Journal of Colloid and Interface Science 593 (2021) 67–78 Contents lists available at ScienceDirect Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis 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 article info abstract 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 hydropho- bic spiro form to the zwitterionic merocyanine form. The nanofibrous coatings prepared by electrospin- ning 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 photo- switching between the two hydrophobic and hydrophilic states as a response to UV and visible light irra- diation. 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