Morphology, miscibility and continuity development in poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends

- The interfacial tension between PLA/PBAT was determined using rheological analysis.
- The PLA/PBAT blend shows a limited one-way partial miscibility of PBAT in the PLA-rich phase.
- The dispersed phase in the PLA/PBAT blend exists in the form of stable fibers.
- PLA/PBAT show a wide and symmetrical region of co-continuity.

In this study, the morphology and miscibility of poly(lactic acid), PLA, and poly(butylene adipate-co-terephthalate), PBAT, blends were studied in detail. Three techniques to examine the morphology of PLA/PBAT are compared: SEM using lower secondary electron imaging (LEI), SEM using low-angle backscattered electrons (LABE) and atomic force microscopy (AFM). The interfacial tension of the system was examined by fitting the Palierne model to the rheological data and the results indicate a very low value of 0.6 ± 0.15 mN/m. The miscibility in PLA/PBAT blends was studied by modulated DSC and the results show a limited one-way partial miscibility of PBAT in the PLA-rich phase with the glass transition temperature of the PLA-rich phase in PBAT showing a drop of up to 10 °C, as compared to the neat PLA. Further investigations reveal that this effect depends significantly on the molecular weight of PBAT and underlines the strong entropic nature of this partial miscibility. The morphology at 1 vol.% of the dispersed phase shows that the dispersed phase exists in a stable fiber form even at these very low concentrations, an observation which is compatible with partial miscibility, with fiber diameters of 300 nm for PLA in PBAT and 150 nm for PBAT in PLA. The influence of composition on the dispersed phase fiber diameter shows a significant increase in fiber diameter with minor phase concentration which is not a result of classic coalescence, but more a result of the partial miscibility phenomenon. The region of dual-phase continuity has been examined by a rheological approach and is determined to be a wide and highly symmetric region with the lower and upper limits located between 30-40 and 60-70 vol.% of PBAT, respectively.