Mesomorphic Intermediate Stages During Brill Transition of Nylon 6/6

The Brill transition in nylon 6/6 is generally perceived as a fully reversible solid–solid transformation between a low-temperature triclinic lattice (α-phase) and a high-temperature pseudohexagonal lattice; however, there is a ca. 7% density mismatch between these two structures, and so one would not expect a complete solid–solid transformation. The kinetic aspects of the Brill transition in nylon 6/6 during (1) cooling from the dominantly pseudohexagonal state and (2) heating of glassy specimen quenched from the molten state were analyzed via in situ small-/wide-angle X-ray scattering (SAXS/WAXS), Fourier-transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). For successfully deconvoluted WAXS profiles upon cooling, two additional reflections located between (100)α and (010/110)α positions are necessary when the temperature is below 200 °C. The FTIR spectra signify the incipient formation of α-crystals at 150 °C, and the SAXS invariant progressively increases from 150 to 110 °C instead of a sigmoidal jump, which indicates a gradual formation process of a high-density α-phase. Thus, the precursory split of the characteristic peak of a high-temperature phase in WAXS profiles can only be attributed to an intermediate form of mesomorphic nature. Upon heating the quenched glassy specimen, the mesomorphic phase content increases at the full expense of the α-phase (with additional contribution from the amorphous phase), followed by the plateaued mesomorphic phase content, which then transformed into pseudohexagonal structures at 180 °C. The 2D correlation maps of N–H stretching bands also give consistent results, which indicate that the mesomorphic phase is generated from both the amorphous phase and α-crystals. Both cooling and heating processes indicate the existence of a mesomorphic intermediate between high-temperature pseudohexagonal and low-temperature triclinic (α) phases; hence, the Brill transition in nylon 6/6 is not a direct solid-to-solid transformation, but a continuous melting–recrystallization process. This bears significance in addressing mechanical properties of nylons in the vicinity of Brill transition temperature.