Enhanced sensing performance of EVA/LDPE/MWCNT piezoresistive foam sensor for long-term pressure monitoring

Conductive polymer composites (CPCs) with piezoresistive properties are smart detecting systems with high sensitivity, fast response, and low energy consumption. To enhance their sensitivity and cope with their high filler content requirements, solid CPCs are normally transformed into blends and foams, which reduces the elastic modulus, enabling larger deformations, wider detection range, and increased sensitivity. However, providing reversible and durable sensing properties remains challenging as it depends on the phase morphology, foam cell structure, and particle dispersion. To this end, we use multi-walled carbon nanotube (MWCNT) as a conductive filler with a high aspect ratio and develop CNT-filled ethylene vinyl acetate/low-density polyethylene (EVA/LDPE) blend foams. We fix the foam structure using chemical foaming and crosslinking agents in a compression molding process. To optimize the piezoresistive behavior, we follow a step-wise tuning approach, studying the blending morphology, the foaming attributes, the particle dispersion and affinity, thereby selecting the most promising formulations for further investigations. The least mechanical and electrical hysteresis is obtained at minimum foam density, which is obtained at a compromised crystallinity and crosslinking density. The higher affinity of MWCNT to EVA and the faster crosslinking of the EVA phase results in uniformly dispersed small cells at EVA-rich formulations and 4 phr MWCNT content. The final foam sensor demonstrates an impressive response and recovery times of less than 200 ms and remarkable durability thanks to the optimized cell morphology and mechanical properties. These findings expand the application of carbon-filled polyolefin foams in pressure monitoring, particularly in wearable sensors for strain and motion sensing.