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

Differential scanning calorimetry (DSC) was performed on aqueous solut ions of poly(N-isopropylacrylamide-co-butyl methacrylate-co-X), with X being hydrophilic, hydrophobic, cationic, or anionic comonomers, to e lucidate 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 se paration, which is related to the breaking of hydrogen bonds between w ater molecules surrounding hydrophobic moieties on the polymer, was a linear, decreasing function of the LCST. This suggests that the hydrop hobic interactions between polymer side groups, which are the major dr iving force for phase separation, are enhanced at elevated temperature s due to a decrease in the structuring of water around hydrophobic sid e groups. It is concluded that the changes in LCST caused by the incor poration of comonomers are due to changes in overall hydrophilicity of the polymer and are not due to a direct influence of comonomer hydrop hilicity or charge on the structuring of water around hydrophobic grou ps.