Brill Transition in Nylons: The Structural Scenario()

Abstract

Pleated and rippled sheet crystal structures of even–even nylons that feature twisted amide group–aliphatic part links were introduced in the previous contribution (Macromolecules, 2021, 54, DOI: 10.1021/acs.macromol.0c02404). Pleated sheets are made of isochiral conformational isomeric stems, and rippled sheets are racemic stereocomplexes of enantiomeric stem conformations. These models are used to reanalyze the various and puzzling manifestations of the Brill transition first reported in 1942. The transition is generally considered to result from the generation of gauche bonds in the aliphatic parts of the nylon α phase which travel to the junctions with the amide groups. Recognition of the pleated/rippled sheet structures suggests a different scenario that takes into account, but challenges, this analysis. The transition takes place in pleated/rippled sheets with twisted chain conformations. It is a dynamic interconversion between the two possible conformers (mirror images) of the stems in the crystal lattice. The interconversion timescale is in the ≈100 picosecond range, as determined by NMR. During the interconversion, the aliphatic part C–C–C plane flips by ≈120° (from + to −60°). The intrasheet H bonds are preserved, but transitory intersheet H bonds may be formed, which accounts for the frequent but not systematic pseudo-hexagonal cell of the Brill structure. This scenario is fully consistent (and more so than the earlier models) with the wide body of experimental data available on the Brill transition. The interconversion between conformers is generic for all types of nylons (even, even–even, etc.) and is therefore a valid scenario for their Brill transitions as well. On this basis, the Brill transition should be defined by a molecular process (the conformation interconversion) rather than by an experimental criterion that lacks general validity (the merging of equatorial reflections).