Studies of Structural, Nonlinear Optical and Thermal Properties of Specific Nonconjugated Conductive Polymers Including a Co-Polymer Styrene-Butadiene-Rubber

Abstract

The structures, quadratic electro-optic effects, thermal properties, and photovoltaic characteristics of specific nonconjugated polymers have been studied. Various tools and techniques such as Optical absorption, Differential Scanning Calorimetry, and FTIR spectroscopy have been used to study these polymers, in particular a nonconjugated co-polymer styrene-butadiene-rubber (SBR). The optical absorption spectrum of styrene-butadiene-rubber at low doping of iodine showed two peaks at 4.27 eV and 3.2 eV due to radical cations and charge-transfer from double bond to dopant respectively. Upon doping, FTIR showed a reduction in the intensity of peaks 964 cm-1 and 910 cm-1 corresponding to =C-H bending vibration-band. This is due to transformation of double bonds into radical cation upon charge transfer. Quadratic electro-optic effect for iodine doped styrene-butadiene-rubber has been measured at specific wavelengths 633 nm and 1550 nm using field-induced birefringence method. The Kerr coefficients measured at 633 nm and 1550 nm were 3.1x10-10 m/V2 and 1.2x10-10 m/V2 respectively. These Large Kerr coefficients and third order susceptibilities have been attributed to the subnanometer-size metallic domains (organic quantum dots) formed upon doping and charge-transfer. Thermal properties of the specific nonconjugated conductive polymers have been studied over a range of temperatures (-550C to 1100C) before and after doping with iodine. It was observed that specific heat capacity of poly(β-pinene) increases upon doping while in the case of 1,4-trans-polyisoprene, SBR and cis-1,4-polyisoprene reduces. The Tg of undoped poly(β-pinene) as observed is at 770C and the Tm of 1,4-trans-polyisoprene has been found to be at 600C in this range respectively. After doping these thermal transitions were not clearly observable. In the case of styrene-butadiene-rubber and cis-1,4-polyisoprene, no significant change in heat flow characteristics and line-shape was found before and after doping with iodine. The refractive indices of iodine doped poly(β-pinene) were calculated using Kramer-Kronig transformations utilizing the optical absorption data for high level of doping. Refractive indices at specific wavelengths (633nm) for doped PBP and SBR have been determined by measuring reflectivity at different angles of incidence. Photovoltaic effect in a composite involving nonconjugated conductive polymer, poly(β-pinene) (PBP) has been studied. Photovoltaic cells were constructed with iodine-doped poly(β-pinene) film sandwiched between two electrodes: 1) titanium dioxide on ITO glass and 2) carbon on ITO glass, and using potassium iodide as an electrolyte. It was observed that as the intensity of the incident light increases the photo-current and photo-voltage increases proportionally (photovoltaic effect). Thus structural, optical, quadratic electro-optic effect, thermal properties and photovoltaic studies of specific nonconjugated conductive polymers including a novel co-polymer (Styrene-butadiene-rubber) have been made.