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 amorphous polymer above Tg. In particular, a random copolymer of isobutyl methacrylate and a disperse red 1-labeled methacrylate (mole ratio 94:6) is quantitatively 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 that SHG is sensitive to the first moment of the orientational autocorrelation function (OACF), or 〈cos θ〉 (where θ is the polar angle between the poling field and the permanent dipole moment directions), and birefringence to the second, or 〈cos2 θ〉, the ability of the models to describe chromophore motion is tested by comparing their predictions of the relationship between the OACFs and the measured properties.

By investigating the same polymer system under equivalent conditions, it is shown that chromophore reorientation is explained by a modified Debye model that takes into account a distribution of relaxation times.