Energetics of phase separation in aqueous solutions of poly(vinyl methyl ether)

Phase separation in aqueous solutions of poly(vinyl methyl ether) was first investigated by high-sensitivity differential scanning calorimetry at normal and high pressures. Thermograms of the phase separation had two singular points: a binodal point (Tt,1) and a point of the three phases coexistence (Tt,2). At normal pressure the temperature Tt,1 decreased slightly and the temperature Tt,2 was constant (36.8 ± 0.1°С) upon increasing the polymer concentration from 0.01 to 0.25%. The transition enthalpy and heat capacity increment did not depend on the polymer concentration (Δth = 88.0 ± 1.2 J g−1; Δtcp = −1.0 ± 0.1 J g−1 K−1). The calorimetric experiments at high pressures were carried out up to pressure 170 MPa in the range of polymer concentrations 0.1–1.1%. When pressure increased the temperature Tt,1 dropped by 10 °C, the temperature Tt,2 increased by 15 °C, the total transition enthalpy reduced 1.5 fold and the transition heat capacity increment remained virtually unchanged. These data reveal that the sequential phase transitions, which the diluted aqueous solutions of poly(vinyl methyl ether) undergo upon heating, have different nature.