Jiayuan Miao

Generalized model for the diffusion of solvents in glassy polymers: From Fickian to Super Case II

The diffusion of small solvent molecules in glassy polymers may take on a variety of different forms. Fickian, anomalous, Case II and Super Case II diffusion have all been observed, and theoretical models exist that describe each specific type of behavior. Here we present a single generalized kinetic model capable of yielding all these different types of diffusion on the basis of just two parameters. The principal determinant of the type of diffusion is observed to be a dimensionless parameter, γ, that describes the influence of solvent-induced swelling in lowering the potential barriers separating available solvent sites. A second parameter, η, which characterizes the effect of the solvent in reducing the potential energy of a solvent molecule when at rest at an available site, only influences the type of diffusion to a lesser extent. The theoretical analysis does not include any effects that are explicitly non-local in time, an example of which is the inclusion of polymer viscosity in the Thomas-Windle model; it thus represents a variant of Ficks second law utilizing a concentration-dependent diffusivity when η is small. To check the significance of time-delayed swelling, a simulation of a modified model was performed that contained a history-dependent term. The results were found to be very similar to those in the time-local model.

Theoretically guided design of efficient polymer dielectrics

We analyze the nature of energy storage in polymer dielectrics, with a focus on the case of polymers containing permanent electric dipoles. We note that the stored energy resides largely in the potential energy of distortion of molecular bonds. In the case of a model polymer dielectric in which the permanent electric dipoles lie perpendicular to the chain axis, we study how the density of cross-linked sites, at which the chain is prevented from rotating, affects the stored energy. If the length of segment of chain that is free to rotate is large, the dipoles rotate freely in unison with little bond strain, and hence little stored energy. If the segment length is small, then the large distortion of bond angles necessary for dipole rotation makes this rotation small, and there is again little stored energy. An optimum cross-link density can thus be found at which the stored energy is maximized. The interaction between dipoles is evaluated, and found to be potentially most significant at high dipole density,...