Modeling welding of semicrystalline polymers

Thermal welding of polymers involves joining two polymeric objects by putting their melted surfaces into close contact. Upon close contact, polymer chains diffuse across the interface, and crystallization also may happen simultaneously in the case of semicrystalline polymers. There is a limited understanding of how processing conditions control the quality of a weld. From a fundamental theoretical standpoint, three coupled phenomena occur during welding of semicrystalline polymers: heat diffusion, mass diffusion, and crystallization. In this work, we develop a theoretical framework to explain these processes and solve the governing differential equations numerically. Our results for a typical welding process of two PEEK polymers show that the final thickness of the interface developed is mainly controlled by the crystallization time scale: the three processes happen simultaneously until the maximum crystallinity is reached, after which mass diffusion stops completely irrespective of temperature. Further, we found that the lower the sample cooling temperature thinner the interface developed, which is consistent with experiments. This study provides a foundation for the development of theoretical models for more complex polymer binding problems in the industry.