Volume-phase transition of N-isopropylacrylamide gels induced by hydrostatic pressure

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Abstract
Pressure-induced volume phase transitions in neutral and ionized N-isopropylacrylamide-water (NIPA) gels were investigated. When the pressure is increased, the gels which shrink at atmospheric pressure undergo two volume phase transitions. One is the continuous phase transition (for neutral gels) or the discontinuous phase transition (for ionized gels) from the shrunken state to the swollen state at low pressures of several tens MPa. Another is the gradual shrinking transition occurred in the high pressure region above 100 MPa. The former transition shows that the derivative of the pressure with respect to the volume at constant temperature, (∂P/∂V)T, is positive. The results are explained by considering the pressure effects on the hydrophobic interaction. The phase transition pressure increases with increase of the temperature, and the critical pressure and temperature exist. The P–T coexistence curve resembling the evaporation curve of a liquid–gas system is obtained. These pressure-induced phase transitions are qualitatively explained by taking account of the pressure effects on the hydrophobic and hydrophilic interactions. It is assumed that the polymer solvent interaction parameter χ is a function of the pressure as well as the temperature, χ(T,P)=χ0(T)+χ1(T)P+χ2(T)P2. χ1 represents the pressure effects on the interaction between water molecules and the hydrophobic parts of NIPA, and it is responsible for the phase transition at low pressure. χ2 represents the pressure effects on the association between water molecules and the hydrophilic parts of NIPA, and it is responsible for the gradual shrinking transition at high pressure.