A fundamental understanding of water transport and morphology is critical for improving ionic conductivity in polymer membranes. In a series of random copolymer anion exchange and cation exchange membranes, we systematically investigate the influence of counterion type, side chain type, and degree of ionic functionalization on water transport using NMR diffusometry. Time-dependent water diffusion measurements reveal micrometer-scale heterogeneity of the hydrophilic network in these random copolymers. We propose a model in which the hydrophilic domain network in these membranes has micrometer-scale distributions of local nanometer-scale dead ends, leading to changes in tortuosity as a function of water content and membrane composition. We furthermore parse tortuosity into two length-scale regimes, one regime from nanometer (local) to bulk and another from micrometer to bulk, offering enhanced discrimination of the multiscale morphological structures that influence bulk transport. This study thus provides new insights into ionic polymer membrane morphology and diffusion behavior, with the ultimate goal of controlling polymeric materials for enhanced fuel cells and other separations applications.

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.

Polymer electrolytes show promise as alternatives to conventional electrolytes in energy storage and conversion devices but have been limited due to their inverse correlation between ionic conductivity and modulus. In this study, we examine surface morphology, linear viscoelastic, dielectric and diffusive properties of molecular ionic composites (MICs), materials produced through the combination of a rigid and charged double helical polymer, poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT), and ionic liquids (ILs). To probe temperature extremes, we incorporate a non-crystallizable IL to allow measurements from −90 to 200 °C. As we increase the PBDT weight percentage, shear moduli increase and do not decay up to 200 °C while maintaining room temperature ionic conductivity within a factor of 2 of the neat IL. We connect diffusion coefficients of IL ions with ionic conductivity through the Haven ratio across a wide temperature range and analyze trends in ion transport based on a relatively high and composition-dependent static dielectric constant. This behavior may result from collective rearrangement of IL ions in these networks. We propose that these properties are driven by a two-phase system in MICs corresponding to IL-rich "puddles" and PBDT-IL associated "bundles" where IL ions form alternating sheaths of cations and anions around each PBDT rod. These polymer-based MIC electrolytes show great promise for use in electrochemical devices that require fast ion transport, high modulus, and a broad thermal window.

To probe local molecular-scale effects of polymer backbone dynamics on the transport of water and salt in desalination membranes, we have studied water diffusion behavior in two chemically similar copolymers using NMR diffusometry. We observe a greater activation energy for water diffusion in a glassy hydroxylated methacrylate membrane as compared to its chemically similar rubbery acrylate counterpart. We also investigate water diffusion in aqueous solutions of both precursor monomers, which serve as close mimics of each membrane’s local environment. Comparison between membranes and solution mimics can further inform on the effects of purely geometric (physical) nanoconfinement versus those effects from water–chain intermolecular interactions. We find that diffusive activation energy differences between the rubbery and glassy membranes originate mainly from differences in the intermolecular interactions of water with the different local polymer structures. We further propose that the more rigid glassy methacrylate backbone introduces configurational restrictions to transport, leading to greater water–salt permeability selectivity compared to the rubbery polymer.

Photoredox ring-opening polymerization of O-carboxyanhydrides allows for the synthesis of functionalized polyesters with high molecular weights, narrow molecular weight distributions, and various tacticities.