Microcalorimetric Study of the Temperature-Induced Phase Separation in Aqueous Solutions of Poly(2-isopropyl-2-oxazolines)

Abstract

The effect of temperature on aqueous solutions of poly(2-isopropyl-2-oxazoline) (PIPOZ) samples of molecular weights ranging from 1900 to 5700 g mol -1 was monitored by turbidimetry, high-sensitivity microcalorimetry (HS DSC), and pressure perturbation calorimetry (PPC) from 10 to 80 °C. The polymers were soluble in cold water and underwent phase separation at T CP ∼ 45−63 °C, depending on their molecular weight. The phase transition was endothermic, with an enthalpy change ranging from 0.36 ± 0.01 to 1.16 ± 0.01 kcal mol -1 . The coefficient of thermal expansion of PIPOZ in water (α pol ), determined by PPC, underwent a sharp increase at the temperature corresponding to the onset of phase transition, reaching a maximum value at T ∼ T CP . Microcalorimetric measurements were carried out with solutions of PIPOZ samples in D 2 O and in aqueous NaCl solutions. The thermodynamic and volumetric changes associated with the phase transition of aqueous PIPOZ solutions are compared to those of aqueous solutions of two related polymers, poly(vinylcaprolactam) and poly( N -isopropylacrylamide) (PNIPAM), a polymer structurally related to PIPOZ that undergoes a phase transition in water at ∼31 °C.