Diffusional behaviour of entangled star polymers

Abstract

The dynamic properties of concentrated solutions and melts of linear polymers are reasonably well understood in terms of the reptation concept, whereby molecules are constrained to curvilinear motion within ‘tubes’ formed by entanglements with their neighbours1–3. Understanding of dynamics of entangled nonlinear polymers, however, is still rather limited. Reptation in such cases is expected to be strongly suppressed, but there is little direct evidence for this, and conflicting models have been proposed4–6. We report here a critical study of the diffusion coefficient D of a series of model linear and three-arm star-branched polymers, diffusing in an entangled linear polymer melt matrix, designed to elucidate dynamic mechanisms for the entangled stars. Measuring D as a function of diffusant degree of polymerization N we find, for the linear molecules, D ∝ N−2, as expected for purely reptative motion1,7–9. For the case of moderately short stars, however, D falls considerably faster than an inverse square power law, and is well fitted by an exponential type relation D ∝ e−αN. There is evidence that for the longest stars used in the present study it is the release of entanglements by reptation of the linear matrix molecules which dominates the dynamics. This is the first direct indication of such ‘tube-renewal’ effects in a system of entangled polymers8.