Nucleation Mechanism for Form II to I Polymorphic Transformation in Polybutene-1

By means of in situ small-/wide-angle X-ray scattering (SAXS/WAXS) techniques, nucleation mechanism for Form I crystals of polybutene-1 (PB-1) has been investigated by heating the early-stage polymorphic transformation specimens (from Form II to Form I) above the nominal melting temperature of Form II crystals. Upon selective melting of Form II crystals at 124 °C, the SAXS curves of the few surviving Form I nanocrystals (volume content about 4.6%) in the melt can be fitted using a form factor for polydisperse cylinders. The geometrical size of Form I cylindrical nanocrystals was around height ≈40 ± 2 nm and radius ≈ 23 ± 1 nm when the aging time reached 12 h. Surprisingly, the height of Form I nanocrystals not only increased with lamellar thickness (dc) of initial Form II crystals but also was always larger than the dc of both initial Form II and pure Form I (i.e., complete transformation). We estimated that the Form I folded-chain nanocrystals consist of two parts, namely, the nuclei of Form I and the epitaxial growth of Form I crystals. Furthermore, our findings suggested that nucleation of Form I has a higher chance to occur at the thicker part of initial Form II crystals. This is probably because the longer parallel-chain stems give rise to a reduction of conformational entropy of the chains, allowing a smaller energy barrier for nucleation of Form I.