Reverse osmosis (RO) is one of the most efficient means of water purification, however improvements to this process are strongly limited by the polymer membranes used as the separation medium. We will present water transport investigations on two series of neutral random copolymer membranes based on polyhydroxymethylmethacrylate-co-methylmethacrylate (poly(HEMA-co-MMA)), in which Tg &gt; room T (glassy dynamics), and polyhydroxyethylacrylate-co-ethylacrylate (poly(HEA-co-EA)), in which Tg &lt; room T (rubbery dynamics). Using NMR diffusometry we measure water diffusion as a function of polymer composition and diffusion encoding time. Such variable time studies reveal clear micron-scale heterogeneity in these polymers. For the first time, we introduce the quantitative separation of material tortuosity into two regimes, averaged over the nm-to-bulk and μm-to-bulk ranges, which allows for separate insights into the influence of multi-scale morphology on bulk transport. When compared with bulk water and salt uptake measurements, we show that water-salt selectivity, water and salt permeation, and nm-to-bulk tortuosity show dramatic differences between the two membrane series, but the μm-to-bulk tortuosity for these membranes is independent of polymer composition, leading to the conclusion that these properties are influenced by backbone dynamics and nanometer-scale interactions. We propose a model for these membranes in which the hydrophilic network has micron-scale heterogeneous arrangements of nm-scale dead ends (see figure). These studies give new perspectives into the morphology of polymeric materials, and provide a means for guiding design of materials for water purification, fuel cells, and other molecular separations applications.

Investigating multi-scale transport in random copolymer membranes for use in molecular separations

255th National Meeting (2018)

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