Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing

TL;DR

This study uses molecular simulations to explore how welding time affects the tensile failure, interfacial entanglements, and fracture energy of polymer interfaces, revealing the critical role of entanglements in interface strength.

Contribution

It provides a detailed molecular-level understanding of how welding time influences failure modes and fracture energy in welded polymer interfaces, linking entanglement density to mechanical strength.

Findings

Interfacial fracture energy increases with welding time as t^{1/2}

Failure mode transitions from chain pullout to chain scission with increased welding time

Interfacial entanglements are crucial for achieving bulk-like fracture energy

Abstract

Large-scale molecular simulations are performed to investigate tensile failure of polymer interfaces as a function of welding time $t$. Changes in the tensile stress, mode of failure and interfacial fracture energy $G_I$ are correlated to changes in the interfacial entanglements as determined from Primitive Path Analysis. Bulk polymers fail through craze formation, followed by craze breakdown through chain scission. At small $t$ welded interfaces are not strong enough to support craze formation and fail at small strains through chain pullout at the interface. Once chains have formed an average of about one entanglement across the interface, a stable craze is formed throughout the sample. The failure stress of the craze rises with welding time and the mode of craze breakdown changes from chain pullout to chain scission as the interface approaches bulk strength. The interfacial fracture energy $G_I$ is calculated by coupling the simulation results to a continuum fracture mechanics model. As in experiment, $G_I$ increases as $t^{1/2}$ before saturating at the average bulk fracture energy $G_b$. As in previous simulations of shear strength, saturation coincides with the recovery of the bulk entanglement density. Before saturation, $G_I$ is proportional to the areal density of interfacial entanglements. Immiscibiltiy limits interdiffusion and thus suppresses entanglements at the interface. Even small degrees of immisciblity reduce interfacial entanglements enough that failure occurs by chain pullout and $G_I \ll G_b$.