Dynamics of Entangled Polymer Chains

Abstract

Polymer melts flow like conventional (viscous) liquids if they are subjected to slowly varying perturbations; but, at somewhat higher frequencies w, they behave like an elastic rubber. These mechanical properties depend strongly on the polymer structure (linear/branched, flexible/rigid, etc). In this review, we concentrate on the best understood case: linear, flexible chains, with a large number (N) of monomers per chain. The crucial feature is that these chain.s are entangled. The effects of entanglements on the mechanical properties of polymers have been recognized and analyzed in some detail. More recently, entanglements have been probed at a more microscopic level: (a) self-diffusion, follow­ ing the motions of one labeled chain; (b) measurements involving spatial effects at distances smaller than the coil size: polymer/polymer welding; spinodal decomposition of blends; neutron measurements on stretched samples; chemical reactions in melts. We discuss all these phenomena within a single model, based on the reptation idea (1). This model is not final, and may well require future adjustments, but it gives a comparatively simple framework with which to unify these very different types of experiments. The viscoelastic properties of polymers have been measured with great care, beginning with the pioneering work of 1. Ferry (2). These properties display a certain characteristic time 7, which increases very rapidly with the molecular weight (or equivalently, with the number N of monomers per chain)