Chain alignment and welding of polymer interfaces

Fused Filament Fabrication (FFF) is an increasingly popular method for manufacturing of materials. In FFF, layers of polymer are extruded from a nozzle on top of previously deposited layers and bond to them by interdiffusion and formation of entanglements across the interface. The extrusion process in FFF is usually fast enough to align the polymer chains, giving rise to non-equilibrium effects that may alter the entanglement structure at the weld. We perform large scale molecular dynamics simulations of polymer welding. Melts of bead-spring FENE chains in equilibrium or after alignment under simple shear are joined and allowed to interdiffuse. We examine how chain retraction and entanglement evolution affect interdiffusion. After welding, we quench the system below the glass transition temperature and simulate tensile fracture, characterizing craze formation in the materials and strength of the interface for different welding times and degrees of alignment. The results provide insight into the molecular mechanisms responsible for fracture in polymer glasses with aligned chains, relating the degree of alignment to the competition between chain pullout and scission.