Water content, relative permittivity, and ion sorption properties of polymers for membrane desalination

The interplay of polymer water content, dielectric relative permittivity, and ion sorption properties was studied using a cross-linked poly(glycidyl methacrylate) (XL-pGMA) polymer. The water content of the base XL-pGMA polymer was increased, by partial hydrolysis of the epoxide ring on the polymer side chain, to prepare a series of hydrolyzed XL-pGMA materials. Microwave dielectric spectroscopy, performed from 45 MHz to 26.5 GHz, revealed lower relative permittivity at a given water content compared to Nafion® 117. This observation suggests that knowledge of polymer water content alone may be insufficient for estimating relative permittivity properties of hydrated polymers. State of water analysis suggested that (for polymers that contain similar total amounts of sorbed water) more water in hydrolyzed XL-pGMA interacts with the polymer compared to that in Nafion® 117, and this result may be related to the observed differences in the relative permittivity properties of the two materials. The hydrolyzed polymers were more water/ion sorption selective, important for desalination applications, compared to many uncharged materials reported in the literature but less selective compared to Nafion® 117. Membrane phase mean ionic activity coefficients suggest that Nafion® 117 is more thermodynamically ideal (i.e., the mean ionic activity coefficients are closer to unity) than the hydrolyzed XL-pGMA materials, and thus, ion exclusion in Nafion® 117 is primarily due to Donnan exclusion. Measured static relative permittivity values were used to estimate, via electrostatic theory, the free energy barrier for ion sorption in each material, and these values were qualitatively consistent with values determined using measured ion sorption data. This study suggests that polymer chemistry, not water content alone, influences the relative permittivity properties of hydrated polymers and that relative permittivity measurements can provide qualitative insight into ion sorption properties, which is important for designing advanced desalination membranes.