Crystallization and melting of model ethylene-butene copolymers

Crystallinity and melting behavior are directly affected by the presence of a noncrystallizable comonomer. Hydrogenated polybutadiene, HPB, emulates a random ethylene-butene copolymer and provides the basis for comparison to the equilibrium theory of Flory. Melting behavior, density (crystallinity), and SAXS long period were measured for HPB's having 12 to 88 ethyl branches per 1000 backbone C atoms. DSC curves calculated from equilibrium theory are compared to experimental traces. It is shown that the equilibrium melting temperature T-m(c) of infinitely thick crystals, while thermodynamically correct, is inaccessible to experiment. Thickest crystals with observable populations melt at the practical final melting temperature T-m(f), which is below T-m(c). The peak melting temperature T-m(p) has no relation to the most populous crystal thickness. Crystallization of molten copolymer chains leads to fewer thick and thin crystals than predicted by theory; the difference is attributed to kinetic factors of secondary nucleation barriers and mass transport. Crystallization at feasible rates is achieved when the melt is at a temperature low enough to undercool a sizable fraction of crystallizable segments. Crystallization prevents the motion of segments required to achieve equilibrium, so solidification proceeds as if the system were quenched, accounting for insensitivity of copolymer morphology to cooling rate. Only the size of the largest crystals which melt at experimental T-m(f) can be established by thermodynamics. There is some evidence that small equilibrium crystallinities are approached in highly branched copolymers.