ELECTRIC-FIELD POLING IN POLYMERIC NONLINEAR-OPTICAL MATERIALS - RELAXATION DYNAMICS, MODEL, AND EXPERIMENT

Simulations of the electric field poling process for second-order NLO- active polymeric materials containing dipolar chromophores were perfor med by modeling the time-dependent dynamics of a dipole interacting wi th an externally applied field and subsequent force-free relaxation, e mploying several modifications of the Smoluchowski equation. The model examines chromophore dipole alignment/relaxation processes in both tw o- and three-dimensional space. The 3-D model predicts that at field-o n equilibrium, the ratio, R, of the second-harmonic coefficients, d(33 )/d(31), approaches 3.0, in accord with static statistical-mechanical models. In contrast, the 2-D model predicts R similar to 6.0. The dime nsionality in which the rotational diffusion process is confined also determines the rate of dipolar alignment/relaxation, with a slower rat e predicted in the 2-D case. Suitability of the rotational diffusion m odel for the alignment and relaxation dynamics of appended NLO chromop hores in poled polymer films is also examined. At temperatures at or a bove the glass transition temperature, T-g, experimentally measured d( 33) relaxation kinetics of a prototypical chromophore-functionalized p olymer, N-(4-nitrophenyl)-(S)-prolinoxy poly(p-hydroxystyrene), (S)-NP P-PHS, are well described by the bi-exponential expression predicted b y the 3-D model. Below T,, however, the dynamics are not well modeled as simple 3-D rotational diffusion, the apparent result of complex dyn amical matrix interactions. Under all conditions examined, the experim ental d(31) relaxation dynamics can be described approximately using t he 2-D model. The temperature dependence of the relaxation rate above T-g is well described by the Williams-Landel-Ferry (WLF) equation, whi le below T-g, the reorientation process is Arrhenius-like. The d(33) g rowth kinetics are found to be accurately approximated using expressio ns derived from the 3-D rotational diffusion model. Below T-g the expe rimental activation energy determined from field-on polarization is id entical within experimental error to that determined for field-off dep olarization.