In-Plane vs Through-Plane Electrical Conductivity of Poly(lactic acid)/Multiwalled Carbon Nanotubes: Impact on Thermoelectric Properties

In this work, the direction of measurement on the thermoelectric (TE) properties of thick and thin films of poly(lactic acid)/multiwalled carbon nanotube (PLA/MWCNT) nanocomposites was investigated at room temperature. The influence of the elaboration process and the crystallinity of the polymer matrix has also been studied. Thick nanocomposite discs, filled with various amounts of carbon nanotubes, were prepared either by melt-blending (up to 10 wt % of MWCNT) or by solution mixing (up to 40 wt % of MWCNT), and the through-plane TE properties were analyzed. All nanocomposites display a positive Seebeck coefficient (S) value, demonstrating a p-type semiconducting behavior. It appears that the elaboration process and the structure of the PLA (amorphous or semicrystalline) have no significant impact on the TE properties. In addition, thin films of PLA/MWCNT nanocomposites, with the same filler content as thick discs, were prepared by drop-casting on dedicated electronic chips, and the in-plane TE properties were measured using a thin film analyzer. The in-plane values of electrical conductivity (σ) and the figure of merit (ZT) appeared to be 100 to 1000 times higher than those obtained from through-plane measurements, revealing a strong anisotropy of electrical conductivity. In contrast, the thermal conductivity and Seebeck coefficient show only small differences between in-plane and through-plane values. The highest ZT achieved in this work was 1.2 × 10–4 for thin films with 40 wt % of MWCNT, which is the best ZT value that can be found in the literature for a thermoplastic filled with multiwalled carbon nanotubes.