STUDY OF POLING AND RELAXATION IN KINK AND LINEAR MAIN-CHAIN-FUNCTIONALIZED POLYMERS FOR 2ND-ORDER NONLINEAR-OPTICAL APPLICATIONS

The rotational dynamics of nonlinear optical chromophores functionaliz ed to polymer main chains mere studied using second harmonic generatio n. Corona poling was used to orient the chromophores into the bulk non centrosymmetric structure required to observe second-order nonlinearit y. In order to detect different microscopic relaxation mechanisms of t he polymers, chromophores were incorporated into the polymer main chai n but positioned in two different ways. It was found that for a kink p olymer, in which the chromophores were directed at an angle away from the major molecular axis of the polymer chain, the motion of the tilte d chromophores may occur through local segmental motion. For a Linear polymer, which had the same chromophore, but placed parallel to the ch ain direction, a large scale main-chain motion was involved in orienta tion. Therefore, the end-to-end vectors of the polymer chains could be detected. The temperature dependence of the second-order nonlinearity in these polymers showed that there was an optimum temperature at whi ch the main-chain chromophores could be relatively easily oriented dur ing poling. The retarded polymer mobility at lower temperatures and th e enhanced rotational Brownian motion at higher temperatures reduced t he degree of the chromophore alignment, and therefore a lower second-o rder signal was observed during poling. Dielectric relaxation spectros copy showed that the bulk conductivity and crystallinity might also co ntribute to the decrease in second-order nonlinearity observed at high temperatures.