Structure and mechanical properties of biobased polyamide 11 specimens subjected to different heat treatments

The macroscopic mechanical properties of polyamides depend on their microscopic structural properties, such as the crystalline phase, lamellar thickness, and long period, which change depending on the heat treatment conditions. In this study, the relationships between the macroscopic mechanical properties and lamellar structures of polyamide 11 obtained from different heat treatments were investigated. Heat treatments include quench, isothermal, and quench–annealing conditions. Wide-angle X-ray scattering measurements indicated that different crystalline phases formed depending on the heat treatment conditions. Polarized optical microscopy and differential scanning calorimetry revealed spherulite and crystal morphologies that were not spherulite. Small-angle X-ray scattering revealed that the lamellar thickness and long period increased with the heat treatment temperature. With increasing the long period of lamellae, the number of stress transmitters, which are stress transfer factors (entanglements, loop chains, etc.) between lamellae, increases. The lamellar thickness and long period of polyamide 11 were ~1.5 times greater than that of polyamide 6 under similar heat treatment. In uniaxial tensile tests, polyamide 6 specimens heat-treated at high temperatures fractured and did not elongate. Polyamide 11 elongated without fracture. The thick lamellar and long-period structure of polyamide 11 obtained from heat treatments in this study might explain its excellent elongation until the late stage of deformation. The relationships between the macroscopic mechanical properties and lamellar structures of polyamide 11 obtained from different heat treatments were investigated. From the tensile tests, the maximum stress and strain at necking increase with the heat treatment temperature. WAXS measurements indicated that different crystalline phases formed depending on the heat treatment conditions. SAXS revealed that the lamellar thickness and long period increased with the heat treatment temperature. Based on these results, we elucidated that the macroscopic mechanical properties correlate with the lamellar thickness and long period.