Correlating the diffusion of water to performance in model reverse osmosis polyamides with controlled crosslink densities

Abstract

We systematically reduce the crosslink density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride crosslinking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (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 15 mol % of the difunctional chain extender, we reduce the crosslink density of the network by a factor of two, which leads to an 80% increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30% increase in the salt passage through the membrane. Surprisingly, this leads to a 30% decrease in the overall permeance of water through the membrane. This conundrum is resolved by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. In the highest and lowest crosslink density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest crosslinked 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 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 (5–6) Å, 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 engineering high-performance RO membranes.