Change of lamellar morphology upon polymorphic transition of form II to form I crystals in isotactic Polybutene-1 and its copolymer

Morphology and domain sizes in isotactic polybutene-1 homopolymer (iPB-1) and (butene-1)-ethylene random copolymer (iPB-1/C2) are studied by spin-diffusion time-domain H-1 NMR and SAXS experiments. Lamellar thickness is smaller for iPB-1/C2 than for iPB-1 and decreases with decreasing crystallization temperature. iPB-1/C2 with form I' crystals, which were directly formed upon crystallization from a heterogeneous melt, has typical for polyolefins lamellar morphology with the crystal-amorphous interface that separates crystal lamellae and interlamellar amorphous domains. Different lamellar structure is observed in iPB-1 and iPB-1/C2 with form I crystals which were obtained upon polymorphic phase transition from form II to form I crystals. The phase transition causes fragmentation of crystal lamellae into small blocks with a wide size distribution as was shown by the NMR method. Two types of the interface are present in these samples: the inner interface that separates crystal blocks within fragmented crystal lamellae, and the crystal-amorphous interface on the surface of fragmented lamellae. The molecular mechanism of polymorphic form II to form I phase transition is proposed. High chain mobility in form II crystals and chain folding structure on the lamellar surface play significant role at the initial and final stages of the restructuring of crystal lamellae during the phase transition, respectively.