The role of entanglement in crystallization and melting of cyclic poly(ε-caprolactone)s

Abstract

The entanglement of cyclic polymers is an open issue. Cyclic poly(ε-caprolactone)s (PCLs) with a number-average molar mass (M_n) ranging from 8 to 37 kg mol⁻¹ were synthesized using a protocol without linear precursors to avoid the influence of linear contaminants. The crystallization and melting behaviors of the synthesized unconcatenated and unknotted cyclic PCLs were investigated by DSC, POM and X-ray. The DSC and POM characterized crystallization behaviors of the cyclic PCLs showed that the half-time of crystallization and spherulite growth rate related to the molecular weights. The rheological results showed that the critical molecular weight of cyclic PCLs is 11 kg mol⁻¹, which is about two times that of the linear PCLs. The coupling effect of ring topology and entanglement leads to the maximum crystallization rate and minimum lamellar thickness at the medium molecular weight. In situ simultaneous SAXS/WAXS probe in heating was performed on the samples after isothermal crystallization, and the results indicated that the ring topology did not change the crystal of PCL, but the evolution of nano-scale crystalline structure is different from that of the linear PCLs. The evolution of metastable crystalline structure of cyclic PCLs is discussed based on the SAXS results, and the entanglements were found to reduce the rate of structural evolution.