posted on 2020-09-08, 16:12authored byXinlei Tang, Petra Pötschke, Jürgen Pionteck, Yilong Li, Petr Formanek, Brigitte Voit
In
conductive polymer composites (CPCs), which can be used as both
strain sensors and materials with self-diagnosis capabilities for
structural health monitoring, the piezoresistive sensitivity can be
tuned by changing the electrical filler network structure, mainly
influenced by the conductive filler content. Typically, the electrical
resistance increases exponentially with strain, and the piezoresistive
sensitivity and linearity cannot be improved simultaneously. In this
work, we report a facile method to tune the piezoresistive behavior
of melt-mixed poly(vinylidene fluoride) (PVDF)/carbon nanotube (CNT,
0.75–2.0 wt %) composites using blending with poly(methyl methacrylate)
(PMMA, 5–30 wt %). PVDF and PMMA are completely miscible in
the melt state regardless of the proportion. For PVDF-rich blends,
the crystallization of PVDF induces separation of the PVDF crystal
region from the miscible PVDF/PMMA amorphous blend part during the
cooling process. Addition of PMMA tuned the piezoresistive strain
behavior and improved the electrical conductivity and toughness at
the same time. The PVDF/PMMA/CNT composites show higher sensitivity
at low strains than their PVDF/CNT counterparts with comparable initial
resistivity. For example, ΔR/R0 at 5% strain is 18.6% for the PVDF(80)/PMMA(20) blend
containing 0.75 wt % CNT versus 11.0% for PVDF containing 1 wt % CNT,
both having a volume resistivity of around 104 Ω·cm.
The PVDF/PMMA/CNT blend composites also show a less steep exponential
increase in the sensing response at higher strains, indicating better
linearity. These differences are due to the altered microstructure
of the composites and the more homogeneous distribution of CNTs between
the smaller and less numerous PVDF crystallites when PMMA is added.
The concept of modifying the composite microstructure by adding another
commercially available miscible polymer offers a simple and effective
way to tune the piezoresistive behavior and improve mechanical properties
of CPC sensor materials.