Effects of Rapid Microwave-Curing on Mechanical and Piezoresistive Sensing Properties of Elastomeric Nanocomposites

The development of multi-functional nanocomposites capable of large deformation and force sensing with high sensitivity and good reliability are of considerable interest in wearable and flexible electronics. Recent studies have shown that the introduction of highly conductive nanoparticles to the microstructures in nanocomposites can significantly improve the sensitivity and linear sensing range. More recently, the control of microstructures in nanocomposites is regarded as an attractive approach to increase the sensing range and improve the sensor performance.

Microwave irradiation is efficient in curing thermoset resins as it utilizes interactions at the molecular level to heat materials from inside the material itself, rather than heating an environment and allowing the heat to propagate from the outside. This heating process leads to significantly reduced curing times due to more efficient and localized energy delivery. The ability of microwave irradiation to quickly cure a thermoset resin is substantially enhanced with the addition of carbonaceous nanofillers. These carbon-based nanoparticles have high microwave absorption, therefore when dispersed within a prepolymer, they can provide localized heat throughout the uncured thermoset to rapidly cure the material. Carbon nanofillers, such as MWCNTs, have been implemented recently to rapidly microwave-cure epoxies and experimental results have shown that MWCNT doped thermosets cured in a microwave have comparable or improved properties over those cured in an oven. Due to the significantly reduced curing times and improved energy efficiency over thermal-curing, it is important that microwave-curing of thermosets containing dispersed carbonaceous nanofillers be fully explored.

In this study, one-step microwave irradiation is used to fabricate porous conductive nanocomposites through simultaneous rapid curing and residual solvent evaporation. One-step microwave-curing is applied to PDMS containing dispersed MWCNTs and residual tetrahydrofuran (THF) to demonstrate the simplicity of fabrication and to investigate the pores produced in the nanocomposites as a result of high internal temperature build-up and subsequent solvent evaporation. It has been reported in the literature that dielectric loss factor and consequently microwave absorption of thermosets decrease due to partial curing. This phenomenon is investigated in this study by precuring the nanocomposite prepolymer before microwave-curing to explore the effects on the curing process, porosity, and electrical conductivity. Additionally, nanocomposites with different loadings of MWCNT are fabricated to compare their resulting mechanical properties and piezoresistive sensing functions. A scanning electron microscope (SEM) is used to qualify the porosity and demonstrate the quality of MWCNT dispersion within the nanocomposites. The nanocomposite with the best piezoresistive performance is further investigated to fully characterize the piezoresistive response of the porous compression sensor.