by EDX and nitrogen-mapping analyses. The prepared photochromic papers have a potential application in the confidential documents, light-sensitive devices, anti-counterfeiting and chemical sensors. Acknowledgment We wish to express our gratitude to Iran Polymer and Petrochemical Institute (IPPI) for financial support of this work (Grant# 24761167). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbpol.2016.05. 009. References Abdollahi, A., Mahdavian, A. R., & Salehi-Mobarakeh, H. (2015). Preparation of stimuli-responsive functionalized latex nanoparticles: the effect of spiropyran concentration on size and photochromic properties. Langmuir, 31(39), 10672–10682. Achilleos, D. S., & Vamvakaki, M. (2010). Multiresponsive spiropyran-based copolymers synthesized by atom transfer radical polymerization. Macromolecules, 43(17), 7073–7081. Babaee, M., Jonoobi, M., Hamzeh, Y., & Ashori, A. (2015). Biodegradability and mechanical properties of reinforced starch nanocomposites using cellulose nanofibers. Carbohydrate Polymers, 132, 1–8. Cayre, O. J., Chagneux, N., & Biggs, S. (2011). Stimulus responsive core-shell nanoparticles: synthesis and applications of polymer based aqueous systems. Soft Matter, 7(6), 2211–2234. Chen, J., Zeng, F., Wu, S., Su, J., & Tong, Z. (2009). Photoreversible fluorescent modulation of nanoparticles via one-step miniemulsion polymerization. Small, 5(8), 970–978. Chen, J., Zhong, W., Tang, Y., Wu, Z., Li, Y., Yi, P., et al. (2015). Amphiphilic BODIPY-based photoswitchable fluorescent polymeric nanoparticles for rewritable patterning and dual-color cell Imaging. Macromolecules, 48(11), 3500–3508. Crano, J. C., & Guglielmetti, R. J. (1999). . pp. 495. Organic photochromic and thermochromic Compounds (1) New York: Kluwer Academic Publisher. Crano, J. C. (2002). Organic photochromic and thermochromic compounds. physicochemical studies, biological applications, and thermochromism (vol. 2) Boston: Kluwer Academic Publishers. Darwish, T. A., Tong, Y., James, M., Hanley, T. L., Peng, Q., & Ye, S. (2012). Characterizing the photoinduced switching process of a nitrospiropyran self-assembled monolayer using in situ sum frequency generation spectroscopy. Langmuir, 28(39), 13852–13860. Feczkó, T., Samu, K., Wenzel, K., Neral, B., & Voncina, B. (2013). Textiles screen-printed with photochromic ethyl cellulose-spirooxazine composite nanoparticles. Coloration Technology, 129, 18–23. Florea, L., McKeon, A., Diamond, D., & Benito-Lopez, F. (2013). Spiropyran polymeric microcapillary coatings for photodetection of solvent polarity. Langmuir, 29(8), 2790–2797. Goldburt, E., Shvartsman, F., Fishman, S., & Krongauz, V. (1984). Intramolecular interactions in photochromic spiropyran-merocyanine polymers. Macromolecules, 17, 1225–1230. Gossweiler, G. R., Brown, C. L., Hewage, G. B., Sapiro-Gheiler, E., Trautman, W. J., Welshofer, G. W., et al. (2015). Mechanochemically active soft robots. ACS Applied Materials & Interfaces, 7(40), 22431–22435.

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