Molecular Composites Based on Rigid Rod Polymers for Electrooptical Applications
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ABSTRACT Novel rigid rod polymers substituted with NLO-active chromophores have been developed towards application in electrooptical signal modulation. The materials are based on rigid polyesters and polyesteramids, in which the chromophores are either covalently linked to the backbone by short flexible spacers or directly incorporated into the main chain. In the bulk these systems form macroscopically ordered structures with layers of rigid rod backbones separated by the side chain segments. The properties and stability of the induced polar order can thus be adjusted by morphological parameters like the layer distance or the orientation of the main chains relative to the substrate. The relaxation of the NLO activity with time is described by a multiexponential decay and shows enhanced time-temperature stability even above 100 0 C. The temperature dependence of the relaxation times exhibits unusual features that distinguishes these systems from conventional NLO side chain polymers.
BASIC CONCEPTS The development of optical communication systems has triggered intense research in second order nonlinear optical (NLO) polymers [1]. In the widely accepted poled polymer approach, NLO-active chromophores are either dissolved in a polymer matrix or linked to a polymer chain, and then oriented in a strong electric poling field. The polar orientational order is then frozen in by cooling the sample into the glassy state of the polymer. However, most of the current materials are based on polymers with flexible main chains. The stability of the polar order in these materials decreases tremendously when the temperature is raised near to the glass transition temperature. Recently a new type of molecular design was introduced in which NLO-active chromophores are linked to rigid rod polyesters and polyesteramids [2-5]. Rigid polymers containing aliphatic side chains have been widely studied [6-9]. When films are cast from solution, layered structures have generally been observed, in which layers of backbone chains are well separated by the flexible side chains. Structural parameters like the layer spacing depend strongly on the substitution pattern and on the length of the side chains. X-ray diffraction experiments parallel and perpendicular to the film surface suggest that the films are laterally isotropic, with the layers oriented parallel to the substrate plane, as shown in Figure la. If now some of the sidechains are replaced by NLO-active chromophores, this anistropic structure should force the chromphores to an upright orientation (Figure lb). Consequently the nonlinear optical properties should differ strongly from those observed in conventional side chain polymers based on a flexible polymer backbone. This is especially true when the chromophores are linked to the main chain by a rigid connection. In the latter case the chomophores will be only allowed to rotate around the main chain axis. 15 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
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