Evaluation of the relationships in the tensile plastic deformations and the nano-micro structures of PA6 and PAMXD10 with different thermal histories

Abstract

Polyamide (PA) is a semi-crystalline polymer in which the main chain is configured by repeating units of amide bonds (NHCO). The strength of PA is achieved by the intermolecular hydrogen bonds between the hydrogen and oxygen atoms of the amide bonds. The thermal history conditions change the crystallization rate and crystalline structure of the PA, which affects its macroscopic mechanical properties. In this study, the relationship between the nano-micro structure of PAs and their macroscopic plastic deformation behavior was investigated. PA6 and PAMXD10 specimens were prepared with different microstructures obtained from two different crystallization processes, called the “isothermal” and “annealing” conditions, respectively. The microstructures were investigated using differential scanning calorimetry, small angle X-ray scattering (SAXS), large angle X-ray scattering and polarized optical microscopy, and the mechanical behaviors were evaluated using uniaxial tensile tests. The maximum stresses of the stress strain curves for PA6 and PAMXD10 increased with temperature under isothermal and annealing conditions. Although the ductility of PA6 and PAMXD10 obtained under isothermal conditions at a higher temperature decreased, both PA specimens obtained under the annealing condition at a higher temperature elongated without fracture across a large strain range. The SAXS measurements revealed that the nano-micro structures of the brittle specimens in PA6 and PAMXD10 consisted of thick lamellae and thick amorphous phases. In contrast, large and small long-period structures co-existed in the specimen obtained under annealing conditions. This indicates that the presence of a composite structure, in which large and small long-period structures are mixed, inhibits the propagation of fractures in the later stages of deformation.