Softening of bulk modulus and negative Poisson’s ratio near the volume phase transition of polymer gels

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Abstract
The static bulk and shear modulus of poly (N‐isopropylacrylamide)/water gel (NIPA gel) have been obtained as a function of temperature by measuring the change of sample size induced by an applied tension. It is deduced that the method gives the static elastic moduli of gels to a good approximation except just above the discontinuous transition temperature T0, where the stress‐induced shift of T0 made the estimation of elastic strain impossible. Two kinds of NIPA gels with different polymer concentration and crosslinking density were used as samples, of which one undergoes a discontinuous volume phase transition and the other a continuous one. In both gels, the bulk modulus softens drastically towards the transition showing that the soft mode of this transition is a uniform and isotropic volume‐deformation mode. The shear modulus, on the other hand, shows only a small anomaly at the transition. The anomaly exhibited by the Poisson’s ratio σ is quite peculiar, i.e., it becomes negative and approaches −1 towards the critical point. It is shown that the results can be explained fairly well on the basis of the Flory‐type phenomenological theory extended to take into account the concentration dependence of the interaction parameter.