Excess entropy and stress-strain behavior of polymeric glasses

The often observed discrepancy between the specific volume data and the thermal data has been explained by recognizing the fact that the free volume fraction is easier to increase than to reduce, because the rate of the “mixing” process depends on the relaxation time associated with the amount of free volume present. Many aspects of tensile behavior of polymeric glasses can and have been explained in terms of strain-induced dilation resulting in the increase of free volume fraction. By introducing the concept that the excess entropy cannot respond quickly to compression, the shear behavior can be attributed to the increase in the free volume fraction and the excess entropy. An equation of state has been introduced which can predict shear behavior from tensile or uniaxially compressive data and vice versa. Effects of temperature, pressure, strain rates, and thermal history can all be accounted for by such an equation of state.