A series of low polydispersity linear and cyclic poly(e-caprolactone) samples with molecular weights in the range of 2-7.5 kg/mol have been studied by Differential Scanning Calorimetry (DSC), Small Angle X-ray Diffraction (SAXS), Transmission Electron Microscopy (TEM), Electron Diffraction, Atomic Force Microscopy (AFM) and Polarized Light Optical Microscopy (PLOM). Single crystals were prepared from dilute solution at the same crystallization temperature. Small differences in the morphology of cyclic and linear single crystals indicate that cyclic PCL crystals were obtained at higher supercoolings than linear ones. The lamellar thicknesses of cyclic PCL single crystals were slightly larger than those obtained for linear PCL lamellae as indicated by AFM. It was found that cyclic PCL crystals (both crystallized from solution and from the melt) exhibit higher melting point values (Tm) than analogous linear PCL lamellae. Cyclic PCLs crystallize from the melt at higher rates than their linear counterparts at identical crystallization temperatures (both the overall crystallization rates and the spherulite growth rates were found to be higher for the cyclic PCL chains).After isothermal crystallization studies from the melt, it was found that although cyclic PCL crystals displayed slightly larger lamellar thickness than the linear ones as determined by SAXS, it is the higher equilibrium melting point that causes the observed higher Tm- values of cyclic PCLs. A modified Thomson-Gibbs equation and a modified Hoffman-Weeks equation were derived to demonstrate that the higher equilibrium melting point 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. Both the melting points and surface free energy for cyclic and linear PCLs were determined.

Crystallization kinetics and morphology of cyclic PCL: The determining role of supercooling

245th National Meeting (2013)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.