Thermal Expansion and Second-Order Transition Effects in High Polymers: III. Time Effects

Abstract

It has been suggested that the so‐called second‐order transition in high polymers is not an equilibrium phenomenon, i.e., a true thermodynamic singularity, but rather a rate effect. Confirmation of this point of view has been obtained by determining the equilibrium volume‐temperature curve of polystyrene. Such equilibrium curves exhibit no transition within the range of from 20° to 140°C, whereas the same material shows a transition at about 82°C for rates of heating usual in thermal expansion measurements. Two facts appear in agreement with the viscous flow interpretation of the thermal expansion process: First, two mechanis ms were found to operate, at markedly different rates. One gave almost instantaneous expansion, even at room temperature, whereas the other was quite temperature‐dependent in this region, being extremely slow at room temperature. Second, the activation energy for the slower mechanism was found to be of the order of magnitude of that for viscous flow, considerably lower than the activation energy for rubber‐like elasticity. Volume‐temperature curves at finite rates of heating are discussed in light of these findings.