| Kinetics of phase separation of a rigid-rod polymer, poly(p-phenylenebenzobisoxazole) (PBO) are examined by time-resolved small angle light scattering and turbidity measurements. Solutions in methanesulfonic acid appear to undergo a spinodal decomposition-like phase separation upon exposure to water vapor, exhibiting the characteristic Cahn-Hilliard kinetics. Transmittance measurements are used to determine the effects of molecular weight and coagulation medium on the rate of coagulation. The effects of processing conditions on the morphology of molecular composite films are examined by optical and electron microscopy. The coagulant and the method of its introduction have been found to exert a strong impact on the final film morphology. A computer simulation program of the coagulation process has been developed to better understand the different structures emerging from coagulation processing of molecular composites. In addition, the optical properties of thin films of PBO are characterized. The indices of refraction, birefringence, absorption spectra, and the third-order susceptibility of PBO are reported. The indices of refraction of 1 micron PBO films are measured at 633 nm using a novel application of the Brewster angle method of Abeles. By preparing both isotropic and highly ordered PBO samples, the birefringence is determined. Absorption spectra are taken over the range of 300 nm to 2000 nm. The magnitude of the third-order susceptibility, {dollar}chisp{lcub}(3){rcub}{dollar}, is determined by third harmonic generation using a Nd:YAG laser (1.06 microns) as the fundamental wave. {dollar}chisp{lcub}(3){rcub}{dollar} for PBO is found to be (1.38 {dollar}pm{dollar} 0.16) {dollar}times{dollar} 10{dollar}sp{lcub}-11{rcub}{dollar} esu. |
