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

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 (alpha-crystal form at 30 degrees C and alpha + gamma-crystal form at 40, 50, 60, and 80 degrees C, respectively), melt slow cooling (gamma-crystal form), and melt quenching (gamma-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 degrees C. The results show that the alpha-crystal form gradually transforms into gamma-crystal form during heating, indicating the occurrence of Brill transition. The gamma-crystal form is stable below T-m, whereas the gamma-crystal form can transform into gamma-crystal form above 110 degrees C. Only gamma-crystal form appears when cooling, and most of the crystallization has been completed before cooling to 170 degrees C. The investigation provides more theoretical supports of phase transitions in PA12 for deep understanding of the modification, optimization, and application.