Structure of crystals of nylon 6,3 and related polymers: Folding is stabilized by thermodynamic interactions

The n,3 polyamides have the structure: -(CH₂)n NHCOCH₂ CONH-]×. Due to the stereochemistry of the malonamide unit, these polymers have a unique hydrogen bonding system with two different orientations at 120°: they do not form hydrogen bonded sheets as in conventional polyamides. We have obtained a very well oriented mat from crystals of this polymer which shows up to ten orders of the lamellar spacing. In this paper we analyze the structure of the fold in the crystal surface of nylon 6,3 and in related polyamides, including polyglycine. The thickness of these lamellar crystals is in agreement with the values determined for other polyamides. These results, taken together with some recent findings with other polymers, indicate that the thickness of polymer lamellar crystals may be thermodynamically controlled. An outline of this hypothesis is presented.