Modeling Thermal Welding of Semicrystalline Polymers

Abstract

Thermal welding involves joining two polymeric layers by heating only their surfaces above their melting point and bringing them into intimate contact. While it is generally accepted that chains interdiffuse across the interface and (co)crystallize as the surfaces cool, there is limited understanding of the driving physics. Here, we develop a theoretical framework incorporating three coupled processes─heat transport, mass transport, and crystallization─and illustrate our approach in the specific case of welding two polyetheretherketone (PEEK) layers. We show that the final thickness of the interpenetration region is primarily controlled by the crystallization time scale: all three processes occur until the crystallinity shuts down the mass transport. Furthermore, the lower the bulk temperature, the thinner the interface. These results rationalize experimental findings that the interface strength increases as the bulk temperature is raised toward the melting point from below.