Coarse-grained molecular dynamics simulations of interdiffusion and thermomechanical properties at the interface of laser powder bed fusion processed thermoplastics

Abstract

The interfacial behaviors of thermoplastic polymers during laser sintering are an important issue in determining the performance of 3D printed products. In this study, by considering the molecular chain microstructure, a coarse-grained molecular dynamic model is developed to simulate the interdiffusion process at the interface of PA 6 particles, and the effects of different degrees of polymerization and fusion temperatures on the fraction-depth tensor of polymer chains are identified. In addition, the impact of varying fraction-depth tensors on the mechanical strength, thermal conductivity, and viscosity of PA 6 are discussed. From the simulation results, it is proposed to divide the microscopic interfacial interdiffusion into four stages according to interfacial behaviors and fraction-depth tensor, i.e., the interface formation stage, the chain movement stage, the interfacial decomposition stage, and the entanglement volume stage. The operating conditions determine the final stage that the interdiffusion process can reach, which leads to the corresponding variation in mechanical strength. When PA 6 has a higher polymerization degree and reaches a higher level of interdiffusion stages, higher mechanical strength can be obtained because of chain self-locking. The viscosity of the polymer, respectively, decreases by 87.76 %, 86.11 %, and 82.05 % for the degree of polymerizations 50, 100, and 200 without chain self-locking; when the chain self-locking occurs, the viscosity decreases by only 66.71 % for the degree of polymerizations 300. The interfacial entanglement has less effects on the thermal conductivity of PA6. This study provides a microscopic understanding of the interdiffusion between laser-sintered thermoplastic particles, which can help screen suitable thermoplastic materials for laser powder bed fusion technology.