Assessing the Branching Architecture of Sparsely Branched Metallocene-Catalyzed Polyethylenes Using the Pompom Constitutive Model

Abstract

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 longchain 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.

Extent of branching from linear viscoelasticity of long‐chain‐branched polymers

“…One artifact of this temperature dependence of relaxation time separation is the minor variation of n with temperature. This can be seen in Table 3 from our analysis of the viscoelastic data for the three commercial resins studied by Doerpinghaus and Baird 22, 23. We noted similar behavior in other branched polyethylenes.…”

“…For the majority of the branched polyethylene materials we examined from the published studies, we scanned and digitized the viscoelastic data to evaluate n and S . The linear viscoelastic data published by Doerpinghaus and Baird22 was provided by the authors. Kokko et al25 determined the power‐law parameters themselves, following our previous study 21…”

Analytical Rheology of Polymer Melts: State of the Art

“…Stadler and Münstedt [88,315] sought to use size-exclusion chromatography with multiangle laser light scattering (SEC-MALLS), and shear rheological measurements to analyze viscosity functions of LCB polyethylenes, by separating the influence of long-chain branching and molecular weight on rheology. Doerpinghaus and Baird [316] parameterized the pom-pom model, which describes the nonlinear rheology of BPs semiquantitatively [53,163,165,166,[317][318][319][320], to shear and extensional rheology for different levels of LCB.…”

“…Some of the more sophisticated model-driven techniques allow us to combine heterogeneous data like rheology and molecular weight distribution obtained from multiple detectors with molecular models by exploiting some a priori information about the sample. I have already mentioned several methods which explicitly employ models (whether molecular-based or phenomenological) to interpret experimental data [63,66,316,327,331,333]. The rest implicitly use a model, even though it may only be specified as an empirical correlation, and not really labeled as a "model."…”

Rheological characterization and modeling of linear and branched metallocene polypropylenes prepared by reactive processing

“…Linear viscoelastic data were used to estimate the terminal relaxation time spectra and a new correlation between modulus separation and relaxation time polydispersity was given. Doerpinghaus and Baird [8] used the Pom-Pom model for the examination of the metallocene polyethylenes. The numbers of arms in the Pom-Pom model agreed qualitatively with the dilute solution measurements.…”