Microcellular Poly (Ethylene-Co-Vinyl Acetate)/Lignin Nanocomposites for Soft Ferroelectrets

This work examines the role of lignin and carbon nanotubes (CNT) in modifying the mechanical, dielectric, and ferroelectret properties of poly (ethylene-co-vinyl acetate) (EVA)-based nanocomposite foams, with the potential to act as a ferroelectret by polarization of the pore regions in the foam. Lignin acts as an effective nucleating agent, improving cell homogeneity and transitioning foam morphology from bimodal to uniform pore distribution at higher loadings. The lignin addition substantially increases the compression properties and energy dissipation when subject to cyclic loading and considerably enhances thermo-oxidative stability and dielectric properties of EVA foams. Finite-element modeling demonstrates that the addition of CNT below the percolation threshold can significantly increase the relative permittivity of the polymer as a result of the enhanced local electric fields within the low permittivity pore space, thereby increasing pore polarization and contributing to a higher piezoelectric d33 coefficient. Experiments confirm these modeling predictions, highlighting both CNT and lignin in effectively enhancing permittivity and piezoelectric response of cellular structures with concentration well below its percolation threshold. Optimizing the lignin and CNT content can achieve balanced enhancements in mechanical, dielectric, and piezoelectric properties, suggesting promising pathways for advanced cellular polymer composites in ferroelectret and energy harvesting applications.