The application of graphene nanoplatelets (GNPs) in polymer composites is a challenge due to their high tendency to agglomerate and restack during processing. In this work, alkyl phosphonium-based ionic liquid was used to assist the dispersion of GNP in an ethylene-vinyl acetate (EVA) matrix, through a melt-mixing procedure. The mechanical properties and creep resistance of the films prepared by the film extrusion process were evaluated. The results demonstrated that the noncovalent treatment of GNP with the ionic liquid (IL) enhanced the electrical conductivity and creep stability of the EVA composites. The microwave absorbing properties were studied in the X-band and Ku-band. A reflection loss (RL) of −15 dB for EVA containing 0.5 wt% of GNP and 1:1 wt% of GNP/IL was achieved. The use of a multi-layered structure containing thin film layers was efficient for enhancing the microwave absorbing performance, with a minimum RL of −24.6 dB and effective absorption bandwidth of 4.3 GHz. This result is attributed to the internal reflection and scattering of the radiation between layers. The use of simple, low-cost materials and procedures, combined with the system’s excellent mechanical and electrical properties, makes it a promising candidate for multifunctional applications as electrostatic dissipative and microwave absorbing materials for electronic packaging and other electronic devices.

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.