Differential scanning calorimetry (DSC) was performed on aqueous solutions of polyCZVisopropylacrylamide-co-butyl methacrylate-co-X), with X being hydrophilic, hydrophobic, cationic, or anionic comonomers, to elucidate the mechanism of temperature-induced phase separation and the effect of comonomer content, hydrophilicity, and charge on the lower critical solution temperature (LCST). The endothermic heat of phase separation, which is related to the breaking of hydrogen bonds between water molecules surrounding hydrophobic moieties on the polymer, was a linear, decreasing function of the LCST. This suggests that the hydrophobic interactions between polymer side groups, which are the major driving force for phase separation, are enhanced at elevated temperatures due to a decrease in the structuring of water around hydrophobic side groups. It is concluded that the changes in LCST caused by the incorporation of comonomers are due to changes in overall hydrophilicity of the polymer and are not due to a direct influence of comonomer hydrophilicity or charge on the structuring of water around hydrophobic groups.

mol %. Copolymerization in dioxane (50 w/v %) with EGDMA as a cross-linker (0.01 mol of EGDMA/mol of monomer) and BPO as an initiator (1.3 X 10-3 mol of BPO/mol monomer) was performed as previously described.10

A new method for separation of molecules of different size is presented. The method is a useful addition to conventional separation methods which depend mainly on gel permeation chromatography using size exclusion. In the new method, hydrogel membranes are used which swelling level can be thermally controlled. In this study, a crosslinked poly(N-isopropylacrylamide-co-butylmethacrylate 95: 5mol%) membrane is used and three solutes of distinct molecular size: two dextrans with molecular weights of 150,000 and 4,400 g/mol respectively and uranine with a molecular weight of 376 g/mol. The swelling of the membranes as function of temperature was measured as well as the influence of the swelling level on the permeability of the three solutes. The influence of the swelling level and the solute size on the permeability was as expected from the free-volume theory. Based on these permeability phenomena, separation was performed in a continuous way by varying the membrane swelling at the appropriate time. A linear relationship between inverse membrane hydration and solute diffusion was found for uranine and dextran (MW= 4,400)) indicating the validity of the free-volume theory.

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