Chromophore-Functionalized Glassy Polymers with Large Second-Order Nonlinear Optical Responses. Transient Dynamics and Local Microstructure of Poly(p-hydroxystyrenes) As Assessed by in-Situ Second Harmonic Generation Techniques

Abstract

Microstructural relaxation of thin films of the poled, chromophore-functionalized amorphous polymer N-(4-nitrophenyl)-(S)-prolinoxypoly(p-hydroxystyrene) has been studied by in-situ second harmonic generation (SHG) techniques. The temporal and temperature dependence of the SHG intensity decay has been analyzed as a function of poling and processing parameters within the framework of the Kohlrausch-Williams-Watts (KWW stretched exponential) model. The average SHG relaxation time, τ, increases rapidly upon reduction of the applied poling field strength, with increasing poling time (physical aging), and with decreasing film temperature. The other KWW parameter, β, which reflects the distribution of relaxation times, decreases (the distribution broadens) moderately with increases in the applied electric field strength and strongly with increases in poling time (physical aging). The observed value of β increases (the distribution narrows) with increasing film temperature. These trends and the variation in KWW parameters yield information regarding reorientation dynamics of the tethered chromophore molecules within the polymer matrix and thus on the nature of the system subspace which is explored during relaxation. Both parameters reveal a strikingly narrower distribution of relaxation times/reduced rotational mobility versus chromophore-doped, guest-host systems and classify the present materials as Angell intermediate glasses. The temperature dependence of the second harmonic signal decay after poling field cessation can be divided into two distinct regions : (i) above T g , where the dynamics are characteristically nonlinear and best described by the Williams-Landel-Ferry (WLF) equation ; (ii) below T g , where the behavior is linear and modeled adequately by the Arrhenius equation. Analysis of the growth of the second harmonic signal as the poling field is applied yields a similar picture ; however, limiting SHG values at temperatures significantly above T g appear to be influenced by both thermal disruption of chromophore alignment and ion conduction/space charge effects.