Long-Chain Branched Polypropylene: Effects of Chain Architecture, Melt Structure, Shear Modification, and Solution Treatment on Melt Relaxation Dynamics

Polymers with large molecular structures like long-chain branched polypropylene, LCB PP, are prone to a disentanglement phenomenon known as shear modification. Extrusion decreases melt viscosity and elasticity, restored by prolonged melt heating (annealing) or a solution treatment. Here, for LCB PPs and blends with linear isotactic polypropylene, L PP, we study chain architecture, branch content, linear viscoelasticity, the changes caused by shear modification, and recovery thereof in solution. Our LCB PPs are cross-linking products of a linear precursor. The architecture and molar mass distribution of the LCB PPs followed random branching according to percolation theory, with deviations explained by a non-negligible fraction of linear chains. A solvent-insoluble fraction, gel, was indicative of large percolation clusters. Shear modification of our LCB PPs was not fully reversible due to breakage of chains in the high molar mass tail or of even larger structures (percolation clusters) not detected by gel permeation chromatography. We also propose shear modification of LCB PP (i) deforms chain conformations, (ii) perturbs the long-range melt order created by the cross-linking reaction, and (iii) affects mixing quality between linear and branched chains. In solution, we propose recovery mechanisms are chain swelling into spherical conformations and a redistribution of linear and branched chains. Our work shows that the understanding shear modification of branched polymers requires knowledge of content and architecture of all chain species, their molecular mixing quality, and consequently their mutually dependent relaxation mechanisms.