Polymorphic transformation in isotactic1-butene/ethylene copolymers

The effect of molecular weight, composition, and aging temperature on the kinetics of the tetragonal-to-hexagonal polymorphic transformation in isotactic (it) 1-butene/ethylene random copolymers has been investigated by differential scanning calorimetry and wide-angle X-ray diffraction. It has been found that, as it occurs in it poly(l-butene) homopolymers, the average molar mass has no effect on the rate of transformation. However, even at very small concentration, the presence of ethylene units highly enhances the rate of the process, to the extent that structural interconversion at room temperature requires only few hours in a copolymer with 5.5 wt % of counits, while more than 10 days is needed to complete the transformation in the homopolymer. By coupling the information obtained from different techniques, it has been found that, together with the usual development of stable hexagonal (form I) crystals with melting temperatures higher than that of the parent metastable tetragonal form (form II), a fraction of low-melting form I crystals are readily generated in the early stages of transformation. Thermal curves recorded at various aging times exhibit multiple endotherms, which also include transient peaks or shoulders. This complex thermal behavior can be tentatively explained by assuming that, before being converted into the hexagonal modification, the highly mobile as-crystallized form II undergoes an isostructural transformation through migration and redistribution of the defects initially trapped in the crystal lattice. The resulting tetragonal crystals, being characterized by widely different defect content, exhibit different transformation rates.