Preparation of functional coating films using breath figure (BF) method and the study of morphological, optical and wettability behavior with varying experimental conditions

Abstract In this work, photoactive polymer film coatings were prepared using the breath figure (BF) method, starting from a poly(tert-butyl acrylate-styrene block), PtBA-block-PS and a photochromic agent, 1-(2-hydroxyethyl)-3,3-dimethylindoline-6-nitrobenzopyran (SP) grafted onto the poly(tert-butyl acrylate) block. The effects upon the composite film behavior in response to the solvent concentration, type of solvent and relative humidity were studied. Films containing homogeneously dispersed micrometre-sized pores were obtained. The optical and morphological behavior were investigated using optical microscopy and scanning electron microscopy. The films obtained have various potential applications as surface coating materials with color changing properties, such as whiteboards, device displays or advertising surfaces. Graphical Abstract


Introduction
Polymers represent the most diverse and widely employed coating materials for particulate systems, and therefore they are indispensable in the fabrication of advanced nanoscale materials based on organic functionalities or inorganic nanoparticles. [1] Depending on the features of the coating material, i.e., its physical properties, chemical composition and surface architecture, different technological applications could be realized for various fields. [1,2] In this context, the integration of photochromic compounds into experimental surface coatings has been carried out for the design and manufacture of variable transmission optical materials. [3,4] The resultant changes in the molecular properties of the coatings can be applied to various photonic devices, such as erasable optical memory media and photo-optical switching components. [3][4][5][6] The change produced by the photochromic substance in the chemical structure of the species, allows it to absorb in its excited state (color-state) in changeable regions of the spectrum (excitation determined modulation), generally in the region of the visible, returning to its basal state (colorless-state) in the presence of a second radiation absorbance, usually in the visible spectrum or induced thermally. [3] The photochromic materials are classified into inorganic, organic and hybrid photochromic materials. [5,[7][8][9] Organic photochromics often stand out above the others, due to their excellent tailor ability and molecular modification functions. [5,8] In relation to this, the exploration of such light-responsive molecules in devices typically requires immobilization on a surface through an appended functionality that does not interfere with the light switching behavior. This has been achieved for photoswitchable molecules by the formation of self-assembled monolayers (SAMs), [10] bilayers, [11] and incorporation into polymer films [12] and nanoparticles. [13] There are many successful methods for constructing ordered honeycomb structures with controllable pores over a large area, thus forming self-assembled mono and bilayers. However, these top-down approaches usually involve multiple complicated and expensive steps, and damage the final pattern after removal of the templates. In 1994, Francois et al. [14] reported a simple one-step method using condensed water droplets as a dynamic template, the so called breath-figure (BF) method, [15,16] which is used to prepare highly ordered honeycomb-patterned films template-free with two-or three-dimensional hexagonal arrays of pores. Moreover, using the BF method, one can obtain nanometric and micrometric porous polymeric films, with high surface homogeneities. [17][18][19][20][21][22][23] The resultant films feature high surface areas and functional reactivities, [24,25] thus amplifying the interactive properties of the films with other species or with the medium. [22][23][24][25] This easy approach for the fabrication of the porous films consists of an ordered array of water droplets, used as a molding pattern, that may be removed by simple evaporation [14] More specifically, a polymer solution is cast on a solid substrate under high humid conditions, and then, rapid evaporation of the volatile solvent causes a quick drop in the surface temperature under the thin layer of dew, which leads to fast condensation and nucleation of water droplets on the surface of the polymer solution. [26] Therefore, the morphology and domain size of the porous patterns are generally controlled by adjusting each block length, while the copolymer molecular weight defines the absolute template dimensions. [27,28] Previously in the literature, photo-responsive honeycomb films have been shown and detailed. [29][30][31] Therein, honeycomb films were prepared using a photo-responsive amphiphilic copolymer containing photochromic spiropyran (SP). Through the photo-responsivity of spiropyran, the molecular polarities provide solubility changes against chloroform vapor after UV irradiation. [29] Subsequently, metallic honeycomb films printed by photo-patterning were obtained. [30] Furthermore, an SP-incorporated honeycomb-structured porous film prepared using the BF method, which combines the attractive features of a honeycomb-patterned porous structure with the excellent responsivity to stimulus of SP has also been reported. [31] In addition, the generation of porous patterns via the BF approach, has also been broadly developed for the synthesis of functional microstructures materials for advanced technologies, such as bio-related materials. [32][33][34] In relation to the above, honeycomb-patterned porous films with good surface wettability also have great potential applications in various areas. [35][36][37][38] For example, honeycomb-patterned porous films with different structures can be generated from polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) using a breath figure technique. CO 2 -induced reversible surface wettability is then achieved by alternating the introduction and removal of CO 2 . Therefore, original hydrophobic surfaces are directly used as scaffolds for cell culture with CO 2 atmosphere to enhance the interaction with cells, and thus resulting in better cell attachment and proliferation. [35] In this work, a unique triple patterned honeycomb film with CO 2 -driven reversible wettability between hydrophobicity and hydrophilicity is presented. The honeycomb film was prepared by directed self-assembly in BF templating of a PolyStyrene-b-Poly(VinylBenzylChloride) PS-b-PVBC block copolymer under a mixed water/ethanol atmosphere. The presence of the triple structures remarkably increased the top surface structuration, resulting in an enhanced contact angle compared with normal honeycomb films. Such a smart bio-inspired honeycomb film based on a biocompatible trigger shows high application potential in bioengineering. [36] However, hydrophilic honeycomb films are difficult to obtain using the direct self-assembly of pure (co)polymers. Such a hydrophilic honeycomb-patterned porous film could find potential applications in systems where pore wetting is required, including tissue engineering, lithography, and nanoparticle embedding. [37,38] The objective of this work focuses on the design and preparation of porous coating films varying the experimental conditions of concentrations and solvents, with a controlled relative humidity percentage (RH) %. These factors were chosen due to their influence on general physical and chemical behavior, and their resultant potential influence upon the optical, and morphological properties, pores size, shape and uniformity of the films. Such factors are decisive for the design and final applications of the films. In this context, photoactive polymer porous films were prepared using the BF method using different solvents and polymer concentrations. The surface morphological behavior was observed via optical micrograph images and scanning electron microscopy (SEM). The effects upon the wettability and optical properties of the films are also reported.

Preparation of porous polymer films
Three series of porous films were prepared according to the following experimental breath figure procedure. In the two first series, the concentration effect (C ¼ 1.0, 3.0 and 5.0 g L À1 ) was studied, with the following solvents CHCl 3 , CS 2 and THF maintained at the same temperature, time and relative humidity conditions. The third series, the concentration effect (C ¼ 1.0, 3.0 and 5.0 g L À1 ) was studied at different RH of 65%, 70%, 75% and 80% at 25 C at 1 h of evaporation time, using a solvent (CHCl 3 /CS 2 /THF). The surface morphological properties were determined by optical micrograph images, SEM and AFM microscopy.

Measurements
The structure of the polymer and photoactive polymers were determined by proton nuclear magnetic resonance ( 1 H-NMR) on a Bruker 400 MHz spectrometer. The FT-IR spectra were recorded on a Perkin-Elmer Spectrum-Two spectrometer with an UATR unit coupled in the range of 4000 to 500 cm À1 with a resolution of 1 cm À1 . The molecular weights of the polymers were determined by size exclusion chromatography (SEC) using a Shimatzu LC 20 instrument using DMF as the solvent (flow rate: 1.0 mL min À1 ), equipped with RI detectors. The samples were measured at 30 C with a concentration of 6 mg/mL, calibration was performed using polystyrene, Column type: Styrene-divinyl benzene (T-, D-series), Shipping solvents THF, DM. Exclusion limits (Da Polystyrene): 1,500 (T-, D-1000), 20,000 (T-, D-2500), 70,000 (T-, D-3000). The UV-Vis absorption spectra was recorded between 200 and 800 nm in a spectrophotometer (Perkin Elmer model Lambda 35) at 25 C. Thermal analysis was performed using a thermogravimetric analyzer (TGA/DSC1 1100 SF, Mettler Toledo, Spain). The measurements were carried out at a heating rate of 10 C min À1 in a nitrogen atmosphere. Water contact angle water (WCA) measurements were performed in a Ram e-hart model 250 (p/n 250-U1) standard goniometer/tensiometer using a sessile drop on the glass substrate with the films. Range: 0-180 , Accuracy: ± 0.1 , Resolution: ± 0.01 . An optical microscope LEICA Model DM2000 LED with an automatic camera (LEICA MFC 170 HD) with an exposure of 500.00 ms, saturation of 120 and gamma 0.00 was used for the acquired images, the aperture was 1/3 and the focus 3/3 under fluorescence. The breath figure method was applied using a chamber Darwin model PH9-DA with relative humidity (RH) control at 25 C. The morphological properties were analyzed by scanning electron microscopy (SEM) using a JEOL/S-5000 model and electron microscope.
The thermal stability is very important for the fabrication of the films, for this reason the influence of the SP on the thermal stability of the photoactive polymer was studied using thermogravimetric analysis (TGA). The TGA and its derivative (DTG) curves, shown in Figure 1 respectively), show the thermal stabilities for P(tBA-SP)-block-PS; PtBA-block-PS and PS homopolymer, also showing different-steps of degradation. The resulting materials have an extrapolated TDT of about 380 C. The TDT were high for the PS homopolymer and P(tBA-SP)-block-PS, exhibiting a degradation that proceeds via single-steps. Whereas, the PtBA-block-PS exhibited a degradation that proceeds via two-step degradation, which leads to a decrease in the onset of TDT in the first step. The first step occurs in the temperature range between 110 and 210 C, this is attributed to the bond breaking reaction of the tert-butyl groups, and the second step above 380 C may be due to chain degradation. The TGA results reveal that the photoactive block copolymers have good thermal stabilities.

Effect of the relative humidity (RH) in CHCl 3
The films exhibited different morphologies corresponding to different relative humidities (70%; 75% and 80% RH in CHCl 3 ), see Figure 2(a-i). The surface exhibited porous morphologies in response to increasing the concentration of CHCl 3 from 1.0 to 5.0 g L À1 , at 70% and 75% RH, with the exception of the case for higher relative humidity, at 80% RH, and at lower CHCl 3 concentrations (1.0 and 3.0 g L À1 ). The surface morphologies varied from large pores disordered with different size of the pores to ordered porous films (see Figure 2(c,f,i), respectively). As a result, the best experimental conditions to obtain an ordered and porous morphology were at 75% RH, at 1, 3 and 5 g L À1 in CHCl 3 . Notably, the different solvents and concentration also affected the surface morphology.

Effect of the concentration and solvent
The differences in the density of the solvents also caused an effect on the surface morphology of the resultant films. The density of the solvents is a necessary condition to avoid the coalescence of the water drops affecting the form and size of the pores. In this context, two series of porous polymer films were fabricated, at C ¼ 1.0, 3.0 and at 5.0 g L À1 in different solvents (THF, CS 2 and CHCl 3 ) at 25 C and for 1h of time, at a relative humidity of 75%. It is known that the pore shape can be affected by solvent density. [45] In this sense, the THF has a smaller density (d ¼ 0.889 g mL À1 ) than CS 2 and CHCl 3 , and exhibits a higher miscibility with the water, see Figure 3(a-c). Moreover, CS 2 has a higher density (d ¼ 1.28 g mL À1 ) than water and exhibits a lower miscibility than the THF, see Figure 3(d-f). Finally, the CHCl 3 has a higher density (d ¼ 1.48 g mL À1 ) than water and exhibits a lower miscibility than the THF with the water, see Figure 3(g-i).
The photoactive P(tBA-SP)-block-PS copolymer exhibited different surface morphologies which varied between a high to low porous formation and between ordered to non-ordered porous structured films, depending on the polymer concentration and solvents. For example, by increasing the concentration from 1.0 to 5.0 g L À1 in THF, the surface morphology exhibited high coalescence. Large pores in the films were found for low concentrations (see Figure 3(a-c)) due to reduced density, resulting in miscibility with water and a low spread of formation (fractioning) of pores in the films. Otherwise, by increasing the concentration from 1.0 to 5.0 g L À1 of CS 2 , the surface morphology exhibited porous structured films (see Figure 3(d-f)), due to CS 2 having a higher density (d ¼ 1.26 g mL À1 ) than water and exhibiting a lower miscibility than the THF. On the other hand, very good results were obtained using CHCl 3 at the three different concentrations (1.0, 3.0 and 5.0 g L À1 ), resulting in ordered porous structured films. The SEM images (see inset), exhibited porous structured films with pore diameter sizes ranging approximately between 0.48 and 0.82 mm in response to decreasing CHCl 3 concentrations. (see Figure 3(g-i), respectively). The good compatibility of the photoactive polymer and the solvent was very important to form porous structured films. Notably, the different concentrations and solvents affected the structured morphology of the films, observing different porous ordered patterns, only for CHCl 3 as solvent, a porous distribution forming a hexagonal pattern is observed. The volatility of solvents, not only influences the pore size formation of the structured film, but also determines whether a regular porous structure can be achieved. These results indicate that the quality and order of the pores in the films was improved using a solvent of lower density (CHCl 3 ), see Figure 3(g-i), favoring the speed of evaporation of solvent which inhibits coalescence with the water.

Wettability of the porous films
The wettability of the surface of the porous films determines the film's potential for a wide variety of practical applications. Moreover, the determination of the water contact angle (WCA) allows one to know the quality of a material surface prior to adhesion processes such as coating, adhesive bonding, etc.. [46] It is important to mention that the degree and pore size of the surface films affects the contact angle values. [47][48][49] The wettability of the films was explored due to their different degrees of uniformity and pore size resulting from a variety of experimental conditions. When the concentration increased from 1.0 to 5.0 g L À1 for the different solvents, a slight increase of the WCA was observed, see Table 1. This behavior is attributed to the size and degrees of uniformity of the pores in the polymer films. It is important to note that all the porous films have WCA values close to 90 , according to these WCA values, the polymeric films exhibited hydrophobic wettability. These small differences in the results suggest that the incorporation of water in the pores of the films decreases due to a decreased size of the pores which enhances the hydrophobicity of the corresponding porous films (higher WCA). Thus, the WCA values obtained varied from 91.8 to 84.8 for the polymer films at different concentrations and solvents, with WCA values remaining close to 90 . The slight increases in the contact angle value suggest a lower size of the pore on the surface of the films which influences their wettability (see Table 1). In accordance with these results it is possible to conclude that the films exhibited hydrophobic properties.
Optical microscopic images corresponding to films prepared with C ¼ 1.0, 3.0 and 5.0 g L À1 CHCl 3 showed similar surface morphologies to the SEM microscopy ( Figure 4(a-f)). The films showed porous ordered morphology depending on the concentration. To determine the effect of the polymer concentration, the pore size distribution was estimated from the SEM images using the software Fiji. [48] The Feret diameter was used as the measure of the pore size. The mean pore sizes estimated from these micrographs are presented in Figure 4(g-i). The pore size dispersion (standard deviation) decreases with the concentration increments, except for CHCl 3 , where the opposite effect is observed. Therefore, although the mean pore size changes with concentration (0.479 ± 0.202 mm, 0.491 ± 0.119 mm and 0.818 ± 0.119 mm at C ¼ 1.0, 3.0 and 5.0 g L À1 respectively), its variation is not significant. Moreover, the porous structured films displayed a higher pore definition for the different concentrations in CHCl 3 .
The presence of minor fraction porosity in THF at 3 g L À1 polymer concentration, compared to a major porosity in CHCl 3 , decreases the surface ratio of this porous surfaces, leading to a WCA of 89.5 (nearly 90 ) higher than that obtained for the films in CHCl 3 (WCA ¼ 85.3 ). This result again suggests that the large pore fraction obtained in CHCl 3 gives rise to the more hydrophobic character for the honeycomb films. The WCA values for the films in CHCl 3 which produced ordered pore arrays can be confirmed from the SEM characterization displayed in Figure 4. The WCA values found to be lower than 90 for the pores' films, suggest that the state of water droplets on the film surface is close to the Wenzel Cassie state, [50,51] in which water drop enter to pores wetting the surface. On the contrary, the pores on the porous polymer films in THF show a CA of 89.5 , implying that the state of water droplets on the film surface is close to the Cassie-Baxter state, [50,51] in which water drop rest on the air entrapped in pores.

Optical properties
The fluorescent properties of the porous films were examined using optical fluorescence microscopy. The photoactive films obtained in CHCl 3 and CS 2 solutions were irradiated with an ultraviolet light lamp at 365 nm (0.5 mW cm À2 ) for 15 min at room temperature. After irradiation, there appears an emission band at 564 nm for CS 2 and another at 560 nm for CHCl 3 (see Figure  5(a,c)) which corresponds to the SP moiety in the ring-opened isomer form. As can be noted, after irradiation, the P(S)-b-P(MMA)-SP switches reversibly between two states (red and colorless) as can be noted by the absorption features in the visible region. After irradiation, the micrographic images exhibited similar electronic properties corresponding to the films, exhibiting red luminescence for all cases, which was attributed to the conversion of SP to merocyanine (MC) of the photochromic moiety, which is chemically linked to the polymer side chain used for the manufacture of porous films, see Figure 5(b,d).

Conclusions
The photoactive polymer films exhibited different surface morphologies depending on the polymer concentrations and solvents. The porous morphologies varied from large pores with disordered distributions to ordered porous structured films with varying characteristic pore sizes. The results indicated that the optical and morphological properties of the photoactive polymer films are strongly altered when the SP compound was grafted in the polymer chain. Reversible variations in UV absorption and the color of the honeycomb porous film's response to UV light was found, demonstrating their photochromic properties that are enhanced by the isomerization of SP. It was observed that the pore shape and distributions are affected by the solvent and its chemical density. In this sense, samples prepared in CHCl 3 , exhibited the superior formation and definition of the pores in the films. Moreover, the WCAs obtained ranged from 91.6 to 84.8 for the polymer films at different concentrations and solvents. This slight variation in WCA was due to increases in polymer concentrations and resultant decreases in pore size. All films displayed strong hydrophobic properties.
The resultant optical properties indicated that the functionalized system emits a red color after irradiation (k ¼ 365 nm). As can be noted, after irradiation, the photoactive films switch reversibly between two states (red and colorless) as can be noted by the absorption features in the visible region.