Polymer Thin Film Necking: Ductility from Entanglements and Plane Stress Condition

The ductility of polymer thin films is critical to many applications such as organic electronics and separation membranes. Large-scale molecular simulations are performed to reproduce the experimentally observed necking, a ductile deformation mode. The simulations show that the morphology of a necked film differs qualitatively from craze fibrils in brittle polymers. The micromechanics of thin film necking are revealed with details transcending the capability of experiments. The free boundary of a thin film promotes the plane stress condition and allows the onset of a neck via strain localization. The underlying entanglement network stabilizes the neck by preventing chain pullout. The strain hardening of entangled polymers in the neck region compensates for the reduction in thickness and supports stable neck propagation under a constant tensile force with no bond breaking. 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. Krupenkin and Fredrickson’s geometric argument based on the extension of entanglement network strands is able to predict the draw ratio, as verified by the topological analysis using the Z1+ package. The ductile thin film necking is consistently observed in the simulations with thicknesses larger than the unperturbed polymer chain size, temperatures below the glass transition, and deformation rates much higher than the limited monomer mobility.