MEASUREMENT OF POLAR ORIENTATION IN POLED SIDE-CHAIN NLO POLYMERS USING INFRARED-SPECTROSCOPY

Infrared spectroscopy has been used to provide an independent estimate of dipole orientation in poled films of a side-chain polymer based on poly(methyl methacrylate) with an oxynitrostilbene side group attache d via an aliphatic spacer. In particular, we used the dichroism of the absorption band assigned to the NO2 symmetric stretching vibration as a measure of the orientation of the nitrostilbene group. This yields [P2(cos theta)], where theta is the angle between the transition dipol e moment and the symmetry axis of the sample. We then used a Langevin- type model to calculate [cos theta] from [P2(cos theta)] and concluded that values of [cos theta] as high as 0.5 are being achieved. We find , however, that [P2(cos 0)] obtained by comparison of normal incidence measurements on unpoled and poled films does not agree with [P2(cos t heta)] obtained from tilted-film measurements on poled films. We propo se that this discrepancy arises because the local field in a poled sam ple affects the polarizability of the molecules. To confirm this, we a pplied high fields to unpoled films in the infrared spectrometer at ro om temperature. The results clearly show a large reduction in absorban ce with the applied field when the infrared electric field vector has a component perpendicular to the plane of the film. A major error ther efore occurs as a consequence of the internal field in poled samples i f [P2(cos theta)] is derived from the tilted film measurements, wherea s [P2(cos theta)] derived from normal incidence measurements is essent ially correct. (C) 1994 John Wiley & Sons, Inc.