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

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.

Small-angle neutron scattering on polymer gels: phase behavior, inhomogeneities and deformation mechanisms

“…This indicates that states of hydrophobic solvation are different in the low-pressure (PoP 0 ) and highpressure (P4P 0 ) regions. Such a reentrant phase diagram was observed not only in water soluble polymers [58][59][60] but also in biomacromolecules. 61 We have discussed the reason for the convexity of the phase diagram in previous work.…”

Thermodynamic study of a pressure‐temperature phase diagram for poly(N‐isopropylacrylamide) gels

“…Recently, the author reported that hydrogels, such as PNIPA gels in water 11 and poly(acrylamide) gels in acetone-water mixtures, 12 underwent volume phase transitions by a change in hydrostatic pressure, the pressure-induced volume phase transition. The pressure-induced volume phase transition can be explained by taking account of the free energy difference derived from the volume change accompanying the hydration of the network polymer chains.…”

Deswelling Microgel Particles Using Hydrostatic Pressure

“…In this context, an alternative to temperature is provided by hydrostatic pressure, which is also known to change the polymer-solvent mixing of pNIPAM gels. [39][40][41][42][43] The influence of pressure over the swelling of microgel particles, however, has never been explored, despite the fact that pressure changes can be achieved in very short times, as they propagate at the speed of sound of the material, and occur homogeneously throughout the sample. The major drawback is, however, the need of a pressure chamber, which often requires substantial technical efforts.…”