Crystallization and Melting of Model Ethylene−Butene Copolymers

Abstract

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.

Crystal thickness distributions from melting homopolymers or random copolymers

“…10). Secondly, kinetic factors limit experimental ␣ m to well below equilibrium values for all T, 6 so melt concentration of ethylene units is closer to x than predicted by equilibrium theory. Hence dT/dl for nonequilibrium HPB-20 will be closer to the dashed line in Figure 6.…”

“…Model ethylene-butene copolymers are hydrogenated polybutadienes, the synthesis and characterization of which were described earlier. 6,19 All polymers are described in Table II. HPB-20 and HPB-49 have 20 and 49 ethyl branches per 1000 backbone carbon atoms, and are the same as EB-20 and EB-49 in Table I.…”

“…Flory equilibrium theory is applied to random ethylene-butene copolymers as in earlier work by Crist and co-workers. 6,7 Thermodynamic parameters are as above for copolymers with 12, 20, and 49 ethyl branches per 1000 backbone C atoms. In Table I the copolymers are designated EB-xx, where the suffix is br/1000 C; mole fraction x of crystallizable ethylene units is given as well.…”

“…6,7 Proceeding nevertheless, we use eq 8 to evaluate dT/dl, leading to an expression analogous to eq 6:…”

Phase Transitions of Bulk Statistical Copolymers Studied by Dynamic Monte Carlo Simulations

“…8,9 Although the predictions of these equilibrium theories 10 are qualitatively correct, it is actually impossible to verify the quantitative predictions with the experimental results of copolymers. 11 First, there is no characteristic melting point for copolymers, since the distribution of sequence lengths gives rise to a distribution of crystallization temperatures, resulting a wide crystallite size distribution and hence, a wide melting temperature range. Second, a true equilibrium distribution of sequence lengths in corresponding crystallites cannot be approached in reality due to the irreversible nature of polymer crystallization.…”