Molecular Interpretations of Observed non-Fickian Penetrant Transport Behavior in Glassy Polymers

The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer's network structure alters both the rate and the mode of penetrant transport. To this end, the effects of the basic network parameters of PMMA, including the degree of crosslinking, polymer mesh size, and the crosslink size, on the integral sorption of methanol were studied utilizing both gravimetric data and \textit{in-situ} ultra-high-resolution X-ray computed tomography studies. The effects of sub-Tg annealing/aging, temperature, and the presence of un-reacted monomer were also investigated. Controlling the relative timescale of the relaxational process by altering the polymer network structure is shown to directly influence the Case II front propagation velocity and the nature of the observed transport behavior.