Direct Formation of Different Crystalline Forms in Butene-1/Ethylene Copolymer via Manipulating Melt Temperature

The crystallization behavior of a butene-1/ethylene random copolymer with 9.88 mol % ethylene counits was investigated by means of differential scanning calorimetry, wide- and small-angle X-ray scattering, and polarized optical microscopy. Unlike in its homopolymer counterpart which crystallizes always into a metastable form II from the melt state, the random copolymer was found to crystallize either into form II or stable form I' directly from its melt state after being cooled down. The occurrence of either crystalline form only depended on the temperature where the crystalline material was molten before cooling down. Even though the material was brought to temperatures higher than the equilibrium melting temperature, heterogeneities of segmental segregation character were preserved which promoted massive nucleation of form I' crystallites, which makes it possible that the material is able to crystallize into pure form I'. Only if when the melt temperature was high enough where all heterogeneities of the above-mentioned character were erased can the material be crystallized into pure form II. The effect is found independent of the previous crystalline form, meaning that the helical conformation of chains in the heterogeneous melt does not affect the nucleation of stable form I'