Structural Origins of Shear Banding in Bidisperse Polymer Melts

LC Ma and YJ Ruan and YY Lu and LJ An, CHINESE JOURNAL OF POLYMER SCIENCE, 43, 2150-2159 (2025).

DOI: 10.1007/s10118-025-3421-8

Shear banding in entangled polymer melts remains a fundamental yet unresolved phenomenon in nonlinear polymer rheology. Here, we perform molecular dynamics simulations of bidisperse entangled melts-comprising equal numbers of chains with lengths N=200 and N=400-to uncover the structural origins and dynamic evolution of shear banding. This bidisperse system amplifies spatial heterogeneities in the entanglement network and facilitates direct comparison with monodisperse melts of N=300, revealing quantitatively consistent steady-state shear stress versus shear rate responses. Notably, a pronounced stress plateau spanning over an order of magnitude in shear rate is observed, within which shear banding emerges reproducibly across independent simulations, as confirmed by systematic velocity profile and interface position analyses. Our findings challenge the prevailing notion that shear banding arises solely from dynamic flow instabilities. Instead, we establish a microstructure-driven framework, demonstrating that shear band nucleation is governed by pre-existing structural heterogeneities- specifically, localized weakening of the entanglement network at short- chain-enriched "soft spots", indicative of a robust microstructural memory effect. During shear start-up, short chains preferentially disentangle and migrate along the shear direction; beyond a critical strain, long chains retract and redistribute away from the fast shear band center to minimize elastic energy. This chain-length-dependent migration dynamically enriches the shear band in short chains, stabilizing its structure and revealing a molecular mechanism that links entanglement heterogeneity to macroscopic flow localization. By bridging molecular-scale structural features with nonlinear rheological responses, this work offers a complementary perspective to classical tube and convective constraint release (CCR) models, highlighting the critical interplay between microstructural heterogeneity and chain migration in the onset and persistence of shear banding.

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