New insights into crystalline transition in nylon 46

Jia-Ru Xu, Shuang Yang, and Er-Qiang Chen

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871, China

Nylon 46 (PA46) is an important engineering plastic. Two crystal structures, monoclinic and triclinic, have been proposed for the α form of PA46. To distinguish which one is more suitable for the melt crystallized PA46, we prepared double oriented samples of PA46 by mechanical rolling at 180 °C followed by annealing at 260 °C. The samples showed the typical α phase diffractions. Combining the two-dimensional (2D) wide-angle X-ray diffraction (WAXD) patterns obtained with the X-ray beam perpendicular and parallel to the chain axis, we concluded that PA46 adopted the monoclinic structure that was determined by Atkins based on the electron diffraction of single crystals grown from solution [1]. On heating, the room temperature α phase firstly transformed into high temperature α phase, and then changed into γ phase which is pseudo-hexagonal [2]. We observed that directly cooling down the sample after high temperature rolling could result in the γ phase retained at room temperature, which was evidenced by 2D WAXD. This room temperature γ phase was a new observation. It was relatively stable and would transform to high temperature γ upon heating. For comparison, we also studied the thermal transition of the melt quenched samples. The obtained amorphous PA46 crystallized in γ phase during heating. The non-oriented γ phase would change into α phase during cooling, showing the typical Brill transition.

[1] Atkins, E.D.T., Hill, M., Hong, S.K., Keller, A. Organ, S. Macromolecules 25, 917 (1992). (link)
[2] Ramesh, C. Macromolecules 32, 3721 (1999). (link)