Modeling the nonlinear shear rheology of linear polymer melts

Details

Supervisors

Van Ruymbeke, Evelyne

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

The rheology of polymer melts is a big concern for the industry. Indeed, contemporary processes as extrusion involve the knowledge of fluid properties like their viscosity or their stress response when a deformation is applied. In this master thesis, we focus on shear flow. The knowledge of the evolution of the viscosity of the fluid to which a constant shear rate is applied is important in order to have an appropriate parametrization of the processes. This is the objective of this work. More specifically, we would like to understand and model the viscoelastic properties of well-defined monodisperse linear polymer melts. To this end, the tube-based Time Marching Algorithm (TMA) model developed at UCLouvain is implemented and further developed in order to capture the specificities of the nonlinear shear regime of deformation. This last year, first attempts to use it in the nonlinear regime have been discussed. The first improvement made by Taghipour et al. concerned the description of the steady state response of entangled melts. The predictions of the TMA model accounting for this new contribution shows a good agreement with the experimental data in the steady regime for PS133k and PS185K. Thus, the first objective of the present work was to reproduce these results. On this base, we further develop the model to capture the overshoot observed in the transient response of the stress growth coefficient. We start from the assumption that the overshoot can be seen as the transition of the chain from fully entangled to partially entangled state. An expression of the disentanglement time is introduced and leads to an equation representing the proportion of entanglements that remain from the initial state of the chain. As a first approximation, this function was considered as a decreasing exponential involving a prefactor. Despite its simplicity, this approach gives good results for the description of the overshoot observed in the transient viscosity of PS133K in a large range of shear rates but not as good results for PS185k at large shear rates. In order to further explore the possible origin of the discrepancy, we test the model developed by Xie and Schweizer, who introduce a force imbalance condition defining the possibility for a polymer chain to retract. As it presents similitude with the previous approach, we implemented it in the TMA model. Unfortunately, the results obtained were not good as both the position and the value of the overshoot were overestimated. Finally, the influence of the different parameters involved in the expression of the probability of entanglement is studied in details. This allows to highlight the importance of the prefactor and to obtain a good description of the overshoot observed in the transient response of the stress growth coefficient.

Citations

• APA

Dalne, M. (2022). Modeling the nonlinear shear rheology of linear polymer melts. https://dial-mem.test.bib.ucl.ac.be/handle/2078.2/26714