Copolymers of halogenated acrylates and methacrylates for the application in optical telecommunication: optical properties, thermal analysis and determination of unsaturation by quantitative FT-Raman and FT-IR spectroscopy

The copolymers of tetrachloroethylacrylate (TeCEA) and pentafluorophenylacrylate (PFPA) and of TeCEA and pentafluorophenylmethacrylate (PFPMA) have been examined with regard to their applicability as core materials for optical waveguides. In particular, their thermal properties as function of polymerisation conditions have been investigated and optimized by addition of crosslinkers. The polymerisation reactions were followed by FT-Raman and FT-IR spectroscopy. Also the optical characteristics like material dispersion and attenuation of the polymers have been studied. The fundamental demands of waveguide polymers are high optical transmission at the near-IR wavelengths windows around 1300 and 1550 nm, respectively, the adjustability of the refractive index of the core polymer versus the cladding polymer, and sufficient thermal stability of at least 70°C. Both the examined copolymers show low absorption at 1300 and 1550 nm of 0.2 and 0.7 dB/cm (best values), respectively. The refractive index (RI) of TeCEA/PFPA copolymer is tunable between 1.464 and 1.518 (1300 nm). TeCEA/PFPMA copolymers are tunable between RI values of 1.469 and 1.518 (1300 nm). Thermal stability as indicated by the glass transition temperature (Tg), which reaches about 81°C for the TeCEA/PFPA copolymer with 5 weight% (wt%) of ethyleneglycoldimethacrylate (EGDMA) crosslinker, whereas for TeCEA/PFPMA copolymer with the same amount of EGDMA a Tg of 105°C is obtained. As determined by quantitative vibrational spectroscopy the conversion in the all-acrylate based copolymer is in the order of 98%. Using also EGDMA as crosslinker and with an additional thermal treatment the unsaturation in photocured TeCEA/PFPMA copolymer can be decreased to values of 4%.