Introduction of stereocomplex crystallites of PLA for the solid and microcellular poly(lactide)/poly(butylene adipate-co-terephthalate) blends

Solid and microcellular poly(l-lactide)/poly(butylene adipate-co-terephthalate) (PLLA/PBAT) blends with or without poly(d-lactide) (PDLA) were prepared via melt blending and batch foaming process with supercritical carbon dioxide, respectively. The introduction of PDLA on the rheological properties, crystallization behavior and dynamic mechanical properties of the PLLA matrix were investigated. The formed PLA stereocomplex between PLLA and PDLA enhanced the storage modulus and complex viscosity of PLLA/PBAT blends efficiently. Interestingly, the addition of 5 wt% or 10 wt% PDLA in the PLLA/PBAT blends was unfavorable for the PLLA crystallization behavior. The potential reason can be sc-PLA crystallites acting as the physical crosslinking points, which constrained the molecular mobility of the PLLA matrix and even blocked the nucleating effect of PBAT domains. Both the enhanced melt strength and decreased crystallinity of the PLLA matrix are favorable for the cell nucleation and growth and the gas adsorption, respectively. The designed partially foaming of PLLA/PBAT with or without PDLA was carried out to investigate the foaming mechanism. The final cell morphology of PLLA/PBAT foams exhibited typical open-cell structure mainly attributed to the soft immiscible PBAT phase as separated domains. With further addition of PDLA in the PLLA/PBAT blends, the microcellular morphology exhibited decreased average cell size and increased cell density. The sc-PLA crystallites networks in the PLLA matrix acted as cell nucleating agents, which meanwhile resisted the force of cell growth and then prevented the cell collapse.

The introduced PLA stereocomplex could enhance the melting strength of PLLA/PBAT blends efficiently. The microcellular morphology of PLLA/PBAT foams with PDLA exhibited decreased average cell size and increased cell density.