Prof.Bernard Lotz from the Centre National de la Recherche Scientifique (CNRS) in the Institute Charles Sadron in Strasbourg was invited to our institute to give an academic lecture, titled “The structure of nylons. A new approach to an 80 years-old puzzle: the Brill transition.” 

The discovery of Brill transition in polyamides can date back to 1942 in nylon-66, showing a reversible polymorphic phase transition from the triclinic to the pseudohex-agonal structure upon heating. The characteristic feature of Brill transition could be monitored by wide-angle X-raydiffraction (WAXD) performed as a function of temperature, wherein the room temperature spacings of 0.44 nm and 0.37 nm gradually get closer and are replaced by one with a spacing of 0.42 nm at the transition temperature. Over the years since its discovery, the Brill transition has been observed and studied among most types of nylons over a broad temperature range from 120 °C to 180 °C. However, the nature of the Brill transition is still not completely understood.The origin of this transition might be related to librational motions of the methylene segments constrained by the amide linkages acting as pinning points.

Relying on X-ray analysis and solid state NMR, Prof. Lotz has proposed a new approach to explain this 80 years-old puzzle: the origin of Brill transition is induced by dynamic interconversion between two enantiotopic isomers of nylon chains. In his dynamic model, the hydrogen bonding sites are fixed and the CH2s neighboring to amide bonds also move very slowly, while the CH2s in between can librate over a wide angle during increasing the temperature. At the transition temperature, the pleated structure features along the nylon chains flip over and produce enantiotopic isomers. The flipping dynamics is fast enough to result in mixed conformations and only one d-spacing during X-ray measurement. The better understating of the Brill transition by this new approach would not only help optimize the nylon polymer processing conditions, but also applied to similar transitions in other kinds of polymers.