A fundamental investigation of non-Fickian and Case II penetrant transport in glassy ploymers
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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, samples of glassy poly(methyl methacrylate), poly(2-hydroxyethyl methacrylate), and poly(vinyl alcohol) were synthesized primarily by an iniferter-mediated, thermally-initiated free radical polymerization procedure. The thermal and mechanical properties of these polymers, as well as the polymer network structure, were varied through crosslinking and confirmed by detailed characterization. The dynamics of small molecule penetrant transport were examined in each polymer, with an emphasis on the occurrence of non-Fickian and Case II transport. The degree of crosslinking and choice of crosslinking molecule were shown to be powerful tools in tuning the observed penetrant transport process. For instance, the transport dynamics were altered from Fickian to Case II by increasing the degree of crosslinking and from Case II to Fickian by increasing the crosslinking interchain bridge length. Within the purely Case II regime, the rate of penetrant transport, or the Case II front velocity, was shown to scale with the square root of the degree of crosslinking in all systems investigated. A novel procedure for the in situ examination of penetrant transport in glassy polymers was developed utilizing high-resolution X-ray computed tomography. This completely nondestructive technique was used to visualize features in the interior of opaque solid objects and obtain digital information on their 3-D structure and properties. In this manner, the time-dependent penetrant concentration profiles throughout a swelling polymer were determined and analyzed.