## 2 Results and Discussion
### Cross-linked thin films  
First the film forming properties of poly(acrylic acid-_co_-allylacrylate)s was investigated. Coatings of the polymers onto SiO\({}_{2}\) substrates were prepared by spin coating of solutions with increasing polymer concentration. Optical microscopy showed that the coating on a SiO\({}_{2}\) substrate was smooth and homogenous before and after UV irradiation (Figure 1). Essentially, coatings prepared from poly(acrylic acid-_co_-allyacrylate) (molar ratio 90/10) and long irradiation time yielded water insoluble coatings. Note that these cross-linking conditions are comparable to those of the bulk material. Secondly, the influence of the copolymer composition, ratio of acrylic acid to allyl acrylate repeating units, on thin film reticulation was investigated. It is expected that the solid/air interface may reduce the cross-linking density near the interfaces, resulting in poor cross-linking efficiency and consequently poor film stability. To this end, we focused on qualitative analysis of the stability of thin films upon water droplet deposition. The variables are the molar content of the reactive groups in the copolymer, the photoinitiator and the accelerator concentration. Thin films of different molar compositions on SiO\({}_{2}\) substrates were prepared by spin coating. The results are summarized in Table 1, in all cases homogeneous films were obtained before and after irradiation (see Figure 1). From Table 1, one can conclude that the optimum results in terms of the shortest irradiation time and the film insolubility are obtained with the urethane dimethacrylate as accelerator and a copolymer with a molar composition of acrylic acid to allyl acrylate of 90/10, respectively. The hydrophilicity of the coating on a model substrate was tested by the sessile droplet method. Only contact angle measurements on water insoluble coatings are reported. The UV cross-linked coating exhibited a contact angle of less than 20\({}^{\circ}\), suggesting a hydrophilic surface as expected for a thin hydrogel layer of polyacrylic acid. These results are in contrast to the literature data [19, 20] on adsorbed or plasma grafted poly(acrylic acid) layers, which show a contact angle of 45\({}^{\circ}\)-50\({}^{\circ}\) and suggest that the cross-linked poly(acrylic acid) coating behaves like thin hydrogel layers. Figure 1: Optical micrograph of the surface of a thin poly(acrylic acid-_co_-allyacrylate) layer. (molar ratio 90/10). The film was cast from 10 wt % THF solution and irradiated with a UV lamp (6 mW/cm\({}^{2}\)) for 4 hours through a shadow mask. The arrow indicates the border between the irradiated and non-irradiated area. A protective coating should preserve the optical transparency of the SCG. At this stage, we believed that coating with a thickness of _circa_ 100 nm or less has minimum impact on the optical properties; the reason is the high refractive index of titanium dioxide layer (2.1-2.7) compared to a thin polymer coatings (1.45) [21]. The film thickness is readily variable through the polymer concentration and the casting processes, e.g., spin coating, dip coating or spraying. Coating processes are not the scope of this work, however, since we were interested in coating large glass surfaces efforts have been taken to show the feasibility of solution spraying as the most suitable technique. Therefore, we qualitatively assessed the influence of the polymer concentration as well as the deposition process mainly spin-coating _versus_ spraying on the layer thickness. Both processes showed a monotonic increase of the thickness with the polymer concentration. As expected, the difference between the two methods is the deposited amounts, which were higher for solution spraying (Figure 2). This is particularly true since the thickness was evaluated from the sprayed volume and the deposited amount varies with the pressure employed during the spraying process. The dependence illustrated in the right graph of Figure 2 should be regarded as tendency since we systematically overestimate the thickness.