Experimental and theoretical investigations of the dielectric and electro-optical properties of polycarbazoles and polysiloxanes

Abstract

Following the availability of sharp fractions of cyclic and linear poly(dimethyl siloxanes), a comparative dielectric study has been undertaken of these materials. The electronic, atomic and orientational polarizations have been determined in the undiluted state for number-average molar masses in the range 160-7700 and dipole moments calculated. For cyclic dimethyl siloxanes containing more than 20 skeletal bonds the dipole moments were found to be the same, within experimental error, as those found for the nearest equivalent linear molecules. However, the dipole moments of smaller cyclic molecules were observed to be substantially less than the dipole moments of the equivalent open chain molecules. Measurements of the static dielectric permittivity of solutions of cyclic and linear poly(dimethyl siloxanes) in cyclohexane indicated a significant specific solvent effect. Theoretical investigations of the conformational statistics of cyclic and linear dimethyl siloxane oligomers and polymers have been undertaken. Rotational isomeric three-state models were used to calculate electric dipole moments, optical anisotropies and molar Kerr constants. Theoretical dipole moments agreed well with the experimentally determined values. Samples of poly(N-vinyl carbazole) possessing different tact­ icities were prepared and carefully characterised. Measurements of their static dielectric and electro-optical properties in solution in l ,4-dioxan have been made. The results of these measurements support the view that the stereostructure of poly(N-vinly carbazole) is sensitive to the mechanism of polymerization. These results, together with carbon-13 N.M.R. data, are discussed in terms of the possible conformations of the polymer chains and the relative orientations of the bulky carbazole side groups.