# 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
Figure 9: Optical loss investigation of the samples of base material and nanocomposites as a function of climatic conditions (temperature and humidity) (black and red lines represent trends). SiO\({}_{2}\)-containing nanomaterials (sourced from Nissan, Evonik, Nyacol and IOLITEC, Heilbronn, Germany) were used for direct mixing with oxirane-containing polymers. The MEK-ST and PMA-ST dispersions from Nissan and the porous SiO\({}_{2}\) NPs from IOLITEC (all ca. 20 nm in diameter) were introduced in these polymers. Before processing, the porous NPs were modified with fluorinated silane to render them dispersible. Temperature-dependent spectroscopic ellipsometry was also used in this case to measure the TOC, as it allows measurement of thinner layers (below 1 \(\upmu\)m; Figure 12, right) in comparison to m-line spectroscopy (usually thicker than 2-3 \(\upmu\)m). Note that TOC should be independent of wavelength. This allowed us to estimate the effect of NP introduction for a broad variety of NPs. A definitive effect of lowering the TOC by the introduction of SiO\({}_{2}\) NPs was registered using the MEK-ST NP dispersion (Nissan; Figure 12). As the next approach, we incorporated NPs into the polymer side chain. For this synthesis, monomer-modified SiO\({}_{2}\) NPs were used as co-monomers in the polymerization. Methacrylate and double-bond-modified NPs, porous NPs modified by 3-(trimethoxysilyl)propyl methacrylate (both from IOLITEC; ca. 20 nm diameter), and S125 (70 nm diameter) and DP5820 NPs (Nyacol; 20 nm diameter) were used in the co-polymerization. Glycidyl methacrylate and trifluoroethyl methacrylate (alternatively,  
Figure 6:0 Comparison of optical propagation losses at 1547 nm between polymer material and polymer composites with MgF\({}_{2}\) NPs (solid lines are exponential fits). Figure 6:1 Schematic representation of co-polymers capable of in situ formation of SiO\({}_{2}\) NPs by sol-gel reaction (**upper route**) and co-polymer containing an NP in a side chain (**lower route**). pentafluorostyrene) were used as co-monomers. The presence of NPs in the polymer matrix was proved by TGA (20-30 wt% of SiO\({}_{2}\) residue was determined). TOCs were measured by m-line spectroscopy and ellipsometry, but there was no clear difference. The values for TOC were similar to the values for the basic fluorinated polymers, and a direct comparison was not possible (Figure 13). However, the suppression of the glass transition in the measured temperature range was detected. Similar polymers without NPs have a \(T_{g}\) of ca. 100-120 \({}^{\circ}\)C (Figure 13). This could be due to the influence of NPs. A co-monomer containing a siloxane residue in a side chain was used to prepare SiO\({}_{2}\) NPs inside polymers in situ (Scheme 9, upper route). This monomer was co-polymerized with trifluoroethyl methacrylate or pentafluorostyrene and glycidyl methacrylate. UV curing of the polymer in the presence of a photoacid generator followed by heating to 130 \({}^{\circ}\)C leads to the opening of the oxirane rings, as exhibited in the general reaction. Also, the photoacid catalyzes the sol-gel reaction of siloxane groups in the presence of water vapors from the air. This finally leads to the formation of silica-like structures during  
Figure 6:2 Temperature dependence of refractive index for direct blending of MEK-ST SiO\({}_{2}\) NPs into polymer matrix (red circles—unfilled polymers; black triangles—nanocomposites) for two different oxirane-containing polymers: **left**—m-line spectroscopy; **right**—ellipsometry. Figure 6:3 Temperature dependence of refractive index (TOCs of corresponding materials are shown as numbers on the plots) for a polymer incorporating SiO\({}_{2}\) NPs in the side chain. Double-bond-modified SiO\({}_{2}\) NPs from IOLITEC were used for the synthesis: **left**—m-line spectroscopy; **right**—ellipsometry. heating via the evaporation of sol-gel reaction products, namely, ethanol and water.