Hydration and Phase Behavior of Poly(N-vinylcaprolactam) and Poly(N-vinylpyrrolidone) in Water

Phase transitions of poly( N -vinylcaprolactam) (PVCL) and poly( N -vinylpyrrolidone) (PVPy) in H 2 O and D 2 O were investigated by turbidimetry, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. The phase diagram of PVCL solution is binodal with a lower critical solution temperature (LCST) of 32.5 °C and a critical concentration of about 5 wt %. PVPy also undergoes a phase separation around 30 °C in aqueous 1.5 M KF solution. The IR spectra of PVCL and PVPy in water critically change at the phase transition temperature ( T p ). Above T p the C−H stretching and the C−H bending bands shift to lower wavenumbers. The amide I band of PVCL is divided into four components, and the intensities of the 1565, 1588, and 1625 cm -1 components increase and that of the 1610 cm -1 component decreases while the phase transition proceeds. Addition of KCl and KF lower T p of PVCL, whereas KI and KBr raise T p . Addition of methanol gradually raises T p up to ca. 40 vol % and steeply raises T p at > 40 vol %. Although these salts do not alter the profiles of IR spectra of PVCL, methanol induces a change in the amide I band.