Nonlinear shear flow of model entangled polymers : experiments and modeling
(2020)
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- Authors
- Supervisors
- Van Ruymbeke, Evelyne
- Abstract
- Nonlinear rheology of polymer matter is of significant relevance to many industrial and biological processes. In response to large deformations over a range of timescales, polymer chains exhibit nonlinear viscoelastic properties. Example of such properties include shear thinning and normal stress differences. Despite theoretical and experimental advances in the field, the molecular origin of such properties is not yet fully understood. It is therefore, valuable from both a scientific and engineering perspective, to provide further molecular insight and be able to predict the nonlinear viscoelastic properties of entangled polymer matter. In this thesis, we investigate both theoretically and experimentally the nonlinear response of linear polymers under shear flow. Their complex nonlinear behavior is difficult to quantify experimentally and is a known challenge for existing molecular constitutive models. Concerning the theoretical part, we first implement three of the most accurate tube-based models for predicting the nonlinear shear properties of entangled polymers, i.e. the Mead-Larson-Doi model, the GLaMM model proposed by Graham et al., and the Rolie-Poly model proposed by Likhtman and Graham, and confront them to recent experimental data available in literature for polymer melts and solutions. This allows highlighting several issues in the models, often related to the way the polymer chains retraction induced by a high shear flow is taken into account. Based on this analysis, we then modify the TMA tube-based model developed at UCLouvain, in order to account for the influence of fast flow on the chain relaxation and be able to predict the steady-state response of entangled melts and solutions. The very good agreement found between the predictions and the experimental data demonstrates the importance of coupling constraint release effect and chain stretching as well as accounting for flow-induced disentanglement. As a perspective, this approach should be extended to also describe the transient regime. Concerning the experimental part, we measure and study the stress growth coefficient of systematic sets of entangled polymer systems under large shear flow, using the state-of-the-art Cone-Partitioned Plate (CPP) geometry. This geometry minimizes edge fracture effects and thus it provides reliable data. Two distinct classes of model polymers are studied. First, by investigating the properties of a series of model viscoelastic solutions composed of a flexible high molecular weight monodisperse, Polystyrene (PS), diluted in Oligomer styrene, we show that the shear thinning properties are strongly dependent on the long chain concentration and thus, on the equilibration time of an entangled segment. Second, by investigating the shear properties of a family of bidisperse blends composed of $10 wt%$ of long polystyrene chains diluted in different linear matrices, we show that while the dynamics of the polymer matrix does not affect the long chain stretching, it strongly affects the way the steady viscosity varies with the shear rate. This result should therefore help us to control and tune the shear thinning properties of entangled polymers.
- Affiliations
Citations
Taghipour, H. (2020). Nonlinear shear flow of model entangled polymers : experiments and modeling. https://hdl.handle.net/2078.5/121937