Cyclic PCL with molecular weight of 7.5 kg/mol was studied by small-angle (SAXS) and wide-angle (WAXS) X-ray scattering, and differential scanning calorimetry (DSC). The cyclic and linear PCL crystallized to form identical crystallographic structure of PCL. The cyclic PCL can melt at a higher temperature than its linear analogue, which may be due to a lower entropy change of the cyclic chains upon melting. Investigations by SAXS have revealed that cyclic PCL displayed slightly larger lamellar thickness than that of linear PCL at a given T_c. As the Gibbs-Thomson and Hoffman-Weeks plots have also been frequently adopted to extrapolate the “equilibrium melting temperature” of cyclic polymer, we attempted to show here that the melting temperature from extrapolation is a characteristic temperature that depends upon the cyclization entropy of the polymer. We have obtained T^*=91.2℃, δ_e= 42.8 mJm^(-2) and 〖ΔS〗_cyc= -11.9 Jkg-1K-1 for cyclic PCL; T_mL^°=80.0℃ and δ_e= 30.3 mJm^(-2) for linear PCL. We also attempted to propose an alternative chain folding model for cyclic polymers.

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