Epitaxy in Polybutene-1 Form II-on-Form I Cross-Nucleation Revealed by Nanofocused X-ray Diffraction on Ad Hoc Morphology

The existence of epitaxy in polybutene-1 form II-on-form I cross-nucleation was investigated by nanofocused synchrotron X-ray diffraction. To this aim, form I hedrites, which show a lamellar stacking with a uniform crystal lattice orientation, were adopted as the substrate. The form II crystals develop a spherulitic transcrystalline layer on the lateral surface of the form I substrate, due to cross-nucleation. Through mapping both the intensity and azimuthal orientation of characteristic planes of the two polymorphs, a clearly defined nucleation area between form II (daughter) and form I (parent) could be identified. Comparing the two-dimensional diffraction patterns in that area, a preferred mutual orientation of the two structure is revealed. In particular, the (200)(II) plane of form II and the (110)(I) plane of form I are reciprocally oriented at a fixed angle of similar to 8.5 degrees. This orientation results in almost parallel (110) planes between the two structures. It is calculated that the mismatch between interchain distances within the common (110) planes is about 4%, while that along the chain axes is less than 10%, both well below the accepted mismatch criterion for epitaxial crystallization. These results provide solid evidence for the existence of an epitaxial relationship in the cross-nucleation between polybutene-1 form II and form I. We note that the (110) contact plane between the two structures in cross-nucleation is the same as the one involved in the well-studied solid-state phase transformation from form II to form I. Moreover, the X-ray nanofocus approach and the proposed data analysis could be effectively applied to other cross-nucleating systems to shed light on the role of epitaxy in this peculiar phenomenon of nucleation between polymorphs.