Copolymer crystallization: Approaching equilibrium

Melt crystallization of random copolymers leads to solids with crystalline fraction wc and final melting temperature Tmf that are substantially below the predictions of Flory's equilibrium crystallization theory. Model ethylene/butene random copolymers, when crystallized as multilayer films by rapid solvent evaporation, exhibit increased wc (50% relative) and Tmf (4 K) compared to melt crystallized values. For a copolymer with 0.92 mol fraction ethylene, the density-derived crystallinity wc=0.6 is the same as that from Flory's theory, although the maximum observable crystal thickness from Tmf remains about 25% of the theory value. These effects are seen because crystallization from solution occurs without many of the constraints to segment dynamics that limit crystalline fraction during melt crystallization. Crystal thickness is dominated by secondary nucleation barriers in both melt and solution. Chain or sequence folding is much more regular in the solution crystallized material, and amorphous layer thickness is reduced from about 8 nm to 3 nm.