## 2 Results and Discussion
### Functionalities of the thin hydrogel layer
This is in agreement with the observed spontaneous contamination of the SCG surface by the siloxane residues (\(\gamma_{\rm TiO_{2}Air}>\gamma_{\rm TiO_{2}Siloxane^{+}}\gamma_{\rm Siloxane/air}\)). Following the same reasoning but replacing the titanium dioxide interface by the protective copolymer layer we get to the same conclusion (S \(>\) 0). Given that the surface energy of the copolymer protective layer is larger than that of the siloxane residue \(\gamma_{\rm PAA/air}\sim\) 50-68 mN/m [30,31]; and the interfacial energy between the siloxane residues and the polyacrylic acid is  
Figure 3: Effect of the sealant on the contact angle of a water droplet measured on protected and unprotected SCG as a function on time. (The origin denoted the time at which the silicone seal was applied.) On the right are photographs of the SCG samples wherein the sealants geometry (black) was varied. Below a side view of a water droplet on protected SCG is shown. below \(\gamma\) pA/Siloxane\(\sim\)0.1 mN/m [32]. Thus spreading of the hydrophobic residues is expected to occur on the protective coating. This was confirmed by coating half of the available glass surface with a protective layer; the layer was prepared as described before. The sealant was deposited only on the protected area in a way that the residual siloxane has to spread over the copolymer layer before contaminating the uncovered part of the surface. This is illustrated in Figure 4, wherein the resulting values of the water contact angle are indicated as a function of time. While the protected area was maintained hydrophilic, the bare SCG surface became hydrophobic after longer time as evidenced by the contact angle values. The effect was assigned to the siloxane residues that spread over the protective layer. These results comply with our understanding of the interfacial interactions, which favors the spreading of the low surface energy hydrophobic residues on both protected and unprotected hydrophilic SCG. More important is the fact that the siloxane residues are more weakly bonded to the copolymer layer (physisorbed) [33, 34] than on the titanium dioxide surface [35, 36]. Formation of a water film provokes desorption of the siloxane from the copolymer interface and favors their spreading on water air surface. This is likely what happened whenever we measured water contact angle on contaminated protective layer. As a consequence, the role of the copolymer layer is two-fold: it enhances the spreading of the hydrophobic residues and reduces their adhesion to the support. Both effects markedly contribute to facilitate removable of the silicone residues. The viability of the approach was also supported by measuring the photodecomposition rate as a function of the radiation time for given film thickness. The rate of photooxidation of the protective layer was estimated by measuring the rate of decrease in the integrated IR absorbance associated with C-H stretching vibration under artificial sun light [37] (6 mW/cm\({}^{2}\)). The results are displayed in Figure 5. It is shown that UV irradiation of a layer of 100 nm lasts up to 40 h. As expected, the protective layer is indeed not permanent. Decomposition occurs quite fast during the first four hours of irradiation. After this period, the concentration decreases almost linearly with time. If the initial amount of polymer is set equal to 100%, the average rate of decomposition can be calculated to be about 2.5% per hour. The accelerated test, however, provides only a hint on how long the layer will last until it photo-decomposes by the underlying TiO\({}_{2}\) layer. From the model  
Figure 4: Partially protected SCG contaminated at the end of the protected area (black sealant): contact angles at different times from contamination. The discontinuous line indicates the end of the protective coating.