Dopant Orientation Dynamics in Doped Second-Order Nonlinear Optical Amorphous Polymers. 1. Effects of Temperature Above and Below T_g in Corona-Poled Films

The dynamics of the rotation of dopants dispersed in amorphous polymers have been studied using second harmonic generation (SHG), a nonlinear optical technique measuring frequency doubling. Corona poling is used to orient the dopants into the noncentrosymmetric structure required to obtain the SHG signal. The SHG intensity is sensitive to the dopant rotation, which is a function of the surrounding local polymer microenvironment, including segmental mobility and local free volume and the electric field across the film.The second harmonic coefficient, d_film, is related to the angle between the applied poling field and dopant axis parallel to the molecular dipole moment and to [I_film]^1/2, where I_film is the SHG intensity of the poled film. The effect of temperature above and below the glass transition on dopant rotation and polymer relaxations has been examined. As temperature increases, the rate of dopant orientation during poling increases. When the applied field is removed, the large SHG intensity drop reported for in situ contact-poled, doped films is not seen in these corona-poled, doped systems. The temporal stability of the SHG intensity following removal of the applied corona decreases with increasing temperature but does not decay in a manner described by a single or stretched exponential equation. Both the maintenance of the dopant orientation and the complicated SHG intensity decay relate to the persistence of the corona-induced field across the polymer film, due at least in part to a time-dependent retention of surface charge on the dielectric. The induced surface charge and the polymer relaxation behavior and their effects on the observed temporal stability of the SHG intensity as a function of temperature must be considered. Surface voltage decay following poling has been measured with an electrostatic voltmeter, and the magnitude and time scale of the surface charge persistence have been determined. A very small amount of charge remains on the film surface following poling but is still of sufficient magnitude to affect significantly dopant orientation in the glassy matrix.