Study on crystal structure and phase transitions of polyamide 12 via wide-angle X-ray diffraction with variable temperature

Abstract

Polyamide 12 (PA12) is a long carbon chain polyamide with plenty of excellent properties. In this work, PA12 samples were prepared in three ways: solution casting (α-crystal form at 30 °C and α + γ-crystal form at 40, 50, 60, and 80 °C, respectively), melt slow cooling (γ-crystal form), and melt quenching (γ-crystal form), which undergoes isothermal crystallization in the range from room temperature to the temperature close to the melting point. The crystallization behaviors, including the crystal transition, d-spacing, and crystallinity, upon heating and cooling were measured by the wide-angle X-ray diffraction (WAXD) from 30 to 235 °C. The results show that the α-crystal form gradually transforms into γ-crystal form during heating, indicating the occurrence of Brill transition. The γ-crystal form is stable below Tm, whereas the γ-crystal form can transform into γ-crystal form above 110 °C. Only γ-crystal form appears when cooling, and most of the crystallization has been completed before cooling to 170 °C. The investigation provides more theoretical supports of phase transitions in PA12 for deep understanding of the modification, optimization, and application.