Flipping the Pressure- and Temperature-Dependent Cloud-Point Behavior in the Cononsolvency System of Poly(N-isopropylacrylamide) in Water and Ethanol

Abstract

It was demonstrated for several organic cononsolvents added to aqueous solutions of poly(N-isopropylacrylamide) that they lead to an increasing solubility of the polymer in the low-temperature/high-pressure region while reducing the solubility of the polymer and therefore its cloud temperature at atmospheric pressure. A complete inversion of the phase-separation behavior regarding both the effects of pressure and temperature was found by increasing the mole fraction of added ethanol over the whole cononsolvency range. As a special case, at a mole fraction of x = 0.23 the system cleared up independent of temperature at a constant pressure of about 1000 bar. The measurements were performed in an optical high-pressure cell up to pressures of 3000 bar and down to temperatures of -20 degrees C. The results are discussed in terms of solvent-solvent and solvent-polymer interactions.