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

Abstract

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.

Local Water Transport in Rubbery versus Glassy Separation Membranes and Analogous Solutions

“…To address current and future global water shortages, polyamide-based membranes are the current “gold standard” in reverse osmosis water purification technology because of their simultaneously high water flux (20–40 L m –2 h –1 ) and salt rejection (>99%). − However, these materials face shortcomings in the form of fouling and oxidative degradation due to the chlorine use necessary for pretreatment of feedwater. − While many different avenues are being explored to produce efficient chlorine-resistant materials, largely through synthesis of polysulfone- and poly­(phenylene oxide)-based membranes, − these materials presently are unable to match the performance of polyamide membranes. Further advances in membrane design require studies into structure–dynamics–property relationships to achieve the combination of desirable properties needed to meet water purification goals of the future. , …”

Mixed-Matrix Membrane Fabrication for Water Treatment

“…Recently, ion and water transport in glassy hydrated polymers has been reported, and has become a topic of interest in the membrane field [ 45 , 46 , 47 , 48 , 49 ]. Xie et al measured water and salt transport in a disulfonated poly (arylene ether sulfone) copolymer (i.e., BPS-32) [ 45 ].…”

“…More recently, Chang et al prepared two chemically similar copolymers, rubbery 2-hydroxyethyl acrylate-co-ethyl acrylate (HEA-co-EA) and glassy 2-hydroxyethyl methacrylate-co-methyl methacrylate (HEMA-co-MMA), to probe the impact of polymer backbone dynamics on ion and water transport properties [ 48 , 49 ]. Both had similar and relatively low water contents (~8% by mass).…”

“…However, the rubbery membrane had salt permeability coefficients roughly 2–3 times higher than those of the glassy membrane. In a later study, Chang et al reported water dynamics and tortuosity in the same membranes over several length scales [ 49 ]. Using pulsed-field gradient nuclear magnetic resonance (PFG NMR), they measured water diffusivity as a function of diffusion encoding time.…”

“…Water-diffusion coefficients decreased with increasing encoding time, plateauing at long times as water-molecule diffusion became increasingly hindered by the polymer segmental obstructions on longer length scales. The long-duration plateau value of water diffusivity was regarded as equivalent to the value observed in measurements of bulk-transport properties [ 49 ]. Salt solubility and diffusivity were measured via equilibrium and kinetic desorption techniques, respectively.…”