Miscibility and Crystallization of Poly(L-lactide)/Poly(ethylene glycol) and Poly(L-lactide)/Poly(ε-caprolactone) Blends

The miscibility and crystallization behavior of two binary blends, poly(L-lactide) (PLLA)/poly(ethylene glycol) (PEG) and PLLA/poly(ε-caprolactone) (PCL), with PLLA as the main component (PLLA composition was not lower than 50 wt%) have been investigated by differential scanning calorimetry (DSC) and optical microscopy. PEG and PCL with moderate molecular weight (MW≍10000) were used as the blending components for PLLA. PLLA and PEG were miscible in the melt over the composition range investigated. The variation of the bulk crystallization rate of PLLA with PEG composition displayed a maximum at 10 wt% PEG. On the other hand, a monotonic increase in spherulite growth rate with increasing PEG composition was observed. Since the bulk crystallization rate is determined by both the nucleation density and the growth rate, the depression of nucleation density coupled with the increase of growth rate with increasing PEG composition may give rise to the observed maximum in the composition variation of the bulk crystallization rate. The crystallizability of PLLA and PEG in the blends was also evaluated. The crystallizability of PEG was reduced by blending with PLLA, while that of PLLA was essentially unaffected by blending with PEG. For the PLLA/PCL system, a phase-separated morphology was identified in the melt. However, the crystallization rate of PLLA could be enhanced upon blending with PCL. Since no preferential nucleation at the domain interface was observed, the promotion of PLLA crystallization rate was attributed to its partial miscibility with PCL.