V.J. Witherspoon, K. Ito, C.R. Snyder, M. Tyagi, T.B. Martin, P.A. Beaucage, R.C. Nieuwendaal, R.S. Vallery, D.W. Gidley, J.D. Wilbur, D. Welsh, C.M. Stafford, C.L. Soles
National Institute of Standards and Technology,
United States
Keywords: membranes, polyamides, desalination, reverse osmosis, neutron scattering, diffusion, solubility, transport, water
Summary:
We systematically reduce the cross-link density of a polyamide (PA) network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride cross-linking agent with a difunctional isophthaloyl analog, that promotes chain extension, in order to elucidate robust design cues for improving the PA separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating the difunctional chain extender, solid-state NMR methods corroborate that we achieve the desired reduction in the cross-link density, which leads to increased swelling of the PA network in liquid water and an appreciable increase in the salt passage through the membrane. However, this is accompanied by a puzzling decrease in the permeance of water through the membrane. This interesting observation is further investigated by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. As the cross-link density of the membrane decreases, and the solubility of water increases, we observe a significant retardation of the microscopic water diffusion coefficient inside the membrane. This is rationalized in terms of an increase in the free or unreacted amines in the reduced cross-link density membrane, and stronger water-polymer interactions, corroborated by the solid-state NMR measurements. In both highest and lowest cross-link density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits a translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest cross-linked density network, consistent with the reduced water permeance. This is interpreted as water molecules interacting more strongly with the increased free amine content. Over longer length scales the water diffusion is less confined, exhibiting mobility that is independent of length scale. The length scales of confinement from the quasi-elastic neutron scattering experiments at which this transition from confined to translational diffusion occurs is on the order of (5 to 6) angstroms, consistent with complementary X-ray scattering, small angle neutron scattering, and positron annihilation lifetime spectroscopy measurements. The confinement appears to come from heterogeneities in the average inter-atomic distances, suggesting that diffusion occurs by water bouncing between chains and occasionally sticking to the polar functional groups. The results obtained here are compared with similar studies of water diffusion through both rigid porous silicates and ion exchange membranes, revealing robust design cues for realizing high-performance RO membranes.