Phase transition in even-even nylon crystals

Temperature dependence of crystal structures is examined with X-ray diffraction at normal pressure or elevated pressures for solution-grown nylons with octamethylene moiety between amino or carboxy groups (nylons 82, 410, 610, 810). Although octamethylene moiety between amino groups has the potential to cause the phase transition, the low melting point veils the transition appearance. Considering this comprehensively with previous results, it is concluded for all even-even nylons, that the phase transition in the two-dimensional methylene layer system can be experimentally observed only in nylon 6Y (Y less than or equal to 8) crystals at normal and/or high pressures, where the hexamethylene moiety between amino groups plays an intrinsic role. The so-called 'Brill transition' widely reported for melt-crystallized nylons is not a phase transition but an abrupt thermal expansion.