From ultralight microcellular to nanocellular poly (lactic acid) foams through tailoring transcrystalline structures of in-situ nanofibrillar composites

Abstract

The foaming properties of poly (lactic acid) (PLA) were enhanced by improving its crystallization behavior and melt elasticity through the incorporation of in-situ fibrillated poly (butylene terephthalate) (PBT). Shear rheological studies of the fabricated isotropic in-situ nanofibrillar composites (NFCs) revealed that the PBT nanofibrils (NFs) established a rheological percolated network at contents below 1 wt%. Elongational rheology further confirmed that the formed network significantly increased the strain-hardening factor and, consequently, improved foamability. Batch foaming was conducted using supercritical CO2 (scCO2) as a physical blowing agent under various temperature–pressure conditions. The crystalline phase morphology was tailored by both the foaming conditions and the PBT NF content, which, in turn, influenced the foam morphology and density. Foam characteristics were analyzed based on high-pressure differential scanning calorimetry (HP-DSC) results and morphological observations of the foam's crystalline phase. The orthogonal patterning of crystallized PLA nanodomains on the surface of PBT NFs led to the formation of two-dimensional nanohybrid shish-kebab (NHSK) superstructures. The incorporation of these fine, flexible NHSK structures within the foam cell walls resulted in ultra-lightweight foams with closed-cell microcellular morphology, a foam density of 47 kg/m3, and a cell density of 1.5 × 109 cells/cm3. At lower saturation temperatures, a thicker but still flexible NHSK network formed a beehive-like framework during expansion, with each compartment containing interconnected nanocellular foam structures. At higher PBT NF contents, the development of a rigid, fan-shaped transcrystalline PLA phase around PBT NF bundles restricted the expansion of highly nucleated samples, yielding a closed-cell nanocellular morphology. Despite the high crystallinity of these nanocellular foams, relative foam densities below 0.5 were achieved, attributed to their exceptionally high cell densities.