Polymer transport membranes that facilitate or inhibit ion transport are critical for fuel cells, flow batteries, ion exchange columns, electrochemical sensors, getter materials for encapsulation, gas separations, and clean water. A common thread that spans nearly all of these applications is that their function is coupled to the transport of water or some other small molecule penetrant through the membrane. There is a significant need to understand the nature by which small molecule penetrants move through the membrane. In this presentation, I will focus on the general case of water diffusion through three types of membrane materials with very different network structures: tightly cross-linked epoxy resins that are used in structural composites and exhibit very little swelling, loosely cross-linked polyamide membranes that are used as the active layer in a reverse osmosis desalination membranes and exhibit moderate swelling upon hydration, and ion containing block copolymer alkaline fuel cell membranes that contain transient ionic cross-links and swell significantly with water. A combination of three techniques will be used to characterize the swollen structure and dynamics of the membrane materials relative to their transport behavior. Positron annihilation lifetime spectroscopy will be used to quantify the intrinsic “free volume” or intermolecular packing efficiency in the material, small angle neutron scattering will be used to quantify the microstructure of the water-rich domains in the hydrated materials, and quasielastic neutron scattering will be used to quantify both the fast polymer dynamics of material and the dynamics of the water molecules diffusing through the membranes. The insights from these measurements will be used to discuss the different mechanisms that are used to describe water transport in these materials ranging from solution-diffusion to pore-flow transport.

The coupling of polymer dynamics and water transport in cross-linked polymer membranes

255th National Meeting (2018)

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