Interactions between crystalline and amorphous domains in semicrystalline polymers: small-angle X-ray scattering studies of the Brill transition in nylon 6,6

The lamellar structures in nylon 6,6 were characterized at several temperatures using synchrotron small-angle X-ray scattering (SAXS) to monitor the changes during the Brill transition at approx. 160°C (T_B). The integrated intensity and the coherence length of the lamellar stacks increase during heating above T_B. In contrast, during cooling, the intensity increases below T_B whereas the coherence length increases up to T_B and remains unchanged upon further cooling. The increase in SAXS intensity observed above T_B during heating is attributed to the enhanced contrast resulting from a decrease in the packing density of the amorphous chain segments. It is suggested that the changes in the packing of the crystalline stems, as they do occur during crystalline phase transitions, provoke a proportionate response in the packing of the amorphous chain segments in the interlamellar regions. The thermal expansion coefficient of the amorphous segments calculated from the X-ray diffraction data above T_B is 5.9 × 10^-4/K (molar thermal expansion coefficient, 854 × 10^-10 m³/(mol K)), and this is in reasonable agreement with the published values.