ROTATIONAL BROWNIAN-MOTION OF CHROMOPHORES AND ELECTRIC-FIELD EFFECTS IN POLYMER-FILMS FOR 2ND-ORDER NONLINEAR OPTICS

Rotational dynamics of nonlinear optical chromophores embedded in amorphous polymer films 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. Electric field effects were examined by simultaneously measuring the second harmonic signal (during and following poling) and surface voltage decay (following poling). It is found that for short times the residual field following poling retards chromophore reorientation. A mathematical model that describes the rotational Brownian motion of chromophores in a polymer matrix is developed to simulate the field-dependent behavior. The electric field effects can therefore be deconvoluted from the Brownian motion to reveal information concerning local mobility in polymers. Further applications of the model in distinguishing the post-poling electric field effects and in computing the local free volume and viscosity are discussed. A first attempt is made to realize the contributions of the residual surface voltage, field-induced bulk charges, and thermally injected charges to the rotational motion of the chromophores. The magnitude of the local free volume and the local viscosity-temperature behavior in a doped poly(methyl methacrylate) system are estimated and compared with those predicted by the DoolittIe-Williams-Landel-Ferry equation.