Probing Morphology, Transport and Local Intermolecular Interactions in Polymeric Materials via NMR Diffusometry and Spectroscopy

Understanding transport of water molecules and salt ions from a molecular level up to macroscopic length scales is critical to the design of novel materials for many applications, including separations membranes for fuel cell and desalination applications, as well as rechargeable battery technology. This work aims to investigate and develop new models correlating the dynamics and structure of polymeric materials, to the transport of small molecules within them, using a variety of Nuclear Magnetic Resonance (NMR) techniques.
We present three studies through which we utilize two chemically similar membranes: hydroxyethyl acrylate-co-ethyl acrylate (HEA-co-EA) and hydroxymethyl methacrylate-co-methyl methacrylate (HEMA-co-MMA), which greatly differ in glass transition temperature, in order to understand the fundamental relationships from polymer chain dynamics and small molecule diffusion. From observations of the micron scale diffusion of these materials we find that the more dynamic, rubbery HEA-co-EA exhibits lower water to salt selectivity than HEMA-co-MMA, and that this difference arises from nanoscale morphology of the materials. From this, we propose a new model for hydrophilic pathways inside polymeric materials consisting of nanometer scale interconnected pathways are interrupted by micron scale arrangements of so-called "dead ends". We also for the first time show the separation of material tortuosity into two regimes, ranging from the nanometer-bulk and micron-bulk length scales. We further separate the contributions of structure from chemical interactions in the chemically similar desalination materials by investigating the local activation energy of diffusion in both materials, as well as aqueous solutions of the hydrophilic monomers analogous to the internal membrane environment. We find that the effects of local geometric confinement are very similar between the two materials, however the intermolecular interactions between water and the hydrophilic monomers, with respect to water transport, are significantly different between the two hydrophilic species. Geometric confinement accounts for a 5 ± 1 kJ/mol increase in diffusive activation energy from solution to membrane for both chemistries, and a 4 ± 1 kJ/mol difference in activation energy is seen between the two chemistries in both solution and membrane form. We propose that the entropic contributions to transport, are strongly impacted by the rigid environment of the HEMA material, and is related to the increased water-salt selectivity, as well as the increasing selectivity with increased ionic size observed compared to the HEA system. Using Dynamic NMR spectroscopy, we further investigate the differences seen in water-monomer intermolecular proton exchange by NMR. We utilize an iterative least-squares solving method to fit our exchange lineshape to a model of an uncoupled, two-site exchange lineshape in order to obtain rate and equilibrium population data from -50 to 70 °C. We find that, similar to the diffusive activation energy, the HEA-water system shows reduced enthalpy and entropy of the transition state compared to HEMA-water, such that there is faster exchange between HEMA and water at all temperatures measured, in addition to more biased populations in the HEA-water system.