Time-dependent nonlinear optical properties of pyroelectric liquid crystalline polymers

Highly oriented pyroelectric liquid-crystalline polymers were prepared by p hotopolymerization under the influence of a static electric field from bina ry mixtures of two acrylate monomers exhibiting chiral smectic C mesomorphi sm. Both monomers contained nitro groups to yield second order nonlinear op tical properties (second harmonic generation) and one of the monomers had t wo functional groups to yield a crosslinked polymer. The room temperature s econd order nonlinear susceptibility of the polymers showed during the firs t two hours a 10 % decrease after which it remained constant during the nex t 48 days. At elevated temperatures there was a significant difference in t he nonlinear optical properties over time between crosslinked and uncrossli nked polymers. The uncrosslinked polymer showed a pronounced loss of second order nonlinear optical activity with time at greater than or equal to 38 degrees C. The crosslinked polymer showed a much smaller and basically a te mperature independent decrease rare in the second order nonlinear optical p roperties at all the ageing temperatures (23-130 degrees C). Both the loss in mesogen order parameter, very evident for the uncrosslinked polymer, and conformational changes occurring within the mesogens (beta mechanism), may account for the observations made.