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The rotational dynamics of nonlinear optical chromophores functionalized to polymer main chains were studied using second harmonic generation. Corona poling was used to orient the chromophores into the bulk noncentrosymmetric structure required to observe second-order nonlinearity. In order to detect different microscopic relaxation mechanisms of the polymers, chromophores were incorporated into the polymer main chain but positioned in two different ways. It was found that for a kink polymer, in which the chromophores were directed at an angle away from the major molecular axis of the polymer chain, the motion of the tilted chromophores may occur through local segmental motion. For a linear polymer, which had the same chromophore, but placed parallel to the chain direction, a large scale main-chain motion was involved in orientation. Therefore, the end-bend 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 which the main-chain chromophores could be relatively easily oriented during poling. The retarded polymer mobility at lower temperatures and the enhanced rotational Brownian motion at higher temperatures reduced the degree of the chromophore alignment, and therefore a lower second-order signal was observed during poling. Dielectric relaxation spectroscopy showed that the bulk conductivity and crystallinity might also contribute to the decrease in second-order nonlinearity observed at high temperatures.

Several new dipolar main-chain (2) nonlinear optical (NLO) polymers were prepared. The NLO-phores, based on alkoxy substituted -cyanoacrylates, were also placed as monomeric guests within a poly(methyl methacrylate) host. Polar alignment of the NLO-phores by corona poling and their relaxation behavior was monitored by detecting the optical signal resulting from the second harmonic generation. The study revealed that two dipolar rigid-rod main-chain NLO polymers (NLOPs), having solubilizing side chains, were not responsive to alignment by corona poling. We found that each main-chain NLOP prepared in the study showed a strong resistance to alignment, whereas virtually the same NLO-phore as a guest within a polymer host responded well to corona poling. When small variations were made in the structure of the NLO-phore, it was found that when hydrogen-bonding groups are rigidly coupled to the NLO-phore they were very effective for retaining polar asymmetry induced by poling and induced no deleterious effects on the alignment process. For main-chain NLOPs, it appears for optimum alignment the bonding axis and the polar axis should not be parallel.

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