Birefringence and second-order nonlinear optics as probes of polymer cooperative segmental mobility: Demonstration of Debye-type relaxation

Two complementary optical techniques, second-harmonic generation (SHG), and electric birefringence are used to observe chromophore dynamics in an amor phous polymer above T-g. In particular, a random copolymer of isobutyl meth acrylate and a disperse red 1-labeled methacrylate (mole ratio 94:6) is qua ntitatively studied over a range of time scales and temperatures. In doing so, two models of reorientation are tested: one considers the chromophores motions as small correlated diffusive steps (Debye rotation) and the other describes the motions in terms of uncorrelated instantaneous jumps. Given t hat SHG is sensitive to the first moment of the orientational autocorrelati on function (OACF), or < cos theta > (where theta is the polar angle betwee n the poling field and the permanent dipole moment directions), and birefri ngence to the second, or < cos(2) theta >, the ability of the models to des cribe chromophore motion is tested by comparing their predictions of the re lationship between the OACFs and the measured properties. By investigating the same polymer system under equivalent conditions, it is shown that chrom ophore reorientation is explained by a modified Debye model that takes into account a distribution of relaxation times. (C) 1999 American Institute of Physics. [S0021-9606(99)51530-4].