Isothermal crystallization of random ethylene-butene copolymers: Bimodal kinetics

Random copolymers of ethylene and butene contain crystallizable sequences of different length l. Isothermal crystallization of model ethylene-butene random copolymers having 21 and 73 branches per 1000 backbone carbon atoms was studied by differential scanning calorimetry (DSC). Two melting peaks, which correspond to two crystal populations, are seen after isothermal crystallization. High-temperature melting is attributed to lamellar crystals formed from the longest sequences in the copolymer. These thicken during crystallization and melt at temperatures as large as 22 degrees C above the crystallization temperature T-c. A second crystal population melts around T-c + 5 degrees C. It is concluded that these crystals are dominated by shorter sequences that solidify with little or no folding and hence resemble fringed micelles. Thickening is suppressed in this set of crystals, so they melt near T-c. Low melting crystals are shown to grow more slowly than the lamellar structures; the difference in transformation rates is attributed to lower undercooling and restricted mass transport for crystallization of short sequences. No comparable evidence for two crystal populations is seen when unbranched polyethylene or a heterogeneous copolymer (19 branches/1000 C) is crystallized isothermally under similar conditions.