Weld Formation Between Polymer Films Prepared at Different Temperatures: Insights from Molecular Dynamics Simulations

Inspired by Fused Filament Fabrication (FFF) Additive Manufacturing (AM), we use Molecular Dynamics (MD) simulations to investigate the early stages of the formation of the weld between two polymer films prepared at different temperatures - one above and one below the dilatometric glass transition temperature. We identify three stages of welding: (i) surface approach and formation of the initial contact, (ii) surface adjustment, and (iii) interdiffusion. Surface interactions affect film roughness, polymer conformation, and interfacial temperature during the initial stage. As the two layers come into contact, heat transfer equilibrates the system in an asymmetric way: the hot film cools down more slowly than the cold film heats up. When the films are allowed to exchange heat with the environment, most of the effects of the temperature difference at the interface terminate during the initial surface adjustment, before polymer interdiffusion begins at around the bulk Rouse time. However, if the films are isolated, the onset of interdiffusion occurs earlier for films prepared at different temperatures compared to films prepared at the same temperature. This indicates the importance of thermal relaxation across the interface between welding films, and suggests mechanisms to improve the weld strength.