POISSON RATIO IN POLYMER GELS NEAR THE PHASE-TRANSITION POINT

Poisson's ratio, which was obtained by measuring swelling curves of fr ee and radially constrained gels, has been investigated as a function of temperature and ionic strength for N-isopropylacrylamide (NIPA) gel s and as a function of acetone concentration for polyacrylamide gels. It was found that the Poisson ratio exhibits a negative dip near the t ransition temperature for both neutral and ionic NIPA gels. The dip be comes deeper and shifts to a higher temperature as the ionic strength increases. The negative Poisson ratio of NIPA gels suggests that the b ulk modulus is much smaller than the shear modulus as gels undergo the phase transition. The Poisson ratio of acrylamide gel, however, was f ound to be positive throughout the measured region.