Poisson’s ratio in polymer gels near the phase-transition point

Poisson's ratio, which was obtained by measuring swelling curves of free and radially constrained gels, has been investigated as a function of temperature and ionic strength for N-isopropylacrylamide (NIPA) gels and as a function of acetone concentration for polyacrylamide gels. It was found that the Poisson ratio exhibits a negative dip near the transition temperature for both neutral and ionic NIPA gels. The dip becomes deeper and shifts to a higher temperature as the ionic strength increases. The negative Poisson ratio of NIPA gels suggests that the bulk modulus is much smaller than the shear modulus as gels undergo the phase transition. The Poisson ratio of acrylamide gel, however, was found to be positive throughout the measured region.