Crystallization and melting of model ethylene-butene random copolymers: Thermal studies

摘要

Crystallization and melting of a copolymer depend on both intrachain and interchain distributions of noncrystallizable comonomer. Hydrogenated polybutadiene (HPB) is a model ethylene-butene (EB) random copolymer with known intrachain distribution and no interchain distribution of branching. Differential scanning calorimetry (DSC) was used to study HPB having 21 to 106 ethyl branches per 1000 backbone carbon atoms. Crystallization during continuous cooling occurs in two stages: a rapid primary transformation over a temperature interval of about 5 degrees C, followed by more gradual crystallization at lower temperatures. The crystalline fraction alpha is below the value from Flory's theory, but the two quantities converge for high branching. Isothermal crystallization of these homogeneous copolymers results in crystals having two distinct melting ranges. This unexpected result is accounted for by a model that partitions crystallizable ethylene sequences into two fractions on the basis of lengths established by crystallization temperature and kinetics. The fraction of the longest sequences creates a kinetically controlled distribution of crystal thickness l similar to that in a homopolymer. Segments of intermediate length form stable, but thinner, crystals that melt at a lower temperature. One ethylene-butene copolymer prepared with a metallocene catalyst behaves like the homogeneous model copolymers. [References: 22]