Influence of cross-linking functionality on the nonlinear optical properties of poled and cross-linked polymer films

We investigate the two potentially competing processes of molecular orientation and network formation in a functionalized guest-host system. We examine a pair of similar guest molecules, one difunctional, the other tetrafunctional. Using second-harmonic-generation data, we determine the average molecular orientation in each sample. At low concentrations of the guest molecules, the molecular orientation is well described by the oriented gas model. However, at concentrations high enough to permit the formation of a long-range network, the molecular orientation of the tetrafunctional molecules is seen to be compromised. This is in contrast with samples containing the difunctional molecules, which show no reduction in the induced orientation. However, the induced orientation is seen to relax more quickly in samples containing difunctional units than in those containing tetrafunctional units. Hence it would appear that increased stability in molecular orientation is achieved at the expense of the degree to which the molecules can be oriented.