Effect of molecular structure of PEG/PCL multiblock copolymers on the morphology and interfacial properties of PLA/PCL blends

The influence of multiblock copolymers derived from epsilon-caprolactone and ethylene glycol on the morphology, interfacial tension and thermal properties of immiscible poly(lactic acid) (PLA)/poly(epsilon-caprolactone) (PCL) blends was studied. Multiblock copolymers of PCL and poly(ethylene glycol) (PEG) with similar molar masses, but different block lengths, were used as compatibilizers. PLA/PCL blends (80/20 wt%) containing 1, 3, and 5 wt% of multiblock copolymers were prepared in a corotating twin-screw extruder. The interfacial tension of the PLA/PCL blends were estimated by shear rheometry using the Palierne model and its morphology and thermal behavior were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The interfacial tension of uncompatibilized PLA/PCL estimated by the Palierne model is 1.51 mN/m. Upon addition of 1 wt% of the multiblock copolymer with longer block lengths (C2000E6000) the interfacial tension decreases to a minimum value of 0.38 mN/m. This minimum value of interfacial tension was only achieved with 3 wt% of multiblock copolymer with shorter block lengths (C550E1500). These results indicate that multiblock copolymers with shorter block lengths require a higher amount to reach the interface saturation. Furthermore, when the interface saturation is reached there is a change in the blend's fracture mode, from inter-particle to trans-particle, corroborating the increase in the interfacial adhesion estimated by the Palierne model. The presence of the C2000E6000 copolymer increased the crystallinity of the PLA matrix and contributed to the adhesion of the PLA/PCL blend.