Polytetrafluoroethylene–nanocellulose composites with negative in-plane thermal expansion and low relative permittivity

Abstract Low-dielectric materials for advancing telecommunication technologies must exhibit a low coefficient of thermal expansion (CTE), low relative permittivity ( ε ), and low dielectric loss tangent (tan δ ) at frequencies > 10 GHz. However, these properties are difficult to achieve simultaneously in polymer-based materials. Polytetrafluoroethylene (PTFE) has desirable dielectric properties but exhibits a large CTE. In this study, PTFE nanoparticle/cellulose nanofiber (CNF) composites were fabricated via an aqueous casting method, with the aim of combining the dielectric properties of PTFE and low thermal expansivity of CNF. The in-plane CTE of a composite comprising 75 wt% PTFE and 25 wt% CNF (PTFE 75 CNF 25 ) was negative (–6.1 ppm⸱K –1 ). It increased as the CNF content increased; however, it remained lower than that of the CNF film and well below that of the PTFE film. Idealized unit-cell modeling suggested that the negative CTE could be attributed to the local mechanical response of the CNFs network induced by the thermal expansion of PTFE nanoparticles, as supported by the stress–temperature behavior under constraint. At 40 GHz, PTFE 75 CNF 25 showed a lower ε (1.96) than the PTFE film (2.04) owing to the presence of air, and ε increased as the CNF content increased. The tan δ of PTFE 75 CNF 25 (0.022) was higher than that of the PTFE film; however, it remained comparable to that of conventional low dielectric materials. These results demonstrate that the combination of a negative in-plane CTE and a low ε originates from this composite architecture.