Mechanical modeling and testing of different polyamides considering molecular chain structure, crystallinity, and large strains

Polyamide (PA) is a semi-crystalline polymer in which the main chain is configured by the repeating units of amide bonds (-NHCO-). The strength of PA is achieved by intermolecular hydrogen bonding between the hydrogen and oxygen atoms of the amide bonds. In this study, the effects of the density of the amide group and crystallinity were evaluated by uniaxial tensile tests of commercial PA11, PA610, PA6, and PA-MXD10. The maximum stress was proportional to the tensile rate, and the stress-strain curves of the PAs with small crystallinity showed a sharp drop after the maximum stress. In contrast, a gradual change in the stress was observed around the maximum stress of the PAs with high crystallinity. Subsequently, a time-dependent nonlinear mechanical model based on the unfixed molecular chain network (MCN) model is proposed to predict the mechanical behavior of PAs. Because this model was proposed for amorphous epoxies, we modified the unfixed MCN model to reproduce the mechanical response of semi-crystalline PAs. In this study, a constant resistance to debonding in the original unfixed MCN model was replaced by the function of the viscoplastic strain. The modified version of the unfixed MCN model reproduced the gradual softening at a large strain range and the effects of the crystallinity on the mechanical response of the PA.