The Brill transition in nylon 6/6 is generally perceived as a fully reversible solid-solid transformation, with the well-separated (100)α and (010)α reflections in the X-ray diffraction profile merging into a fully overlapped peak of (100/010)γ reflections. However, there is an 8% mismatch between densities of γ and α crystals, from which one would not expect complete conversion via solid-solid transformation. Here we report observations via small/wide-angle X-ray scattering (SAXS/WAXS), Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) on kinetic aspects of the Brill transition in nylon 6/6 during (1) cooling at 10 °C/min from a γ-dominated state at 250 °C to room temperature, (2) reheating at 10 °C/min of a “glassy” specimen quenched from the melt state at 280 °C and (3) isothermal annealing at 130 °C for an α-containing specimen cooled (at 10 °C/min) from the γ-containing melt at 250 °C. For successful deconvolution of WAXS profiles upon cooling, two additional diffraction peaks located in between of (100)α and (010)α positions are needed below 200 °C. Two new IR bands, assigned to symmetric and asymmetric CH2 stretching modes, are observed to emerge at 150 °C, which further grow with decreasing temperature. This signifies the incipient formation of α crystals at 150 °C. The SAXS invariant QSAXS continuously increases from 150 to 110 °C instead of a sigmoidal jump, indicating that the formation of high-density α phase is gradual rather than instantaneous. As both FTIR and SAXS evidences indicate formation of α crystals below 150 °C, the precursory split of the characteristic WAXS peak of the γ phase at q ≈ 1.54 Å−1 below 210 °C can only be attributed to an intermediate form of mesomorphic nature; this is also supported by the anomalous temperature dependence of the two WAXS peaks of the mesophase. Via fitting of SAXS profiles to a model of approximately FCC-packed ellipsoids, the nanodomain shape was observed to change from prolate (250–190 °C) to oblate (180–30 °C) which, in combination with the domain size determined from WAXS profiles, suggests growth along the c-axis due to the gauche-to-trans conformation change during γ-to-mesomorphic phase change. For the as-quenched “glass” at room temperature, deconvolution of the WAXS profile indicates coexistance of mesophase and small α crystals. Upon heating, the mesophase content increases at the full expense of the α phase (with additional contribution from the amorphous phase) from 30 to 90 °C, followed by pleaued mesophase content and then trasformation to γ crystals at 180 °C. In the overlapping temperature range of 30 to 110 °C, 2D correlation maps of N-H stretching bands indicate an increase of disordered H-bonds occurring after the decrease of free H-bonds and ordered H-bonds, consistent with the generation of mesophase from both the α crystals and the amorphous phase. During isothermal annealing at 130 °C of the specimen cooled at 10 °C/min from 250 °C, the two WAXS peaks (contributed from both α crystals and the mesophase) shift outward in opposite directions, suggesting increased (100)α and (010)α intensities upon meso-to-α phase transformation. Quantitative deconvolution results show that the increase of α phase content is contributed by both the decreased mesophase and amorphous phase contents. To summarize, observations in this study indicate that there is always a mesomorphic intermediate during transformation between α and γ forms, hence the Brill transition is not a direct solid-to-solid transformation.

The Brill transition in nylon 6/6 is generally perceived as a fully reversible solid-solid transformation, with the well-separated (100)α and (010)α reflections in the X-ray diffraction profile merging into a fully overlapped peak of (100/010)γ reflections. However, there is an 8% mismatch between densities of γ and α crystals, from which one would not expect complete conversion via solid-solid transformation. Here we report observations via small/wide-angle X-ray scattering (SAXS/WAXS), Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) on kinetic aspects of the Brill transition in nylon 6/6 during (1) cooling at 10 °C/min from a γ-dominated state at 250 °C to room temperature, (2) reheating at 10 °C/min of a “glassy” specimen quenched from the melt state at 280 °C and (3) isothermal annealing at 130 °C for an α-containing specimen cooled (at 10 °C/min) from the γ-containing melt at 250 °C. For successful deconvolution of WAXS profiles upon cooling, two additional diffraction peaks located in between of (100)α and (010)α positions are needed below 200 °C. Two new IR bands, assigned to symmetric and asymmetric CH2 stretching modes, are observed to emerge at 150 °C, which further grow with decreasing temperature. This signifies the incipient formation of α crystals at 150 °C. The SAXS invariant QSAXS continuously increases from 150 to 110 °C instead of a sigmoidal jump, indicating that the formation of high-density α phase is gradual rather than instantaneous. As both FTIR and SAXS evidences indicate formation of α crystals below 150 °C, the precursory split of the characteristic WAXS peak of the γ phase at q ≈ 1.54 Å−1 below 210 °C can only be attributed to an intermediate form of mesomorphic nature; this is also supported by the anomalous temperature dependence of the two WAXS peaks of the mesophase. Via fitting of SAXS profiles to a model of approximately FCC-packed ellipsoids, the nanodomain shape was observed to change from prolate (250–190 °C) to oblate (180–30 °C) which, in combination with the domain size determined from WAXS profiles, suggests growth along the c-axis due to the gauche-to-trans conformation change during γ-to-mesomorphic phase change. For the as-quenched “glass” at room temperature, deconvolution of the WAXS profile indicates coexistance of mesophase and small α crystals. Upon heating, the mesophase content increases at the full expense of the α phase (with additional contribution from the amorphous phase) from 30 to 90 °C, followed by pleaued mesophase content and then trasformation to γ crystals at 180 °C. In the overlapping temperature range of 30 to 110 °C, 2D correlation maps of N-H stretching bands indicate an increase of disordered H-bonds occurring after the decrease of free H-bonds and ordered H-bonds, consistent with the generation of mesophase from both the α crystals and the amorphous phase. During isothermal annealing at 130 °C of the specimen cooled at 10 °C/min from 250 °C, the two WAXS peaks (contributed from both α crystals and the mesophase) shift outward in opposite directions, suggesting increased (100)α and (010)α intensities upon meso-to-α phase transformation. Quantitative deconvolution results show that the increase of α phase content is contributed by both the decreased mesophase and amorphous phase contents. To summarize, observations in this study indicate that there is always a mesomorphic intermediate during transformation between α and γ forms, hence the Brill transition is not a direct solid-to-solid transformation.

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