Influence of the Polymer Structure on the Achievement of Polar Orientation in High Glass Transition Temperature Nonlinear Optical Polyimides by Photo-Assisted Poling ZOUHEIR SEKK AT, 1 * ANDRE ´ KNOESEN, 2 VICTOR Y. LEE, 2 ROBERT D. MILLER 2 1 Department of Electrical and Computer Engineering, University of California – Davis, California 95616 2 IBM Almaden Research Center, 650 Harry Road, K17F / 801, San Jose, California 95120-6099 Received 21 July 1997; revised 22 December 1997; accepted 30 December 1997 ABSTRACT: We have used combinations of light, heat, and electrostatic fields to investi- gate the orientation of nonlinear azo-chromophores chemically incorporated 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 chromo- phore and the polymer main chain was systematically altered to determine to what extent this steric interaction influences the orientation of the nonlinear chromophore. Chromophores in polymers may be oriented by a number of methods: (a) polarized light at room temperature ( i.e., photo-induced orientation or PIO ) , ( b ) polarized light and electric fields ( i.e., photo-assisted poling or PAP ) at temperatures ranging from room temperature to the polymer T g , and (c) electric fields at T g (thermal poling). While thermal poling and PIO are usually possible, PAP depends strongly on the molecular structure of the polymer. Previously we have shown that PIO can be accom- plished at room temperature in a system where the nonlinear chromophore is embedded into the polyimide main chain via the donor substituent, and this orientation can only be thermally erased at temperatures approaching T g . In this article we show that, whereas photoisomerization can efficiently depole donor-embedded polyimides in a mat- ter 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 polyim- ide, in which the azo chromophore is tethered via a flexible linkage to the polymer backbone. In this case some PAP occurs even at room temperature, while no PAP is observed for a donor-embedded system with a similar T g . We suggest that the orienta- tion during PAP below T g in the side-chain polyimide is primarily due to the movement of the azo side chains, and there is a very little coupling of this motion to the main chain.  1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1669–1677, 1998 Keywords: photo-assisted poling; polymer dynamics; polymer structure; nonlinear op- tical polyimides; high glass transition temperature INTRODUCTION for various applications. For example, nonpolar orientation of the dipolar chromophores leads to In the last few years there has been great interest birefringence with potential application in optical in orienting polar chromophores in polymer hosts storage. 1,2 A noncentrosymmetric polar order of nonlinear chromophores results in second-order * Present address: Department of Applied Physics, Osaka nonlinearities with application in electro-optical University, Suita, Osaka 565, Japan; permanent address: De- modulators and harmonic generators. partment of Physics, Faculty of Sciences, University Sidi Mo- hamed Ben Abdellah, BP 1796, Atlas-Fes, Morocco Thermal poling is the most effective method of Correspondence to: Z. Sekkat, Department of Applied Phys- inducing polar order in a polymer. In this process, ics, Osaka University, Suita, Osaka 565-0871, Japan polar chromophores are oriented in a static elec- Journal of Polymer Science: Part B: Polymer Physics, Vol. 36, 1669–1677 (1998)  1998 John Wiley & Sons, Inc. CCC 0887-6266/98 / 101669-09 tric field, at temperatures near the glass transi- 1669 7025 / 8q5d$$7025 04-17-98 13:19:20 polpal W: Poly Physics