coatings Preparation and Characterization of Some Sol-Gel Modiﬁed Silica Coatings Deposited on Polyvinyl Chloride (PVC) Substrates

: Transparent and antireﬂective coatings were prepared by deposition of modiﬁed silica materials onto polyvinyl chloride (PVC) substrates. These materials were obtained by the sol-gel route in acidic medium, at room temperature (25 ◦ C), using different alkoxysilanes with various functional groups (methyl, vinyl, octyl or hexadecyl). Physicochemical and microstructural properties of resulted silica materials and of thin coatings were investigated through Fourier Transforms Infrared Spectroscopy (FTIR), UV-Vis spectroscopy, Thermal Gravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), Atomic Force Microscopy (AFM) and ellipsometric measurements. Wetting behaviors of the silica coatings were evaluated by measurement of static contact angle against water. FTIR spectra of materials conﬁrmed the high degree of cross-linking that result from the formation of the inorganic backbone through the hydrolysis and polycondensation reactions together with the formation of the organic network. These sol-gel silica coatings showed a reduction in the reﬂectance (10%) compared with uncoated PVC substrate. AFM reveals that the ﬁlms are uniform, and adherent to the substrate, but their morphology is strongly inﬂuenced by the chemical composition of the coating matrices. These silica coatings can be useful for potential electronic and optical devices.


Introduction
Many useful synthetic and/or processing strategies have been developed to prepare transparent hydrophobic and super-hydrophobic coatings. Coatings are mainly applied on substrates (e.g., glass, plastic or textiles) for decorative, protective or functional purposes. It was observed that, in most cases, it is a combination of these objectives [1]. Many techniques have been carried out to prepare hybrid coatings including plasma etching [2], layer-by-layer film formation [3], electrospinning [4], chemical vapor deposition (CVD) [5] and the sol-gel process [6]. Compared with the mentioned methods, the sol-gel process is the most used method to produce the hybrid silica coatings because: (a) it allows the control of the product's chemical composition; (b) it is attractive for coating polymers that have melting points between 150 and 300 • C; (c) uniform coatings can easily be fabricated by dipping or spin coating; (d) is low cost and a cost-effectiveness technique and (e) ceramic, glass, metal and polymer substrates can be easily coated [7]. Sol-gel route is considered an adequate method to produce smart and green coatings that can be suited to applications in optoelectronics, self-cleaning solar cells, photovoltaics and sensors [8,9]. Chen et al. [10] fabricated the multifunctional coating with superhydrophilic, high transparency, antireflective and self-cleaning properties via a chemical vapor deposition method. They demonstrated that the antifogging and transparency properties were due to the low water contact angles and the decrease of refractive index, respectively. Lin et al. [11] realized sol-gel silica antireflective coatings with amphiphobic property and good transmittance. Gao et al. [12] fabricated antireflective superhydrophobic coatings using three silica-based sols: silica sol prepared in acidic medium, silica nanoparticle suspension prepared according to the Stöber method and mesoporous silica nanoparticle suspension, followed by chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane. Authors obtained coatings presenting a transmittance of 95.3% at the wavelength of 630 nm. Ellis-Terrell et al. [13] reported the preparation of silica nanoparticle spray coatings on stainless steel and aluminum coupons. The results revealed that functional silica nanoparticles coating exhibited both superhydrophobic and oleophobic wetting properties (water contact angle ≥ 150 • and oil contact angle ≥ 140 • ), at lower temperatures (100 • C). Tasleem et al. [14] synthesized transparent hybrid silica hydrophobic coatings using the chemical surfactant trimethoxyoctylsilane and green surfactants (Nelumbo nucifera). They showed that the hybrid silica coatings displayed good transparency, low surface energy and superhydrophobic property Eshaghi immobilized the nano-hybrid coatings on polycarbonate (PC) substrates through the sol-gel method, using silica nanoparticles, 3-glycidoxypropyltrimethoxysilane (GPTMS) and perfluorooctyltrichlorosilane (PFTS) [15]. Maghrebi reports the preparation of polyimide-silica hybrid films with nanostructure using the sol-gel technique by hydrolysis-polycondensation of tetraethoxysilane (TEOS) in the polyamic acid solution [16]. Wang et al. [17] obtained superhydrophobic transparent film on a glass substrate by hydrolyzing the tetraethoxysilane (TEOS) in an acidic environment and then reacted with hexamethyldisilazane (HMDS). Islam et al. [18] reported that nanoparticle optical coating of silica and titania on polymer substrate PMMA can be prepared via a sol-gel method, at room temperature. Ortelli et al. [19] reported the preparation of organic-inorganic hybrid compositions (ceramers) through the sol-gel process, using different alkoxysilane precursors (trimethoxymethylsilane (TMEOS), (3-aminopropyl) trimethoxyalkoxysilane (APTMS) and tetraethoxysilane (TEOS)). The obtained results showed that the ceramer coatings exhibit thermal stability and water repellency properties. Fasce et al. [20] prepared the poly (ethylene oxide)/silica hybrid coatings (PEO-Si/SiO 2 ), deposited onto a PVC substrate, obtaining coatings with uniform thickness. Al-Bataineh et al. [21] demonstrated that the polyvinyl chloride (PVC)/polystyrene (PS) hybrid films doped with silica nanoparticles with high transmittance (>80%) can be achieved by using the dip-coating method. Abdel-Baset et al. [22] showed that the silica nanoparticles (SiO 2 NPs), synthesized by the sol-gel method, could be well dispersed on the PVC films surface in order to obtain transparent PVC-SiO 2 nanocomposite films [22]. Sutar et al. [23] reported the preparation of superhydrophobic coating by applying the multiple layers of PVC/SiO 2 NPs on glass substrate.
In the present study, the transparent and antireflective coatings were prepared by depositing the silica materials on the plastic substrates (polyvinyl chloride (PVC)). Silica materials were synthetized via a sol-gel process, in acidic medium, at room temperature (25 • C), using different alkoxysilanes as silica sources: tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), vinyltrimethoxysilane (VTMES), octyltriethoxysilane (OTES) and hexadecyltrimethoxysilane (HDTMES). This method was chosen because it is feasible for low cost, cost-effectiveness and large-scale production. Physicochemical and microstructural properties of resulted silica materials and of coatings were investigated through FTIR and UV-Vis spectroscopy, TGA and DMA analysis, AFM, ellipsometric and water contact angle measurements. To our best knowledge, there are only a few studies that demonstrated the hydrophobic and antireflective properties of the silica coatings on the PVC substrate. The chemicals were used as received. The plastic standard (polyvinyl chloride (PVC)) substrate was purchased from FabTech (Bucharest, Romania) and was chosen because is a low cost, flexible and chemically nonreactive material.

Preparation of Sol-Gel Silica Materials and of Coatings
Different silica materials were synthetized via a two-step sol-gel process, in acidic medium, at room temperature (25 • C), in a similar way to that previously reported [24]. These silica materials were deposited on plastic substrates (PVC) in order to obtain coatings with hydrophobic and antireflective properties.
The first step of the process involved the hydrolysis of TEOS (2.1 mL) with alkoxysilanes (MTES, VTMES, OTES and/or HDTMS) in an acidic aqueous solution containing isopropyl alcohol (7.5 mL) and HCl (0.1 N; 0.23 mL). These solutions were magnetically stirred for 1 h, 400 rpm, at room temperature (25 • C). As a second step, complexing agent (MA) (0.05 g) was added to the solution and after complete dissolution the crosslinking agent (TIP, 0.2 mL) was added dropwise to the solution. In order to continue the hydrolysis and condensation reactions, the second portion of HCl (0.1 N; 1 mL) was added and the mixtures were stirred for another hour, at 25 • C. The molar ratio between the TEOS and alkoxysilanes was 1:1. The molar ratios between the alkoxysilanes were of 1:1 for MTES/OTES and MTES/VTMES and of 1:0.4 for MTES/HDTMES. Three silica mixtures were obtained: TEOS/MTES/OTES (S1), TEOS/MTES/VTMES (S2) and TEOS/MTES/HDTMES (S3). Silica mixture S2 was photo-polymerized under UV lamp irradiation (λ = 365 nm, 10 min) to initiate VTMES-radical polymerization. The obtained samples were characterized both as powders (placed into plastic vials, left to dried at room temperature and analyzed after solvent evaporation) and as films (deposited onto PVC substrate by draw down sample coating with the manual applicator) (see Scheme 1). All final materials (powders and coatings) were dried and kept (overnight) at room temperature (25 • C). The hydrophobic, optical and mechanical properties of the silica coatings were studied to explore their potential multifunctional applications. Scheme 1. Sol-gel synthesis of organically modified silica materials in order to obtain thin films on plastic substrate (PVC).

Fourier Transforms Infrared Spectroscopy (FTIR)
Fourier Transform Infrared (FTIR) spectra of thin films (obtained by deposition of sol-gel silica materials on PVC substrate) were carried out on a Jasco FT-IR 6300 instrument (JASCO Int. Co., Ltd., Tokyo, Japan), with an Attenuated Total Reflectance (ATR) accessory of diamond crystal. Data were collected at room temperature, in a wavenumber interval of 400-4000 cm −1 (30 scans at a resolution of 4 cm −1 ).

Thermogravimetric Analysis (TGA)
A TA TGA Q500 IR instrument (TA Instruments, New Castle, DE, USA) was employed for the thermogravimetric analysis (TGA) of sol-gel silica materials (obtained as powders). Samples of 6-9 mg were placed in aluminum pans and heated from 40 to 700 • C, in nitrogen atmosphere (heating rate of 10 • C/min).

Dynamic Mechanical Analysis (DMA)
Rectangular films with the length × width × thickness of 12.75 × 7.0 × 0.14 mm 3 were analyzed using a DMA Q800 (TA Instruments) operating in the strain module, ramp 3 • C/min to 120 • C, in air, with an oscillation frequency of 1 Hz, and 5 µm amplitude. Three tensile test specimens were used for each silica films. The presented values represent their arithmetic mean, at a temperature of 30 • C.

Atomic Force Microscopy (AFM)
In order to exhibit the morphology and to evaluate the roughness of the deposited samples (thin films), atomic force microscopy (AFM) analysis was made with an XE-100 microscope (Park Systems, Suwon, Korea). The non-contact working mode was chosen as to preserve the tip shape and to minimize the tip-sample interaction. We used for all AFM measurements sharp tips, NSC36 type from Mikromasch, with a radius of curvature of 8 nm, spring constant 2 N/m and vibrating frequency of 130 kHz. The raw AFM micrographs were processed with the XEI program from Park Systems in order to present the images in the tridimensional view mode and to calculate the roughness.

Spectroscopic Ellipsometry Measurements
Spectroscopic ellipsometry measurements were carried out in air at room temperature, using a variable angle spectroscopic ellipsometric (VASE) (J.A. Woollam Co., Lincoln, NE, USA) in the 300-1000 nm spectral range. The data analysis was done using commercially available WVASE32™ software (version 3.920) package in order to evaluate the thickness of the coatings.

Contact Angle (CA) Measurements
CA measurements of the silica coatings were collected using a CAM 200 contact angle tensiometer (KSV Instruments, Helsinki, Finland) equipped with a high resolution camera (Basler A602f, Ahrensburg, Germany) and an autodispenser. All CA measurements were carried out in a static regime, at room temperature. The water drop volume selected to measure the contact angles was of 6 µL. The contact angles values were calculated from deionized water drop images (average of ten liquid droplets placed in different regions of the silica coatings).

FTIR Analysis
FTIR spectra of thin films covered with sol-gel silica materials were recorded in order to identify the functional groups existent in the silica thin films (Figure 1). In the FTIR spectrum of PVC substrate, two peaks were identified at 700 and 963 cm −1 characteristics for CH 2 rocking vibrations and trans-CH wagging vibrations, respectively. The peaks that appeared at 2911 and 2850 cm −1 were assigned to C-H stretching vibrations. The band at 1732 cm −1 could be attributed to the carbonyl stretching (CO-stretching). The band at 1330 cm −1 was designated to CH 2 deformation vibration [25]. A band also could be observed at 610 cm −1 and was characteristic of C−Cl stretching vibration [26].
Analyzing Figure 1, in the case of samples S1, S2 and S3, respectively, it can be observed that the Si-O-Si group gave a strong absorption band at 1020 cm −1 (stretching vibrations), corresponding to linear and branched structures. Contributions of the C-H bonds and Si-C bonds were detected in the silica thin films and identified in the range of 2980-2853 cm −1 and 1280-1270 cm −1 , respectively. The presence of the Si-O group was detected at 910 cm −1 (bending vibrations) [27]. The presence of these peaks confirms the formation of a network structure inside the silica materials. According to the literature, the C-H bonds are observed at 770 cm −1 (out-of-plane vibration) and at 1275 cm −1 (symmetrical deformation vibration) [28]. The characteristic absorption band at 3280 cm −1 (stretching mode) was due to the hydroxyl polar (O−H) group.
For sample S3, the peaks at 2925 and 2852 cm −1 , attributed to −CH 3 and -CH 2 groups, were much more intense compared with the same peaks observed in samples S1 and S2, because of hexadecyl groups grafted on the silica coating.
For the vinyl silicates (sample S2), the CH 2 in-plane bending deformation for SiCH-CH 2 was detected at 1410 cm −1 . The peak observed at 1603 cm −1 was assigned to C=C bond. In case of the samples S1 and S3, the CH 2 group was identified at 1460 cm −1 (bending deformation) [29].

TGA Analysis
TGA analysis was performed on these silica materials (obtained as powders) in order to establish the main thermal events. The thermal behavior is shown in Figure 2. The TGA curve obtained for silica samples shows three weight loss steps at 30-90 • C, 90-290 • C and 290-700 • C temperature regions. Each sample shows a mass loss below 200 • C that could be attributed to a loss of water and/or isopropanol. This is followed by a mass loss that occurred between 290 and 700 • C. The significant mass loss above 200 • C varied from sample to sample and could be assigned to the decomposition of organic groups and loosely bound Si-(OR) groups (see Table 1). It can be seen that only 17.71 wt.% weight loss was found for sample S3 below 700 • C, while 26.82 and 43.6 wt.% weight loss were observed for samples S1 and S2, respectively. The third mass loss detected in the range of 290-700 • C was due to the organosilicate framework degradation [30].
Lu et al. [31] demonstrated that the coatings based on silica nanoparticles has good thermal behavior when the treatment temperature is lower than 360 • C. This fact could be due to the heat-resistance performance of silica nanoparticles and to the cross-linked Si-O-Si networks in the silicone sealant.

UV-Vis Spectroscopy
Diffuse reflectance of the prepared coatings was measured by a UV-visible spectrometer and evaluated at 550 nm ( Figure 3). Coatings based on silica materials showed a reduction in the reflectance (10%) compared with an uncoated plastic substrate (PVC) (10.7%). The slight difference of coating S2 compared with coatings S1 and S3 can be due to the generated nanopores on the functional material, which reduces the antireflective effect. This result is in good agreement with a previous paper that reveals that the silica is an adequate material to achieve the antireflective properties [10]. Furthermore, nano scale geometries give a guarantee for the antireflective properties. Additionally, it was demonstrated that the homogeneous stable coatings with low reflectance (<2%) and controllable refractive index properties of this method make it feasible to obtain antireflective coating on plastic with optimal performance at low temperature [18]. Zhang et al. [32] developed a novel nanocomposites coating with a closed-pore structure by a sol-gel dip-coating method, silica sol synthetized in acidic medium. The results indicated that the nanocomposite coating showed a reduction in the reflectance (6.29%). Wang et al. [33] reported the surface functionalization of silica nanoparticles on polycarbonate (PC) and polymethylmethacrylate (PMMA) glass substrates. The obtained results showed that the coated PC/PMMA substrates exhibit high optical transmission (98%), superior abrasion resistance after 100 cycles and contamination resistance performed at room temperature.

Dynamic Mechanical Analysis (DMA)
DMA analysis of rectangular films covered with sol-gel silica materials is shown in Figure 4. Interactions between components of the hybrid coating and adhesion forces between surfaces of the coating and respectively the PVC sheet are the main factors that determine the viscous and the elastic response of the composites. In our study, differences between storage modulus of the coated PVC sheets, as a measure of recoverable elastic component are not significantly high, which indicate weak interactions between films and the substrate. Variation of the loss modulus influenced the loss factor only slightly because of the decrease in the storage modulus (SM).
The presence of organic groups in the inorganic silica network usually lead to a large volume of the structure, which is to be occupied and the mobility of the composites will be fine-tuned, probably by changes in the crosslinking possibilities.
The higher values of dissipation component (loss modulus) mean a more flexible system, due to more energy dissipated S2 > S1 > S3. As expected, stiffness increased in the order S2 < S1 < S3, as can be observed from Table 2.  The loss factor (Tanδ) is the tangent of the phase angle (δ) between storage modulus (E ) and loss modulus (E") and provides a measure of damping in the composite material. Tanδ was around 1 in the case of S1 and S2, while in the case of S3 was significantly lower. This is an indication that the higher alkyl chain of the organic group in the hybrid coating and the higher interactions at the surface with PVC chains by van der Waals forces made the composites lose energy through these interactions.

Atomic Force Microscopy (AFM)
The surface morphology of the S1-S3 films was investigated by AFM analysis (Figure 5), which presents the tridimensional (3D) images, registered at the scale of 1 × 1 µm 2 . The AFM images from Figure 5 can be understand as tridimensional topographic maps of the samples. Besides the root mean square (RMS) roughness, commonly used in AFM description, representing the standard deviation of the height value in the selected region, a much more intuitive parameter is the peak-to-valley, Rpv, which represents the heights difference between the lowest and the highest points of the scanned. For comparison reason, first of all it was scanned the bare plastic substrate (Figure 5a) over an area of 0.25 × 1 µm 2 . Figure 5a shows a flat surface, slightly wavy, with a RMS roughness value of 1.67 nm, and a peak-to-valley, Rpv, parameter analyzed of 12.5 nm, mostly due to its natural curvature or random protuberances. Figure 5b shows the 3D image of the S1 coating (TEOS/MTES/OTES) with hills and valleys aspect (corrugated aspect). However, the height difference between the hills and valleys in Figure 5b is of the order of only few nanometers, as the Rpv parameter do not exceeds 2.42 nm. Therefore, it can be presumed that the substrate irregularities are smoothed by the deposition of S1 film (TEOS/MTES/OTES), in agreement with the roughness decrease from 1.67 nm (PVC substrate) to 0.41 nm (S1 coating). In other words, the deposited solution "fills" the irregularities of the plastic substrate. The introduction of the VTMES in the (TEOS-MTES) coatings composition (Sample S2- Figure 5c) increase the uniformity of the film, as the height differences between neighbor protuberances and valleys is decreased, and the diameters of the depressed areas (in comparison with S2 film). The S2 surface is characterized by a RMS roughness of 0.26 nm and an Rpv parameter of 3.6 nm. Nevertheless, nanometric sized grains can be easily observed on the surface of the S2 film.  Figure 5d presents the 3D AFM image of the coating obtained after HDTMES addition to (TEOS-MTES) matrix. As can be observed from Figure 5d the morphology of the S3 film was completely changed in comparison with S1 and S2 films, by the appearance of separated surface parcels (bushes-like) that did not exceed 10 nm in height but having hundreds of nm in x-y directions (in plane). This aspect can be most probably related to the surface segregation induced by the presence of hydrophobic functional group from HDTMS. The S3 film exhibits a RMS roughness of 2.51 nm and an Rpv parameter of 12.4 nm. As a general remark, smooth, adherent and continuous silica-based films were obtained by sol-gel on PVC substrate.
Alam et al. [34] demonstrated that the single and double layer coatings with antireflective property (transmission of 98%) and a roughness of 17.5 nm and 28.5 nm, respectively, can be achieved by deposition of the silica nanoparticles on the glass substrate.
Spectroscopic ellipsometry (SE) was used to estimate the thickness of the deposited S1-S3 films on the PVC substrate. The lowest mean squared error (MSE) values that show the goodness of the fit were obtained using a three layer model (roughness/film/substrate) and are presented in Table 3. The roughness values obtained from AFM analysis were used as input parameters. The film layer was fitted using a general oscillator model with a couple of Tauc-Lorentz oscillators. The film thickness varied from 1288 up to 3500 nm. Small thickness non-uniformity was observed in sample S2, reflected also in the slightly higher value of the MSE parameter.
It was observed that, with an increasing thickness of the silica coatings, the roughness of coatings increased, resulting in a decrease of material hardness.

Contact Angle Measurements
The surface wettability was characterized by the measurement of static contact angle against water ( Figure 6). The preparation of hierarchical structure has a significant influence on the surface wetting behavior [35]. Surface modification led to an increase of the water contact angle from 83 • for uncoated substrate to 106 • for organically modified silica coatings. The surface wettability was changed from hydrophilic to hydrophobic. Silica coatings S1 and S3 present the water contact angle of 104 • ± 1.4 • and 106 • ± 1.6 • , respectively, which indicated that the contact angle of the silica hybrid surfaces was affected by the polarity of the matrix. The small angle was determined for silica coating S2 (88 • ± 2.4 • ) that contain sol-gel mixture TEOS/MTES/VTMES. The hydrophobic character of the silica thin film increased with the increase of the length of the functional group, following the order: VTMES < OTES < HDTMES. This phenomenon takes place probably because the octyl and hexadecyl groups are more hydrophobic than the vinyl moiety. Coating S2 has a low water contact angle, comparing with other coating S1 and S2 and this result can be due to the existence of the similar interaction between the vinyl groups (of VTMES) and the hydroxyl groups from the inorganic network being only partially condensed. This coating has hydrophilic property because the forces of interaction between water and the substrate are nearly equal with the cohesive forces of bulk water, and water does not cleanly drain from the substrate. The contact angle decrease due to the not stable siloxane groups (Si-O-Si) or to the presence of free silanols groups (Si-OH) that make the film surface polar and reactive, allowing interaction with water molecules physically adsorbed or bounded by hydrogen bonds [36].

Conclusions
In conclusions, transparent thin films with hydrophobic and antireflective properties were prepared by coating the plastic substrate (PVC) with silica materials obtained by the acid-catalyzed sol-gel process. FTIR spectra indicated the absorption bonds at 1020 and 910 cm −1 characteristic for Si-O-Si and Si-O groups, respectively (vibrational stretching) that were formed during the hydrolysis and polycondensation of alkoxy groups in the presence of TEOS. It was shown that the mechanical properties of the hybrid coatings could be tailored by varying the sol-gel silica materials. DMA analysis indicated that the higher alkyl chain of the organic group in the hybrid coating and the higher interactions at the surface with PVC chains by van der Waals forces made the composites lose energy through these interactions. AFM investigations evidenced smooth and continuous surfaces, with particular morphology depending on the chemical composition. For ellipsometric measurements, it was observed that the film thickness varied from 1288 up to 3500 nm. The TGA results showed that the mass loss could be attributed to condensation of surface hydroxyl groups and contributions from solvent or other materials trapped within the microstructure. It was seen that the hydrophobic character of the silica thin film increased with the increase of the length of the functional group, following the order: VTMES < OTES < HDTMES. The hydrophobic character of coatings S1 (TEOS/MTES/OTES) and S3 (TEOS/MTES/HDTMES), respectively, could be related to their polarity nature, rather than topographic aspects. The diffuse reflectance of coatings based on silica materials was reached as 10% at the wavelength of 550 nm, whereas the water contact angle was about 107 • . These silica layers probed to be hydrophobic without any post-growth treatment or chemical functionalization, contributing to the durability of antireflective coatings. Such antireflective hydrophobic coatings have promising potential as electronic and optical devices.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.