Thermal Expansion and Second-Order Transition Effects in High Polymers: PART II. Theory

The nature of the thermal expansion anomaly in high polymers, known as the second-order transition, is examined in some detail. It is suggested that below the transition temperature, Tm, polymer chains can expand sidewards but not parallel to their length. At Tm lengthwise expansion becomes prominent, thus accounting for the sudden increase in thermal expansion. Experimental results are presented showing the anisotropic expansion of oriented polymers below Tm. The transition effect is then treated as a problem in viscous flow, which gives rise to various semi-empirical plots connecting Tm with applied forces, plasticizer content, and time effects. The brittle point, Tb, involves highly elastic deformation, and is shown to be a fundamentally different test, although Tm and Tb are sometimes numerically equal. Various factors influencing the brittle point are reviewed briefly.