# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 2 Fluorinated Polymers for Passive Optical Waveguides
### Composite Waveguiding Materials Containing SiO\({}_{2}\) and MgF\({}_{2}\) Nanoparticles
Optical cladding is prepared from a UV cross-linked polymer. This special polymer has high resistance to high optical power and high thermal stability. The material has such requirements because these glass fibers are used, for example, for transmission of laser radiation in surgical lasers and material processing technology, where the transmitted laser power could be up to 100 W. In addition, the fibers for medical lasers can be autoclaved several times with hot steam for healthcare applications [9]. The C-F bonds show high thermal and environmental stability. Therefore, fluorinated polymers (a well-known example being PTFE) typically exhibit increased thermal and media stability compared to their non-fluorinated counterparts [32]. Inorganic materials in the form of NPs could introduce additional stability to the nanocomposites. Therefore, we investigated the influence of harsh environmental conditions (high humidity and high temperature) on the optical propagation losses. The comparison of the original acrylate formulations and the nanocomposites prepared using these formulations for stability in conditions of elevated temperature and/or humidity is shown in Figure 9. The nanocomposites showed better performance at a high temperature (125 \({}^{\circ}\)C; Figure 9, right), while under high-humidity and high-temperature conditions (Figure 9, left) the behavior was not good. We can attribute this fact to the probable absorption of water vapor by silica NPs and a consequent decrease in the concentration of C-F bonds. MgF\({}_{2}\) nanoparticles were prepared from Mg(OEt)\({}_{2}\) by reaction with HF in dry methanol [3]. The NPs were modified with trifluoroacetic acid to render them compatible with optical polymers. Stabilized in this way, MgF\({}_{2}\) NPs were also introduced into monomeric mixtures. Again, the introduction of inorganic NPs led to a decrease in TOC, as shown in Figure 10, left. It is also clear that the low-refractive index MgF\({}_{2}\) NPs decreased the refractive index of the resulting polymer films (Figure 10). However, despite visual transparency, the optical propagation loss increased significantly (Figure 11) for the two different acrylate formulations. The light scattering by NPs as a reason for the increase in optical propagation  
Figure 8: Representations of optical propagation loss measurements for polymer nanocomposites with SiO\({}_{2}\) NPs from different sources: **left–**solvent dispersion for core material (\(n=1.5\)) containing 25 wt% SiO\({}_{2}\) (MEK-ST from Nissan); **right–**cladding material containing 30 wt% SiO\({}_{2}\) (Nanopol C764 from Evonik) (red lines represent exponential fits). losses is hardly plausible (the NP diameter was ca. 12 nm [33]). Therefore, the absorption of the organic shell consisting of trifluoroacetic acid at 1547 nm could have been responsible for the increase in optical propagation losses. Next, we considered nanocomposite materials based on the oxirane-containing polymers described in Section 2.2[1; 3]. Also, low- and extremely low-refractive index MgF\({}_{2}\) and SiO\({}_{2}\) NPs were used (see Scheme 8). In the case of SiO\({}_{2}\) NPs, we attempted to introduce them into the polymer matrix using three different methods, namely, direct blending of the NP dispersion with a polymer solution (similar to monomer mixtures; Scheme 8, upper route), co-polymerization of monomer-modified SiO\({}_{2}\) NPs with other co-monomers (Scheme 8, lower route) and in situ sol-gel formation of similar SiO\({}_{2}\) structures during UV curing and annealing. The formation of similar SiO\({}_{2}\) structures was catalyzed by a photoacid generator using a co-polymer with siloxane groups [34]. The in situ sol-gel formation of similar SiO\({}_{2}\) structures and polymers prepared with monomer-modified NPs is illustrated in Scheme 9. Figure 10: Temperature dependencies of refractive index of acrylate formulation and nanocomposite films (**left**) and polymer and polymer-and-nanocomposite films and (**right**) measured by m-line spectroscopy.