Characterization of sodium chloride and water transport in crosslinked poly(ethylene oxide) hydrogels

Three series of crosslinked poly(ethylene oxide) (XLPEO) hydrogel materials were synthesized via UV-photopolymerization of aqueous solutions containing (1) poly(ethylene glycol) diacrylate (PEGDA) (n = 10), (2) PEGDA (n = 13), and (3) mixtures of PEGDA (n = 13) and poly(ethylene glycol) acrylate (PEGA) (n = 7), where n is the number of ethylene oxide groups. The water content in the prepolymerization mixture was varied from 0 to 80 wt.% and resulted in XLPEO hydrogels having equilibrium water contents ranging from 0.3 to 0.8 (v/v). These hydrophilic XLPEO hydrogels are highly water permeable. The NaCl transport properties of XLPEO were studied using direct permeation and kinetic desorption methods, and good agreement between these two methods was observed. Generally, NaCl permeability in XLPEO increased from less than 0.1–2 (×10−6 cm2/s) as prepolymerization water content increased from 0 to 80 wt.%. NaCl permeability also increased with increasing PEGDA chain length and was higher in samples prepared with PEGA in the prepolymerization solution, presumably due to decreases in effective crosslink density. There is a tradeoff between water permeability and water/salt selectivity: materials with high water permeability typically exhibit low water/salt selectivity, and vice versa. NaCl permeability and diffusivity were strongly correlated with free volume in the hydrogels. Free volume was characterized based on both equilibrium water content and positron annihilation lifetime spectroscopy (PALS). In these samples, the equilibrium water content was proportional to the fractional free volume from the PALS measurements.