Photoswitchable Surface Wettability of Ultrahydrophobic Nanofibrous

Journal of Colloid and Interface Science 593 (2021) 67–78

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Regular Article Photoswitchable surface wettability of ultrahydrophobic nanoﬁbrous coatings composed of spiropyran-acrylic copolymers ⇑ Erfan Nezhadghaffar-Borhani a, Amin Abdollahi a, Hossein Roghani-Mamaqani a,b, , ⇑ Mehdi Salami-Kalajahi a,b, a Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box: 51335-1996, Tabriz, Iran b Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran graphical abstract

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Article history: Hypothesis: Light-controlling of surface characteristics in polymeric coatings has been a significant Received 23 October 2020 research area because of its potential application in development of smart surfaces. Wettability of Revised 1 March 2021 light-responsive polymeric coatings based on spiropyran photochromic compound could be tuned by Accepted 2 March 2021 light irradiation. This is mainly because of spiropyran isomerization between the hydrophobic and hydro- Available online 12 March 2021 philic states. Experiments: Light-responsive latex nanoparticles containing spiropyran moieties were synthesized by Keywords: semi-continuous emulsion copolymerization of acrylate monomers, which have different chain flexibility Photochromism depending on the copolymer composition. Photochromic properties of spiropyran in stimuli-responsive Spiropyran Latex nanoparticles latex nanoparticles displayed dependence of photochromism intensity and its kinetics to flexibility of Nanofiber the polymer chains in addition to the polarity of media. Photoswitchable surface wettability of the Ultrahydrophobic surface spiropyran-containing acrylic copolymer coatings was investigated, where the photo-responsive coatings Wettability were prepared by solution casting and electrospinning methods. Findings: The photoswitchable coatings prepared by solution casting and electrospinning methods showed significant differences in their physical characteristics and especially surface wettability. The

⇑ Corresponding authors at: Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box: 51335-1996, Tabriz, Iran. E-mail addresses: [email protected] (H. Roghani-Mamaqani), [email protected] (M. Salami-Kalajahi). https://doi.org/10.1016/j.jcis.2021.03.012 0021-9797/Ó 2021 Elsevier Inc. All rights reserved. E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

polymeric coatings displayed water droplet contact angles in the range of 60-93°, which could reversibly be switched to 55-86° upon UV light (365 nm) illumination as a result of isomerization of the hydrophobic spiro form to the zwitterionic merocyanine form. The nanofibrous coatings prepared by electrospinning method displayed higher contact angles in the range of 120-136°, which was switched to 78-105° upon UV light irradiation. The developed photo-responsive coatings displayed highly-efficient photoswitching between the two hydrophobic and hydrophilic states as a response to UV and visible light irradiation. The photoswitchable nanofibrous coatings displayed ultrahydrophobic characteristics, where the colored water droplets were stable on their surface and could easily be adsorbed by a cellulosic tissue. In summary, the photoswitchable nanofibrous coatings could be applied for design and development of ultrahydrophobic materials with the ability of photo-controlling of surface wettability by light irradiation with tunable intensity. Ó 2021 Elsevier Inc. All rights reserved.

1. Introduction preparation of photo-responsive polymer coatings. Some of the common and important photochromic compounds (organic) in Hydrophobic and hydrophilic coatings with the ability of sur- the preparation of light-responsive polymers and surfaces are face wettability control in response to external triggers has been azobenzene, spiropyran, spiroxazine, diarylethene, and fulgide one of the most interesting research topics in the coating industries [41–45]. These photochromic compounds could be isomerized [1–6]. Smart polymers with reversible switching between the between two forms with different characteristics upon light irradi- hydrophobic and hydrophilic states are the most significant cate- ation, which cause considerable variations in the chemical and gory of such advanced materials,[7,8] which have advanced appli- physical properties of the surrounding media or polymer matrix, cations in anti-fogging, [9] self-cleaning, [10] anti-icing,[11] drug such as wettability, polarity, surface energy, and refractive index delivery, [12] sensors, [12,13] and also smart membranes with [46–49]. Spiropyran as an important photochromic compound dis- switchable permeability [14]. The coatings with water contact plays coloration upon UV light by cleavage of the spiro CAO bond angle of above 90° are well-defined as hydrophobic surfaces and isomerization from a ring-closed and non-polar spiro (SP) form according to the Young’s equation, where the super-hydrophobic to a ring-opened and zwitterionic merocyanine (MC) form [18,48]. surfaces have contact angles of larger than 150° [15,16]. The coat- Light-induced reversible isomerization of spiropyran between the ings with water contact angles of lower than 10° are known as two SP and MC structures could be used in the preparation of smart super-hydrophilic surfaces. [17] Super-hydrophobic surfaces have polymer coatings with the ability of photoswitchable surface wet- mainly been observed in natural sources, such as non-smooth tability. Su and coworkers reported a photochromic hydrophobic plant leaves and also fruits and animals skin. Volger defined that cellulosic paper modified with spiropyran-containing acrylic surfaces display water contact angle of more than 65° are copolymer based on fluorinated acrylic monomers prepared via hydrophobic, and also surfaces with contact angle of below 65° emulsion polymerization [50]. Optical properties of the photo- could be considered as hydrophilic materials. [18] Surface geome- responsive hydrophobic coatings displayed photoswitchable sur- try, morphology, and chemistry (functional groups) are highly face wettability upon UV and visible light irradiation because of important factors in controlling of surface wettability. [12] Func- light-induced isomerization of spiropyran molecules between the tionalization of surfaces with polar groups induces hydrophilicity hydrophobic SP and hydrophilic MC forms. Such smart behavior by increasing interfacial tension, however decrease of interfacial and the corresponding change of contact angle could significantly tension is achieved by functionalization with nano-polar or be affected by the concentration of spiropyran groups and fluori- hydrophobic functional groups [19–21]. According to the Wenzel nated comonomer. Wang and coworkers prepared a light- and Cassie-Baxter theories, induction of surface roughness is an responsive surface by grafting of spiropyran on etched silicon sub- essential requirement to enhance hydrophobicity and hydrophilic- strates using atom transfer radical polymerization, which induced ity of different coatings. The hydrophobic and hydrophilic coatings roughness to silicon surface and led to increase of surface could be prepared by several methods, such as phase separation, hydrophobicity [12]. Investigation of surface wettability of the sil- [22] self-assembly, [23] casting of polymer solution, [24] electro- icone substrate under UV and visible light irradiation displayed spinning, [25] and layer-by-layer deposition [26]. that water contact angle was changed from 138.8 ± 1.3 to 42.7 ± 1 Design and development of smart polymer coatings with .7° after UV irradiation (365 nm) for 5 min, and it was reversed to hydrophobic, super-hydrophobic, and super-hydrophilic charac- its original state upon visible light irradiation of about 20 min. teristics, which could be controlled by different stimuli, are very Such a reversible surface wettability variation accompanied by a important in different applications related to the coating indus- color change between yellow and purple was related to the tries. Controlling of surface-wettability in response to different hydrophobic and hydrophilic surfaces, respectively. Asatekin and external stimuli, such as pH, [27] temperature, [28] solvents, [29] coworkers developed a photo-responsive self-cleaning membrane and light [30] has largely been studied. Light has been considered via coating of a thin layer of comb-shaped graft copolymers com- as an important stimulus because of its fast and facile accessibility, posed of polyacrylonitrile backbones and spiropyran functional controllability out of the systems, and ease of application [31–34]. groups on porous support membrane [14]. The roughness induced Light-responsive coatings have extensively been used for photo- on the membrane surface by comb-shaped light-responsive switching of surface wettability. [18,32] Photo-controlling of sur- copolymer chains and its reversible photoswitchable wettability face wettability in polymer coatings could be carried out by two resulted in removing predeposited foulant layers by light- types of organic and inorganic light-responsive materials. [18,35] induced surface morphology transition under UV or visible light Metal oxides, such as titanium dioxide [36], zinc oxide [37], tung- irradiation. Wang and coworkers developed polymeric films with sten oxide [38], silicon dioxide [39], and stannic oxide [40] are the photoswitchable surface wettability based on acrylamino most significant category of photo-responsive inorganic materials. spiropyran-methyl methacrylate copolymer and also using electro- On the other hand, organic photochromic compounds are spinning technique to form a rough surface [46]. The results dis- light-responsive materials that have extensively been used in the played decease of surface contact angle in response to UV light

68 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78 irradiation due to isomerization of the spiropyran from hydropho- the previous mixture dropwise at 15 min. Afterward, appropriate bic SP to the hydrophilic MC forms accompanied with a local color amounts of the SPEA monomer (3 and 5 wt% with respect to the change. Therefore, important role of surface roughness on increase final monomer content) were dissolved in 1 mL of DI water and of water contact angle in stimuli-responsive polymeric coatings then added dropwise to the mixture for surface functionalization containing spiropyran with the ability of light-controlling of sur- of the latex nanoparticles with spiropyran, which could signifi- face wettability by concentration of the spiropyran and UV light cantly improve the final optical properties. After the addition of dosage was comprehended. SPEA solution, the polymerization reaction was continued for 4 h In this study, stimuli-responsive polymer coatings were pre- to obtain photochromic latex nanoparticles with monomer conver- pared from styrene and butyl acrylate copolymer containing differ- sion of above 95% and coagulation content of below 0.5% (using the ent contents of spiropyran photoswitch. The copolymer samples methods presented in Supporting Information). Total amount of were designed in a wide range of chain flexibility with different the SPEA incorporated to the latex nanoparticles was determined glass transition temperatures (Tg) by using different styrene and by coagulation of the latex samples and absorption measurement butyl acrylate ratios according to the amounts obtained from the of the remaining serum by UV–Vis analysis with respect to the Flory-Fox equation using semi-continuous emulsion polymeriza- standard solution and the corresponding calibration curve. It was tion. Photoswitchability of the surface wettability was carried out found that more than 95% of the added SPEA was incorporated to upon UV and visible light irradiation by isomerization of spiropy- the nanoparticles. Chemical structure, molecular weight (MW), ran between the hydrophobic SP and hydrophilic MC forms. The and number-average degree of polymerization (DP) of the samples 1 main idea was studying the surface roughness effect on the photo- were investigated by H NMR (CDCl3, 300 MHz). The corresponding switchable surface wettability of the smart polymer coatings in spectra are displayed in Figure S1 (Supporting Information). addition to the effect of spiropyran concentration and also copolymers flexibility. Higher surface area and also surface roughness of 2.4. Preparation of photochromic coatings the electrospun nanofibrous polymer coatings might have considerable effects on the water contact angle and photoswitchable sur- The photochromic latex samples (20 mL) were coagulated by face wettability of the polymer coatings. acid solution, and then the polymer aggregates were washed with water and ethanol for several times (4 times in both water and ethanol, 20 mL) and dried at 50 °C for 24 h by using a vacuum oven. 2. Experimental To prepare the photochromic polymer coatings, a solution of pure copolymers (0.5 g) in toluene (15 mL) was prepared and transferred 2.1. Materials to a petri dish for slow evaporation of solvent at ambient condition for 48 h. For complete removal of solvent, all of the photochromic Butyl acrylate (BA), styrene, acryloyl chloride (AC), sodium coatings were put in vacuum oven at 40 °C for 48 h. Finally, the pre- hydrogen carbonate (NaHCO ), ammonium persulfate (APS), 3 pared photochromic polymer coatings were used for characteriza- sodium dodecylsulfate (SDS), dimethylformamide (DMF), toluene, tion and investigation of their smart characteristics. and all of the solvents were supplied from the Merck Chemical Company. 2,3,3-Trimethylindolenin, 2-bromoethanol, and 2- 2.5. Electrospinning of the photochromic polymer solutions hydroxy-5-nitrobenzaldehyde were purchased from Sigma- Aldrich and used for the synthesis of (R/S)-2-(30,30-dimethyl-6-nit Concentration of the polymer solution is an important factor in ro-30H spiro[chromene-2,20-indol]-10-yl)ethanol (SPOH). Distilled- electrospinning process and final morphology of the nanofibers. deionized (DI) water were also used in the experiments. All the Therefore, a solution of pure photochromic copolymer in DMF materials were used without further purification. was prepared with a concentration of 50 wt% after dissolving in ambient condition for 24 h. The obtained copolymer solutions 2.2. Synthesis of SPOH and SPEA were used for electrospinning on glass and aluminum foil surfaces.

SPOH as hydroxyl-functionalized spiropyran photochro- 2.6. Characterization mophore was prepared in three steps from 2,3,3- trimethylindolenin according to the procedure reported by Raymo Chemical structures of the spiropyran derivatives and the corre- and coworkers [51]. The purple SPOH crystals were modified to the sponding copolymers were characterized by proton nuclear mag- spiropyran ethyl acrylate (SPEA) monomer as a yellow powder, netic resonance (1H NMR) spectroscopy using a Bruker DPX according to the previously reported methodology [52]. Chemical 400 MHz apparatus in CDCl3. Differential scanning calorimetry structures of SPOH and SPEA were characterized by 1H NMR spec- (DSC) was used for determination of Tg in N2 atmosphere and also troscopy (CDCl3 and DMSO, respectively, 400 MHz), and the under 10 °C/min heating rate by using of NETZSCH Instruments Co. resulted spectra were reported in our previous studies [30,53]. (DSC 200, F3Maia, Germany) within the temperature range of 100 to 150 °C. The samples for DSC analysis were prepared by coagula- 2.3. Synthesis of photochromic latex nanoparticles tion of the latex samples, its purification by washing with water and ethanol for several times to remove non-polymeric additives Semi-continuous emulsion polymerization was used for the including salty compounds, and finally draying in vacuum oven synthesis of light-responsive latex nanoparticles containing at 40 °C for 24 h. Size of the latex nanoparticles and its distribution spiropyran groups with a solid content of about 10 wt%. In a typical were studied by using ZETASIZER NANO ZSP dynamic light scatter- procedure, a mixture of NaHCO3, SDS, and DI water (with the ing (DLS, Malvern, United Kingdom) at 25 °C. To prepare the sam- amounts presented in Table S1) was transferred to a three- ples for DLS analysis, the initial latex samples were diluted with DI necked round bottom flask equipped with a condenser and nitro- water to reach the concentration of about 1 mg/mL (a clear col- gen gas inlet under magnetic stirring at 400 rpm. To initiate the loidal sample). Morphology and size of the latex nanoparticles polymerization reaction, an aqueous solution of APS (0.08 g) was and nanofibers were microscopically studied using field-emission added to the resulted mixture and temperature was set at 80 °C. scanning electron microscopy (FE-SEM) by using of a Tescan Mira Immediately, a mixture of styrene and BA (with the amounts pre- III (Czech Republic). Typically, a drop of diluted latex samples (with sented in Table S1 for each sample with different Tg) was added to a concentration about 1 mg/mL) was placed on the sample holder

69 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78 and dried in vacuum at 25 °C to prepare the samples for FE-SEM oven to obtain highly pure polymer powders. The polymer pow- microscopy. Finally, the samples were put in vacuum, evacuated, ders were used for characterization of chemical structure and also and a layer of gold was deposited under flushing with argon by measurement of molecular weight. The powder samples were dis- using EMITECH K450x sputter-coating (England). The optical prop- solved in CDCl3 for 24 h, and used for investigation of chemical erties including photochromic properties, kinetics of pho- structure and molecular weight by 1H NMR (300 MHz). Measure- tochromism, and photofatigue resistance for all the samples were ment of DP and MW were carried out by integration of the hydro- investigated by UV–Vis analysis by using EU2200 UV–Vis ONLAB gen peak area h (chain end group, as indicated in chemical Instruments (China). For investigation of optical properties of the structure of Figure S1) and hydrogen b (as the variable value with photochromic latex nanoparticles, excitation of the spiropyran molecular weight of the samples) using Equations (1) and (2). The molecules was done by a UV lamp (365 nm, 6 W/m2), CAMAG hydrogen peak area b is integration of the characteristic peak (a/3) 12VDC/VAC (50/60 Hz, 14VA, Switzerland), and the source for vis- and (j,k)/6 for BA and styrene contents, respectively. ible light (back isomerization) was a common LED lamp (8 W/m2). DP=(b)/(h/6) (Equation (1)) For this purpose, UV and visible light irradiation for 5 min were MW = [DP M (molecular weight of styrene or BA mono- used for all the samples. To investigate hydrophilicity and o mer)] + 228.18 (molecular weight of the terminal group resulted hydrophobicity of the polymer coatings and nanofibrous tissues, from dissociation of APS as initiator) (Equation (2)) contact angle of water droplet on their surface was measured using Chemical structure of the copolymer samples characterized by a KRUSS G10 (Germany) at room temperature, 25% relative humid- 1HNMR spectroscopy and the related spectra are shown in Fig- ity, with a water droplet volume of 0.1 mL. Morphology of the pho- ure S1. Lower concentration of spiropyran in the copolymer sam- tochromic nanofibers (0.2 mg/mL) was studied by using Olympus ples has resulted in the absence of its characteristic peaks in the BX50 Microscope with UV Narrow filter, where excitation and 1H NMR spectra. DP values of the copolymer samples were mea- emission were performed in 360–370 and 420 nm, respectively. sured using Equation (1) and (2), and the results are presented in An electrospinning instrument CO881007NYI (Asia Nanostruc- Figure S1. Accordingly, the observed DP values are close to the ture/Iran) was used to prepare the photo-responsive nanofibers. monomers ratio used in polymerization procedure, which confirms Syringe rate of 0.7–0.9 mL/h, fixed electrical voltage of 16–19 KV, high efficiency of the copolymerization method for preparation of and also tip to collector distance (glass and aluminum foil surfaces) the random copolymers. The molecular weight for all the copoly- of 20 cm were used in the electrospinning experiments. mer samples is in the range of 16000–22000 g/mol, which is suitable for preparation of the polymer coatings by film formation and 3. Results and discussion electrospinning methods.

Hydrophobicity and hydrophilicity of the surface functional 3.2. Characterization of thermal properties groups are the most effective factors for controlling water droplet contact angle in polymer coatings. Therefore, incorporation of The prepared copolymers based on styrene and BA monomers smart molecules into polymer coatings with the ability of polarity with different ratios are expected to show different chain flexibility variation in response to an external trigger has been an attractive and Tg values. Photochromic properties of the spiropyran mole- research subject in the recent years. [54–56] In smart polymer coat- cules could significantly be affected by the characteristics of poly- ings containing photochromic compounds, water contact angle or mer matrix especially polarity and chain flexibility, where surface wettability could be changed upon light irradiation (UV or increasing polarity and chain flexibility could facilitate isomeriza- visible) by isomerization of the chromophores between two polar tion of the spiropyran molecules upon UV and visible light irradia- and non-polar structures. Spiropyran as one of the most significant tion between the SP and MC forms [59,60]. Therefore, the photochromic compounds has extensively been used in the prepa- copolymer samples were studied by DSC analysis to investigate ration of light-responsive polymer coatings [50,52,53,57,58]. This their Tg values. In the synthesis recipe of the copolymers, the photochromic compound displays light-induced isomerization comonomers feed ratio was determined for a specific Tg value between the non-polar, discolored, and non-fluorescent SP isomer (15, 30, 45, 60, 80, and 95 °C) by the Flory-Fox equation. As shown and colored, highly polar, zwitterionic, and fluorescent MC isomer. in Figure S2 (Supporting Information), the experimentally This light-induced isomerization reaction is a powerful tool for extracted Tg values from the DSC thermograms are very close to light-controlling of surface wettability by UV irradiation on various the values theoretically indicated by the Flory-Fox equation. On substrates. Isomerization of the SP molecules to MC structure the other hand, these findings confirmed efficiency of the emulsion results in increase of hydrophilicity and also decrease of water con- copolymerization reaction of styrene and BA. Incorporation of 3 wt tact angle on the surface. Here, stimuli-responsive photochromic % of spiropyran photoswitches with an aromatic structure to the latex nanoparticles were prepared by copolymerization of styrene, copolymer backbones was led to increase of Tg values from 58 to BA, and SPEA in emulsion system, according to the procedure 62 °C for SBNPs-60–3%. Increase of the spiropyran concentration reported in Table S1 and schematically presented in Fig. 1. The pre- from 3 to 5 wt% in SBNPs-60–5% was resulted in decrease of Tg pared latex nanoparticles have different Tg values (chain flexibility) from 62 to 56 °C. This shows that further increase of spiropyran and contain 3 and 5 wt% of chemically incorporated spiropyran concentration (above 3 wt%) was led to increase of free volume photoswitch. After complete characterization of the photochromic by expansion of the distance between polymer chains as a result copolymers, all of the samples were used for preparation of poly- of increasing the spiropyran pendant groups, which is similar to mer coatings by two different strategies of solution casting and the effects of long chain pendant groups on chain flexibility of electrospinning. Such stimuli-responsive polymer coatings have polymers. photoswitchable surface wettability characteristics upon UV irradiation due to the presence of the spiropyran photochromophore. 3.3. Investigation of size and morphology of the latex nanoparticles

3.1. Characterization of the copolymers by 1H NMR Emulsion polymerization generally yields latex nanoparticles with a size in the range of 50–300 nm and a narrow size distribu- All of the latex samples were coagulated and washed with tion [30,52,59]. As discussed in the related researches, particle size water and ethanol for several times, and then dried in vacuum and its distribution in the latex samples could signiﬁcantly be

70 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

Fig. 1. Typical procedure for the synthesis of stimuli-responsive copolymer latex nanoparticles containing spiropyran by semi-continuous emulsion polymerization and the photochromic properties.

influenced by a wide range of parameters, such as ionic strength, would not be very effective in final applications of the prepared concentration of ionic surfactant, and also spiropyran amount latex samples as polymer coatings and electrospinning fibers. [30,52,53,57,59]. The spiropyran photoswitch is isomerized to a highly polar zwitterionic MC form when dissolved in water in 3.4. Photochromic properties of the latex nanoparticles emulsion polymerization; therefore, increase of its concentration could be resulted in higher ionic strengths and correspondingly Photochromic behavior and photoluminescence emission of the increase of particle size in addition to a broad particle size distribu- latex nanoparticles containing spiropyran photoswitches were tion [30,52]. The prepared latex samples with different concentra- studied by UV–Vis spectroscopy to investigate the time- tions of spiropyran molecules were studied by DLS analysis to dependent characteristics, kinetics of photochromism, and also investigate the nanoparticles size and its distribution. For this pur- photofatigue resistance of the spiropyran molecules upon several pose, the latex samples were diluted from 10 to 0.1 wt% and ana- light (UV and visible) irradiation cycles. Spiropyran molecules lyzed in ambient conditions (Fig. 2). The latex nanoparticles with show light-induced reversible isomerization between the SP (dis- different Tg values showed a narrow size distribution with particle colored and non-polar) and MC (colored and polar) forms upon size in the range of 40–55 nm, as shown in Fig. 2A. Size of the latex UV and visible light irradiation that could tune surface wettability nanoparticles was slightly increased after incorporation of 3 wt% of of the polymer coatings. Investigation of photo-responsivity and the spiropyran molecules (Fig. 2B). Similar results were observed kinetics of SP M MC isomerization is essential to study photo- for the latex nanoparticles containing 5 wt% of spiropyran photoswitchable surface wettability of the stimuli-responsive polymer switch, as shown in Fig. 2C. A significant trend for the variation coatings. For this purpose, the initial photochromic latex samples of PDI was not observed in all the samples. Only a slight increase were diluted from 10 to 0.5 wt% and studied by UV–Vis spec- in PDI was shown by incorporation of spiropyran molecules to troscopy after 5 min of UV light irradiation (365 nm) for several the polymer nanoparticles. The results displayed that, in spite of times in 20 s intervals. Upon UV light irradiation, the spiropyran the previous studies, [30,52] colloidal instability (aggregation) photoswitch was isomerized from the SP to MC form, which was and increase of particles size were not observed by increasing characterized by a broad absorbance peak in the wavelength range spiropyran concentration in this study, which is a confirmation of 400–700 nm due to photochromic coloration. The maximum for efficiency of the utilized emulsion polymerization strategy. absorbance wavelength and its intensity display significant depen- To investigate morphology of the latex nanoparticles by FE-SEM dency to the polarity and flexibility of the polymer matrix, because microscopy, the initial latex samples were diluted from 10 to polar and flexible media could stabilize the MC form and increase 0.01 wt% and mixed with a solution of phosphotungstic acid intensity of photochromism [59,60]. Higher chain flexibilities

(0.1 wt%) with a ratio of 1/9 of acid solution to the diluted latex. (lower Tg values) at higher BA contents was resulted in lower pho- The phosphotungstic acid solution was added to increase stability tochromism intensities by decreasing polarity [59]. Photochromic of the nanoparticles morphology under the electron beam. As properties of the prepared latex nanoparticles were investigated shown in Fig. 3A-A‘‘ for the SBNPs80 sample and also in Fig. 3B- for all of the samples with different chain ﬂexibilities (Tg values) B” for the SNPs95 sample, the prepared latex nanoparticles have at two different spiropyran concentrations (3 and 5 wt%). The a spherical morphology with a size of about 50 nm, where aggrega- time-dependent UV–Vis spectra for the photoluminescent latex tion of the nanoparticles was resulted from their high surface area. nanoparticles are shown in Fig. 4. Accordingly, increase of the

On the other hand, absence of a non-ionic steric surfactant in the absorbance intensity at kmax were observed for the samples con- polymerization procedure could be resulted in increase of taining higher concentration of spiropyran and also the samples nanoparticles aggregation. Such a low content of aggregation with Tg of about 30 °C. There are two main effective factors for

71 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

Fig. 2. DLS curves of the light-responsive latex nanoparticles with a concentration of 0.1 wt%: (A) without and (B) with 3 and (C) with 5 wt% of spiropyran.

Fig. 3. FE-SEM images of the acrylic latex nanoparticles with a concentration of 0.01 wt%: (A-A’’) SBNPs80 and (B-B’’) SNPs95. controlling of photochromic properties including flexibility and chain flexibility. These two factors displayed a synergetic effect in polarity of the polymer chains, where the optimum polarity and an optimum copolymer composition, which was observed for the chain flexibility was observed for the SBNPs-30–3% and SBNPs- sample with Tg of 30 °C. 30–5% samples with the maximum photochromic intensity. A significant approach for investigation of effective factors on Increasing polarity and chain flexibility were observed by increas- the photochromic properties of spiropyran is study of the kinetics ing the BA content. On the other hand, increase of photochromism of SP M MC isomerization under UV and visible light irradiation. intensity could be observed by increasing both of the polarity and Different characteristics of the surrounding media, such as polarity,

72 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

Fig. 4. Real time UV–Vis spectra of the photochromic latex samples with a concentration of 0.5 wt% under different UV light (365 nm) illumination times (time intervals are various) for (A) SBNPs-15–3%, (B) SBNPs-30–3%, (C) SBNPs-45–3%, (D) SBNPs-60–3%, (E) SBNPs-80–3%, (F) SNPs-3%, (G) SBNPs-15–5%, (H) SBNPs-30–5%, (I) SBNPs-45–5%, (J) SBNPs-60–5%, (K) SBNPs-80–5%, and (L) SNPs-5% pH, and flexibility in addition to light intensity could significantly irradiation time intervals (20 s) for investigation of kinetics of influence kinetics of the SP M MC isomerization. [30,57,58,60– the SP M MC isomerization by measurement of the absorbance

62] The photo-responsivity rate of the spiropyran molecules in at a speciﬁc kmax through UV–Vis spectroscopy. Before plotting polymer coatings indicates the kinetics of the surface wettability the measured absorbance values as a function of irradiation time, variations in response to light irradiation, which is a signiﬁcant all of the results were normalized with respect to the absorbance parameter in photoswitchable polymer coatings. Therefore, the value obtained for the samples before excitation with UV and vis- latex samples were diluted to 0.5 wt% and exposed to UV ible light irradiation. The normalized results were plotted as a (wavelength of 365 nm) and visible light irradiation at different function of irradiation time (UV and visible) to obtain the kinetic

73 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78 curves for the SP M MC isomerization (Figure S3 (Supporting Infor- acrylic copolymers in concentration of about 3 wt% to prepare mation)). Origin software (Professional 2017) was used for fitting the polymer coatings by solution casting method. Figure S5 (Sup- the obtained neat data with suitable curves to extract the kinetic porting Information) displays surface wettability of the acrylic equations and parameters (kc and T1/2). Hence, the SP to MC iso- copolymer coatings based on the samples without spiropyran, merization kinetic curve was fitted by using the BoxLucas1 model, which was investigated by measuring the contact angle of a water and the MC to SP isomerization (back reaction) curve was fitted by droplet on the coatings surface. The contact angle was greatly using the ExpDec1 model, according to the Equations (1) and (2). increased by increasing the styrene feed ratio because of increasing hydrophobicity or T of the coatings, as shown in Fig. 6A-F. The Normalized absorbance ðtÞ g ¼ ð ð ÞÞ ð Þ results indicated the suitable surface wettability of polymer coat- ð1Þ A 1 exp kc1t 1 Normalized absorbance ings in a wide range of hydrophobic and hydrophilic nature with a contact angle ranging from 35 to 100° for the neat block copoly- Normalized absorbance ðtÞ mer latex samples. ¼ A0 þ Aexpðkc2tÞð2Þ Normalized absorbance ð1Þ As shown in Fig. 5, surface wettability of the photochromic polymer coatings containing 3 wt% of spiropyran molecules with where, A and A0 are the constant values, and kc1 and kc2 represent different Tg values were investigated under visible and UV the rate constants for the SP to MC (Equation (1)) and MC to SP (365 nm) light irradiation. A comparison of the results shown in (Equation (2)) photo-isomerization upon UV and visible light irradi- Fig. 5A and Figure S5 (Supporting Information) shows increase of ation, respectively. The normalized absorbance values at time t and contact angle or hydrophobicity of the copolymer coatings after 1 were extracted from Fig. 4. It should be noted that k and T are c 1/2 incorporation of spiropyran to the acrylic copolymers, which is the most significant parameters for monitoring photo-responsivity due to the hydrophobic nature of the SP form under visible light of the photochromic latex samples to UV and visible light irradia- irradiation. Upon UV light irradiation and isomerization of the SP tion. The resulted kinetic equations and parameters are presented to the MC forms, a remarkable decrease of contact angle in the in Table S2 (Supporting Information). Different results were range of 5-12° and also increase of hydrophilicity due to the pres- observed for the samples as a function of chain flexibility and also ence of the zwitterionic MC molecules on the coatings surface were polarity of the latex nanoparticles. Comparison of the k and T c1 1/2 observed. All of the photoswitchable coatings displayed reversible parameters for the SP ? MC isomerization displays increase of surface wettability variation in response to UV and visible light the responsivity rate by increasing the T value for all of the sam- g irradiation in several repeating times. In the case of photoswitch- ples, where the SBNPs-60–3% and SBNPs-60–5% samples show the able coatings containing 5 wt% of spiropyran, a negative pho- maximum k and also minimum T values that indicate fast c1 1/2 tochromism was observed because of higher concentration of responsivity of these samples to UV light irradiation. High stability spiropyran, which has resulted in stability of the MC form and cor- of the MC structure in these samples resulted in faster isomeriza- respondingly decrease of surface hydrophobicity of the coatings. tion of the SP molecules to MC. The MC ? SP isomerization has These results confirmed the photoswitchable surface wettability not significantly influenced by the chain flexibility and polarity. of the photochromic coatings prepared by solution casting method. The kinetics study results showed that photochromism and isomer- To increase the contact angle of water droplet on the coatings sur- ization of spiropyran in the latex nanoparticles could be controlled face, preparation of nanofibrous photochromic coatings by electro- by the characteristics of polymer matrix especially chain flexibility spinning strategy is proposed, which is investigated in the and also polarity. following. A significant characteristic of the photo-responsive coatings is reversibility of their behaviors under several UV/Vis irradiation cycles, which could be investigated by photofatigue resistance dia- 5. Photochromic nanofibrous coatings prepared by grams obtained from UV–Vis spectroscopy. For this purpose, all of electrospinning the latex samples were diluted to 0.5 wt% and exposed to UV and visible light irradiations for 5 min, and then the absorbance inten- Preparation of nanofibrous polymer coatings with the aid of sity was measured at the corresponding k of each samples at 20 max electrospinning method is an efficient method to increase cycles of UV/Vis light irradiation. As shown in Figure S4 (Support- hydrophobicity and water contact angle by induction of roughness ing Information), all of the samples displayed a high reversibility to the surface. All of the copolymer samples were dissolved in DMF for the spiropyran photochromic behavior resulted from the SP to with a concentration of 10 wt%, and then nanofibrous coatings MC isomerization, which is a confirmation for maximum photofa- were prepared from the copolymer solutions by using the electro- tigue resistance of the photoswitchable latex samples containing spinning method. Both of the sample series containing 3 and 5 wt% spiropyran moieties. These photochromic latex samples could be of spiropyran were used for this purpose. The samples containing used for the preparation of photoswitchable coatings with the abil- 5 wt% of spiropyran did not display negative photochromism ity of reversible surface wettability variation upon UV and visible (compared to the polymer coatings prepared by solution casting), light irradiations for several times. and also their photochromic properties were higher than the samples containing 3 wt% of spiropyran. Therefore, some of the sam- 4. Photoswitchable surface wettability of the photochromic ples including SBNPs-60–5%, SBNPs-80–5%, and SNPs-5% were polymer coatings selected for morphological investigation by FE-SEM and fluorescence microscopy and also photoswitchability of the surface wet- Efficient photoswitchability of the latex samples upon UV and tability by water contact angle method. As shown in Fig. 6, the visible light irradiation was confirmed by investigation of their FE-SEM images from the surface of the prepared coatings show optical properties. The photochromic polymer coatings prepared uniform nanofibers with narrow size distribution, where diameter from these latex samples could reversibility be switched between of the nanofibers is different depending on the copolymer compo- the two hydrophobic and hydrophilic states in response to UV sition or Tg of the samples. For example, both of the SBNPs-60–5% and visible light irradiation. Therefore, all of the photochromic and SBNPs-80–5% samples displayed a diameter of about 500 nm, latex samples were coagulated and washed with ethanol and water as respectively shown in Fig. 6A-A‘‘ and B-B”. The SNPs-5% sample for several times to remove the salty additives, and the resulted showed a much lower fiber diameter of below 100 nm with a nar- polymer powders were dissolved in toluene as a good solvent for row size distribution and uniform morphology (Fig. 6C-C‘‘). The

74 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

Fig. 5. Investigation of photoswitchable surface wettability (contact angle of water droplet with a volume of 0.1 mL) of the photochromic coatings under visible and UV (365 nm) light irradiation for (A) SBNPs-15–3%, (B) SBNPs-30–3%, (C) SBNPs-45–3%, (D) SBNPs-60–3%, (E) SBNPs-80–3%, (F) SNPs-3%, (A´ ) SBNPs-15–3%, (B´ ) SBNPs-30–3%, (C´ ) SBNPs-45–3%, (D´ ) SBNPs-60–3%, (E´ ) SBNPs-80–3%, and (F´) SNPs-3%

Fig. 6. FE-SEM images of the nanofibrous coatings prepared by electrospinning of polymer solutions including (A-A’’) SBNPs-60–5%, (B-B’’) SBNPs-80–5%, and (CAC’’) SNPs-5% results displayed that electrospinning of the copolymer solution to fluorescence emission of the spiropyran moieties under UV irradi- photochromic nanofibers was successfully carried out, and these ation (365 nm). Figure S6 (Supporting Information) displays the nanofibrous coatings could be further used for photoswitchable fluorescence images of the nanofibrous coatings containing 5 wt% surface wettability investigations. of spiropyran, which have bright and highly-intense red fluores- The photoswitchable nanofibrous coatings were investigated by cence emission because of formation of the conjugated MC form fluorescence microscopy because of the highly-intense red by a ring-opening isomerization from the SP structure in response

75 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78 to UV light irradiation. Photoswitchable surface wettability of the 6. Conclusion nanofibrous coatings was investigated by measurement of the water droplet contact angle under UV and visible light irradiation, According to the previously developed approaches and the results are presented in Fig. 7 for the samples containing [12,14,29,65], this study presents an applicable methodology for 5 wt% of spiropyran. Contact angles in the range of 120-136° were the preparation of photoswitchable polymer coatings by solution observed for the nanofibrous coatings under visible light irradia- casting and electrospinning methods, which both of them have tion, which are significantly higher compared to the polymer coat- potential applications in smart coating technologies. Induction of ings prepared by solution casting (60-93°). Such high contact roughness on a coating surface by electrospinning method was angles could originate from the surface roughness of the nanofi- led to remarkable improvement in photoswitching of surface wet- brous coatings. The prepared nanofibrous coatings could be cate- tability between the hydrophilic and ultrahydrophobic states upon gorized as ultrahydrophobic surfaces, because the contact angle UV and visible light irradiation. For this propose, the photoswitch- of water droplets is in the range of 120-150° [15,16,63]. The con- able latex nanoparticles with different chain flexibility and tact angle of water droplets on these nanofibrous coatings was sig- spiropyran contents were prepared by semi-continuous emulsion nificantly decreased to 78-105° upon UV light irradiation because polymerization, and used for preparation of coatings with photo- of isomerization of the SP to MC forms upon UV irradiation (in a switchable wettability by solution casting and electrospinning reversible and repeatable manner). As schematically illustrated in methods. Investigation of light-responsivity displays that optical Figure S7A, the prepared nanofibrous coatings are photoswitchable properties of the photochromic latex nanoparticles were signifi- between two ultrahydrophobic and hydrophilic states under visi- cantly affected by the chain flexibility and also copolymer compo- ble and UV light irradiation, respectively. Formation of the zwitte- sition. Study of surface properties indicates that the coatings rionic MC molecules has resulted in increase of hydrophilicity and prepared by solution casting have a water droplet contact angle also decrease of water droplet contact angle on nanofibrous coat- in the range of 60-93°, which can switch to 55-86° upon UV light ings. Such photoswitchable surface wettability properties could (365 nm) illumination as a result of reversible isomerization of be used in development of smart surface coatings with light- hydrophobic SP form to the zwitterion MC. These coatings were controllable hydrophobicity and hydrophilicity. categorized as hydrophobic surfaces with the ability of displaying To evaluate the ultrahydrophobic characteristics of the coatings, photoswitchable surface wettability in response to light irradiation some of the colored methylene blue and methyl orange aqueous [18,54,66,67]. On the other hand, it was surprisingly observed that solutions were used for dropping on the nanofibrous coatings sur- nanofibrous coatings prepared by electrospinning method display face, according to the Figure S7B. The colored water droplets on the higher contact angle in the range of 120-136°, which was switched ultrahydrophobic nanofibrous coatings were stable and completely to 78-105° upon UV light irradiation. These nanofibrous coatings cleaned after adsorption by cellulosic tissues (a movie related to are classified as ultrahydrophobic surfaces that could be photo- this process is presented in Supporting Information). The devel- switched between two ultrahydrophobic and hydrophilic states oped photoswitchable nanofibrous coatings could display ultrahy- in response to UV and visible light irradiation. Photoswitchable drophobic properties, where the fibers surface could be switched surface wettability of the developed photochromic polymeric films between two hydrophobic and hydrophilic states in response to and nanofibrous coatings were resulted from light-responsivity of the visible and UV light irradiation, which is a powerful tool to spiropyran that can be switched between two non-polar design smart surfaces in the future [64]. hydrophobic SP and polar hydrophilic MC forms in a reversible and repeatable manner. Dropping of some colored water drops containing methylene blue and methyl orange on the photoswitchable nanofibrous coatings displayed their ultrahydrophobicity, where all of the droplets were completely cleaned by desorbing from hydrophilic cellulosic tissue.

7. Supporting Information

A schematic illustration and a movie related to the photoswitchable surface wettability of the nanofibrous coatings and their ultrahydrophobic characteristics, synthesis procedures, 1H NMR spectra, DSC thermograms, kinetic curves and data, photofatigue-resistance curves, contact angle images, and fluorescence images of the nanofibrous coatings were reported as Sup- porting Information.

CRediT authorship contribution statement

Erfan Nezhadghaffar-Borhani: Investigation, Methodology, Writing - original draft. Amin Abdollahi: Investigation, Methodol- ogy, Writing - review & editing, Writing - original draft. Hossein Roghani-Mamaqani: Supervision, Conceptualization, Writing - review & editing, Funding acquisition. Mehdi Salami-Kalajahi: Validation, Formal analysis, Writing - review & editing.

Fig. 7. Photoswitchable surface wettability (contact angle of water droplet with a Declaration of Competing Interest volume of 0.1 mL) of the nanoﬁbrous coatings under visible and UV (365 nm) light irradiation for (A and A’) SBNPs-60–5%, (B and B’) SBNPs-80–5%, and (C and C’) SNPs-5% The authors declared that there is no conﬂict of interest.

76 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

Acknowledgment [22] J. Liu, X. Xiao, Y. Shi, C. Wan, Fabrication of a superhydrophobic surface from porous polymer using phase separation, Appl. Surf. Sci. 297 (2014) 33–39, https://doi.org/10.1016/j.apsusc.2014.01.053. Iran National Science Foundation (INSF) is greatly appreciated [23] J. Xie, Y. Yang, B. Gao, Y. Wan, Y.C. Li, D. Cheng, T. Xiao, K. Li, Y. Fu, J. Xu, Q. for its financial support (Grant Number: 99001094). Zhao, Y. Zhang, Y. Tang, Y. Yao, Z. Wang, L. Liu, Magnetic-Sensitive Nanoparticle Self-Assembled Superhydrophobic Biopolymer-Coated Slow- Release Fertilizer: Fabrication Enhanced Performance, and Mechanism, ACS Nano. 13 (2019) 3320–3333, https://doi.org/10.1021/acsnano.8b09197. Appendix A. Supplementary data [24] J.A. Kharraz, A.K. An, Patterned superhydrophobic polyvinylidene fluoride (PVDF) membranes for membrane distillation: Enhanced flux with improved Supplementary data to this article can be found online at fouling and wetting resistance, J. Memb. Sci. (2019), https://doi.org/10.1016/j. memsci.2019.117596. 117596. https://doi.org/10.1016/j.jcis.2021.03.012. [25] J. Guo, B.J. Deka, K.-J. Kim, A.K. An, Regeneration of superhydrophobic TiO2 electrospun membranes in seawater desalination by water flushing in membrane distillation, Desalination 468 (2019), https://doi.org/10.1016/j. References desal.2019.06.020. 114054. [26] Y. Li, F. Liu, J. Sun, A facile layer-by-layer deposition process for the fabrication of highly transparent superhydrophobic coatings, Chem. Commun. (2009) [1] A. Abdollahi, H. Roghani-Mamaqani, B. Razavi, M. Salami-Kalajahi, 2730, https://doi.org/10.1039/b900804g. Photoluminescent and Chromic Nanomaterials for Anticounterfeiting [27] F. Jiang, S. Chen, Z. Cao, G. Wang, A photo, temperature, and pH responsive Technologies: Recent Advances and Future Challenges, ACS Nano 14 (2020) spiropyran-functionalized polymer: Synthesis, self-assembly and controlled 14417–14492, https://doi.org/10.1021/acsnano.0c07289. release, Polymer (Guildf). 83 (2016) 85–91, https://doi.org/10.1016/j. [2] S.L. Oscurato, F. Borbone, P. Maddalena, A. Ambrosio, Light-Driven Wettability polymer.2015.12.027. Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts, [28] J.W. Kim, Y. Jung, G.W. Coates, M.N. Silberstein, Mechanoactivation of ACS Appl. Mater. Interfaces 9 (2017) 30133–30142, https://doi.org/10.1021/ spiropyran covalently linked pmma: Effect of temperature, strain rate, and acsami.7b08025. deformation mode, Macromolecules 48 (2015) 1335–1342, https://doi.org/ [3] V. Singh, C.-J. Huang, Y.-J. Sheng, H.-K. Tsao, Smart zwitterionic sulfobetaine 10.1021/ma502555d. silane surfaces with switchable wettability for aqueous/nonaqueous drops, J. [29] S. Samanta, J. Locklin, Formation of photochromic spiropyran polymer Mater. Chem. A 6 (2018) 2279–2288, https://doi.org/10.1039/C7TA09475B. brushes via surface-initiated, ring-opening metathesis polymerization: [4] W. Sun, S. Zhou, B. You, L. Wu, A facile method for the fabrication of Reversible photocontrol of wetting behavior and solvent dependent superhydrophobic films with multiresponsive and reversibly tunable morphology changes, Langmuir 24 (2008) 9558–9565, https://doi.org/ wettability, J. Mater. Chem. A 1 (2013) 3146, https://doi.org/10.1039/ 10.1021/la8017387. c2ta01293f. [30] A. Abdollahi, K. Sahandi-Zangabad, H. Roghani-Mamaqani, Light-Induced [5] M. Ghasemlou, F. Daver, E.P. Ivanova, B. Adhikari, Bio-inspired sustainable and Aggregation and Disaggregation of Stimuli-Responsive Latex Particles durable superhydrophobic materials: from nature to market, J. Mater. Chem. A Depending on Spiropyran Concentration: Kinetics of Photochromism and 7 (2019) 16643–16670, https://doi.org/10.1039/C9TA05185F. Investigation of Reversible Photopatterning, Langmuir 34 (2018) 13910– [6] Y. Wang, X. Wang, C. Lai, H. Hu, Y. Kong, B. Fei, J.H. Xin, Biomimetic Water- 13923, https://doi.org/10.1021/acs.langmuir.8b02296. Collecting Fabric with Light-Induced Superhydrophilic Bumps, ACS Appl. [31] Z. Tajmoradi, H. Roghani-Mamaqani, M. Salami-Kalajahi, Cellulose Mater. Interfaces 8 (2016) 2950–2960, https://doi.org/10.1021/ nanocrystal-grafted multi-responsive copolymers containing cleavable o- acsami.5b08941. nitrobenzyl ester units for stimuli-stabilization of oil-in-water droplets, [7] S. Das, S. Kumar, S.K. Samal, S. Mohanty, S.K. Nayak, A Review on Chem. Eng. J. (2020), https://doi.org/10.1016/j.cej.2020.128005. 128005. Superhydrophobic Polymer Nanocoatings: Recent Development and [32] Z. Tajmoradi, H. Roghani-Mamaqani, M. Salami-Kalajahi, Stimuli-transition of Applications, Ind. Eng. Chem. Res. 57 (2018) 2727–2745, https://doi.org/ hydrophobicity/hydrophilicity in o-nitrobenzyl ester-containing multi- 10.1021/acs.iecr.7b04887. responsive copolymers: Application in patterning and droplet stabilization in [8] M. Kuang, J. Wang, L. Jiang, Bio-inspired photonic crystals with heterogeneous media, Polymer (Guildf). 205 (2020), https://doi.org/10.1016/j. superwettability, Chem. Soc. Rev. 45 (2016) 6833–6854, https://doi.org/ polymer.2020.122859. 122859. 10.1039/C6CS00562D. [33] A. Abdollahi, H. Roghani-Mamaqani, A. Herizchi, H. Alidaei-Sharif, A. Enayati, S. [9] Q. Liu, J. Locklin, Transparent Grafted Zwitterionic Copolymer Coatings That Sajedi-Amin, Light-induced spherical to dumbbell-like morphology transition Exhibit Both Antifogging and Self-Cleaning Properties, ACS Omega 3 (2018) of coumarin-functionalized latex nanoparticles by a [2p+2p] cycloaddition 17743–17750, https://doi.org/10.1021/acsomega.8b02867. reaction: A fast and facile strategy to anisotropic geometry, Polym. Chem. 11 [10] F. Li, Q. Li, H. Kim, Spray deposition of electrospun TiO2 nanoparticles with (2020) 2053–2069, https://doi.org/10.1039/d0py00078g. self-cleaning and transparent properties onto glass, Appl. Surf. Sci. 276 (2013) [34] Z. Abousalman-Rezvani, P. Eskandari, H. Roghani-Mamaqani, H. Mardani, M. 390–396, https://doi.org/10.1016/j.apsusc.2013.03.103. Salami-Kalajahi, Grafting light-, temperature, and CO2-responsive copolymers [11] S. Zheng, C. Li, Q. Fu, W. Hu, T. Xiang, Q. Wang, M. Du, X. Liu, Z. Chen, from cellulose nanocrystals by atom transfer radical polymerization for Development of stable superhydrophobic coatings on aluminum surface for adsorption of nitrate ions, Polymer (Guildf). 182 (2019), https://doi.org/ corrosion-resistant, self-cleaning, and anti-icing applications, Mater. Des. 93 10.1016/j.polymer.2019.121830. 121830. (2016) 261–270, https://doi.org/10.1016/j.matdes.2015.12.155. [35] B. Xin, J. Hao, Reversibly switchable wettability, Chem. Soc. Rev. 39 (2010) [12] D. Wang, P. Jiao, J. Wang, Q. Zhang, L. Feng, Z. Yang, Fast photo-switched 769–782, https://doi.org/10.1039/B913622C. wettability and color of surfaces coated with polymer brushes containing [36] N. Stevens, C.I. Priest, R. Sedev, J. Ralston, Wettability of Photoresponsive spiropyran, J. Appl. Polym. Sci. 125 (2012) 870–875, https://doi.org/10.1002/ Titanium Dioxide Surfaces, Langmuir 19 (2003) 3272–3275, https://doi.org/ app.36254. 10.1021/la020660c. [13] A. Abdollahi, J.K. Rad, A.R. Mahdavian, Stimuli-responsive cellulose modified [37] J. Han, W. Gao, Surface Wettability of Nanostructured Zinc Oxide Films, J. by epoxy-functionalized polymer nanoparticles with photochromic and Electron. Mater. 38 (2009) 601–608, https://doi.org/10.1007/s11664-008- solvatochromic properties, Carbohydr. Polym. 150 (2016) 131–138, https:// 0615-0. doi.org/10.1016/j.carbpol.2016.05.009. [38] T. Jiang, Z. Guo, Robust superhydrophobic tungsten oxide coatings with [14] P. Kaner, X. Hu, S.W. Thomas, A. Asatekin, Self-Cleaning Membranes from photochromism and UV durability properties, Appl. Surf. Sci. 387 (2016) 412– Comb-Shaped Copolymers with Photoresponsive Side Groups, ACS Appl. 418, https://doi.org/10.1016/j.apsusc.2016.06.125. Mater. Interfaces 9 (2017) 13619–13631, https://doi.org/10.1021/ [39] X. Yang, L. Zhu, Y. Chen, B. Bao, J. Xu, W. Zhou, Preparation and acsami.7b01585. characterization of hydrophilic silicon dioxide film on acrylate polyurethane [15] J. Kijlstra, K. Reihs, A. Klamt, Roughness and topology of ultra-hydrophobic coatings with self-cleaning ability, Appl. Surf. Sci. 349 (2015) 916–923, https:// surfaces, Colloids Surfaces A Physicochem. Eng. Asp. 206 (2002) 521–529, doi.org/10.1016/j.apsusc.2015.05.007. https://doi.org/10.1016/S0927-7757(02)00089-4. [40] D. Dhar Purkayastha, M.G. Krishna, V. Madhurima, Molybdenum doped tin [16] N.F. Himma, N. Prasetya, S. Anisah, I.G. Wenten, Superhydrophobic membrane: oxide thin films for self-cleaning applications, Mater. Lett. 124 (2014) 21–23, progress in preparation and its separation properties, Rev. Chem. Eng. 35 https://doi.org/10.1016/j.matlet.2014.03.032. (2019) 211–238, https://doi.org/10.1515/revce-2017-0030. [41] B. Razavi, A. Abdollahi, H. Roghani-Mamaqani, M. Salami-Kalajahi, Light-, [17] X.-M. Li, D. Reinhoudt, M. Crego-Calama, What do we need for a temperature-, and pH-responsive micellar assemblies of spiropyran-initiated superhydrophobic surface? A review on the recent progress in the amphiphilic block copolymers: Kinetics of photochromism, responsiveness, preparation of superhydrophobic surfaces, Chem. Soc. Rev. 36 (2007) 1350, and smart drug delivery, Mater. Sci. Eng., C 109 (2020), https://doi.org/ https://doi.org/10.1039/b602486f. 10.1016/j.msec.2019.110524. 110524. [18] S. Wang, Y. Song, L. Jiang, Photoresponsive surfaces with controllable [42] A. Abdollahi, H. Roghani-Mamaqani, B. Razavi, M. Salami-Kalajahi, The light- wettability, J. Photochem. Photobiol. C Photochem. Rev. 8 (2007) 18–29, controlling of temperature-responsivity in stimuli-responsive polymers, https://doi.org/10.1016/j.jphotochemrev.2007.03.001. Polym. Chem. 10 (2019) 5686–5720, https://doi.org/10.1039/c9py00890j. [19] W. Schirmer, Physical Chemistry of Surfaces, Zeitschrift Für Phys. Chemie. [43] C. Hu, W. Xu, C.M. Conrads, J. Wu, A. Pich, Visible light and temperature dual- (1999), https://doi.org/10.1524/zpch.1999.210.part_1.134. responsive microgels by crosslinking of spiropyran modified prepolymers, J. [20] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces Sixth Edition, 1997. Colloid Interface Sci. 582 (2021) 1075–1084, https://doi.org/10.1016/j. [21] G. Cao, Y. Wang, Physical Chemistry of Solid Surfaces, Nanostructures jcis.2020.08.081. Nanomater. (2011), https://doi.org/10.1142/9789814340571_0002.

77 E. Nezhadghaffar-Borhani, A. Abdollahi, H. Roghani-Mamaqani et al. Journal of Colloid and Interface Science 593 (2021) 67–78

[44] N. Murase, T. Ando, H. Ajiro, Synthesis of spiropyran with methacrylate at the [56] G.B. Demirel, N. Dilsiz, M. Çakmak, T. Çaykara, Molecular design of benzopyran moiety and control of the water repellency and cell adhesion of its photoswitchable surfaces with controllable wettability, J. Mater. Chem. 21 polymer film, J. Mater. Chem. B 8 (2020) 1489–1495, https://doi.org/10.1039/ (2011) 3189, https://doi.org/10.1039/c0jm03528a. C9TB02733E. [57] A. Abdollahi, A. Herizchi, H. Roghani-Mamaqani, H. Alidaei-Sharif, Interaction [45] A. Abdollahi, H. Roghani-Mamaqani, M. Salami-Kalajahi, B. Razavi, K. Sahandi- of photoswitchable nanoparticles with cellulosic materials for Zangabad, Encryption and optical authentication of confidential cellulosic anticounterfeiting and authentication security documents, Carbohydr. papers by ecofriendly multi-color photoluminescent inks, Carbohydrate Polym. 230 (2020), https://doi.org/10.1016/j.carbpol.2019.115603. 115603. Polym. 245 (2020), https://doi.org/10.1016/j.carbpol.2020.116507. 116507. [58] A. Abdollahi, A. Mouraki, M.H. Sharifian, A.R. Mahdavian, Photochromic [46] Y. Zhang, S. Wang, Preparation of Smart Poly(SPAA-co-MMA) Film Materials properties of stimuli-responsive cellulosic papers modified by spiropyran- for Regulating Wettability and Humidity by Electrospinning, Polym. Eng. Sci. acrylic copolymer in reusable pH-sensors, Carbohydr. Polym. 200 (2018) 583– 59 (2019) E279–E286, https://doi.org/10.1002/pen.24932. 594, https://doi.org/10.1016/j.carbpol.2018.08.042. [47] Q. Chen, D. Zhang, G. Zhang, X. Yang, Y. Feng, Q. Fan, D. Zhu, Multicolor Tunable [59] A. Abdollahi, H. Alidaei-Sharif, H. Roghani-Mamaqani, A. Herizchi, Emission from Organogels Containing Tetraphenylethene, Perylenediimide, Photoswitchable fluorescent polymer nanoparticles as high-security and Spiropyran Derivatives, Adv. Funct. Mater. 20 (2010) 3244–3251, https:// anticounterfeiting materials for authentication and optical patterning, J. doi.org/10.1002/adfm.201000590. Mater. Chem. C 8 (2020) 5476–5493, https://doi.org/10.1039/d0tc00937g. [48] M. Qin, Y. Huang, F. Li, Y. Song, Photochromic sensors: a versatile approach for [60] A. Abdollahi, Z. Alinejad, A.R. Mahdavian, Facile and fast photosensing of recognition and discrimination, J. Mater. Chem. C 3 (2015) 9265–9275, https:// polarity by stimuli-responsive materials based on spiropyran for reusable doi.org/10.1039/C5TC01939G. sensors: A physico-chemical study on the interactions, J. Mater. Chem. C 5 [49] B. Razavi, A. Abdollahi, H. Roghani-Mamaqani, M. Salami-Kalajahi, Light- and (2017) 6588–6600, https://doi.org/10.1039/c7tc02232h. temperature-responsive micellar carriers prepared by spiropyran-initiated [61] A. Abdollahi, H. Roghani-Mamaqani, B. Razavi, Stimuli-chromism of atom transfer polymerization: Investigation of photochromism kinetics, photoswitches in smart polymers: Recent advances and applications as responsivities, and controlled release of doxorubicin, Polymer (Guildf). 187 chemosensors, Prog. Polym. Sci. 98 (2019), https://doi.org/10.1016/j. (2020), https://doi.org/10.1016/j.polymer.2019.122046. 122046. progpolymsci.2019.101149. 101149. [50] Y. Yang, T. Zhang, J. Yan, L. Fu, H. Xiang, Y. Cui, J. Su, X. Liu, Preparation and [62] A. Abdollahi, H. Roghani-Mamaqani, M. Salami-Kalajahi, B. Razavi, Encryption Photochromic Behavior of Spiropyran-Containing Fluorinated Polyacrylate and authentication of security patterns by ecofriendly multi-color Hydrophobic Coatings, Langmuir 34 (2018) 15812–15819, https://doi.org/ photoluminescent inks containing oxazolidine-functionalized nanoparticles, 10.1021/acs.langmuir.8b03229. J. Colloid Interface Sci. 580 (2020) 192–210, https://doi.org/10.1016/j. [51] F.M. Raymo, S. Giordani, A.J.P. White, D.J. Williams, Digital processing with a jcis.2020.06.121. three-state molecular switch, J. Org. Chem. 68 (2003) 4158–4169, https://doi. [63] J.P. Youngblood, T.J. McCarthy, Ultrahydrophobic Polymer Surfaces Prepared org/10.1021/jo0340455. by Simultaneous Ablation of Polypropylene and Sputtering of Poly [52] A. Abdollahi, A.R. Mahdavian, H. Salehi-Mobarakeh, Preparation of Stimuli- (tetrafluoroethylene) Using Radio Frequency Plasma, Macromolecules 32 Responsive Functionalized Latex Nanoparticles: The Effect of Spiropyran (1999) 6800–6806, https://doi.org/10.1021/ma9903456. Concentration on Size and Photochromic Properties, Langmuir 31 (2015) [64] R. Blossey, Self-cleaning surfaces—virtual realities, Nat. Mater. 2 (2003) 301– 10672–10682, https://doi.org/10.1021/acs.langmuir.5b02612. 306, https://doi.org/10.1038/nmat856. [53] A. Abdollahi, K. Sahandi-Zangabad, H. Roghani-Mamaqani, Rewritable [65] G. Joseph, J. Pichardo, G. Chen, Reversible photo-/thermoresponsive structured Anticounterfeiting Polymer Inks Based on Functionalized Stimuli-Responsive polymer surfaces modified with a spirobenzopyran-containing copolymer for Latex Particles Containing Spiropyran Photoswitches: Reversible tunable wettability, Analyst. 135 (2010) 2303–2308, https://doi.org/10.1039/ Photopatterning and Security Marking, ACS Appl. Mater. Interfaces 10 (2018) c0an00263a. 39279–39292, https://doi.org/10.1021/acsami.8b14865. [66] R. Fürstner, W. Barthlott, C. Neinhuis, P. Walzel, Wetting and Self-Cleaning [54] N. Wagner, P. Theato, Light-induced wettability changes on polymer surfaces, Properties of Artificial Superhydrophobic Surfaces, Langmuir 21 (2005) 956– Polymer (Guildf). 55 (2014) 3436–3453, https://doi.org/10.1016/j. 961, https://doi.org/10.1021/la0401011. polymer.2014.05.033. [67] N. Verplanck, Y. Coffinier, V. Thomy, R. Boukherroub, Wettability Switching [55] P.F. Driscoll, N. Purohit, N. Wanichacheva, C.R. Lambert, W.G. McGimpsey, Techniques on Superhydrophobic Surfaces, Nanoscale Res. Lett. 2 (2007) 577– Reversible Photoswitchable Wettability in Noncovalently Assembled 596, https://doi.org/10.1007/s11671-007-9102-4. Multilayered Films, Langmuir 23 (2007) 13181–13187, https://doi.org/ 10.1021/la7018204.