A major challenge of designing and fabricating electrodes and sensors for wearable and flexible electronic devices is the ability to make conductive composites that has a good dispersion of metal nanoparticles in a matrix that is often insulative. Poly(dimethylsiloxane) (PDMS) is one of the most commonly used elastomeric matrices used since it conforms well to a given shape leading to good contact between the conductive electrode sensor and sensing surface. Consequently, we can integrate both adhesion and conducting functions by creating a conductive PDMS composite that are loaded with conductive fillers such as Ag nanoparticles. The conductivity of these composites which dictates its sensing performance depends on intrinsic properties of filler and matrix, filler loading and filler-matrix interaction. A continuous electrical pathway is needed for these conductive filler-polymer networks so as to achieve a sufficiently low resistivity and reliable sensing. Therefore, the filler amount would need to exceed the percolation threshold concentration. For our Ag-particle based composite, the percolation threshold is estimated to be ~7-8 vol% based on existing literature. Generally, the percolation threshold limit depends on the filler morphology, strength of interaction of individual particles within the agglomerate and dispersion characteristics of the filler agglomerates. Although higher aspect ratio for fillers leads to lower percolation thresholds, a potential benefit of spherical nanoparticles is that the higher mobility of spherical nanoparticles would allow conductive pathways that were lost upon deformation to be more easily recovered. Polyacrylic acid (PAA) is typically used as a particle stabilizer for inks used for direct writing. However, PAA-modified Ag nanoparticles do not mix with PDMS due to incompatible interfacial chemistries, which also leads incomplete PDMS curing as the carboxylic functional groups interfered detrimentally with the catalyst used for PDMS curing. Thus, the modification of the Ag-PAA particles surface chemistry was needed to improve its dispersion and enable the curing of Ag-PDMS mixture.
In this presentation, a single-step process for modifying the existing PAA-functionalized Ag nanoparticles through an inter-polymer complexation between PAA and PVP was presented and the interfacial characteristics of the PAA-PVP-modified nanoparticles system were investigated. The formation of intermolecular hydrogen bond between PAA and PVP, and the increased thermal stability and intermolecular interactions as PAA complexed with PVP were also verified. This surface modification via complex formation allowed the mixture to cure for Ag loading as high as 25 vol% to form Ag-PDMS composites, whilst the unmodified Ag-PAA PDMS system cannot be cured once it exceed 7 vol% of Ag. With the higher Ag loading, we were able to lower electrical resistivity from ~110 Ω.cm (at 7 vol%) to ~6 Ω.cm (pass percolation threshold).