Molecular dynamics simulations of electric field poled nonlinear optical chromophores incorporated in a polymer matrix
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abstract
The electric field poling process of nonlinear optical chromophores embedded in an amorphous polymer matrix was studied using molecular dynamics (MD) simulations. Three systems were considered, consisting of a poly(methyl methacrylate) matrix doped with the following chromophores: N,N-dimethyl-p-nitroaniline (DPNA), 4-(dimethylamino)-4-nitrostilbene (DMANS), and N,N-di-n-propyl-2,4-dinitro-1,5-diaminobenzene (DPDNDAB). The cooling process in the presence of a poling electric field was simulated at constant NPT conditions using simulated annealing. The rotational dynamics of the dopants was investigated in the unpoled and poled states above T g and in the poled state below Tg. The short-time behavior with respect to the back-relaxation to the unpoled state following removal of the poling field was examined for the systems below Tg and was found to deviate from the single-exponential model. The electric field effects, during and following poling, were examined by computing the angle between the dipole moment of the chromophores and the external electric field. MD simulations at temperatures in the vicinity of Tg revealed that during the simulated phase transition from the liquid state to the glassy structure the degree of alignment remained constant. The dependence of back-relaxation to the unpoled glassy state on the chromophores was investigated. DPNA molecules were found to be in closer proximity to the side groups than to the backbone units of the polymer at both temperatures, in contrast to DMANS at both temperatures and to DPDNDAB in the glassy state. The radial distribution functions for all systems are typical of amorphous structures. The reorientation of chromophores exhibits a higher degree of correlation with the facile motion of the PMMA side groups than with the configurational motion along the polymer backbone. The degree of chromophore alignment depends on its size and distance from the side groups of the polymer.
