Relaxational diffusion in guest–host polyphosphazene thin films: Temperature dependence

The relaxation of poled guest–host polyphosphazene (PPZ) films containing the 4-[(4^′-nitrophenyl)azo]phenoxy (NPAP) azo-dye chromophore $(Tg=144 °C)$ has been investigated by measuring the time decay of the nonlinear optical susceptibility tensor in the 80–120 °C temperature range. The kinetic data have been analyzed on the basis of the Legendre polynomial decomposition of the orientational distribution function. In the measuring time range, two components (fast and slow) of the decay have been found together with a constant component that disappears at temperatures close to $Tg.$ This component accounts for the nonreorientable fraction of the molecules at each temperature. The temperature dependence of the two lifetimes has been determined and fitted to an Adam–Gibbs law. At a high temperature, (120 °C) the ratio of lifetimes closely follows the predictions of the orientational diffusion equation. This allows one to determine the two nonlinear components $βzzz=−88.10−30 esu$ and $βzxx=71.10−30 esu$ of the quadratic hyperpolarizability tensor in our rodlike $(C∞v$ symmetry) molecules, showing the non-negligible contribution of the nondiagonal components.