The phase structures of nylon 6.6 as studied by temperature-modulated calorimetry and their link to X-ray structure and molecular motion

The phase behavior of semicrystalline, dry nylon 6.6 is analyzed on the basis of differential scanning calorimetry, DSC, and quasi-isothermal, temperature-modulated DSC, TMDSC. The data were collected over the temperature range from below the glass transitions to above the isotropization. Based on the contributions of the vibrational motion to the heat capacity, as is available from the ATHAS Data Bank, and the multifaceted new calorimetry, as well as on information on X-ray diffraction, molecular dynamics simulation of paraffin crystals, and quasi-elastic neutron scattering, the following observations are made: (a) beginning at the glass transition temperature of the mobile-amorphous phase (Tg = 323 K), a broadened transition of the semicrystalline sample is observed which reaches to 342 K (Tg = 332.7 K). An additional rigid-amorphous phase, RAF, undergoes its separate, broad glass transition immediately thereafter (340–400 K, Tg ≈ 370 K). (b) The transition of the RAF, in turn, overlaps with increasing large-amplitude motion of the CH2 groups within the crystals and latent heat effects due to melting, recrystallization, and crystal annealing. (c) From 390 to 480 K the heat capacity of the crystals increasingly exceeds that of the melt due to additional entropy (disordering) contributions. Above 440 K, close to the Brill temperature, the heat capacity seems to drop to the level of the melt. (d) If observation (c) is confirmed, some locally reversible melting is present on the crystal surfaces. (e) The increasing large-amplitude motion is described as a glass transition of the crystals, occurring below the melting point, at 409 K. The assumption of a separate glass transition in the ordered phase was previously successful in analyzing aliphatic poly(oxide)s and mesophases. The full description of the globally metastable, semicrystalline phase structure of nylons, thus, needs information on the glass transitions of the two amorphous phases and the ordered phase and the various irreversible and locally reversible order/order transitions and their kinetics.