Effects of ionization of incorporated imidazole groups on the phase transitions of poly(N-isopropylacrylamide), poly(N,N-diethylacrylantide), and poly(N-vinylcaprolactam) in water

The effects of ionization of N-vinylimidazole (VIm) units that are incorporated into poly(N-isopropyl-acrylamide) (PiPA), poly(N,N-diethylacrylamide) (PdEA), and poly(N-vinylcaprolactam) (PVCL) on their phase behaviors in water have been investigated by means of turbidimetry, Fourier transform infrared spectroscopy, and differential scanning calorimetry (DSC). The phase transition temperatures (Tp) of these copolymers increase with the degree of ionization of VIm units, which in turn is dependent on the pH of the solutions. Apparent values of pK(a) for the VIm units that are determined from the pH dependencies of T-p are 5.2, 4.5, and 6.7 for PiPA-VIm, PdEA-VIm, and PVCL-VIm, respectively. Changes in Tp arising from the incorporation of an ionized form of VIm unit into these polymers.(DeltaT(p) =(T-p,T-copolymer - T-p,T-homopolymer)/x where x is the comonomer content in mol %) are calculated to be 1.7, 2.2, and 1.5 degreesC/mol % for PiPA-VIm, PdEA-VIm, and PVCL-VIm, respectively. DSC thermograms for each copolymer measured at a different pH show a linear relationship between the enthalpy of the transition (AH) and the T-p probably because the hydrogen-bonding structure of water that surrounds the hydrophobic moieties of the copolymer is gradually broken with increasing temperature. IR measurements show that profiles of IR bands due to the amide and alkyl groups of these polymers exhibit critical changes at T-p, indicating that the hydration states of these groups change upon the phase separation.