Premelting Crystalline Relaxations and Phase Transitions in Nylon 6 and 6,6

Variable-temperature XRD and NMR measurements show that nylon 6 (N6) undergoes crystalline relaxations between the glass transition temperature and the melting point. These relaxations bring about a crystalline transition between 80 and 170 °C from a monoclinic structure to a new crystalline structure, which is also most likely monoclinic. We thus demonstrate, for the first time, the presence of a Brill transition in N6, a phenomena that has been extensively studied in nylon 6,6 (N6,6). In both N6 and N6,6, the integrity of the Η-bonded sheets is maintained during this transition and up to the melting point. The low- and high-temperature crystalline phases coexist during this transformation in both polymers, and the transition from the low-temperature to the high-temperature phase is diffuse, occurring over a temperature range of ca. 100 °C. Accelerated structural changes observed in N6 upon annealing above 170 °C, and the maxima in shrinkage and shrinkage stresses observed at ~18O °C in N6 and N6,6, are attributed to the onset of relaxation processes which produce these observed transitions. The relaxation is attributed to small torsional relaxations of the methylene segments pinned by the amide moiety. The possible high-temperature structures resulting from such oscillations are discussed.