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An Examination of Structural Parameters Affecting the Dynamics of Penetrant Transport In Glassy Polymers

Adam K. Ekenseair1, Richard A. Ketcham2, and Nicholas A. Peppas1. (1) Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, (2) Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712

Glassy polymers are increasingly used in the construction and fabrication of high-tech devices. These devices are often used in applications, such as microfluidics, membrane separations, food packaging, artificial tissues, bones, and organs, and controlled drug delivery, where the polymers come into contact with fluids capable of penetrating the polymer network. To examine the behavior of such systems both on the macroscopic and molecular level, it is important to study the penetrant diffusion process. In such transport, the macromolecular chains rearrange toward new conformations, where the rate of relaxation depends on the penetrant concentration. The relative rates of penetrant diffusion and macromolecular chain relaxation determine the nature of the transport process and lead to a wide variety of penetrant transport phenomena, such as Fickian, anomalous (non-Fickian), and Case II sorption behavior.

With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for both a qualitative and quantitative understanding of the manner in which a polymer's network structure alters not only the rate, but also the mode of penetrant transport. To this end, poly(methyl methacrylate) (PMMA) was synthesized in an iniferter-mediated radical polymerization process to produce a variety of well-characterized polymer network structures. The effects of basic network parameters, including the degree of crosslinking, polymer mesh size, and the crosslink size, on the transport process were studied. The effects of sub-Tg annealing/aging, temperature, and the presence of un-reacted monomer were also investigated.

Utilizing both gravimetric sorption data and in-situ ultra-high-resolution X-ray computed tomography studies, the transport of methanol in PMMA discs was investigated. Changing the polymer network structure is shown to not only influence the rate of sorption but also the nature of the transport process.