The static bulk and shear modulus of poly (Nhyphen;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 temperatureT0, where the stresshyphen;induced shift ofT0made 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 volumehyphen;deformation mode. The shear modulus, on the other hand, shows only a small anomaly at the transition. The anomaly exhibited by the Poissonrsquo;s ratio sgr; is quite peculiar, i.e., it becomes negative and approaches minus;1 towards the critical point. It is shown that the results can be explained fairly well on the basis of the Floryhyphen;type phenomenological theory extended to take into account the concentration dependence of the interaction parameter.
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