Electrical dual-percolation in MWCNTs/SBS/PVDF based thermoplastic elastomer (TPE) composites and the effect of mechanical stretching

Dielectric thermoplastic elastomers (TPEs) offer a number of advantages over traditional dielectric elastomers or rubbers in terms of tailorable mechanical and electrical properties, higher mechanical strain, and ease of processing and shaping. Such a combination of properties has attracted increasing attention in flexible energy harvesting and storage applications. The combination of styrene butadiene-styrene (SBS) and poly(vinylidene fluoride) (PVDF) has the potential to provide a combination of high elongation to break and increased relative permittivity, however the immiscibility between SBS and PVDF results in polymer blends with poor stretchability and processing properties. In this work, a dual percolated structure was created in a thermoplastic elastomer of SBS/PVDF (50/50 wt%), by coupling EVA as a compatibiliser for SBS/PVDF with multi-walled carbon nanotubes (MWCNTs) as a conductive filler that created an electrical percolation network. The elongation at break of SBS/PVDF was significantly enhanced by adding 20 wt% of EVA due to the reduced phase dimensions and enhanced interfacial adhesion. The addition of MWCNTs enabled the formation of a percolated network at 1 wt% in the SBS phase, followed by a second percolation at 3 wt% in both PVDF and SBS phases. The relative permittivity of the composite increased to 22.5 at 1 wt% MWCNT with a tan delta of 0.5, and further increased to 34.9 for a 2 wt% of MWCNT concentration while the tan delta remained constant. In-situ electrical testing for the SBS/PVDF thermoplastic elastomer under strain showed that, at 1 wt% MWCNT, the non-percolated PVDF islands acted as variable capacitors whose capacitance increased with degree of stretching. For the dual percolated structure formed at 3 wt% MWCNT, the capacitance and conductivity of the composites were unaffected up to 30% strain. The high relative permittivity and strains of over 100% means that the SBS/PVDF based thermoplastic elastomer is readily suitable for vibration control sensors, variable impedance devices, energy harvesters and artificial muscles and actuators.