Aliphatic polyamides derived from odd diamine or odd dicarboxylic acid units cannot adopt a conventional sheet structure when molecular chains have an all trans conformation. However, typical fiber diffraction patterns of this sheet structure were observed in several polyamides derived from odd units such as nylons 65 and 56. Consequently, a new structure based on the establishment of intermolecular hydrogen bonds along two different directions was postulated. Real-time temperature dependence of X-ray diffraction patterns for nylons 65 and 56 was studied by synchrotron radiation to gain understanding of the Brill transition usually occurring in polyamides. Significant differences were found between the temperature dependence of both nylons, although a transition towards a monoclinic structure characterized by a single equatorial reflection around 0.425-0.420 nm was observed to occur at high temperatures. This transition was reversible for nylon 65 only. In this case, a characteristic hysteresis effect was found.

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