Large electric dipole moments on polydiacetylenes in toluene: Results from birefringence experiments

Abstract

We report the results of a series of electric field induced birefringence experiments on polydiacetylenes (PDA) in toluene which demonstrate the presence of large electric dipole moments when the molecule is in the rod-like conformation. We show that an induced dipole moment cannot account for the saturation of the birefringence which was observed at relatively low values of the applied electric field. This result is in conflict with earlier models of the alignment process. Using a sinusoidally varying field, we demonstrate a frequency dependence in the birefringence signal which is the characteristic signature of orientation by a permanent dipole moment. The magnitude of the dipole moment is calculated from the value of the saturation birefringence. The degree of orientation of the polymers is estimated from a measurement of the Kerr constant. The degree of orientation is found to be rather low. The apparent permanent dipole moment is thought to arise from an anisotropic net charge which is found on the extended conformation of the PDAs in solvent solutions.