Enhanced Dielectric and Microwave-Absorbing Properties of Poly(Lactic Acid) Composites via Ionic Liquid-Assisted Dispersion of GNP/CNT Hybrid Fillers

Poly(lactic acid) (PLA)-based nanocomposites containing a mixture of graphene nanoplatelets (GNP) and carbon nanotube (CNT) as hybrid fillers were prepared using a solution-assisted sonication process followed by melt processing. The effects of the filler dispersion on dielectric properties and microwave absorbing (MWA) performance were systematically investigated. Two ionic liquids (ILs), trihexyl-(tetra-decyl)phosphonium bis (trifluoromethanesulfonyl)imide (IL1) and 11-carboxyundecyl-triphenylphosphonium bromide (IL2), were employed as dispersing agents for the carbonaceous fillers. Incorporation of IL-treated fillers resulted in enhanced dielectric permittivity and improved MWA performance of the PLA composites. The MWA properties were evaluated in X- band and Ku-band. A minimum reflection loss (RL) of −34 dB and an effective absorption bandwidth (EAB) of 2.1 GHz were achieved for the composite containing GNP/CNT/IL2 (HB3) at a weight ratio of 2.5:0.5:0.5 wt% with one 3 mm thick layer. The superior performance of IL2 is attributed to π-π and π-cation interactions between its phenyl-containing cation and the carbonaceous fillers, as well as improved compatibility with the PLA matrix due to carboxyl groups. Additionally, three-layered composite structures, combining PLA/GNP as the outer layer with IL-assisted hybrid fillers in the core and PLA/CNT at the bottom layer, achieved an extended EAB of 4.5 GHz for GNP/HB2/CNT arrangement and 4.35 GHz for the GNP/HB3/CNT arrangement, driven by enhanced scattering and internal reflection of microwaves. These results demonstrate the potential of IL-assisted hybrid filler dispersion in PLA for developing biodegradable materials with multifunctional applications as charge storage capacitors and microwave absorbing materials for sustainable electronics.