INFLUENCE OF THE POLYMER STRUCTURE ON THE ACHIEVEMENT OF POLAR ORIENTATION IN HIGH GLASS-TRANSITION TEMPERATURE NONLINEAR-OPTICAL POLYIMIDES BY PHOTO-ASSISTED POLING

We have used combinations of light, heat, and electrostatic fields to investigate the orientation of nonlinear azo-chromophores chemically i ncorporated into high glass transition temperature (T-g) polyimides. A number of nonlinear optical polyimides have been synthesized in which the interaction between the nonlinear optical chromophore and the pol ymer main chain was systematically altered to determine to what extent this steric interaction influences the orientation of the nonlinear c hromophore. Chromophores in polymers may be oriented by a number of me thods: (a) polarized light at room temperature (i.e., photo-induced or ientation or PIG), (b) polarized light and electric fields (i.e., phot o-assisted poling or PAP) at temperatures ranging from room temperatur e to the polymer T-g, and (c) electric fields at T-g (thermal poling). While thermal poling and PIO are usually possible, PAP depends strong ly on the molecular structure of the polymer. Previously we have shown that PIO can be accomplished at room temperature in a system where th e nonlinear chromophore is embedded into the polyimide main chain via the donor substituent, and this orientation can only be thermally eras ed at temperatures approaching T-g. In this article we show that, wher eas photoisomerization can efficiently depole donor-embedded polyimide s in a matter of few minutes at room temperature, PAP does not induce any polar order. This behavior is in marked contrast to a structurally related, side-chain, nonlinear polyimide, in which the azo chromophor e is tethered via a flexible linkage to the polymer backbone. In this case some PAP occurs even at room temperature, while no PAP is observe d for a donor-embedded system with a similar T-g. We suggest that the orientation during PAP below T-g in the side-chain polyimide is primar ily due to the movement of the azo side chains, and there is a very li ttle coupling of this motion to the main chain. (C) 1998 John Wiley & Sons, Inc.