Modelling elongational viscosity overshoot and brittle fracture of low-density polyethylene melts

TY - JOUR

T1 - Modelling elongational viscosity overshoot and brittle fracture of low-density polyethylene melts

AU - Wagner, Manfred H.

AU - Narimissa, Esmaeil

AU - Poh, Leslie

AU - Huang, Qian

N1 - Publisher Copyright: © 2022, The Author(s).

PY - 2022/5

Y1 - 2022/5

N2 - The Hierarchical Multi-mode Molecular Stress Function (HMMSF) model predicts the elongational and multiaxial extensional viscosities of polydisperse linear polymer melts based exclusively on their linear viscoelastic characterization and a single nonlinear material parameter, the so-called dilution modulus GD. For long-chain branched (LCB) polymer melts such as low-density polyethylene (LDPE), the HMMSF model describes quantitatively the elongational stress growth coefficient up to the maximum of the elongational viscosity but fails to predict the existence of the maximum and the following steady-state viscosity. By taking into account branch point withdrawal in elongational flow of LCB melts, we extend the HMMSF model and show that the maximum of the elongational viscosity can be characterized by a single additional parameter, the characteristic stretch λ¯ m, while the steady-state tensile stress and the elongational viscosity depend only on the dilution modulus GD as in the case of linear polydisperse melts. Comparison of predictions of the Extended Hierarchical Multi-mode Molecular Stress Function (EHMMSF) model to experimental data of 5 LDPE melts with widely different molecular weights, polydispersities and densities, and a model polystyrene pom-pom polymer shows good agreement within experimental accuracy in constant elongational-rate flow as well as stress relaxation after steady and reversed elongational flow. For the LCB melts considered, we report differences in the specific Hencky strain at the maximum of the tensile stress as quantified by the characteristic stretch λ¯ m, and we discuss correlations between polydispersity, dilution modulus GD, and strain hardening potential of the LDPE melts. We also extend the fracture criterion for brittle fracture of monodisperse polymer melts to the case of polydisperse polymers and find reasonable agreement with experimental evidence.

AB - The Hierarchical Multi-mode Molecular Stress Function (HMMSF) model predicts the elongational and multiaxial extensional viscosities of polydisperse linear polymer melts based exclusively on their linear viscoelastic characterization and a single nonlinear material parameter, the so-called dilution modulus GD. For long-chain branched (LCB) polymer melts such as low-density polyethylene (LDPE), the HMMSF model describes quantitatively the elongational stress growth coefficient up to the maximum of the elongational viscosity but fails to predict the existence of the maximum and the following steady-state viscosity. By taking into account branch point withdrawal in elongational flow of LCB melts, we extend the HMMSF model and show that the maximum of the elongational viscosity can be characterized by a single additional parameter, the characteristic stretch λ¯ m, while the steady-state tensile stress and the elongational viscosity depend only on the dilution modulus GD as in the case of linear polydisperse melts. Comparison of predictions of the Extended Hierarchical Multi-mode Molecular Stress Function (EHMMSF) model to experimental data of 5 LDPE melts with widely different molecular weights, polydispersities and densities, and a model polystyrene pom-pom polymer shows good agreement within experimental accuracy in constant elongational-rate flow as well as stress relaxation after steady and reversed elongational flow. For the LCB melts considered, we report differences in the specific Hencky strain at the maximum of the tensile stress as quantified by the characteristic stretch λ¯ m, and we discuss correlations between polydispersity, dilution modulus GD, and strain hardening potential of the LDPE melts. We also extend the fracture criterion for brittle fracture of monodisperse polymer melts to the case of polydisperse polymers and find reasonable agreement with experimental evidence.

KW - Low-density polyethylene

KW - HMMSF model

KW - Elongational viscosity

KW - Viscosity overshoot

KW - Branch point withdrawal

KW - Brittle fracture

UR - http://www.scopus.com/inward/record.url?scp=85127479291&partnerID=8YFLogxK

U2 - 10.1007/s00397-022-01328-1

DO - 10.1007/s00397-022-01328-1

M3 - 文章

VL - 61

SP - 281

EP - 298

JO - Rheologica Acta

JF - Rheologica Acta

SN - 0035-4511

IS - 4-5

ER -