A Fresh Look at the Structures of Nylons and the Brill Transition

The common knowledge of the crystal structures of nylons dates back to the early 1950s with the work of Bunn and Garner.It describes the packing of sheets made of hydrogen-bonded stems in an extended chain conformation.These alpha phase structures have a specific powder X-ray diffraction pattern with reflections at 4.4Åand 3.7Å.On heating,reflections trans-form progressively and merge at≈4.2Åat the so-called“Brill transition”.Other diffraction patterns have been recorded for different types of nylons and thermal histories.These patterns were interpreted only as indicating the existence of“variants”of the alpha phase.However,neither their structure,nor the origin of the Brill transition were established.Recent structural analyses and molecular modeling approaches have provided new insights in these long-standing structural puzzles.The“variants”feature chain conformations that are“pleated”.The Brill transition does not involve the standard extended chains of the alpha phase but corresponds to a dynamic interconversion(≈10^(10)/s)between mirror conformations of these pleated stems.