The influence of small amounts of linear polycaprolactone chains on the crystallization of cyclic analogue molecules

Abstract

Cyclic poly(ε-caprolactone) model chains (C-PCL) were synthesized by a ring closure click chemistry technique. Identical linear precursors (L-PCL) of equivalent molecular lengths were also prepared. The effect of adding small amounts of linear (L-PCL) to cyclic analogs (C-PCL) was studied by preparing blends (C/L) in solution of the following composition range: 95/5, 90/10 and 80/20. Two sets of blends with PCL samples of different number average molecular weights (3 and 12 kg mol⁻¹) were studied. The blends were analyzed by polarized light optical microscopy (PLOM) and advanced Differential Scanning Calorimetry (DSC) techniques that included non-isothermal and isothermal crystallization studies and thermal fractionation by SSA (Successive Self-nucleation and Annealing). The results show that addition of small amounts of linear chains (i.e., 5 and 10 wt%) to cyclic PCLs produces synergistic decreases in crystallization and melting temperatures, crystallinity degrees and isothermal crystallization rates (for both 3 and 12 kg mol⁻¹ samples). When the amount of linear chains reaches 20 wt%, a significant recovery of a simple mixing law behavior is obtained. Thermal fractionation demonstrated that the addition of linear chains can reduce the annealing capacity of cyclic chains. The behavior of the C/L blends can be explained by the threading of ring molecules by linear chains. This threading effect amounts to an increase in entanglement density that reduces chain diffusion and hence crystallization rate.