# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 3 Fluorinated Polymers as Cladding for Optical Glass Fibers  
The experience in the development of materials for optical waveguides [1; 2; 3; 4; 5] also provides useful knowledge for the development of polymer cladding materials for optical glass fibers [4; 5; 6]. The materials are also based on fluorinated monomers/oligomers cured with UV light. However, in this case, optical polymers with an extremely low refractive index of only 1.39 at 1547 nm are required. Glass fibers coated with a polymer as optical cladding (polymer-clad fibers) are intended for applications in material processing technology and medical technology [35; 36]. Hence, only highly fluorinated monomers or oligomers should be taken into consideration. And these monomers/oligomers should be at least partially multifunctional (containing more than one double bond) to enable crosslinking. So, Scheme 5 is valid in this case as well, only the concentration of fluorine atoms should be higher to yield a refractive index much lower than 1.45 (cladding in polymer waveguides for optical microchips). The importance of environmental stability for polymer-clad glass fibers is described above in Section 2.3, in the discussion of the influence of harsh environmental conditions on the composite materials. In addition to the refractive index, the viscosity of the formulation is also a very important parameter, since the coating of glass fibers is a continuous process under the application of UV light, leading to photochemical radical cross-linking. Viscosity determines the thickness of UV-cured cladding and the speed of the continuous process. Therefore, the developed monomer/oligomer formulations required optimization for those two parameters. It is possible to achieve different viscosities by mixing lowly viscous but highly fluorinated polyether urethane acrylates with highly viscous acrylates or methacrylate oligomers. Fluorolink(r) from Solvay SA (Brussels, Belgium), which is a UV-curable perfluoro-polyether (PFPE)-urethane methacrylate resin, is the most widely deployed and researched product. Resins can be readily mixed with conventional acrylate monomers/oligomers. As is to be expected, the resulting refractive index depends on the mixture composition determining the fluorine content and on the concentration of the radical photo-initiator, which is a non-fluorinated compound. Therefore, a modular system was developed. It contains fluorinated monomers (high fluorine content, low viscosity) and fluorinated oligomers (low fluorine content, high viscosity). The addition of other modifiers like multifunctional acrylates or other cross-linkers to enhance cross-linking density would change the processing properties (adhesion to the substrate). Highly fluorinated monoacrylates were added to obtain a very low refractive index. A mixture of a bifunctional perfluoropolyether (PFPE)-urethane methacrylate oligomer (Fluorolink(r) MD700) and a tetra-functional PFPE-urethane acrylate (Fluorolink(r) AD1700), both Solvay Specialty Polymers (Scheme 10), was found to be practical. To increase the adhesion between the glass fibers and the cladding,silanes were used as adhesion promotors. For better applicability, these materials were co-polymerized with the fluorinated monomers to reduce migration out of the finally cured cladding. Scheme 10 depicts the chemical structures of components according to the technical data sheets of the manufacturers and our NMR spectra. Furthermore, Scheme 10 also depicts the best radical photo-initiator, Omnirad 2100, selected for effective UV-LED curing and cross-linking agents containing silane groups. A comparison of the viscosity and the refractive indices of different fluorocarbylate and oligomer formulations is shown in Table 2. The reference was always a commercially available low-refractive index resin system from the glass fiber producer j-fiber GmbH, Jena, Germany, currently used to produce silica-clad glass fibers with curing with a UV lamp.