Modelling of Elongational Flow of HDPE Melts by Hierarchical Multi-Mode Molecular Stress Function Model.

Leslie Poh,Manfred H Wagner,Esmaeil Narimissa

doi:10.3390/polym13193217

Abstract

The transient elongational data set obtained by filament-stretching rheometry of four commercial high-density polyethylene (HDPE) melts with different molecular characteristics was reported by Morelly and Alvarez [Rheologica Acta 59, 797–807 (2020)]. We use the Hierarchical Multi-mode Molecular Stress Function (HMMSF) model of Narimissa and Wagner [Rheol. Acta 54, 779–791 (2015), and J. Rheology 60, 625–636 (2016)] for linear and long-chain branched (LCB) polymer melts to analyze the extensional rheological behavior of the four HDPEs with different polydispersity and long-chain branching content. Model predictions based solely on the linear-viscoelastic spectrum and a single nonlinear parameter, the dilution modulus for extensional flows reveals good agreement with elongational stress growth data. The relationship of dilution modulus to molecular characteristics (e.g., polydispersity index (PDI), long-chain branching index (LCBI), disengagement time ) of the high-density polyethylene melts are presented in this paper. A new measure of the maximum strain hardening factor (MSHF) is proposed, which allows separation of the effects of orientation and chain stretching.

Highlights

Polyethylene is the most broadly used polymer, with applications ranging from plastics packaging found in daily lives to engineering plastics
The transient elongational data set obtained by filament-stretching rheometry of four commercial high-density polyethylene (HDPE) melts with different molecular characteristics was reported by Morelly and Alvarez [Rheologica Acta 59, 797–807 (2020)]
Results show that the polydispersity of the HDPE samples has a direct relation with their wi2; i.e., an increase in PDI leads to the growth of the mass fraction

Summary

Introduction

Polyethylene is the most broadly used polymer, with applications ranging from plastics packaging found in daily lives to engineering plastics. With processing behaviors of polymers being influenced by their molar mass (Mw), polydispersity (PDI), and long-chain branching (LCB) [8], studying the effects of molecular characteristics and chain architecture on shear and extensional rheological behavior of polyethylene is important in optimizing its processability. Dilution rheology [13,14] can provide quantitative signs of LCB but requires the determination of the molecular weight distribution by SEC, and the method is restricted to long-chain branched polymers with well-defined topography, such as metallocene HDPE. Shroff and Mavridis [24] proposed a long chain branching index (LCBI) based on the ratio between the zero-shear viscosity and the intrinsic viscosity of linear polyethylene. Using the rule-of-thumb relation proposed by Garcia-Franco et al [21], Morelly and Alvarez [23] proposed a simple qualitative index for LCB

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Conclusion