Hierarchical structure of Polybutene-1 in crystal blocks resulting from the Form II to I solid-to-solid transition as revealed by small-angle X-Ray scattering

Polybutene-1 (PB-1) exhibits excellent mechanical properties, while an unavoidable polymorphic transition from a metastable form II to the stable form I occurs after solidification from the molten state. This transition goes along with a deformation of the crystal lattice corresponding to a lateral shrinkage and normal elongation of about 20% and 12%, respectively. Here we study the semicrystalline morphology before and after the transition using small-angle X-ray scattering (SAXS) and differential scanning calorimeter (DSC). Scattering curves of samples with form II crystals could be well described by a model of layer-like stacked lamellar crystals. However, the scattering signal of form I samples showed additional scattering contributions at higher scattering vectors, indicating the generation of fine structures during the transition, much smaller than lamellar long spacing. The lamellar thickness increased less than the factor of 1.12, expected from the elongation of the lattice. The results indicate that the large lamellar crystals split up into blocks during the transition creating new interstitial spaces within the lamellae. This process is caused by the fact that the laterally extended lamella of form II cannot shrink as a whole due to the connectivity of the chains emanating from the crystals and due to possible overcrowding of the chain segments at the crystalline/amorphous interfaces.