Revisiting the Thermal Transition of β-Form Polyamide-6: Evolution of Structure and Morphology in Uniaxially Stretched Films

Structure and morphology evolution of the uniaxially stretched polyamide-6 (PA6) film with the β-form upon heating were investigated mainly by synchrotron two-dimensional (2D) wide-angle X-ray diffraction and small-angle X-ray scattering. For comparison, thermal transitions of the oriented PA6 samples with the α- and γ-form were also monitored, which undergo "incomplete Brill transition" and direct melting, respectively. Using the results of oriented α- and γ-PA6 as the references, we confirm that the stretched β-PA6 is mesomorphic, which can be viewed as a solid mesophase with smectic B-like structure. We identify that upon heating the stretched β-PA6 reorganizes dominantly into γ-form crystal with the chain orientation unchanged, accompanied by the formation of a little amount of α-PA6 crystallites. Further heating to above the melting temperature of γ-PA6, the α-PA6 crystals grow through a recrystallization process. The lamellae resulting from reorganization exhibit a distribution of orientations, and the α-PA6 lamellae formed at high temperatures have the lamellar basal surface normal parallel to the stretched direction. We consider that the abundant hydrogen bonds in the stretched β-PA6 film construct a network, providing the confinement effect to reduce the chain mobility and thus favor the formation of γ-form. The lamellae with the basal surface normal tilted relative to the stretched direction can also be attributed to the hydrogen-bonded network of oriented chains.