Structure‐property studies in polymer melt rheology

Various continuum theories of viscoelastic materials have predicted that extrudates issuing from a die will swell and this phenomenon is well known experimentally. One such continuum theory has been formulated by Pao in terms of relaxation times which can be evaluated independently in other tests. Since relaxation times are qualitatively understood in terms of molecular mechanisms, the swelling of extrudates can be given a molecular interpretation. We have found that the phenomenon originates from entanglement and those molecular motions characterized by relaxation times in the terminal zone of the viscoelastic spectrum. Extrudate swelling has been associated also with the capacity of materials to store energy. Since stored energy is also related to relaxation times, there must exist a functional relation between stored energy and swelling. For this same reason there must also exist a relation between swelling and recoverable strain. We have also presented in this paper some data concerning this latter relation.