Polymorphism in nylon 66

Abstract

The molecular structure of room temperature (RT) nylon 66 and its transition to the high temperature (HT) form have been examined by X-ray crystallographic techniques. An initial literature survey on the RT form revealed a number of anomalies in the spatial relationships of the molecules within the unit cell, namely, poor ethylenic segment packing, distortion of the amide group from a planar configuration and lack of similarity on the projected cell base between RT nylon 66 and RT nylon 6. These anomalies have been resolved, and models proposed which give a better fit between calculated and observed data. The remainder of the paper describes the examination of the HT form with the object of determining whether or not a three-dimensional hydrogen bonded network could exist beyond 180° C. By determining unit cell dimensions and measuring intermolecular hydrogen bond distances it was possible to eliminate certain directions for hydrogen bonded planes. On the basis that any such distance must not exceed about 0.31 nm, which is the maximum hydrogen bond distance reported in the literature, it was concluded that a three-dimensional network could not exist. The cause of the structural transition is attributed to progressive changes in ethylenic segment torsion angles.