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 attached via an aliphatic spacer. In particular, we used the dichroism of the absorption band assigned to the NO 2 symmetric stretching vibration as a measure of the orientation of the nitrostilbene group. This yields 〈 P 2 (cos θ)〉, where θ is the angle between the transition dipole moment and the symmetry axis of the sample. We then used a Langevin‐type model to calculate 〈cos θ〉 from 〈 P 2 (cos θ)〉 and concluded that values of 〈cos θ〉 as high as 0.5 are being achieved. We find, however, that 〈 P 2 (cos θ)〉 obtained by comparison of normal incidence measurements on unpoled and poled films does not agree with 〈 P 2 (cos θ)〉 obtained from tilted‐film measurements on poled films. We propose that this discrepancy arises because the local field in a poled sample affects the polarizability of the molecules. To confirm this, we applied high fields to unpoled films in the infrared spectrometer at room temperature. The results clearly show a large reduction in absorbance with the applied field when the infrared electric field vector has a component perpendicular to the plane of the film. A major error therefore occurs as a consequence of the internal field in poled samples if 〈 P 2 (cos θ)〉 is derived from the tilted film measurements, whereas 〈 P 2 (cos θ)〉 derived from normal incidence measurements is essentially correct. © 1994 John Wiley & Sons, Inc.