Influence of Rubbery versus Glassy Backbone Dynamics on Multiscale Transport in Polymer Membranes

To determine the effects of polymer backbone dynamics on water and salt permeation in water purification membranes, we investigate the fundamental transport and sorption properties of two series of chemically similar copolymers: methacrylate-based copolymers that are glassy at room temperature and acrylate-based copolymers that are rubbery at room temperature. Water diffusion measurements made as a function of diffusion time using pulsed-field-gradient NMR diffusometry provide information about hydrophilic network heterogeneity in the copolymers. These time-dependent measurements enable us to parse tortuosity into two regimes, the nanometer-to-bulk and micrometer-to-bulk ranges, enhancing insight into the influence of copolymer morphology on bulk transport. Combining NMR diffusometry and water and salt sorption and transport measurements, we find that the glassy methacrylate copolymers exhibit greater water–salt selectivity than the acrylate copolymers. These differences likely arise from sub-micrometer polymer morphological and dynamical differences, and we propose multiscale models for heterogeneities of the hydrophilic networks in these copolymers.