Side Chain Polymers for Electro-Optic Applications
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SIDE CHAIN POLYMERS FOR ELECTRO-OPTIC APPLICATIONS
RONALD N. DeMARTINO, DIANE E. ALLEN, RICHARD KEOSIAN, GARO KHANARIAN AND DAVID R. HAAS Hoechst Celanese Corporation, Research Division, 86 Morris Avenue, Summit, NJ 07901 ABSTRACT Organic and polymeric materials have emerged in recent years as candidates for advanced device and systems applications. This interest has arisen from the promise of extraordinary optical, structural, and mechanical properties of certain organic materials, and from the fundamental success of molecular design performed to create new materials. Our approach to the problem is to develop an understanding of the molecular basis of non-linear optical activity, incorporate the most promising candidates into tractable polymers, and evaluate these materials in device format. This paper will review some of the recent developments in the NLO materials effort at Hoechst Celanese. INTRODUCTION In recent years, there has been significant progress made in the development of organic side chain polymers with non-linear optical properties [1,2]. Meredith [3] reported the first investigations on NLO polymers which involved stilbene dyes dissolved in a polymer matrix. However, these guest/host solutions have a number of disadvantages, such as low packing densities, low breakdown fields and poling stabilities. These problems can be overcome by direct attachment of the NLO active dye to a polymer backbone [4]. This not only increases the active unit density, but also prevents phase separation that commonly occurs in guest/host systems. For these materials to possess second order activity, it is necessary to align the dipoles, in the solid state, in a non-centrosymmetric fashion. This involves the process known as electric field poling [5]. The polymer is heated to its glass transition temperature whereupon a strong electric field is applied. The chromophores subsequently align due to their interaction with the field. After a few minutes (to reach equilibrium) the polymer is cooled to room temperature at which point the field is removed. This process freezes in the non-centric alignment and produces NLO polymers with second order activities. SIDE CHAIN NLO POLYMERS Our initial side chain polymers were based on a methacrylate backbone containing an oxy/nitro biphenyl chromophore [6]. We quickly realized that this polymer system suffered from a number of disadvantages: low activity of the
Mat. Res. Soc. Symp. Proc. Vol. 228. c 1992 Materials Research Society
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unit; low Tg resulting in an unstable poled state; and the presence of a liquid crystalline phase. Our attention turned to the more active oxy/nitro stilbene unit and this was similarly incorporated into a methacrylate polymer. However, the active monomer was copolymerized with methyl methacrylate to discourage LC formation and, at the same time, raise the Tg sufficiently to produce polymers with a stable poled state [7]. The structure is depicted in Figure 1.
FIGURE 1. OXY/NITRO STILBENE COPOLYMER WITH MMA Although this polymer system seemed adequat
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