## 2 Results and Discussion
### Synthesis and Characterization of Polymer and Polymer Hybrid Films  
Polymer films were prepared by the photopolymerization of poly(propylene glycol) dimethacrylate (PO-UDMA) and N,N-(diisopropylcarbamoyloxy)ethyl methacrylate (N-MA) in a percent weight ratio of 60:40, by using 1.5 wt.% Irg819 as photoinitiator, at room temperature. The same method was applied for the synthesis of hybrid polymer films when the monomers were photopolymerized in the presence of ZnO or ZnO-SnO\({}_{2}\) particles (Ps) (Scheme 1), according to the formulations presented in Table 1. The irradiation was carried out with a 365 nm UV lamp at a light intensity of 40 mW/cm\({}^{2}\). Figure 4: The spectrum of the cube-like ZnO–SnO\({}_{2}\) nanoparticles and mapping images for zinc and tin. The conversion degree of the characteristic methacrylate double bond in the obtained formulations was assessed on the basis of FTIR spectroscopy; the related data is presented in Table 1. As could be seen in Figure 5, the C=C double bond stretching vibration at 1638 cm\({}^{-1}\) fell with the enhancement of the UV exposure time, the conversion degree being approx. 98.15% after 60 s of irradiation in the case of sample L1. Under the same regime of lightning, the methacrylate double bonds (C=C) of samples L2 and L3 were transformed into C-C bonds with a conversion degree of 92.94% and 93.10%, respectively [59]. These results were better compared to our previous reports in which the PO-UDMA monomer was used in combination with another synthesized monomer [38] or with a commercially available one [39]. The results indicated that the kinetics of the photopolymerization processes was scarcely affected by the incorporation of the inorganic particles. However, the low difference obtained for the conversion degree could be explained in terms of the restriction effect manifested by ZnO or ZnO-SnO\({}_{2}\) particles (Ps) on the mobility of the reactive centers, which led to the decrease of the methacrylate double-bond conversion degree [60]. In addition, the lower conversion values for both samples incorporating Ps could be due to the high ability of ZnO to capture the UV light [61, 62]. Figure 6 illustrates the UV-Vis transmittance spectra of L1, L2, and L3 thin films. Accordingly, all films exhibited good transparency in the region stretching on the visible range. While the transmittance of L1 film was almost 85% for wavelengths beyond 350 nm, the same parameter for the films with 1 wt.% ZnO and 1 wt.% ZnO-SnO\({}_{2}\) was beyond 80% and 70%, respectively. The slightly lower value of UV-Vis transparency for L3 film could be explained by the electron transitions between the valence and conduction bands. Also, the lower transparency of the film containing doped particles could be due to the UV and visible photons captured by the particles incorporated into the film [63]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Composition (wt.\%)**} & \multirow{2}{*}{**Conversion**} \\ \cline{2-2} \cline{4-6}  & & **PO-UDMA** & & **N-MA** & **ZnO** & **ZnO-SnO\({}_{2}\)** \\ \hline _L1_ & 60 & 40 & - & - & 98.15 \\ \hline _L2_ & 60 & 40 & 1 & - & 92.94 \\ \hline _L3_ & 60 & 40 & - & 1 & 93.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data for **L1**, **L2**, and **L3** formulations. Figure 5: Modification of the double bond FTIR absorption band for L1 formulation during the irradiation with UV light. To thoroughly examine the morphology of the obtained polymeric films, they were broken in liquid nitrogen and then analyzed in the fracture section. The film homogeneity and particle distribution in the hybrid composites were assessed by scanning electron microscopy, as shown in Figure 7. The SEM image taken in fracture for the L1 sample, which had no particles in the composition (Figure 7a), indicated a homogenous structure without pores, the noticed irregularities being attributed to the way the film was broken.