Brill transition and melt crystallization of nylon 56: An odd-even polyamide with two hydrogen-bonding directions

Brill transition and crystallization behaviour of nylon 56, a representative polymer of odd-even polyamides, were investigated by simultaneous WAXD and SAXS synchrotron radiation. Nylon 56 crystallized from solution into a peculiar structure where hydrogen bonds were established along the two directions. Nylon 56 experimented on heating a Brill transition that lead to a pseudohexagonal packing and lately to a monoclinic unit cell where neighbouring molecular segments were shifted along the chain axis direction. In disagreement with conventional polyamides, the Brill transition of nylon 56 was not reversible since on cooling the pseudohexagonal arrangement was mainly attained. Optical microscopy studies performed under both isothermal and non-isothermal conditions demonstrated that nylon 56 spherulites had different optical properties than even-even nylons having conventional sheet structures. The birefringence sign changed in the sequence positive-negative-positive when crystallization temperature was decreased.