Tailoring the crystalline structures and mechanical properties of polyamide 1012 specimens by material extrusion-based additive manufacturing

In this work, the evolution of the crystalline structures of MEAM-printed polyamide1012 (PA1012) parts with the nozzle temperatures (T-n) and thermal annealing temperatures (T-a) was investigated. It is found that T-n plays an important role in determining the crystalline structure of PA1012 parts. The freshly-printed PA1012 parts crystalize as gamma crystals at T-n lower than 230 degrees C. With T-n increasing to above 230 degrees C, a small amount of alpha phase coexists with gamma phase in the PA1012 parts. Also, the thermal annealing treatment can tailor the crystalline structure of the MEAM-printed PA1012 parts. Specifically, the crystalline structure exhibits a transformation from gamma phase to alpha crystal, after annealing the PA1012 parts at a T-a higher than 90 degrees C for 10 min. Due to the presence of a high content of gamma phase, the Brill transition occurs at a low temperature for the MEAM-printed PA1012 parts, implying a close relationship between the H-bond sheet structures and the formation of gamma phase. Moreover, the crystalline structure exerts a significant influence on the mechanical properties of the PA1012 parts. The samples with only gamma phase exhibit much higher tensile strength and Young's modulus, but much lower ductility, compared to those containing only alpha crystal. Hence, this work provides effective pathways for regulating crystalline structures as well as mechanical properties of MEAM-printed long-chain polyamide materials.