Rheology and Structure of Entangled Telechelic Linear and Star Polyisoprene Melts

We investigate the linear viscoelastic response of model telechelic linear and star (of varying functionality) polyisoprene melts with different molar masses above the entanglement limit in relation to their structure. We find that these systems self-assemble as a result of the strong dipolar interactions and form clusters that seem to depend primarily on the number of dipolar groups per star. The dynamics is rather complex, but some pertinent features are observed: the segmental dynamics is affected by the telechelic functionalization, especially for short arm lengths; this reflects the change of microstructure (and thus glass-transition temperature) with functionalization. The terminal relaxation is much slower compared to similar nonionic stars, reflecting the relaxation of clusters. Linear semitelechelic polymers (with only one end functionalized) aggregate in a star-like fashion. We further develop a tube model based on the time-marching algorithm for stars and linear chains, where we incorporate the association status of the chains via the dipolar interactions at each time step. The agreement of the predictions with the data, using two adjustable parameters (the average times when two dipolar pair remain associated or free, respectively), is remarkable and suggests design criteria for forming desired supramolecular assemblies.