Structure development during stretching and heating of isotactic propylene-1-butylene random copolymer: From unit cells to lamellae

Structure changes in propylene-1-butylene (P-B) random copolymer having low content of butylene concentration (5.7 mol %) during stretching and heating were investigated by combined wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. On the basis of 2D data analysis, two crystal modifications with three orientation modes were determined: γ-phase lamellae with "tilted cross-β" configuration, aligned α-phase lamellae (with respect to the stretching direction) and α-phase daughter lamellae. It was found that γ-phase was unstable and could be transformed into α-phase by stretching. The early stage "four-point" SAXS pattern was attributed to the coexistence of both α- and γ-phase lamellae. The c-axis in the γ-phase was parallel to the lamellar long axis, where corresponding polymer chains were perpendicular to this axis. On the other hand, the c-axis in the α-phase was aligned to the stretching direction. After yielding, the SAXS pattern changed into a "two-point" pattern along the meridian, and was superimposed with two tilted streaks near the equator. The origin of the scattering streak might be due to tilted microfibrils with a mean diameter of about 8 nm, which probably served as nucleation sites for epitaxial growth of lamellar crystals. Lamellar fragmentation was observed from comparison of the lamellar lateral width and crystal size derived from SAXS and WAXD results, respectively. During heating, the SAXS pattern exhibited a longer lifetime than the corresponding WAXD pattern, indicating the existence of two-step melting processes, i.e., the disappearance of lattice coherence in block crystals followed by the diffusion of chains in molten microfibrils to the surrounding matrix. The nonmonotonic behavior of scattering invariants of meridional peaks and equatorial streaks provided some insights into stretching-induced structure changes. The results indicated that when lamellae melted away, microfibrils became more visible to X-ray, causing the increase in streak intensity.