EFFECT OF COMONOMER HYDROPHILICITY AND IONIZATION ON THE LOWER CRITICAL SOLUTION TEMPERATURE OF N-ISOPROPYLACRYLAMIDE COPOLYMERS

Differential scanning calorimetry (DSC) was performed on aqueous solutions of poly(N-isopropylacrylamide-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.