Effects of Chain Parameters on Kinetics of Photochromism in Acrylic

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Effects of chain parameters on kinetic of photochromism in acrylicspiropyran copolymer nanoparticles and their reversible optical data storage Mohammad Hossain Sharifian, Ali Reza Mahdavian, and Hamid Salehi-Mobarakeh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01869 • Publication Date (Web): 23 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Effects of chain parameters on kinetic of photochromism in acrylic-spiropyran copolymer nanoparticles and their reversible optical data storage

Mohammad Hossain Sharifian, Ali Reza Mahdavian*, Hamid SalehiMobarakeh

Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran Tel: +9821 4478 7000, Fax: +9821 4478 7023 Email: [email protected]

* Corresponding author

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Abstract Chemical bonding between photochromic compounds and polymer matrices will induce several privileges such as photostability, photoreversibility, elimination of dye aggregation and undesirable negative photochromism. In such polymeric systems, the switching rate and photoisomerization are closely dependent on several parameters like chain flexibility, steric restrictions, polarity and even proticity of the surrounding media. Here, the copolymerization of a synthesized spiropyran-based monomer with butyl acrylate (BA) and methyl methacrylate (MMA) comonomers was carried out with various compositions of MMA and BA through emulsion polymerization and their corresponding photoisomerization kinetics were studied. Particles size and their distributions were analyzed by DLS and the morphologies were investigated by SEM and TEM analyses. The results showed they are spherical with diameter of 62-82 nm. DSC thermograms were employed to determine Tg of the prepared copolymers, which were from -23 to 93 oC. The kinetic of photoisomerization was then studied by UV-Vis spectroscopy. Finally, the latex containing spiropyran/acrylic copolymer with Tg of about 0 oC and optimum rate of coloration and decoloration was considered in the reversible optical data storage studies due to its fast photochromism and good film formation properties.

Keywords: Spiropyran, Acrylic, Photochromic, Nanoparticle, Latex Introduction Photochromic compounds undergo reversible color changes by versatile triggers such as heat or a pursuant irradiation1.These materials are extensively employed in cell labeling2,3, biotechnology4,5, optical data storage6–8, security documents9,10, chemical sensors11,12, dualcolor systems13,14and ophthalmic lenses15,16by considering their fast response. They can be introduced into a polymeric matrix by physical (i.e. simple doping)16,17 or chemical (covalent 2 ACS Paragon Plus Environment

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bonding)18,19 methods, but chemical methods are preferred due to their advantages like prevention of the dye leaching and migration20, dye aggregation20, preventing negative photochromisms21, reducing photodegradation22, improved biocompatibility23, and ease of processability24. The isomerization rate during photochromism in a polymer matrix depends remarkably on the surrounding environment such as free volume of the matrix, rigidity, steric restrictions, polarity and protic characteristics25. This returns to the structural changes in the photochromic moiety with a required free volume and flexibility of the polymeric chains with respect to the geometry of isomers. Spiropyran (SP) derivatives convert from colorless to the colored merocyanine (MC) zwitterionic isomer accompanied by ring-opening reaction through cleavage of C-O bonds upon UV irradiation at about 365 nm, and also through inclusion in a polar media which is called negative photochromism26. SP derivates could be incorporated into polymer matrices based on methyl methacrylate27, butyl acrylate28, styrene-butadiene-styrene copolymer29, liquid crystal polymers30, N-isopropyl acrylamide31 and siloxane15. Addition of a plasticizer in the obtained copolymer will result in the fast isomerization rate, because of the ease of polymer chains mobility16. In a substrate where polar groups are present, MC zwitterion is predominantly stabilized and the rate of reverse isomerization (MC to SP) will be reduced32. Zhao et al. studied color changes of naphthodipyran, which was doped in polymers with different polarities33. The observed blue shift in maximum absorption wavelength (λmax) was attributed to the inter-dipole interactions between the chromophores and surrounding polymer chains. The rate of photocoloration-decoloration of naphthodipyran in such thin films was found to be dependent on Tg of the corresponding polymers. Kojima et al. studied optical behavior of spirooxazine derivates in polystyrene and copoly(styrene-allyl alcohol)34. They found that λmax of the colored form of spirooxazine was shifted to higher wavelengths in the polar matrices according to the stabilization of meta-stable zwitterionic isomer by polar


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interactions with hydroxyl groups in the polymers. In another attempt, the effects of polyurethane-acrylate block copolymers containing SP-derivatives on photochromic and photomechanical properties were studied35,36. It was found that the speed of color fade was inversely depended on rigidity of the polymeric matrix. Recently, we have reported the preparation of novel stimuli-responsive papers by modification of cellulosic fibers with the functionalized acrylic nanoparticles containing a spiropyran derivative24,37. The smart papers were immersed into some polar and non-polar solvents and different color changes were observed under UV irradiation, revealing the role of surrounding polarity. Our results demonstrated that the application of a polymer substrate for the photoactive group will prevent occurrence of undesired negative photochromism. Also, glass transition temperature (Tg) of the polymeric matrix influenced on photochromic isomerization rate seriously. Abe group has developed a photochromic polymeric film for the real-time dynamic holography by doping a bridged imidazole dimer in the polymethyl methacrylate film that had been plasticized by tricresyl phosphate16. They showed enhancement in the fading speed of the photochromic dye with the increase in plasticizer content, relating to the growth in free volume with the decrease in Tg of the surrounding medium. Moreover, they investigated the decoloration kinetic of doped 3H-naphthopyran in block copolymers of MMA and BA to reach to the desired mechanical properties and high fading speed38. Thermal fading kinetic of the doped spiropyran and fulgide derivates in PMMA and styrene-butadiene-styrene copolymer was studied by Lin et al29. It was concluded that the fading speed was accelerated when the temperature reached to above Tg of the matrix. It is worth mentioning that the concern about leakage of the doped material still exists. In another work, the effect of chain length of conjugated PBA to a spirooxazine derivative on the kinetic of optical changes was investigated39. Increase in molecular weight of PBA changed isomerization rate of the


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photochromic group and decoloration speed was reduced consequently. Hence, studies on kinetic of photoisomerization and the affecting parameters in polymer-based substrates is a fascinating issue and could lead us to attain fast photochromic devices with advanced properties. In this study, spiropyran ethyl acrylate (SPEA) was prepared and used as a comonomer with covalently introducing into the backbone of polymer chains containing different ratios of methyl methacrylate (MMA) and butyl acrylate (BA) through emulsion polymerization. Thermal properties of the obtained latex nanoparticles were evaluated. The effect of various Tgs and polarities of the obtained copolymers on the rate of SP ⇄ MC isomerization in SPEA and the kinetic of photoisomerization were studied by UV-Vis spectroscopy. The optimizations were conducted to reduce limitations of photochromic polymers, i.e. slow photoisomerization and poor photofatigue resistance. To the best of our knowledge, this is the first report on rate of photoisomerization of SPEA chromophores in the latex nanoparticles with gradient changes in flexibility and polarity of the acrylic copolymer chains for better understanding of the affecting parameters in such systems. Also, we demonstrated the capability of reversible optical data storage by using of such fast photochromic polymers containing spiropyran derivate. Experimental Materials 2,3,3-trimethylindolenin and sodium dodecyl sulfate (SDS) were supplied by Sigma-Aldrich. All of the solvents, 2-bromoethanol, acryloyl chloride, potassium persulfate (KPS), sodium hydrogen carbonate (NaHCO3), triethylamine, Triton X-100, methyl methacrylate (MMA) and butyl acrylate (BA) were purchased from Merck Chemical Co. All chemicals were utilized without further purification and deionized (DI) water was used in all recipes.


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Preparation of the photochromic nanoparticles 1'-(2-acryloxyethyl)-3',3'-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2'-indoline) (SPEA) was synthesized according to our previously reported procedure27. Photochromic latex nanoparticles were prepared using emulsion polymerization. The amounts of components in preparation of each photoresponsive latex with a pre-defined theoretical glass transition temperature ((Tg)theo) (from Fox equation40) have been given in Table 1.Typically, the reactor was charged with an aqueous solution of 0.06 g SDS (as ionic surfactant) and 0.03 g Triton X-100 (as non-ionic surfactant), 0.03 g sodium hydrogen carbonate (as buffer) and 0.03 g potassium persulfate (as initiator). The temperature reached to 75ºC and stirred under continueous flow of nitrogen gas at 450 rpm. Afterward, an aqueous solution of 0.1 g SPEA in 3 mL DI water and also MMA and BA monomers were added simultaneously from two separate dropping funnels into the reactor within 15 min dropwise. Emulsion polymerization progressed under the aforementioned condition for 3 h and the monomer conversion was monitored by gravimetric method. Final conversions for all the samples were above 95% and coagulation amounts were below 1 wt%. Table 1. Preparation of the photochromic latexesa

Sample

PLNB0

PLNB20

PLNB40

PLNB60

PLNB80

MMA (g)

3

2.4

1.8

1.2

0.6

BA (g)

0

0.6

1.2

1.8

2.4

BA:MMA

0:1

0.25:1

0.67:1

1.5:1

4:1

(weight ratio) a

Total amount of DI water in each recipe was 27 mL.

Charecterization Differential scanning calorimetry (DSC) analyses were accomplished from -50 to 120ºC (scanning rate of 10 ºC/min) by using a Maia 200-F3 Netzsch (Germany) calorimeter under a 6 ACS Paragon Plus Environment

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flow of nitrogen gas. Particle size and particle size distribution (PSD) of each sample were determined by dynamic light scattering (DLS). These were performed on a Malvern Zetasizer ZEN3600 (UK) with the laser angle of 90º and the samples were diluted 100 folds with an aqueous solution of surfactants with similar concentrations to the polymerization recipe. SEM images were captured by a Tescan Vega II (Czech Republic). One drop of the diluted latex was placed on a sample holder and dried under vacuum at 25ºC and then, sputter coated by gold powder by using of EMITECH K450x sputter-coater (England). Transmission electron microscopy (TEM) was performed on Philips CM30 electron microscope (Netherland) operating at an acceleration voltage of 150 kV. To prepare TEM samples, a drop of the diluted latex was placed onto a copper grid and left to dry. The photochromic behavior of specimens was studied by Shimadzu-UV2550 UV-Vis spectrophotometer (Japan). For this reason, the initial latex was diluted with DI water to about 0.2 wt%. To evaluate photochromic properties, the excitation was done by a UV lamp (365 nm), CAMAG 12VDC/VAC (50/60 Hz, 14VA, Switzerland). Also the source for visible light was a 15 W LED tube lamp with white light. Results and disccution The major limitations in utilization of photochromic polymeric products relate to the enhancement in fast photoresponsiveity along with stability and photofatigue resistance. For the fast isomerization rate, free volume and surrounding polarity are crucial parameters and covalently bonding of the photoresponsive moieties is essential to reach to adequate stability and improved fatigue resistance. Here and in order to study the role of influencing factors on such isomerization, latex polymer particles containing various BA:MMA weight ratios were prepared in the presence of SPEA via emulsion polymerization (Scheme 1).


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Scheme 1. Schematic representation for preparation of polymer particles containing spiropyran moieties

Size and morphological studies Size of photochromic latex nanoparticles has to be less than half of the wavelength of incident beam to cause minimum light scattering plus the desirable transparency41. For reasonable photochromic properties in this work, small size (blow 100 nm) and narrow particle size distribution (PSD) is preferred. DLS analysis and SEM micrograph of each sample was thus recorded and shown in Figure 1. All latex nanoparticles are spherical and below 100 nm, ranging from 64 to 82 nm with a smooth surface and narrow size distribution. This means that the variation in comonomers composition had no substantial effect on the particles size due to the employed optimized procedure. For better illustration, TEM micrograph of PLNB60 nanoparticles was taken (Figure 1-f) and it demonstrates that the dried latex particles are in the range of what we observed from DLS and SEM analyses.


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Figure 1. SEM micrographs and DLS analysis (insets) of (a) PLNB0 (b) PLNB20 (c) PLNB40 (d) PLNB60 (e) PLNB80 and (f) TEM image of PLNB60 nanoparticles (PSD represents particle size distribution)


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Thermal analysis In order to investigate the effect of comonomers ratio on glass transition temperature of the photochromic latex particles, differential scanning calorimetry (DSC) was run from -50 to 120 ºC. Tgs of polybutyl acrylate and polymethyl methacrylate homopolymers lay at about -50 and 105 ºC, respectively42. DSC thermograms of the samples have been shown in Figure 2 accompanied by the extracted Tgs. The results depict a decreasing trend in Tg of the prepared copolymers with the increase in BA:MMA weight ratio. The difference between experimental and theoretical Tgs returns to some deviation in copolymer compositions during emulsion polymerization. Generally, by controlling the changes in mobility of the surrounding medium of a photochromic compound, one could provide an appropriate situation for facile structural changes during photoisomerization. Hence, this isomerization rate would be manipulated by properties of the host copolymer matrix. The obtained photochromic nanoparticles, covering a wide range of Tgs from -22 to 93ºC (according to (Tg)exp), seem to be appropriate for investigation of the effect of flexibility and corresponding free volume on the isomerization rate of photochromic SPEA moiety in the prepared latex nanoparticles.


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Figure 2. DSC thermograms of the obtained photochromic latex nanoparticles (left) and numerical data for Tgs of the samples (right)

Photochromic properties Colorless spiropyran undergoes photo-induced ring-opening reaction under UV irradiation to yield its isomeric merocyanine form which has a strong visible absorption band in the range of 500-600 nm. The speed of color changes in photochromic compounds is remarkably slower in a solid or polymeric matrix relative to solution state. The slow switching rate in the solid media returns to the inhibition for free rotations of the chromophores in the host matrix. Thus, developments in application of polymer-based chromophores will be restricted seriously. On the other hand, undesirable phenomena such as low photofatigue resistance, negative photochromism, dye leaching and aggregation are usually observed in the physically-adsorbed chromophores in a solid substrate. Here, photochromic behavior of all the prepared samples was investigated using UV-Vis analysis. Figure 3 shows absorption spectra of the photochromic latexes, before and after UV irradiation at 365 nm. It is notable that all the samples were diluted to 0.2 wt% to have comparable results prior to UV-Vis analysis. There is no absorption band in the range of 400700 nm for all the samples before UV irradiation, while the appeared absorption in the range of 500-650 is ascribed to SP to MC isomerization after irradiation at 365 nm. An interesting observation relates to the absorption intensities with the variation in copolymer composition, while SPEA content has been kept constant in all the samples. However, the role of particle size could logically be ignored as all the particles sizes are below 100 nm. Increase in flexibility and decrease in polarity of the copolymer chains with the increase in BA:MMA weight ratio are the two competing factors here. The former provides ease of free rotations in spiropyran during isomerization and ring opening-closure cycle. The later causes destabilization of the zwitterionic MC isomer and resists against SP to MC interchange upon


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UV irradiation. Therefore, considering these two affecting parameters simultaneously is of high importance. An increasing trend in the absorption intensity at 500-650 nm after UV irradiation (Amax) from PLNB0 to PLNB60 in Figure 3 demonstrates the facile MC formation and its higher concentration in the latex particles. However, more increase in BA content had opposite effect and Amax decreased from PLNB60 to PLNB80. To show this variation more precisely, the obtained Amax was normalized by its subtracting from the absorption intensity of each sample before UV irradiation (Asp) and the quantities have been given in Figure 3f. Also, different colors of the prepared latexes after UV irradiation (as a macroscopic evidence) exhibited the relevancy of absorption intensity to BA:MMA weight ratio (inset pictures in Figure 3). Beside the absorption intensity, maximum absorption wavelength (λmax) is another parameter that could be considered. Polarity of the meta-stable colored merocyanine moieties under UV irradiation is obviously higher than spiropyran form and λmax for MC depends on its surrounding environment according to its stabilization and interaction. Either polarity or capability of hydrogen bond formation between MC and the polymer matrix can shift λmax and cause some changes in the latex color. This was observed for all the latexes because of a slight red shift from PLNB0 to PLNB80 (Figure 3f). Actually, this could be attributed to the variation in MMA content that affects on polarity of the polymer matrix significantly.


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Figure 3. UV-Vis spectra for (a) PLNB0 (b)PLNB20 (c)PLNB40 (d)PLNB60 (e)PLNB80 before (…) and after (‫ )ــــ‬UV irradiation at 365 nm and the corresponding picture (the insets) of each latex before (left) and after (right) UV irradiation. The extracted numerical data from UV/Vis spectra have been summarized in (f).

Real-time UV-Vis analysis


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In continuum and to investigate the effect of chain flexibility and polarity on photochromic behavior, the diluted latex could be excited by irradiation at 365 nm in different time intervals. Typically, Figure 4 displays real-time analysis for PLNB60 after UV irradiation in several 10 s intervals and totally for 100 s. It is evident that there is a direct relationship between the absorption intensity and irradiation time according to the formation of MC isomers with the increase in exposure time at 365 nm.

Figure 4. Variation in absorption intensity of PLNB60 latex after irradiation at 365 nm in different time intervals

In order to investigate the effect of chains flexibility and surrounding polarity on switching rate of SP to MC (forward) and MC to SP (reverse) isomerization, absorption peak area of MC form was normalized by its subtracting from the absorption peak area before UV irradiation. The values of normalized absorption peak area (NAPA) were plotted versus irradiation time in Figure 5. The specimen spectrum was recorded after every 10 seconds irradiation at 365 nm until NAPA value became plateau (approximately 430 s for the forward


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and 120 s for the reverse isomerization).The results showed an exponential dependency of color changes in both directions with irradiation time for PLNB-series samples.

Figure 5. Normalized absorption peak area (NAPA) of the prepared latexes versus irradiation time for SP to MC (a) and MC to SP (b) isomerization


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Variation of NAPA demonstrated a steep slope for all the samples in each direction (Figure 5a and 5b) at early stage of irradiation before reaching to plateau. The kinetic of photoactivation of latexes could be well-described by Eq. 1, extracted from Figure 5a. ( ) ( )

= 1 − exp (− )

Eq.1

where kC represents the rate constant of SP to MC isomerization and NAPA(t) and NAPA(∞) are the normalized absorption peak areas at time t and infinity. Also, the reverse isomerization kinetic is fitted to Eq. 2, extracted from Figure 5b, where kD is MC to SP isomerization rate constant. ( ) ()

= (− )

Eq. 2

However, kC and kD are the estimates of ability of the material to undergo coloration after UV and decoloration upon visible irradiations, respectively. The time required reaching to half of the final absorption peak area for coloration, (T1/2)C, and decoloration, (T1/2)D, and also the values of kC and kD have been given in Table 2. Table 2. Coloration and decoloration rate parameters for the prepared latexes

Sample

kC (s-1)

kD (s-1)

(T1/2)C (s)

(T1/2)D(s)

PLNB0

0.031

0.048

22

13

PLNB20

0.032

0.05

24

16

PLNB40

0.035

0.052

28

19

PLNB60

0.06

0.075

14

11

PLNB80

0.05

0.113

23

7

As mentioned before, glass transition temperature and chains polarity affect on optical properties of the incorporated spiropyran simultaneously and in opposite directions. Hence, 16 ACS Paragon Plus Environment

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separate investigation about the effect of these two parameters on the photochromic properties was not possible. The calculated kC would be a good indication to quantify the role of these parameters on the isomerization rate. The increase in BA content in the copolymer chains makes them more hydrophobic and also more flexible (by lowering Tg). Table 2 implies that kC shows a drastic growth from PLNB0 to PLNB60 (by factor of 2) and it descends from PLNB60 to PLNB80. In the other words, lowering of Tg and increase in chain flexibility is the dominating factor primarily, despite the decrease in chain polarity. On the other hand, the surrounding polarity showed a prominent role and controlled the isomerization rate in PLNB80. Figure 5b demonstrates decoloration process of PLNB-series latexes under visible light irradiation after every 10 seconds. The comparison of kD values (Table 2) confirms the above observations. Likewise, kD is dependent on both chain flexibility and polarity. This illustrates that PLNB80 is the fastest sample in terms of color fading under visible light due to the highest non-polarity and chain flexibility. So these two factors are consistent with each other and result in different kinetic behavior for the forward and reverse isomerizations. However, t1/2 values for both directions (SP MC) do not correspond with variations in kC and kD. This returns to the essentiality of simultaneous irradiation and UV-Vis spectroscopy in such studies, because of different isomerization rates in such polymeric samples. Therefore, t1/2 values will not be meaningful in these investigations. Photofatigue resistance Although SP-based systems have shown multiple benefits in various applications, they still suffer from low photofatigue resistance. Some reports express that immobilization of SP groups in a polymer matrix can dramatically improve this weakpoint43. In order to investigate this characteristic in the prepared photochromic latexes, each sample was repeatedly


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irradiated with UV and visible lights at ambient conditions for 10 cycles and the corresponding absorptions were measured immediately after each irradiation with 3 min time interval between the two cycles (Figure 6). The absorption intensity of MC isomers started to decrease after 10 cycles slightly. The results depict that the absorption efficiency is still convincing during these UV-Vis irradiation cycles for such photoresponsive systems.

Figure 6. Variation in absorbance for PLNB-series upon alternative UV (at 365 nm for3 min) and visible (for1 min) irradiations


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Optical data storage Based on the observed kC and kD, photochromic and photofatigue properties, PLNB60 was selected as the optimum sample with desirable characteristics. Therefore, a thin film of PLNB60 latex was prepared by simple casting and employed to store two different patterns in order to demonstrate its reversible optical data storage-ability (Figure 7). After excitation by UV light at 365 nm, the finished logo of the recorded pattern became visible. The film revealed two various colors: irradiated zone of the uncovered section, showing a strong purple color and also a yellow non-irradiated zone (Figure 7-right). By resuming UV irradiation, a fully purple film was obtained. Next, a second pattern was used to create an image on this film under visible light (Figure 7-left) and the persistent visible irradiation without masking, turned it to the primary yellow film. This experiment revealed the potentiality of the prepared transparent thin film from PLNB60 latex nanoparticles for optical data storage induced by UV-Vis irradiation.

Figure 7. Optical images of a patterned PLNB60 thin film after UV (365 nm) and Vis irradiations


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Conclusion Rigid to flexible photochromic polymer nanoparticles were prepared to illustrate the effect of chain flexibility and surrounding polarity on the photochromic property of a spiropyran derivative. Poly(MMA-co-BA-co-SPEA) latex nanoparticles with different BA:MMA weight ratios, blow 100 nm with narrow particles size distributions were prepared through emulsion polymerization and spherical morphology of the particles was observed by SEM and TEM. Wide range of Tgs for the prepared copolymers was confirmed by DSC analysis, from 93 to -22 ºC. UV-Vis spectroscopy and corresponding kinetic studies demonstrated that flexibility and polarity of the polymer chains have significant effects on SP MC photoisomerization in

such nanoparticles. Consequently, these parameters revealed that PLNB60 and PLNB80 were the fastest samples for the forward and reverse isomerizations, respectively. Next, photofatigue resistance of the prepared photochromic latexes were investigated for 10 cycles of UV and visible irradiations. Due to the existing covalent bonds between spiropyran moieties and the polymer chains, good photostability was observed for all the samples. Also, PLNB60 with good film formability and fast photochromism was employed to study its reversible optical data storage properties and the results confirmed its reasonable capability. Acknowledgment We wish to express our gratitude to Iran Polymer and Petrochemical Institute (IPPI) for financial support of this work (Grant No. 24761176).


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For Table of Contents Use Only

Effects of chain parameters on kinetic of photochromism in acrylicspiropyran copolymer nanoparticles and their reversible optical data storage

Mohammad Hossain Sharifian, Ali Reza Mahdavian*, Hamid Salehi-Mobarakeh

Polymer Science Department, Iran Polymer & Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran Tel: +9821 4478 7000, Fax: +9821 4478 7023; Email: [email protected]


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Effects of Chain Parameters on Kinetics of Photochromism in Acrylic

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