New insights on the crystallization and melting of cyclic PCL chains on the basis of a modified Thomson-Gibbs equation

Low polydispersity linear and cyclic poly(ε-caprolactone) with molecular weights of 2-7.5 kg/mol were synthesized. Differences in the morphology of cyclic and linear single crystals (obtained from solution at identical temperatures) indicate that cyclic PCL crystals were obtained at higher supercoolings than linear ones. Cyclic PCL crystals (both crystallized from solution and from the melt) exhibited higher melting point values (Tm) than analogous linear PCL lamellae. In addition, cyclic PCLs crystallize from the melt at higher rates than linear PCLs at identical crystallization temperatures. Results showed that cyclic PCLs possess a higher equilibrium melting point (Tm0) than linear PCL crystals. A modified Thomson-Gibbs equation and a modified Hoffman-Weeks equation have been derived in this work to demonstrate that the higher Tm0 of the cyclic polymer stemmed from the negative cyclization entropy associated with the absence of chain ends and the more collapsed conformation of the cyclic chain in the melt state.