Light-Induced Aggregation and Disaggregation of Stimuli-Responsive

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Light-Induced Aggregation and Disaggregation of StimuliResponsive Latex Particles Depending on Spiropyran Concentration: Kinetics of Photochromism and Investigation of Reversible Photopatterning Amin Abdollahi, Keyvan Sahandi-Zangabad, and Hossein Roghani-Mamaqani*

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Department of Polymer Engineering, Sahand University of Technology, P.O. Box: 51335-1996, Tabriz 51368, Iran S Supporting Information *

ABSTRACT: Light-controlling the physical and chemical properties of smart polymers by using photochromic compounds has been an interesting research subject. Incorporation of spiropyran (SP) on the surface of particles can induce photoswitchable aggregation/disaggregation to stimuli-responsive colloids. Herein, we developed a novel class of stimuli-responsive latex particles bearing SP with different contents (0, 0.5, 1, 3, and 5 wt %) by semicontinuous emulsifier-free emulsion copolymerization, which is able to change the particle size by light-induced aggregation/disaggregation in response to ultraviolet (UV) irradiation and visible light. The scanning electron microscopy images revealed the spherical morphology of the latex particles, with the size in the range of 400−900 nm. Light-induced aggregation and disaggregation of stimuliresponsive latex particles were investigated by dynamic light scattering and also confirmed by variation of transmittance during UV illumination time using ultraviolet−visible spectroscopy. The range of the light-induced shift in the particle size is about 200−600 nm (depending on the concentration of SP), where the reduction of transmittance upon UV irradiation (and conversely upon visible light) confirms the ability of latex particles for displaying reversible photoswitchable aggregation/disaggregation and also lightcontrolling the particle size. The kinetics of SP to merocyanine (MC) and MC to SP isomerizations were experimentally investigated and fitted by exponential equations. The photochromic latexes displayed remarkable photoswitchability and photofatigue resistant properties under alternating UV and visible light irradiation cycles. Additionally, these stimuli-responsive latexes displayed potential applications such as anticounterfeiting inks in erasable and rewritable writings on cellulosic papers for increasing safety in security documents.

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emulsion polymerization28,36,37 or self-assembly38−42 approaches. Among different methodologies, emulsifier-free emulsion polymerization technique is an interesting method for the preparation of photochromic polymer particles because of probable growing of the particle size above 500 nm and also the absence of ionic surfactants. In the recent years, different photochromic compounds such as azobenzene, diarylethene, spiroxazine, and spiropyran (SP) were used for the preparation of photochromic polymers. In these photochromic compounds, SP is the most significant and interesting chemical, whose incorporation into polymer particles has been extensively studied.28,43−46 According to the literature,28,33,47,48 SP is the most applicable and sensitive photochromic compound which requires protection against high-polar interactions and environmental degradation by incorporation into hydrophobic polymer particles. These

INTRODUCTION Stimuli-responsive polymers are a new class of smart materials that has received a great deal of attention in the recent decades. In this context, temperature, pH, and light have been considered as the most interesting stimuli.1−9 Photochromic polymers, as light-responsive materials, have attracted a large deal of attention because of the advantages of light-triggered smart polymers, such as fast and facile responsivity, response reversibility, controllability out of the system, accessibility, and nondestructivity of light as the stimulus.10,11 Photochromic polymers can display different stimuli-chromic properties such as photochromic,6,12−14 mechanochromic,15−18 acidochromic,19−24 solvatochromic,25−28 and ionochromic29−31 in response to the induced light, stress, pH change, polarity variation, and addition of ions, respectively. Generally, photochromic polymers are prepared by chemical or physical incorporation of photochromic compounds into polymer matrices by polymerization or doping methods, respectively.26,32−35 Photochromic particles are the most important class of photochromic polymers, which is prepared by © XXXX American Chemical Society

Received: July 7, 2018 Revised: October 14, 2018

A

DOI: 10.1021/acs.langmuir.8b02296 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration for the preparation of stimuli-responsive latex particles containing different concentrations of SP by semicontinuous emulsifier-free emulsion polymerization.

particle size in gold (Au) and silver (Ag) nanoparticle colloidal dispersion by incorporation of thiol-functionalized SP molecules on the surface of nanoparticles.61,63,66,67 Size of the decorated Au nanoparticles increases by UV irradiation as a result of attractive interactions between the MC molecules, which is fully reversible by visible light irradiation for several times. Recently, Zhang and co-workers prepared SP-decorated SiO2−platinum (Pt) Janus particles and investigated their light-induced dynamic self-assembly and disassembly behaviors.39 Similar results to the previous studies were observed in Zhang’s study, where the Janus particles undergo a dynamic assemble into multiple motors upon UV irradiation (365 nm), and also, it can quickly disassemble into mono motors when the light irradiation is switched to green light (λ = 520 nm). In addition to light-controlling the particle size by these mechanisms, viscosity of the polymer solutions can be modulated by light irradiation. For instance, Dou and coworkers prepared a photo- and thermo-gelling polymer containing SP in the center of its chain.68 Upon UV irradiation and isomerization of the SP to MC form, the polymer chains can interact together via electrostatic and π−π stacking interactions between the MC chain centers. Such interactions can lead to increase of viscosity as a result of increase of the molecular weight by physical cross-linking of polymer chains. It is notable that light-induced self-assembly and disassembly of polymer particles has not been reported till date. Herein, we developed novel stimuli-responsive latex particles, containing various contents of SP, by semicontinuous emulsifier-free emulsion polymerization and investigated the reversible aggregation and disaggregation of the particles under UV and visible light irradiations. For this purpose, acrylate derivative of SP (SPEA) was copolymerized with methylmethacrylate (MMA) in aqueous media without using ionic surfactants. The photochromic latex particles were prepared with different SPEA contents (0, 0.5, 1, 3, and 5 wt %). Morphology and size of the latex particles were investigated by

polymer particles act as a carrier for the introduction of SP into high-polar substrates.26,27,32,33 SP displays different stimulichromic properties by isomerization between the colorless ring-closed SP and colored ring-opened merocyanine (MC) forms in response to the induced triggers. The conjugated structure of the zwitterionic MC form can be affected by polar−polar and hydrogen-bonding interactions; therefore, it displays negative photochromism in high-polar surroundings, especially the protic media.26,27,49,50 Hence, incorporation of SP into the hydrophobic polymer carriers is an essential requirement for the design and development of different smart materials such as chemosensors,19,51,52 optical memories,53,54 photoswitchable fluorescent nanoparticles,32,55,56 and also anticounterfeiting and smart inks.57,58 Application of SP in the development of security documents and also in anticounterfeiting and rewritable inks (smart inks) has been poorly investigated in the recent years, and a few of these studies have focused on such applications.26,33,47,57,59 In the recent decades, a wide range of studies have focused on light-controlling of self-assembly and disassembly or aggregation and disaggregation of different particles by means of physical interactions between the MC moieties.39,60−66 The MC form is a zwitterion, which can physically interact with the other MC molecules by electrostatic attractions and also π−π stacking interactions. Therefore, incorporation of SP on the particle surface can result in the formation of a colloidal dispersion that displays reversible selfassembly/disassembly or aggregation/disaggregation upon ultraviolet (UV) and visible irradiation. After UV irradiation and formation of the MC isomer, interparticle interactions (electrostatic attractions and π−π stacking) lead to growth of the particle size (self-assembly or aggregation). Also, visible light converts the MC to SP form, where the particle size returns to the initial state by disassembly or disaggregation. Related to this research area, Klajn and co-workers developed different classes of such systems for light-controlling the B


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Table 1. Procedure for Preparation of Stimuli-Responsive Latex Particles Containing Different Concentrations of SP sample

water (mL)

NaHCO3 (g)

KPS (g)

NaCl (g)

SPAN 80 (g)

SPEA (g)

CSPEA × 10−3 (mol/L)

MMA (g)

LB LB-SP0.5% LB-SP1% LB-SP3% LB-SP5%

36 36 36 36 36

0.15 0.15 0.15 0.2 0.3

0.06 0.06 0.06 0.06 0.06

0.04 0.04 0.04 0.04 0.04

0.045 0.045 0.045 0.045 0.045

0 0.0225 0.045 0.135 0.225

0 1.10 2.21 6.64 11.07

4.3 4.3 4.3 4.3 4.3

and stirred for 24 h for complete incorporation of the nonionic surfactant onto the latex particles. The total coagulation and conversion are less than 1 wt % and more than 95% (measured by the method described in Section S2 of the Supporting Information), respectively. The total amount of incorporated SPEA in the latex particles 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 was incorporated in these samples. Final photochromic latex samples were obtained after three cycles of centrifugation in 7500 rpm for 15 min and dispersion in DI water to obtain the final latex with 10 wt % solid content. Characterization. Measurement of particle size and its distribution and also zeta potential was carried out by a Zetasizer Nano ZSP and DLS (Malvern, United Kingdom) system at 25 °C, using the initial latexes diluted up to 100 folds in DI water. SEM micrographs were taken by a TESCAN Mira III system (Czech Republic). A drop of diluted latex was placed on the sample holder and dried under vacuum at 25 °C. Then, it was put under vacuum and evacuated, and a layer of gold was deposited under flushing with argon by using an Emitech K450x sputter-coating system (England). Photochromic properties of the samples were investigated using UV−vis analysis using an i3 UV−vis spectrophotometer (Hanon Instruments, China) by dilution of the initial latex to about 0.1 wt %. To evaluate the photochromic properties, 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 light-emitting diode lamp with white light. The UV and visible light irradiation time was set at 5 min for all samples.

scanning electron microscopy (SEM). Light-induced aggregation and disaggregation of latex particles upon UV irradiation were examined by dynamic light scattering (DLS) and also confirmed by variation of the transmittance in ultraviolet− visible (UV−vis) spectra. Photochromic properties, kinetics of SP to MC and MC to SP isomerizations, photofatigue resistant characteristics, and also photoswitchability were studied by UV−vis spectroscopy. Finally, the photochromic latexes were used as rewritable and anticounterfeiting inks for writing on the cellulosic papers for applications in security documents to increase safety. Using of the emulsifier-free emulsion polymerization resulted in preparation of latex particles with size in the range of 400−900 nm. These acrylic-based latex particles can significantly be established on the surface of the paper by physical hydrogen-bonding interactions. In addition, these latex particles act as hydrophobic carriers for introduction of SP as an ink into the cellulosic papers and also for protection of SP toward environmental degradation such as negative photochromism due to high density of hydrogen bonding in high-polar media. To the best of our knowledge, this is the first report on the preparation of such photochromic latex particles by emulsifier-free emulsion polymerization with the mentioned specifications and applications.

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EXPERIMENTAL SECTION

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Materials. 2,3,3-Trimethylindolenin, 2-bromoethanol, and 2hydroxy-5-nitrobenzaldehyde were purchased from Sigma-Aldrich, which were used in the synthesis of (R/S)-2-(3′,3′-dimethyl-6-nitro3′H spiro[chromene-2,2′-indol]-1′-yl)ethanol (SPOH) and SPEA. Acryloyl chloride, MMA, potassium persulfate (KPS), SPAN 80, sodium hydrogen carbonate (NaHCO3), sodium chloride (NaCl), and all of the solvents were supplied by Merck Chemical Company. Distilled deionized (DI) water was used in all recipes, and all of the materials were used without further purification. Synthesis of SPOH and SPEA. SPOH was synthesized according to the reported procedure by Raymo.69 Also, SPOH was modified to the polymerizable SPEA monomer by the method previously mentioned by Abdollahi and co-workers.33 The chemical structures of SPOH and SPEA were characterized by 1H nuclear magnetic resonance (NMR) spectroscopy, and the corresponding spectra are reported in the Supporting Information (Figures S1 and S2). Preparation of the SP-Containing Stimuli-Responsive Latex Particles. Photochromic latex particles with a solid content of about 10 wt % were prepared by the semicontinuous emulsifier-free emulsion polymerization method, as shown in Figure 1. According to Table 1, the mentioned amounts of KPS, NaCl, and NaHCO3 were dissolved in DI water (36 mL) and transferred into the reactor under a continuous flow of nitrogen gas. Also, MMA (4.3 g) was simultaneously added, and the temperature was increased to 75 °C and left under stirring for 3 h for reaching a turbid solution. Then, an aqueous solution of SPEA (0.5, 1, 3, and 5 wt % with respect to the total amount of MMA) was added dropwise at 30−45 min, and the reaction was continued for 3 h to reach the monomer conversion of above 95%. The resulting samples are photochromic latex particles (LB-SP) containing 0.5, 1, 3, and 5 wt % of SP (LB-SP0.5%, LBSP1%, LB-SP3%, and LB-SP5%, respectively). According to Table 1, an appropriate amount of SPAN 80 was added to the prepared latex

RESULTS AND DISCUSSION Light-responsive polymers are one of the most important classes of smart materials, which have been widely studied in the recent years because of their unique responsivity toward light irradiation. Photochromic polymer particles were generally prepared by chemical incorporation of photochromic compounds into the polymer matrix by using emulsion polymerization26,28,32,70,71 and controlled radical polymerization.36,72−74 In the current study, a new strategy was developed for the preparation of photochromic latex particles based on SP by emulsifier-free emulsion polymerization. Lack of ionic surfactant, controlled size of the latex particles in the range of 400−900 nm, and narrow particle size distribution of the latex particles are the main advantages of this method. Additionally, the prepared latex particles can be purified with a facile and fast methodology by centrifugation several times and redispersion in DI water. The latex particles were stabilized by electrostatic repulsive forces because of the presence of negative charge due to SO4− functional groups of the KPS initiator at the particle surface. On the other hand, the size of the particles can reversibly be increased upon UV irradiation as a result of electrostatic attractions and π−π stacking forces between the MC groups on the surface of the particles. Then, the size of the particles can turn back to the initial value under visible light irradiation. Most importantly, these latex particles can be used as an anticounterfeiting ink for writing on the cellulosic papers or security documents. For this purpose, the C


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Figure 2. SEM images of the latexes with different concentrations of SPEA: (A−A‴) LB, (B−B‴) LB-SP0.5%, (C−C‴) LB-SP1%, (D−D‴) LBSP3%, and (E−E‴) LB-SP5%.

Emulsifier-free emulsion polymerization is a significant method for the preparation of polymer particles, with size in the range of 400−900 nm and a narrow size distribution. Absence of an ionic surfactant increases the surface tension on the particle surface and results in the growth of the particles.75−77 It should be pointed that using an ionic surfactant in normal emulsion polymerization or miniemulsion polymerization resulted in nanoparticles with size in the range of 40−200 nm.33,71,78,79 Additionally, the presence of an ionic surfactant in emulsion and miniemulsion polymerizations can result in high stability of the colloids because of high repulsive forces between the particles. In these cases, the size of the latex particles cannot be changed in response to UV irradiation by the interaction of the MC molecules on the particle surface. In addition, the presence of ionic surfactant can limit applications of the latexes, especially in their use as ink for security marking and also photopatterning. Figure 2 displays the SEM images of the prepared photochromic latexes containing different contents of SP. SEM images of the LB sample (Figure 2A− A‴) display spherical particles with size in the range of 500− 700 nm and also a narrow size distribution. Figure 2B−B‴ shows a similar size, size distribution, and morphology for LBSP0.5% compared to the LB sample (SP content of 0 wt %). Other samples display a decrease in the particle size and increase in the size distribution by the addition of SP

required materials, according to Table 1, were used for the preparation of latex particles containing different concentrations of SP by semicontinuous emulsifier-free emulsion polymerization, as shown in Figure 1. For incorporation of SP on the surface of the latex particles, the SPEA solution in water was added dropwise to the reaction mixture after 3 h from the start of polymerization. The presence of SP groups on the particle’s surface results in the induction of reversible lightcontrolling of the particle size under UV and visible light irradiation. It was observed that increase of SP concentration resulted in the reduction of colloidal stability and increase of coagulation at the end of polymerization process, as schematically shown in Figure 1. By dissolution of SP in water, it was converted to the colored MC form, and the zwitterionic nature of this form resulted in variation of the pH and ionic strength of the media. Therefore, as observed in Table 1, the amount of NaHCO3 as buffer was increased by increase of the SP concentration for reaching a good colloidal stability. It should be pointed that the effect of SP concentration on the ionic strength and particle size was recently studied by Abdollahi and co-workers, and the results display an increase of ionic strength and particle size by increase of SP concentration.33 In the next sections, different properties of the photochromic latex particles are studied by SEM, DLS, and also UV−vis spectroscopy. D


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Figure 3. DLS and zeta potential analysis of the stimuli-responsive latexes with different SPEA concentrations before and after UV irradiation at 365 nm: (a) LB, (b) LB-SP0.5%, (c) LB-SP1%, (d) LB-SP3%, and (e) LB-SP5%.

strength. Such a trend was previously observed by Abdollahi and co-workers.33 Light-Induced Aggregation and Disaggregation of Latex Particles. To determine the average particle size, zeta potential, and also particle size distribution, the prepared samples were analyzed by DLS, and the results are presented in Figure 3. All of the latex particles displayed a negative zeta potential in the range of −30 to −40 mV because of the presence of SO4− on the surface of the particles originating from the KPS initiator. The results obtained from DLS confirmed the observation from SEM images, where the addition of SP content resulted in the decrease of the particle size by the decrease of surface tension. According to the literature,61,63,66,67 the size of the particles can be modulated by light irradiation when SP molecules are incorporated on the surface of the particles. Actually, upon UV irradiation and

concentration (Figure 2C‴−E‴). As mentioned in the previous section, conversion of SP to the zwitterionic MC form after dissolving in water leads to a change of ionic strength of the media. Here, the decrease of the particle size is due to the presence of MC molecules on the surface of the particles during the polymerization reaction and the corresponding decrease of the surface tension. Because of the absence of a surfactant in our strategy, the surface charges resulted from the MC form increase the electrostatic interactions and therefore decrease the surface tension. This is similar to the results obtained by Jin and co-workers for selfassembly of SP block copolymers under UV irradiation, where the size of the micelles was remarkably reduced after UV irradiation as a result of decrease of surface tension.80 A slight increase in the particle size and its distribution was observed by the addition of SP concentration as a result of increase of ionic E


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Transmittance change is a key point for investigation of the light-induced aggregation and disaggregation in colloidal systems, where this parameter decreases by increase of the particle size as a result of increase of the reflective index. Hence, we investigated the change of transmittance by time under UV and visible light irradiations by UV−vis spectroscopy at λmax for the MC form. The diluted latex samples (100 folds, 0.1 wt %) were exposed to UV irradiation (365 nm), and the transmittance was measured at each 20 s, and the results are shown in Figure 5. Accordingly, the transmittance decreased when UV light was switched on, and it returned to the initial value upon visible light irradiation. As mentioned in the previous section, the size of the particles increased upon UV irradiation and formation of MC molecules, which consequently resulted in the increase of the light reflectance and also decrease of transparency. According to Figure 5, all of the samples display a decrease in transmittance during UV illumination depending on the illumination time, where the transparency decrement was dramatically increased with the irradiation time. These observations are due to the growth of particle size upon UV irradiation in all of the samples. Also, the intensity and value of the light-induced aggregation/ disaggregation of latex particles increased for samples containing higher SP content. On the other hand, transmittance increased upon visible light irradiation and displayed time dependency. The MC form is converted to the SP form under visible light, which leads to a decrease of the particle size by disassembly of the assembled particles because of the dissociation of the interparticle interactions. The results obtained from transparency changes under UV and visible illumination by UV−vis spectroscopy confirmed the results of DLS analysis for light-induced aggregation/disaggregation of particles. Also, this responsivity is fully reversible under alternated UV and visible light irradiation. Photochromic Properties of the Stimuli-Responsive Latex Particles. SP, as an important class of organic photochromic compounds, has attracted a great deal of attention in polymer matrices because of its unique photochromic properties.81−84 Figure 6 displays the reversible color change of the prepared photochromic latexes before, after, and under UV irradiation (365 nm). The high-intensity color change can be attributed to the latex particles with higher SP content (LB-SP3% and LB-SP5%). For investigation of the photochromic behavior of the latex particles with different SP contents, the initial latex particles were diluted to 0.1 wt % (100-fold), and then absorption spectra were obtained by UV−vis spectroscopy before and after UV illumination in different times, as shown in Figure 7. SP is a nonpolar and discolored molecule, which is stabilized under visible light irradiation. It is isomerized from the SP to MC form upon UV irradiation, where the MC form with a high-polar zwitterionic structure is colored, as seen in Figure 6. The MC form was characterized by the wide absorption peak in the wavelength of 400−700 nm and also its macroscopically observable colored nature. The results obtained from the photochromic properties of all latexes are shown in Figure 7, where the spectra were obtained in UV irradiation time interval of 20 s. As observed in UV−vis spectra, all of the latex samples displayed an absorption band in the wavelength of 450−650 nm after UV illumination as a result of isomerization of the SP to MC form, where the intensity of the absorption peak increased by the irradiation time and also by increase of the SP concentration. These

formation of the MC form, particles are assembled or aggregated by physical attractive interactions because of the formation of electrostatic and π−π stacking interactions between the MC molecules on the particle surface. Therefore, we decided to investigate the particle size, zeta potential, and particle size distribution of the prepared latex particles before and after UV light irradiation at 365 nm. For this purpose, the diluted latex samples were exposed to UV irradiation for 5 min and then analyzed and compared with the results obtained before UV irradiation, as shown in Figure 3. As expected, a remarkable change in the particle size was observed for all of the stimuli-responsive latexes containing different SP contents after UV irradiation. As shown in Figure 4, the latex particles were assembled or aggregated by the

Figure 4. Color change of the stimuli-responsive latex (LB-SP3%) under UV and visible light irradiations and also schematic illustration of the corresponding light-induced aggregation and disaggregation of latex particles.

electrostatic and π−π stacking interactions of surfacedecorated MC molecules upon UV illumination, and they returned to the initial stable colloid solution by disaggregation upon visible light. Variation of the particle size for stimuliresponsive latex samples is different, where it increased by the increase of the SP concentration. Size enlargement for LBSP0.5%, LB-SP1%, LB-SP3%, and LB-SP5% is 280, 390, 420, and 570 nm, respectively. This can be attributed to the increase of MC concentration on the surface and the corresponding increase of the interactions between particles with different sizes. On the other hand, the zeta potential of the samples displays a slight increment after UV illumination, which confirms the interactions between the surface MC molecules and also increase of the particle size after UV illumination. As seen in Figure 4, the MC molecules were stacked together by packing the positive and negative charges in their zwitterionic structure. For this reason, zeta potential undergoes a slight increment after UV irradiation, and it can be claimed that the surface charge has not been affected by the UV irradiation and formation of MC molecules because of interactions of opposite charges with each other in the MC structure. In addition, we investigated the reversibility of the light-induced aggregation/ disaggregation for the LB-SP5% sample by DLS analysis before and after UV irradiation (365 nm) for two cycles. As shown in Figure S3, this phenomenon is fully reversible under UV and visible light irradiations, which confirms the photoswitchability and reversibility of particle size variation upon UV and visible light irradiations. F


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Figure 5. Variation of transmittance with time for stimuli-responsive latexes under UV and visible light irradiations: (a) LB-SP0.5%, (b) LB-SP1%, (c) LB-SP3%, and (d) LB-SP5%.

obtained for photochromic latex particles with the particle size lower than 100 nm, as a result of minimum light reflection. Therefore, the size of the particles, the SP concentration, and also the UV irradiation time are the most effective factors on the photochromic properties of the stimuli-chromic latex particles containing SP. Photochromic behavior of SP in the polymeric matrices depends on UV and visible light illumination time. Hence, the kinetics of the SP to MC and MC to SP isomerizations, respectively, under UV and visible light irradiations were studied for all of the photochromic latexes by UV−vis analysis. For this purpose, the absorbance intensities were measured at specific λmax and different time intervals of UV (365 nm) and visible light irradiations. Therefore, the primarily measured absorbance values were converted to normalized ones (with respect to the absorbance of each sample before UV and visible light irradiation) and plotted versus irradiation time, as shown in Figure 8. Origin software (Professional 9) was used for fitting the resulted curves to extract the kinetics equations and rate constants as the most important kinetics characteristics. For this purpose, the SP to MC isomerization kinetics was fitted by using the BoxLucas 1 model, and also the MC to SP isomerization (the back reaction) was fitted by using the ExpDec1 model (exponential equations presented in eqs 1 and 2, respectively).

Figure 6. Color changes and photochromic properties of the stimuliresponsive latexes with different SP contents before, after, and under UV irradiation (365 nm).

results are in agreement with the images taken from photochromic latexes after UV irradiation (Figure 6). Indeed, the intensity of the MC absorption peak is dependent on the UV irradiation time and also the number of MC molecules.26,27,32,33 The number of MC molecules dramatically increased by the increase of UV illumination time; however, the rate of MC formation can be influenced by different factors that will be investigated in the next section. The number of MC molecules shows a direct relation to the concentration of incorporated SP, where the intensity of the absorption peak increased largely for samples containing higher SP concentration, as seen in Figure 7 for LB-SP3% and LB-SP5% compared to LB-SP0.5% and LB-SP1%. The large size of the latex particles (450−850 nm) and also its large reflection toward light resulted in such nonsmooth spectra. According to the literature,28,33 the most smooth UV−vis spectra are

Normalized absorbance (t ) = A(1 − exp(−kc1t )) Normalized absorbance (∞) G

(1)


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Figure 7. Real time UV−vis spectra of the stimuli-responsive latex samples under different UV light (365 nm) illumination times (time intervals of 20 s): (a) LB-SP0.5%, (b) LB-SP1%, (c) LB-SP3%, and (d) LB-SP5%.

Normalized absorbance (t ) = A 0 + A exp( −kc2t ) Normalized absorbance (∞)

result from the higher stability of the MC form and its lower population after UV irradiation in LB-SP0.5% compared to other samples, which shows that the variation of absorbance is performed very slow. The LB-SP1% and LB-SP3% samples display minimum kc1 and also maximum T1/2 values because of lower stability of the MC form in comparison with the SP form and also higher population of SP molecules that absorbed the UV light and converted to the MC structure. Therefore, a maximum irradiation time is required. These two samples display a maximum kc2 and a minimum T1/2 for the MC to SP isomerization. This can be attributed to the lower stability and large population of MC molecules, which finally resulted in the fast isomerization of the MC to SP form. Higher increase of the SP concentration, as in the LB-SP5% sample, resulted in increase of the SP to MC isomerization rate (the largest kc1 and less T1/2, compared to the LB-SP1% and LB-SP3% samples) and also decrease of the MC to SP isomerization rate (the less kc2 and highest T1/2, compared to the LB-SP1% and LB-SP3% samples). High population of the SP molecules converted to the MC form under UV irradiation, and also higher stability and more population of the MC form after UV illumination required further visible light illumination time for completion of the MC to SP isomerization. It should be pointed that the UV-induced interparticle interactions leading to a variety of particle sizes can display a significant role in control of the MC to SP isomerization kinetics, which requires further investigation. Finally, it can be concluded that the kinetics of the SP to MC and MC to SP isomerizations was affected by two main factors: concentration of SP in the latex particles and also the stability of the MC form. However, the kinetics and photochromic properties can significantly be

(2)

where A and Ao are the constant values and kc1 and kc2 represent the rate constants of the SP to MC (eq 1) and MC to SP (eq 2) photoisomerizations upon UV and visible light irradiations, respectively. The normalized absorbance values at times t and ∞ were extracted from Figure 8. It is noteworthy that kc could be a good indication of susceptibility or responsivity of the photochromic latex particles toward UV and visible light irradiations during photoisomerization. Concentration of SP in the polymer particles is the main effective factor on the isomerization rate. As can be observed in Figure 8, both of the SP to MC and MC to SP isomerization kinetic curves were fitted as well by eqs 1 and 2. The extracted parameters such as R2 (regression), kc, and T1/2 (the time required to reach half of the final absorbance for coloration) are presented in Table 2. The R2 values reveal that the kinetics of the SP to MC and MC to SP isomerizations conforms to eqs 1 and 2. According to the results shown in Table 2, the maximum kc1 and minimum T1/2 and also minimum kc2 and maximum T1/2 are assigned to the LB-SP0.5% sample with the minimum concentration of SP for both SP to MC and MC to SP isomerizations, respectively. Fast photoresponsivity of the LBSP0.5% sample toward UV irradiation is due to the presence of a low number of SP molecules in the latex particles because a few SP molecules received a high intensity of UV irradiation and were rapidly converted to the MC form. Consequently, a minimum energy or UV illumination time is required for the SP to MC isomerization. The minimum kc2 and maximum T1/2 H


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Figure 8. Normalized and exponential fitting curves for the kinetics of SP to MC and MC to SP isomerizations for the stimuli-responsive latexes at the corresponding λmax under UV and visible irradiation: (a) LB-SP0.5%, (b) LB-SP1%, (c) LB-SP3%, and (d) LB-SP5%.

Table 2. Coloration and Discoloration Rate Equation and the Corresponding Kinetic Parameters for the Stimuli-Responsive Latexes Containing Different SP Contents under UV and Visible Light Irradiations SP to MC

MC to SP 2

samples

equation

R

LB-SP0.5% LB-SP1% LB-SP3% LB-SP5%

0.028 − 0.028 exp(−0.015t) 0.058 − 0.058 exp(−0.003t) 0.1085 − 0.1085 exp(−0.0047t) 0.02812 − 0.02812 exp(−0.015t)

0.98 0.99 0.99 0.98

−1

kc1 (s )

T1/2 (s)

0.0150 0.0030 0.0047 0.0150

46.84 215.50 145.22 142.63

a

equation

R2

kc2 (s−1)

T1/2 (s)a

0.0115 + 0.0212 exp(−0.00454t) 0.0064 + 0.0635 exp(−0.01022t) 0.00213 + 0.15434 exp(−0.011137t) 0.01147 + 0.02124 exp(−0.004547t)

0.98 0.99 0.99 0.98

0.0045 0.0102 0.0111 0.0046

322.20 78.08 63.51 298.26

The time required to reach half of the final absorbance for the coloration.

a

bility.32,33 Here, we investigate the photoswitchability and photofatigue resistance of the prepared photochromic latexes by UV−vis analysis under alternating UV and visible light irradiations. For this purpose, diluted latex samples (0.1 wt %) were used for measurement of the absorbance intensity after 5 min of UV irradiation (365 nm) and after 5 min of visible light irradiation for 15 cycles, and the obtained results (absorbance vs cycle number curves) are displayed in Figure 9. The corresponding absorbance intensity was immediately measured after each irradiation within a 5 min interval between each cycle. As expected, photochromic latex particles displayed excellent photoswitchability after each cycle and also significant photofatigue resistance after 15 UV and visible light irradiation cycles. Additionally, it can confirm the reversibility of light-induced aggregation/disaggregation of stimuli-responsive latex particles for a long time. All these advantages resulted from the role of the polymer carrier, chemical incorporation of SP, and also hydrophobicity of the polymer matrix for protection of SP toward environmental

affected by interactions of the MC molecules with each other and with the surrounding media.27,85 As a key point, stability of the MC structure in a media results in the fast SP to MC and slow MC to SP isomerizations. However, stability of the SP form leads to the fast MC to SP and slow SP to MC isomerizations. The MC and SP structures can be stabilized in the polar and nonpolar environments, respectively. Photoswitchability and photofatigue resistance are two of the most important photochromic properties of the stimulichromic polymers that remarkably affect the applications of the final product. SP is a very sensitive chromophore toward environmental interactions such as chemical reactions, polarity, pH, and temperature. Hence, its protection toward environmental degradation is a very important requirement for application of the final polymer in advanced systems such as chemosensors.47,51,86,87 Recent studies showed that incorporation of photochromic compounds such as SP into the hydrophobic polymer particles can lead to significant improvement in photostability, photoswitchability, and also reversiI


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Figure 9. Photofatigue resistance of the stimuli-responsive latexes upon alternating UV (365 nm for 5 min) and visible light (5 min) irradiation: (a) LB-SP0.5%, (b) LB-SP1%, (c) LB-SP3%, and (d) LB-SP5%.

Figure 10. Use of stimuli-responsive latex particles (LB-SP3%) as an anticounterfeiting and rewritable smart ink in security documents.

degradation and negative photochromism, especially in highpolar media. Application of the Photochromic Latex Particles in Security Marking and Photoswitchable Patterning. The prepared photochromic latex particles have large particle sizes and polar acrylic groups, which make them stable on the surface of papers for induction of the photochromic properties. Hence, these latex particles were used for writing on paper as anticounterfeiting and smart inks to increase the security of important documents. As observed in Figure 10, the LB-SP5% latex (10 wt %) was used for writing the SP and MC phrases on the blue circles in a model certificate document, where the invisible SP and MC phrases undergo visibility after UV illumination and coloration. This developed smart ink is a water-based product, which is prepared in a fast and facile

strategy; in addition, it has high security because of the red fluorescence emission of SP under UV irradiation and also its reversible coloration after UV illumination. Photopatterning is a significant and unique application of photochromic polymers based on SP, which was carefully studied in the recent years.57,71 To investigate the application of the prepared photochromic latex particles in photopatterning, photochromic papers were prepared by spraying the latex (10 wt %) on the paper surface, and the corresponding photochromic properties were investigated before and after masking upon UV irradiation (365 nm) by taking the photos, as shown in Figure 11. Latex-coated photochromic papers displayed excellent reversible photopatterning behavior under different masks and UV illumination. In addition, the effect of SPEA concentration on the J


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theoretical kinetic studies on the SP to MC and MC to SP isomerizations revealed that kc and T1/2 were affected by the SP concentration and stability of the MC form upon UV irradiation. The fastest SP to MC kinetics (the largest kc and the less T1/2) is accompanied by the slow MC to SP kinetics (the less kc and the largest T1/2). The prepared photochromic latex particles displayed high photofatigue resistance and also high photoswitchability under alternating UV and visible light irradiation cycles. The prepared photochromic latex particles can be used as anticounterfeiting smart inks in security documents that display red fluorescence emission under UV irradiation and also were converted from invisible text to a visible one upon UV illumination. It is notable that the prepared photochromic papers by latex spraying displayed reversible photopatterning by using a mask under UV irradiation. This is the first report on reversible light-induced aggregation/disaggregation of stimuli-responsive latex particles, which can be used as an anticounterfeiting smart ink. Such stimuli-responsive latex particles have potential applications in the design and preparation of advanced optical devices such as smart printing, optical data storage, optical chemosensors, and also holography, which are now under investigation in our lab with the name of Research Group for Innovation in Smart Polymers (RGISP).

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Figure 11. Photopatterning of the stimuli-responsive latex particles (LB-SP3%) on paper under masking and UV illumination (365 nm).

ASSOCIATED CONTENT

S Supporting Information *

photopatterning behavior was investigated for the prepared photochromic papers based on stimuli-responsive latexes containing different SPEA contents (LB-SP0.5%, LB-SP1%, LB-SP3%, and LB-SP5%), as shown in Figure S4. The observed photopatterns for the photochromic papers under UV irradiation and mask show that the coloration intensity increased by increasing the SPEA concentration in the stimuliresponsive latex samples. Maximum coloration intensity was observed for LB-SP3% and LB-SP5%, which is a result of high concentration of SPEA. Lower amounts of SPEA resulted in minimum coloration for LB-SP0.5% and LB-SP1%. Therefore, the anticounterfeiting properties can significantly be influenced by the concentration of SPEA in the stimuli-responsive latex samples. These properties are fully reversible and also repeatable for several UV and visible light irradiation cycles.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02296. 1 H NMR spectra of SPOH and SPEA, further experiments for determination of the solid content of the latexes, DLS experiments for investigation of reversibility of the aggregation/disaggregation behavior, and also additional images for photopatterning experiments as a function of SP concentration in different latex samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

CONCLUSIONS Herein, we have developed novel stimuli-responsive latex particles by incorporation of different contents of SP into the PMMA latex particles by the semicontinuous emulsifier-free emulsion polymerization strategy. The particle size and morphology were evaluated by SEM images, where the size of the latex particles is in the range of 400−900 nm, depending on the SP concentration. Decrease of the particle size by increase of the SP concentration was also observed. Measuring the particle size by DLS and transparency by UV−vis spectroscopy showed reversible light-induced aggregation/ disaggregation and change of the particle size in the range of 200−600 nm upon UV and visible light irradiations. The sample containing 5 wt % SP undergoes reversible lightinduced aggregation from 550 to 1120 nm upon UV irradiation. The amount of light-induced aggregation/disaggregation of latex particles is dependent on the SP concentration. Also, the intensity of photochromic properties and coloration were dramatically increased by increase of SP concentration in the latex particles. The experimental and

Amin Abdollahi: 0000-0002-6415-7547 Keyvan Sahandi-Zangabad: 0000-0003-1980-8731 Hossein Roghani-Mamaqani: 0000-0001-6681-7679 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Iran National Science Foundation (INSF) is greatly appreciated for its financial support (grant number: 97003824).

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REFERENCES

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