Transmission of water and ions through crosslinked hydrophilic membranes

In order to develop better semipermeable membranes for reverse osmosis, the mechanism of water transport must be determined. An evaluation of existing theories led to the hypothesis that degree of crosslinking and polar group content of a polymer were important factors in determining its semipermeability.

This hypothesis was tested by preparing a series of crosslinked reinforced mem branes by polymerization in a thin film between two plates. These membranes were copolymers of ethylene glycol monomethacrylate (EGM), ethyl methacrylate (EM), and ethylene glycol dimethacrylate (EGD). High-pressure reverse osmosis experiments with a sodium chloride solution and water sorption tests were then conducted and the results were correlated with the degree of crosslinking and hydroxyl content of the membranes.

For copolymers of EGM and EGD, the degree of salt rejection rose as the membrane was more tightly crosslinked, then dropped off sharply. The rise was attributed to a decrease in translational motion of the polymer chain segments relative to one another, with associated hindrance to ion diffusion. The fall was due to monomersolvent incompatibility during polymerization with attendant precipitation of the polymer forming under these conditions, resulting in macrovoids in the membrane structure. As a result, water flux increased as the membrane was more highly crosslinked. Crosslinking had a great effect on water sorption.

Copolymers containing EM showed virtually none of the semipermeability of EGM polymers. Water flux dropped off very rapidly, and salt rejection was not measurable. Perhaps most striking was the fact that water sorption tests showed a direct proportionality between hydroxyl content of the polymer and water sorption capacity.