Flow-induced reduction of the monomeric friction coefficient using a branched environment in linear isotactic polypropylene melt

Abstract

In this work, the effect of the stretch Weissenberg number, WiR, on the monomeric friction coefficient is investigated for three entangled linear isotactic polypropylenes (L-PP) and three entangled long-chain branched polypropylene miscible blends (LCB-PP), both groups of polymer melts with a comparable range of the number of Kuhn segments of the whole chain, the number of Kuhn segments between entanglements, the number of entanglements per chain, and the polydispersity index, via free volume measurements using the Williams-Landel-Ferry equation and temperature-dependent high-strain rate rheology (covering the extensional strain rate range of 2 × 104 1/s to 2 × 106 1/s at a Hencky strain of 6.8) via entrance pressure drop measurements in an abrupt contraction flow using the Gibson method. The ratio of the equilibrium monomeric friction coefficient defined by Doi and Edwards, ζeq, to the monomeric friction coefficient for fully aligned chains due to strong uniaxial extensional flow, ζaligned, was found to increase in the following order: L-PPs (5.234 ± 0.8489) < 10 wt % LCB-PP blend 64k (6.249 ± 0.3950) < 20 wt % LCB-PP blend 70k (8.061 ± 0.1851) < 30 wt % LCB-PP blend 78k (11.104 ± 0.8995). The presence of a high-molecular-weight branched environment in low-molecular-weight linear PPs caused a more intense decrease in the monomeric friction coefficient (due to the increased free volume indicating a higher coalignment ability of the macromolecules in the LCB-PP blends compared to that of pure linear PP macromolecules), leading to a decrease in the Trouton ratio at WiR, in which the polymer chains become fully aligned.