Flexible electronics from hybrid nanocomposites and their application as piezoresistive strain sensors

Poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) is one of the most commonly used styrene-based thermoplastic elastomer (TPE) with unique properties including high resilience and stretchability. In the case of incorporation of conductive fillers into SEBS matrix piezoresistive strain sensors can be fabricated. In this study, conductive flexible polymer nanocomposites were prepared by combination of solvent casting and compression molding by using SEBS matrix with conductive fillers including carbon black (CB), vapor grown carbon nanofibers (VGCNFs), and their mixtures (CB:VGCNF, 1:1) at various filler loadings from 0.5 to 6.5 wt%. Two different SEBS with different block ratios (styrene to ethylene-butylene ratio (S/EB): 13/87 and 19/81) were used in order to observe their effects on morphological, mechanical, thermal, electrical, and electromechanical properties. In all cases, single and hybrid fillers were found to show good dispersion in the matrix. However, polymer type was found significant in terms of mechanical, electrical, and electromechanical properties. SEBS with lower EB content (S/EB:19/81) led to formation of conductive network at lower concentrations for fillers. Although both polymers and all fillers can be used for strain sensors, hybrid composites generally tend to show higher sensitivity. The strain-reversibility of the composites was directly affected by filler type, filler ratio, and polymer type. The most sensitive and stable response was obtained from 3.5 wt% CB-VGCNF filled SEBS with S/ EB:19/81 block type. Test conditions (strain %, test speed) were found significant for both mechanical and electromechanical properties. Generally, for both polymers, lower filler ratio, higher strain ratio, and lower test speed resulted in higher sensitivity.