ACTIVATION VOLUMES ASSOCIATED WITH CHROMOPHORE REORIENTATION IN CORONA-POLED GUEST-HOST AND SIDE-CHAIN POLYMERS

The decay of the second-order optical susceptibility chi((2)) as a function of temperature and pressure has been studied in a variety of corona poled guest-host and side-chain polymeric materials using second harmonic generation (SHG). The specific systems studied include the side-chain copolymer poly(disperse red 1 methacrylate-co-methyl methacrylate) (DR1-MMA) as well as the series of guest-host materials formed by individually dissolving the dyes Disperse Red 1 (DR1), Disperse Orange 3 (DO3), and N,N dimethyl-p-nitroaniline (DpNA) in poly(methyl methacrylate) (PMMA), polycarbonate (PC), and polystyrene (PS). In each of these systems, the observed relaxation of chi((2)) can be represented by a Kohlrausch-Williams-Watts stretched exponential, from which the decay time tau and decay distribution width beta are determined. For pressures up to approximately 1000 atm, the natural log of the pressure shift: factor is seen to vary linearly with applied pressure, yielding the activation volume for rotational reorientation of the chromophores in each system. The activation volumes are loosely correlated with dopant size in a given polymer host, but are not the same for a given dopant in different hosts. Modeling the chromophores as rotating cylinders, we show that the measured activation volumes do not correspond to the average volume swept out by the dye molecules as they reorient. On the other hand, the activation volumes for each of the three dyes dissolved in PS are seen to be in agreement with the measured activation volumes for the molecular motions associated with volume recovery in neat PS. Moreover, the activation volumes for DRI and DpNA dissolved in PS are seen to correlate with the proposed couplings between the rotational reorientation of DR1 and the alpha-relaxation dynamics of PS and the slight decoupling of DpNA from the alpha-transition motion of PS. This correlation suggests a possible relationship between the activation volumes for chromophore reorientation and the size of the components of the host polymer or the volume swept through by the polymer components during structural reconfiguration. We demonstrate that assuming activation volumes for chromophore reorientation to be related to the size or motion of the polymer host constituents yields a consistent interpretation of the observed trends in the measured activation volumes. (C) 1995 John Wiley & Sons, Inc.