Solute permeation through the polyurethane-NIPAAm hydrogel membranes with various cross-linking densities

A series of thermo-sensitive polyurethane-NIPAAm hydrogel membranes with various cross-linking densities have been obtained by the UV-curable method. Sucrose, β-cyclodextrin and dextran were used as the model compounds to evaluate the solute permeability through the hydrogel membranes. The solute permeability depends on the solute size, the hydration and the cross-link density of the hydrogel. Sucrose permeability has been found following the free-volume theory but with different permeating behaviors between the high- and low-hydration hydrogels. The water structure in the hydrogel has been analyzed by the DSC method. It has been found that the high-hydration hydrogels contain relatively more freezing water, while the low-hydration hydrogels are occupied mainly by the nonfreezing bound water. The different water structures between the high- and low-hydration hydrogels are considered to be responsible for their discrepancy in the permeability.

The solute permeability through the thermosensitive hydrogels depends on the solute size, the water content and the cross-linking density of the hydrogel. The linear relationship of ln (Dm) against the inverse of the hydrogel hydration proves the validity of the free-volume theory. The different behaviors in Kd and ln (Dm) between the highand low-hydration hydrogels indicate that sucrose permeates through both hydrogels with a different mechanism. The water structure in the hydrogel is analyzed by the DSC method. It has been found that the high-hydration hydrogels contain relatively more freezing water including the free water and the freezing bound water, while the low-hydration hydrogels are occupied mainly by the nonfreezing bound water. It is believed that the solute should be solved easily in the freezing water region and prefers diffusing through the freezing water channel in the hydrogel. Acknowledgments The work is supported by National Basic Research Program of China (2003CB615701) and National Natural Science Foundation of China (20476045).