Microstructural development and mechanical performance of PLA/TPU blends containing geometrically different cellulose nanocrystals

Abstract

Microstructural development and mechanical performance of poly(lactic acid) (PLA)/thermoplastic polyurethane (TPU) (80/20) blends containing single but geometrically different cellulose nanocrystals (CNCs) were investigated. The CNCs used in this study include cylindrical CNCs (ultrasonic treated), spherical CNCs (chemical treated) and rod-like CNCs (commercial), all characterized by particle size analyzer, atomic force microscopy (AFM) and X-ray diffraction. The melt viscoelastic results obtained for the melt compounded CNCs with PLA/TPU blend have implied a fine dispersion of both spherical and cylindrical CNCs which are preferentially localized in the PLA matrix and/or interface, while poorly dispersed rod-like CNCs were mostly accumulated in the TPU phase. These results were supported by AFM micrographs. More evidence was provided by scanning electron microscope micrographs which showed a decrease in TPU particle size in the blends filled with spherical and cylindrical CNCs, as a result of the nanoparticles’ localization in the PLA matrix and/or interface. The results of dynamic mechanical thermal analysis have revealed that the addition of spherical and cylindrical CNCs into the blends not only has compensated the reducing effect of TPU on the storage modulus but also has enhanced the modulus up to values even higher than that of PLA matrix. In contrast, the rod-like CNCs did not enhance the storage modulus of the blend significantly. More interestingly, the blend nanocomposite containing 3 wt% spherical CNCs exhibited a highly enhanced yield stress as well as significant elongation at break. This behavior could be explained by the induction of an interconnected type microstructure of spherical CNCs in which the nanoparticles are localized in PLA matrix, allowing the overlapping of stress counters around TPU particles and facilitating the stress transformation throughout the loaded sample. A similar enhancing effect but to a lesser extent, was obtained for those blend nanocomposites filled with cylindrical CNCs. In contrast to the other two types of CNCs, rod-like CNCs have weakened the mechanical performance of the blends. The results of fracture toughness obtained from single edge notch bending experiments have shown almost similar enhancing effect of the three types of CNCs on toughening. However, the sensitivity of the fracture test was not high enough to distinguish the effect of different CNCs on mechanical behavior of CNCs-filled blend nanocomposites.