Effects of Ionization of Incorporated Imidazole Groups on the Phase Transitions of Poly(N-isopropylacrylamide), Poly(N,N-diethylacrylamide), and Poly(N-vinylcaprolactam) in Water

摘要

The effect 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 thir phase behaviors in water have been investigated by means of turbidimetry, Fourier transform infrared spectroscopy, and differential scanning calorimetry (DSC). The phase transition temperatures (T_p) 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 T_p arising from the incororation of an ionized form of VIm unit into these polyers (#DELTA#T_p=(T_p,copolymer-T_p,homopolymer)/x, where x is the comonomer content in monl %) are calculated to be 1.7,2.2, and 1.5 deg C/mol % for PiPA-VIm, PdEA-VIm, and PVCL-VIm, respectively. DSC thermograms for each copolymer measured a a different pH show a linear relationship betweeen the enthalpy of the transition (#DELTA#H) 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.