(181p) Polylactic Acid/Multiwalled Carbon Nanotube-Polybutylene Adipate Terephthalate Bio-Nanocomposites with Novel Segregated Chayote Squash-like Domain Morphology

In this study, polylactic acid (PLA), PLE 005-1 grade by NaturePlast, and polybutylene adipate terephthalate (PBAT), PBE-006 grade by NaturePlast, were melt-mixed at a previously optimized 80/20 PLA/PBAT ratio, and modified with multiwalled carbon nanotubes (MWCNT, CheapTubes®) at five different concentrations 0.01, 0.05, 0.10, 0.5, 1.00 wt.%. This was conducted by employing a two-step compounding technique: (1) pre-dispersing the MWCNTs in the minor PBAT phase via solution processing master-batching; (2) followed by twin-screw extrusion of the PBAT masterbatch into the major PLA phase. The initial phase of nano-dispersion was accomplished via a suspension transfer process, involving four sequential steps: (1) dispersion of MWCNTs in acetone via ultra-sonication to create a transferable suspension of exfoliated MWCNTs; (2) dissolution of PBAT in dichloromethane (DCM) to form a polymeric solution; (3) suspension transfer into the polymeric solution with continued sonication and stirring; and (4) solvent evaporation under continuous stirring to prevent the gravity settling and re-agglomeration to obtain well-dispersed MWCNT/PBAT masterbatches at 0.05, 0.25, 0.5, 2.5 and 5 wt.% concentrations suitable for the extrusion compounding step.

Microstructural analysis by scanning electron microscopy (SEM) revealed a highly dispersed and evenly distributed microstructure in the masterbatches with MWCNTs all over the PBAT matrix. However, upon the extrusion melt-blending of the PBAT masterbatch into the PLA matrix, a phase-separated morphology was formed with the MWCNTs preferentially localized in segregated PBAT domains forming novel and interesting chayote squash-like domains within the PLA continuous matrix. Furthermore, SEM revealed a great deal of interfacial cavitation between the domains and the PLA matrix. In some places, however, the spiky MWCNTs in the PBAT domains appeared as bridges in those cavities generating a sort of interfacial bridging. In most places, spiky and rough surface features were observed on the domains, which may be responsible for some kind of friction between the phases. In any case, such interfacial bridging and/or interfacial friction may be triggering and contributing to novel micro-mechanisms responsible for the significant performance in the ductility, strength, and toughness of these bio-nanocomposites at certain concentrations. Another interesting observation was that the nearly spherical domains of PBAT in the PLA/PBAT blends were turned into elongated and sometimes even interconnected domains in the bio-nanocomposites.

The neat PLA (the control, tested dried as molded) exhibited a Young’s modulus, an elongation-at-break, and a toughness value of 3.955±0.398 GPa, 2.93±0.26 %, and 1.380±0.19×10⁶ J/m³, respectively. Interestingly, at 1wt.% MWCNT, the bio-nanocomposites displayed values of 3.364±0.138 GPa, 133±38 %, and 58.63±19×10⁶ J/m³, respectively. Relative to the pristine PLA matrix, these values correspond to an enhancement of 4500% and 4200% in elongation-at-break and toughness, respectively. Furthermore, the bio-nanocomposites displayed greater flexibility than the pristine PLA. In this opportunity, we intend to present the latest work on this exciting and relevant sustainability topic.