Studies of Structures, Quadratic Electro-Optic Effect and Rechargeable Battery Characteristics in Specific Nonconjugated Conductive Polymers including Trans-1,4-Polyisoprene

Abstract

The structures, quadratic electro-optic effect and rechargeable battery characteristics in specific nonconjugated conductive polymers have been studied. FTIR spectroscopy, x-ray diffraction, optical absorption, differential scanning calorimeter and optical microscopy have been used to study structures of nonconjugated conductive polymers including in particular, trans-1,4-polyisoprene. This is partly because trans-1,4-polyisoprene is semicrystalline in structure. Quadratic electro-optic effect has been measured at different wavelengths using field-induced birefringence method. Rechargeable batteries involving doped nonconjugated conductive polymer as an electrode were studied for enhanced efficiency. The optical absorption spectrum of trans-1,4-polyisoprene at low doping of iodine shows two peaks: one at 4.2eV due to radical cation and the other at 3.2eV due to charge-transfer. Doping leads to a reduction of the intensity of =C-H bending vibration-band due to formation of radical cations upon charge-transfer. X-ray diffraction results showed the γ-phase crystal structure for undoped trans-polyisoprene. Upon iodine-doping intensities of specific diffraction peaks slightly increased but the d-spacings of all the peaks remained essentially unchanged. This indicates that the crystal structure remains essentially the same (γ-phase) except iodine atoms are deposited close to specific lattice planes. Quadratic electro-optic measurements have been made using field-induced birefringence method at 633 nm and 1550 nm. A modulation depth of 0.15% has been observed at 1550 nm for an applied field of 1.15 V/µm for a 0.37µm thick film. The modulation depth had a quadratic dependence on applied field. The Kerr coefficients as measured 3.5x10-10 m/V2 at 633nm and 2.5x10-10 m/V2 at 1550nm are exceptionally large and has been attributed to the subnanometer size metallic domains (quantum dots) formed upon doping and charge-transfer. Thermal properties of the nonconjugated polymer trans-1,4-polyisoprene have been studied before and after doping with iodine using a differential scanning calorimeter (DSC) over the temperature range of -50 ◦C to 110◦C. The Tm of undoped trans-1,4-polyisoprene has been found to be at 60 ◦C. After doping the Tm transition was not clearly observable. Rechargeable batteries have been constructed with sodium chloride (NaCl) dissolved in water with polyvinyl alcohol as additive used as electrolyte, along with two electrodes: 1) stainless steel strip and 2) nonconjugated conductive polymer, iodine doped cis-polyisoprene (natural rubber). The batteries have been characterized measuring currents and voltages with many cycles of charging and discharging. A voltage of 1.5 volts and a maximum current of have been measured. Discharge rate of current has been recorded at definite intervals of time. Polyvinyl alcohol (PVA) has been added to electrolyte solution at 10-50 % weight of NaCl. Gradual slow down in discharge rate – significant increase in capacity has been observed with increase in concentration of PVA in the electrolyte solution.