Electric Poling and Relaxation of Thermoset Polyurethane Second-Order Nonlinear Optical Materials: Role of Cross-Linking and Monomer Rigidity

Abstract

A high μβ isophorone-derived phenyltetraene chromophore (denoted CLD-5) was synthesized. The chromophore was modified with a hexyl group at the middle of the π-conjugate bridge to improve solubility and processability and was dihydroxy-functionalized for covalent incorporation into various cross-linked PU polymer systems. Its electrical poling and relaxation behavior in PU polymer thin films were studied. First, the chromophore was incorporated into conventional TDI/TEA polyurethane, and an electrooptic (EO) coefficient of 57.6 pm/V at 1.06 μm was obtained, which is 28% higher than that obtained from nonhexylated chromophore (CLD-2) in the same polyurethane system. Two new polyurethane systems, poly[(phenyl isocyanate)-co-formaldehyde] (PPIF)/triethanolamine (TEA) and PPIF/bisphenyl-1,1‘-dimethanol (BPDM), were designed to study the influence of cross-link density and monomer rigidity on electrical field poling of chromophore dipoles and relaxation behavior of poling-induced chromophore alignment. CLD-5/PPIF/TEA polymer has the highest cross-link density (3.07 mmol/g) among all the polyurethanes studied here. It gains 38 °C in thermal stability but loses 50% of EO activity as compared with the CLD-5/TDI/TEA polyurethane material, which has a cross-link density of 1.91 mmol/g. A higher EO coefficient (41 pm/V), lower optical loss of 2.56 dB/cm at 1.3 μm, and the highest dynamic stability (133 °C) were obtained for the CLD-5/PPIF/BPDM polyurethane EO material, which has the lowest cross-link density (1.27 mmol/g) and the most rigid monomers. The results indicate that excessive cross-linking deteriorates electric poling of long chromophores in a cross-linked polymer thin film. Therefore, cross-linking by itself is not necessarily a good approach to achieving high thermal stability of electrical field-induced chromophore alignment in polyurethane systems.