Demixing and Remixing Kinetics of Poly(2-isopropyl-2-oxazoline) (PIPOZ) Aqueous Solutions Studied by Modulated Temperature Differential Scanning Calorimetry

The demixing and remixing kinetics of aqueous solutions of poly(2-isopropyl-2-oxazoline) (PIPOZ) with four different chain lengths (number-average molar mass of 3300-13000 g mol(-1)) is studied by means of modulated temperature differential scanning calorimetry (MTDSC) in both nonisothermal and quasi-isothermal modes. The nonisothermal measurements show that the aqueous solutions of all four PIPOZ samples follow the lower critical solution temperature (LCST) phase behavior. Both the LCST and the corresponding polymer weight fraction (f(w)) decrease with increasing molar mass: 33.8 +/- 0.1 degrees C at 29.8 wt %, for 3300 g mol(-1) shifts to 26.2 +/- 0.1 degrees C at 19.8 wt % for 13000 g mol(-1). This typical type I phase behavior of aqueous solutions of PIPOZ is in contrast to the typical type II phase behavior of poly(N-isopropylacrylamide) (PNIPAM) which has isomeric repeat units. The demixing and remixing kinetics throughout the phase transition is studied by quasi-isothermal heat capacity measurements. Overall, the response for PIPOZ solutions is markedly faster than for PNIPAM solutions.