Flow Behavior of Sparsely Branched Metallocene-Catalyzed Polyethylenes
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The effects that sparse long-chain branching (Mbranch > Mc) has on the shear and extensional rheological properties are analyzed in two separate treatments. The first focuses on the shear rheological properties of linear, sparsely branched, and highly branched PE systems. By employing a time-molecular weight superposition principle, the effects of molecular weight on the shear rheological properties are factored out. The results show that as little as 0.6 LCB/104 carbons (<1 LCB/molecule) significantly increases the zero-shear viscosity, reduces the onset of shear-thinning behavior, and increases elasticity at low deformation rates when compared to linear materials of equivalent molecular weight. Conversely, a high degree of long-chain branching ultimately reduces the zero-shear viscosity. The second treatment focuses on the relationship between long-chain branching and extensional strain-hardening behavior. In this study, the McLeish-Larson molecular constitutive model is employed to relate long-chain branching to rheological behavior. The results show that extensional strain hardening arises from the presence of LCB in polyethylene resins, and that the frequency of branching in sparsely branched metallocene polyethylenes dictates the degree of strain hardening. This observation for the metallocene polyethylenes agrees well with the proposed mechanism for polymerization.
The presence of long-chain branching profoundly alters the melt fracture behavior of commercial polyethylene resins. Results obtained from a sparsely branched metallocene polyethylene show that as few as one long-chain branch per two molecules was found to mitigate oscillatory slip-stick fracture often observed in linear polyethylenes. Furthermore, the presence and severity of gross melt fracture was found to increase with long-chain branching content. These indirect effects were correlated to an early onset of shear-thinning behavior and extensional strain hardening, respectively. Conversely, linear resins exhibiting a delayed onset of shear-thinning behavior and extensional strain softening were found to manifest pronounced slip-stick fracture and less severe gross melt fracture. The occurrence of surface melt fracture appeared to correlate best with the degree of shear thinning arising from both molecular weight distribution and long-chain branching.
The ability to predict the flow behavior of long-chain branched and linear polyethylene resins was also investigated. Using the benchmark 4:1 planar contraction geometry, pressure profile measurements and predictions were obtained for a linear and branched polyethylene. Two sets of finite element method (FEM) predictions were obtained using a viscoelastic Phan-Thien/Tanner (PTT) model and an inelastic Generalized Newtonian Fluid (GNF) model. The results show that the predicted profiles for the linear PE resin were consistently more accurate than those of the branched PE resin, all of which were within 15% of the measured vales. Furthermore, the differences in the predictions provided by the two constitutive models was found to vary by less than 5% over the range of numerical simulations obtained. In the case of the branched PE resin, this range was very narrow due to loss of convergence. It was determined that the small differences between the PTT and GNF predictions were the result of the small contraction ratio utilized and the long relaxation behavior of the branched PE resin, which obscured the influence of extensional strain hardening on the pressure predictions. Hence, it was expected that numerical simulations of the 4:1 planar contraction flow for the mildly strain hardening metallocene polyethylenes would not be fruitful.
- Doctoral Dissertations