Nucleation Enhancement Effect in Poly(l-lactide) (PLLA)/Poly(ϵ-caprolactone) (PCL) Blend Induced by Locally Activated Chain Mobility Resulting from Limited Miscibility

Abstract

The enhancement effect of nucleation in immiscible blend systems has recently attracted interest. Although several authors have reported that the effect occurs at the phase interface, little is known about the mechanism involved. We focused on poly(l-lactide) (PLLA)/poly(ϵ-caprolactone) (PCL) immiscible blend systems in which the presence of PCL enhanced the nucleation of PLLA at low temperature. We investigated the nucleation behavior of PLLA during aging at temperatures below Tg. Generally, neat polymers, including PLLA, seldom generate nuclei below Tg due to restrictions in chain mobility. However, through DSC analysis of the crystallization behavior following an aging process, we revealed that the nucleation of PLLA occurs during aging even at temperatures below Tg in the PLLA/PCL blend. Since the nuclei density became saturated with increasing aging time, the nucleation behavior was regarded as heterogeneous nucleation. The asymptotic density of nuclei depended on the PCL content, indicating that dispersed PCL acted as active sites for nucleation. The nucleation rate R was almost independent of the aging temperature, suggesting that the marked decrease in chain mobility due to the glass transition is locally evaded at the active sites. Nucleation was observed even at temperatures as much as 40 °C lower than Tg following the addition of only 1 wt % PCL, while the Tg obtained by a DSC heating scan showed a subtle decrease. This suggests that the limited miscibility of PLLA/PCL leads to the aggregation of PCL and induces local and deep depression of Tg at the interface of the PCL domains, resulting in marked enhancement of PLLA nucleation.