Melt density, equilibrium melting temperature, and crystallization characteristics of highly pure cyclic poly(epsilon-Caprolactone)s

A series of cyclic and linear poly (epsilon-caprolactone)s (c-PCLs: c-PCLn, n = 30-200, average degree of polymerization; l-PCLs: l-PCLn, n = 30-200) in extremely high purity have been quantitatively investigated in terms of melt density, equilibrium melting temperature crystallization nature in connection to molecular topology effects. Synchrotron X-ray reflectivity analysis determined higher melt densities for c-PCLs than those of the linear counterparts. c-PCL was determined to exhibit 8.4 degrees C higher equilibrium melting temperature than that of the linear counterpart. Such higher equilibrium melting temperature may originate from a cooperative effort of thermodynamic factors (larger equilibrium heat of fusion and smaller equilibrium entropy) and morphological factors (thicker crystalline layer and higher fold surface energy). Comparing with l-PCL, c-PCL revealed apparently faster crystallization behavior at a chosen T-c but slower crystallization behavior at a same degree of supercooling because of the higher equilibrium melting temperature. Both c-PCL and linear counterpart were observed to follow heterogeneous nucleation and growth mechanism in crystallizations. Overall, this study achieved comprehensive insights into cyclic topology effects on the melt density, equilibrium temperature, and crystallization behavior of PCL.