Rigid-rod-like main chain polymers with rigidly attached chromophores: A novel structural concept for electrooptical materials .2. Theory and electrooptical measurements

A theoretical approach to describe the electrooptical properties of rigid-like polymers with NLO-active chromophores rigidly attached to the main chains is developed and the results are compared with the experimental data. Due to the particular structure of the polymer, the motion of the chromophores is restricted to the two dimensional rotation of the chromophore around the main chain. Theoretical computations based on Boltzmann-weighted distribution functions predict a pronounced effect of this dimensional restriction on the nonlinear optical properties. In particular, the relaxation of the polar order induced by a strong electrical poling field is shown to be significantly slowed down in comparison to conventional flexible and three-dimensionally mobile main chain systems. Str.uctural investigations on spin-cast films of the polymers designed to realize this concept show a layered morphology with the polymer main chains oriented parallel to the substrate plane. Due to the anisotropic structure, the ratio of the nonzero tensor components chi(222)((2)) and chi(xxx)((2)), as determined by attenuated total reflection, is well above three, in agreement with the theoretical predictions. The relaxation of the polar order is described by a multiexponential decay. The decay follows an Arrhenius-type time-temperature superposition law, which reflects a relaxation mechanism typical for locally activated processes.