Structural relaxation in glassy small molecule and polymer systems
Abstract
A critical analysis of a variety of mobility models has been undertaken, where the predictions are compared to experimental data over a wide range of temperatures and pressures. Various experimental mobility data, including viscosity, dielectric relaxation, light scattering, nuclear magnetic resonance, mechanical measurements, have been unified to obtain a single mobility surface in temperature-specific volume space. The mobility surfaces are constructed for ortho-terphenyl, di(n-butyl)phthalate, di(2-ethylhexyl)phthalate, glycerol, n-propanol, poly(styrene), polyvinyl acetate), poly(methyl methacrylate), poly(vinyl chloride). The mobility data are analyzed in terms of the following models: the modified Vogel-Tamman-Fulcher (VTF) function, the configurational entropy model of Scherer, the free volume model, the Adam-Gibbs configurational entropy and energy models, the modified free volume approach, and the shoving model of Dyre. Critical analyses of the mobility models with the experimental data indicate: (1) The free volume and the Scherer's model cannot describe the pressure dependence of mobility; (2) The modified VTF function and the configurational entropy and energy models are able to describe the mobility data in a moderate pressure range. However, they are unable to describe the data in the wider pressure range; (3) The modified free volume approach can describe the mobility data in a wide temperature and pressure range, although the parameters are phenomenological; (4) The shoving model, where the molecular mobility depends upon the glassy shear compliance of environmental liquid, can describe the mobility data in a wide temperature and pressure range with physically meaningful parameters. The second harmonic generation (SHG) signal decay in a nonlinear optical polymer system was investigated experimentally and theoretically. It was assumed that disorientation of a chromophore depends on the structural relaxation of surrounding polymer matrix in which a chromophore is embedded. The nonexponential nature of signal decay can be predicted by accounting for the spatial and temporal heterogeneity of polymer matrix. A stochastic model that acknowledges the fluctuation of local specific volume can qualitatively predict the experimental data of SHG signal relaxation for various thermal histories. Allowing for a partial decoupling between the chromophore mobility and relaxation of polymer matrix, the stochastic model can quantitatively describe the experimentally observed decay of SHG signal.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering
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