Molecular Simulations of Polymer Thin Film Necking: Ductility From Entanglements and Plane Stress Condition

Document Type

Conference Proceeding

Publication Date



Polymer Science and Engineering


Recent advances in the nanotechnology of fabricating and characterizing polymer thin films have enriched conventional thermoplastic mechanics. One intriguing observation is a polymer that is brittle in the bulk state, such as polystyrene, exhibits ductility in the thin film state via the shear deformation resulting from necking. To reveal the microscopic picture of necking in a glassy polymer thin film, large-scale coarse-grained molecular simulations are performed. The simulations demonstrate that stable necking relies on an entanglement network that prevents the catastrophic chain pullout. The neck propagates under a constant tensile force, for which strain hardening in the necked region results in higher local stress that compensates for the reduction in the thickness. The necked film is perforated with voids, exhibiting a morphology different from fibrils in crazing, which is a brittle deformation mode unique to glassy polymers. The replacement of crazing with necking and thereby an enhanced ductility is facilitated by the free boundary that promotes plane stress. Despite the critical role of entanglements, the width of the neck is much larger than the entanglement spacing. The Considère construction predicts well the onset of necking but not the draw ratio of necked polymers, where voids break down the conservation of volume. A simple geometric argument based on the extension of entanglement strands is able to relate the draw ratio in necking to that in crazing.

Publication Title

APS March Meeting 2024