Assessing the branching architecture of sparsely branched metallocene-catalyzed polyethylenes using the pompom constitutive model

The model parameters for the multimode differential pompom constitutive equation were determined for densely branched, sparsely branched, and linear polyethylene resins. The versatility and robustness of the pompom model is demonstrated through good rheological predictions in both shear and extensional deformations. The model parameters obtained for the sparsely branched materials indicate that the frequency of long-chain branching dominates the degree of strain hardening observed in uniaxial extension. This finding agrees well with the proposed mechanism of long-chain branching using constrained-geometry metallocene catalysts. The model parameters agree qualitatively with the long-chain branch content determined from dilute solution measurements. Furthermore, the values of the number of pompom arms (i.e., the priority) were found to be quite high even for the sparsely branched materials but were only associated with the longest relaxation times. This suggests that branching for the metallocene systems may be highly concentrated on the longest chains rather than randomly distributed on all of the chains.