Molecular Weight Dependence of the Viscosity of Highly Entangled Polyisobutylene

An unsolved problem in the viscoelastic properties of entangled linear polymers is the viscosity–molecular weight relationship. Experiments indicate a power law, η0 ∝ Ma, with a = 3.5 ± 0.2, whereas the theoretical prediction for “pure reptation” is an exponent equal to 3.0. This discrepancy is ascribed to dynamic modes that compete with reptation so that a cubic dependence on M is observed only at sufficiently high molecular weights. That is, almost all existing experimental data are for polymers with too few entanglements to exhibit η0 ∝ M3. To address this issue, we carried out experiments on polyisobutylene having as many as 430 entanglements per chain. Because reentanglement of polymer melts prepared from solution takes extraordinarily long times (another unsolved problem in rheology), annealing was carried out for 5 years prior to the viscosity measurements. We determined that at all temperatures, η0 was less than the extrapolation assuming the M3.5 dependence of lower molecular weight polyisobutyle...