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Structural disorders in the α and γ forms and the iodine complex of nylon-6 and the mechanisms underlying their transitions

Polymer Journal volume 58, pages 137–148 (2026)Cite this article

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Abstract

Nylon-6 exhibits several forms of crystal modification. The α form converts to the iodine complex when it is immersed in a highly concentrated KI/I2 solution. After deiodization in a hypo (sodium thiosulfate) solution, the iodine complex changes to the γ form. The phase transition mechanism from the α to the γ form through the iodine complex has remained a challenging issue because of the lack of established crystal structures. As previously reported (Polymer Journal, 2025), a quantitative analysis of 2D wide-angle X-ray and neutron diffraction data necessitated a revision of the crystal structure of the α form by incorporating up/down chain packing disorder. This paper reports that a similar up/down chain disorder is also present in the crystal lattices of the γ form and the iodine complexes. In both the α and γ forms, the crystal lattice is composed of stacked hydrogen-bonded sheet planes. The aforementioned up/down chain packing disorder can be expressed in terms of the stacking disorder of sheets or the disordered slippages of sheets along the a axis. Thus, the notion of stacking disorder of sheet planes allows for a systematic and logical interpretation of the transition behaviors among these crystalline forms of nylon-6.

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Acknowledgements

This work was performed with support from the staffs of the BIX-3 and i-BIX facilities at JAEA, Tokai, Japan. The TOF experiments were performed under the proposals 2022PX3002, 2023PX3002, and 2024PX3001 of the Ibaraki Prefecture Project. This study was supported by the Grant-in-Aid for Scientific Research (B) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 22H02151).

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Authors and Affiliations

  1. Department of Future Industry-oriented Basic Science and Materials, Toyota Technological Institute, Nagoya, Japan

    Kohji Tashiro

  2. Comprehensive Research Organization for Science and Society (CROSS), Ibaraki Quantum Beam Research Center, Ibaraki, Japan

    Kohji Tashiro, Katsuhiro Kusaka & Terutoshi Sakakura

  3. Aichi Synchrotron Radiation Center Knowledge Hub Aichi, Seto, Aichi, Japan

    Kohji Tashiro

  4. Institute for Quantum Life Science, National Institutes for Quantum Science and Technology, Ibaraki, Japan

    Kazuo Kurihara

  5. Institute for Quantum Life Science, National Institutes for Quantum Science and Technology, Chiba, Japan

    Taro Tamada

  6. Center of Quantum Life Science for Structural Therapeutics, Chiba University, Chiba, Japan

    Taro Tamada

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Correspondence to Kohji Tashiro.

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Tashiro, K., Kurihara, K., Tamada, T. et al. Structural disorders in the α and γ forms and the iodine complex of nylon-6 and the mechanisms underlying their transitions. Polym J 58, 137–148 (2026). https://doi.org/10.1038/s41428-025-01110-z

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