Use of the Lognormal Distribution Function To Describe Orientational Relaxation in Optically Nonlinear Polymers

Second harmonic generation decay has been used to follow the orientational relaxation of dipolar chromophores either dissolved in (guest-host) or chemically attached to (side chain) an amorphous polymer host. The guest-host system investigated is the chromophore lophine-1 in a polyimide host and the side-chain system is the dye molecule Disperse Red 1 chemically attached to a poly(methyl methacrylate) backbone. The relaxation at any given temperature is characterized by a functional form arising from a lognormal distribution of relaxation times. From the temperature dependence of the parameters in this function, we can obtained activation enthalpies and entropies for the relaxation process in these two polymer systems. These quantities can be related to other quantities such as the isobaric coefficient of thermal volume expansion and the activated volume. Also investigated is the effect of aging on the relaxation process.