# Fluorinated Polymers for Photonics--From Optical Waveguides to Polymer-Clad Glass Optical Fibers
## 3 Fluorinated Polymers as Cladding for Optical Glass Fibers
For comparison reasons, the neat Fluorolink AD 1700 and Fluorolink MD 700 were added in Table 2. The reference system is normally cured by mercury lamps at ambient temperature. The application of UV-LEDs instead of mercury vapor lamps containing mercury or noble gases is currently gaining in importance. Formulation No. 6 (Table 2), developed by us, consists of Fluorolink AD 1700 and Fluorolink MD 700, as well as a liquid photo-initiator. The No. 6 formulation and a commercially available low-index coating material (j-fiber GmbH, Jena, Germany) were cured in a nitrogen atmosphere as films on glass slides. The resulting samples were an object of investigation of the influence of the exposure time (drawing speed) and the type of light source on the degree of conversion, the cross-linking density of the cured polymers and the consequent thermal properties. Commercial glass-fiber drawing processes, for example, those performed by j-fiber, run at speeds of up to 2000 \(\mathrm{mm}\mathrm{i}\mathrm{n}^{-1}\). Therefore, the investigation of conversion and curing behavior is essential for process development. Hence, acrylate formulations with different compositions and developed by employing different radical photo-initiators were cured for different exposure times (different drawing speeds and UV doses) and by using different UV sources (Hg lamp and 390 nm UV-LEDs). The experimental setup is exhibited in Figure 17.  
The conversion was determined using FTIR spectroscopy by monitoring the intensity of the 1640 cm\({}^{-1}\) peak (C=C bond stretching vibrations in the methacrylate group). It was demonstrated that there was almost no difference between the commercial material and the developed formulations at high exposure times over 1 s. With short exposure times, there were significant differences between curing with a Hg lamp and with UV-LEDs. However, the differences between the different formulations and the reference system were only minor (Figure 18). The conversion can reach nearly the value achieved with Hg lamps by increasing the UV intensity of the UV-LEDs. Thus, the highly fluorinated thermosets provide excellent curing behavior (a high degree of conversion) with both UV-LEDs and Hg lamps. Dynamic Mechanical Analysis (DMA) was also used to investigate the dependence of the conversion of resin formulation No. 6 on the exposure time with UV-LED light. For this mechanical investigation, coatings prepared on glass slides were separated from the substrate to obtain free-standing films. The curves of the storage modulus (\(E^{\prime}\)) and the loss factor (\(tan\delta\)) are shown in Figure 19. As expected, an increasing exposure time (UV-LED, 4 Wcm\({}^{-2}\), 385 nm) leads to a higher degree of conversion and thus to a higher network density, resulting in a higher modulus of the rubbery plateau. The modulus of the rubbery plateau at the curve is proportional to the cross-linking density [37]. Additionally, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No. \({}^{*}\)**} & \multirow{2}{*}{**Resin**} & \multicolumn{2}{c}{**Viscosity**} & \multicolumn{4}{c}{**Refractive Index (20 \({}^{\circ}\)C)**} \\  & & **[mPas, 26 \({}^{\circ}\)C]** & **400.0 nm** & **486.1 nm** & **598.3 nm** & **653.0 nm** & **704.0 nm** \\ \hline
1 & Low-index reference from j-fiber & 3088.89 & 1.4074 & 1.3991 & 1.3940 & 1.3920 & 1.3908 \\
2 & Fluorolink AD 1700 & 22,440.00 & 1.4428 & 1.4337 & 1.4281 & 1.4259 & 1.4246 \\
3 & Fluorolink MD700 & 193.80 & 1.3561 & 1.3506 & 1.3471 & 1.3456 & 1.3448 \\
4 & Fluorolink AD 1700/Fluorolink & 990.00 & 1.3877 & 1.3805 & 1.3761 & 1.3744 & 1.3733 \\
5 & Fluorolink AD 1700/Fluorolink & 6677.80 & 1.4048 & 1.3980 & 1.3933 & 1.3914 & 1.3903 \\
6 & Fluorolink AD 1700/Fluorolink & 3216.87 & 1.3954 & 1.3885 & 1.3839 & 1.3821 & 1.3810 \\ \hline \hline \end{tabular} \({}^{*}\)Formulations 2 to 6 were prepared utilizing only solvent-free Fluorolink AD1700 and contained 5% Omnirad 2100 photo-initiator.  
\end{table}