Thermodynamics of statistical copolymer melting

The melting temperature of a crystal formed of sequences of a statistical A/B copolymer depends not only on crystal composition and lamellar thickness, but on the composition of the coexisting melt as well. Hence analytical expressions are derived for the final melting temperature, where the melt composition equals the known overall composition, generally expressed as mole fraction xA of crystallizable monomer. Of the various treatments for final equilibrium melting of A crystals from which B units are excluded, only Floryʹs Tmc is correct, but it is unobservable. The Sanchez–Eby theory extends Floryʹs treatment of final melting temperature to metastable folded sequence crystals of arbitrary thickness lmax, and also allows for incorporation of B units in the crystal. The Gibbs–Thomson method for extrapolating final melting temperatures when lmax→∞ leads to a limiting Tm∗ that can be compared to theory for determining if B units are excluded from the crystals. Extrapolation to equilibrium melting by the Hoffman–Weeks method will yield Tmf<Tmc, confusing attempts to verify comonomer exclusion or inclusion.