Tuning\nthe Piezoresistive Behavior of Poly(Vinylidene\nFluoride)/Carbon Nanotube Composites Using Poly(Methyl Methacrylate)

Abstract

In\nconductive polymer composites (CPCs), which can be used as both\nstrain sensors and materials with self-diagnosis capabilities for\nstructural health monitoring, the piezoresistive sensitivity can be\ntuned by changing the electrical filler network structure, mainly\ninfluenced by the conductive filler content. Typically, the electrical\nresistance increases exponentially with strain, and the piezoresistive\nsensitivity and linearity cannot be improved simultaneously. In this\nwork, we report a facile method to tune the piezoresistive behavior\nof melt-mixed poly­(vinylidene fluoride) (PVDF)/carbon nanotube (CNT,\n0.75–2.0 wt %) composites using blending with poly­(methyl methacrylate)\n(PMMA, 5–30 wt %). PVDF and PMMA are completely miscible in\nthe melt state regardless of the proportion. For PVDF-rich blends,\nthe crystallization of PVDF induces separation of the PVDF crystal\nregion from the miscible PVDF/PMMA amorphous blend part during the\ncooling process. Addition of PMMA tuned the piezoresistive strain\nbehavior and improved the electrical conductivity and toughness at\nthe same time. The PVDF/PMMA/CNT composites show higher sensitivity\nat low strains than their PVDF/CNT counterparts with comparable initial\nresistivity. For example, Δ<i>R</i>/<i>R</i>₀ at 5% strain is 18.6% for the PVDF(80)/PMMA(20) blend\ncontaining 0.75 wt % CNT versus 11.0% for PVDF containing 1 wt % CNT,\nboth having a volume resistivity of around 10⁴ Ω·cm.\nThe PVDF/PMMA/CNT blend composites also show a less steep exponential\nincrease in the sensing response at higher strains, indicating better\nlinearity. These differences are due to the altered microstructure\nof the composites and the more homogeneous distribution of CNTs between\nthe smaller and less numerous PVDF crystallites when PMMA is added.\nThe concept of modifying the composite microstructure by adding another\ncommercially available miscible polymer offers a simple and effective\nway to tune the piezoresistive behavior and improve mechanical properties\nof CPC sensor materials.