Fracture of Polymer Glasses with Various Molecular Weights

The molecular origins of the fracture behavior of glassy polymers remain a fundamental problem in polymer science, underpinning the strengthening and toughening of polymer materials. Herein, the fracture of glassy poly(4-tert-butylstyrene) (PTBS) thin films is induced by thermal stress upon rapid cooling of molten films, and the brittle-to-ductile transition is investigated across different molecular weights (M). Results indicate that cracks form under low stress in marginally or nonentangled polymers with M ⩽ 2Me (Me: molecular weight of chain strands between two neighboring entanglement points); craze fibrils form under high stress in moderately entangled polymers with 4Me ⩽ M < 10Me, with craze failure occurring when fibrils break down through chain disentanglement and pull out from the craze–bulk interface; and dense arrays of stable craze fibrils form in highly entangled polymers with M ⩾ 10Me, with failure not occurring until the polymer chains undergo scission. The study provides an experimental demonstration of the mechanism underlying the fracture of glassy polymers, showing that chain length defines the fracture characteristics of the polymers through entanglements.