Quadratic Electro-Optic Effect in the Nonconjugated Conductive Co-polymer Iodine-doped Styrene-Butadiene-Rubber Measured at 633 nm and 1550 nm

The quadratic electro-optic effect in the nonconjugated conductive \textit{co-polymer} film of styrene-butadiene-rubber (SBR) has been measured using field-induced birefringence method. Thin films of styrene-butadiene-rubber have been prepared on various substrates from a chloroform solution and characterized using optical absorption spectroscopy, FTIR and DSC before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at 4.27 eV and the other at 3.2 eV corresponding to the radical cation and charge-transfer transition. FTIR data indicate =C-H vibration bands (964 cm$^{-1}$ and 910 cm$^{-1})$ of polybutadiene decrease upon doping due to transformation of the double bonds into radical cations. The Kerr coefficients as measured at 633 nm and at 1550 nm are 3.1x10$^{-10}$ m/V$^{2}$ and 1.3x10$^{-10}$ m/V$^{2}$ respectively. These exceptionally large values have been attributed to the subnanometer metallic domains formed upon doping and charge-transfer involving isolated double-bonds.