Influences of Local Anisotropy on Molecular Reorientational Dynamics of Side Chain Nonlinear Optical Polymers

Second harmonic generation measurement has been setup to characterize polymer nonlinear optical (NLO) properties and to study molecular reorientational dynamics. Multi -channel data acquisition and analysis software was developed for NLO applications. NLO-active, amorphous polymers with high glass transition temperatures (Tg) were synthesized through polymer substitution reactions. The NLO chromophore reorientational dynamics were investigated in both an amorphous polymer system and a liquid crystalline system. A nonlinear optical response, second harmonic generation (SHG), was used to probe the induction and decay of chromophore orientational order. A two-mode growth of the SHG signal induced by the poling field at temperatures above Tg and a subsequent two-mode SHG relaxation in the absence of the poling field at temperatures below Tg were observed in the amorphous polymer system. A qualitative mechanism that distinguishes between the motion of the side chain chromophores decoupled from polymer segmental movement and the motion of the chromophores coupled to the polymer segmental movement was proposed to explain the observed two-mode poling and relaxation phenomena. Under the influence of an electric field the polymer contour rearranges itself and creates an anisotropic local environment to accommodate the chromophore anisotropic orientation. As a result, the chromophore orientational order is stabilized. Based on this mechanism, a new liquid crystalline system comprised of a cyclic siloxane co-oligomer containing covalently-linked steroidal mesogens and NLO chromophores was developed. Mechanically shearing the smectic melt of the material (at 200^circ C) induced a homeotropically-aligned lamella structure with all the side chains (mesogens and chromophores) aligned perpendicular to the substrate. The homeotropic smectic morphology persists in the solid state after quenching the melt to room temperature as evidenced by x-ray reflection measurement. The film has high optical clarity and exhibits a Tg at 62^circC Electric field induced a second order NLO coefficient (d _{33}=10+/-/V) which is comparable to that in "state of the art" inorganic NLO crystals. Unlike the amorphous polymer system, the magnitude of d _{33} remains unchanged above the Tg on a time scale of tens of hours. Finally, the SHG relaxation behavior of lyotropic and thermotropic polypeptide systems were also investigated.