| Abstract
| - The effects of ionization of N-vinylimidazole (VIm) units that are incorporated into poly(N-isopropylacrylamide) (PiPA), poly(N,N-diethylacrylamide) (PdEA), and poly(N-vinylcaprolactam) (PVCL) on theirphase behaviors in water have been investigated by means of turbidimetry, Fourier transform infraredspectroscopy, and differential scanning calorimetry (DSC). The phase transition temperatures (Tp) ofthese copolymers increase with the degree of ionization of VIm units, which in turn is dependent onthe pH of the solutions. Apparent values of pKa for the VIm units that are determined from the pHdependencies of Tp are 5.2, 4.5, and 6.7 for PiPA−VIm, PdEA−VIm, and PVCL−VIm, respectively. Changesin Tp arising from the incorporation of an ionized form of VIm unit into these polymers (ΔTp = (Tp,copolymer− Tp,homopolymer)/x, where x is the comonomer content in mol %) are calculated to be 1.7, 2.2, and 1.5 °C/mol% for PiPA−VIm, PdEA−VIm, and PVCL−VIm, respectively. DSC thermograms for each copolymermeasured at a different pH show a linear relationship between the enthalpy of the transition (ΔH) andthe Tp probably because the hydrogen-bonding structure of water that surrounds the hydrophobic moietiesof the copolymer is gradually broken with increasing temperature. IR measurements show that profilesof IR bands due to the amide and alkyl groups of these polymers exhibit critical changes at Tp, indicatingthat the hydration states of these groups change upon the phase separation.
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