Stability of nonlinear optical characteristics and dielectric relaxations of poled amorphous polymers with main-chain chromophores

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Abstract
We have studied second‐order nonlinear optical properties and dielectric relaxationcharacteristics for two amorphous epoxy polymers based on bisphenol‐A with nitroaniline‐type nonlinear optical moieties covalently attached as part of the main chain. The nonlinearities, rather large immediately after corona poling, undergo a slow decay at ambient temperature (e.g., ∼29% reduction in the nonlinear coefficient d 33 in 28 days), even though the nominal glass transition temperature T g is ∼80 °C. In dielectric relaxationmeasurements, both the unpoled and poled samples exhibit two relaxation modes: an α relaxation attributed to glass transition and a β relaxation due to local segmental motions of the main chain. Surprisingly, the critical temperature T ∞ associated with the glass transition, obtained by a WLF [M. L. Williams, R. F. Landel, and J. P. Ferry, J. Am. Chem. Soc. 7 7, 3701 (1955)] fit of the α relaxationcharacteristics, shows a significant decrease (as much as 40 °C) with increasing poling field. Moreover, the relaxation times of temporal decay of field‐induced alignment at ambient temperature, as determined from the stretched exponential function fit of birefringence changes, are found to be consistent with extrapolations from the the dielectric α relaxationcharacteristics of poled samples.