Organic, Flexible, Polymer Composites for High-Temperature Piezoelectric Applications

Industrial use of piezoelectric polymers is currently limited by low piezoelectric response and large performance losses at elevated operating temperatures. Leading polymers such as poly(vinylidene fluoride) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) possess piezoelectric d33 constants around -31 pC/N, and drop rapidly above 100°C operating temperatures. In this work, we fabricate a composite of P(VDF-TrFE) and carbon nanoparticles that possess a d33 piezoelectric constant of -40 pC/N at room temperature, 29% higher than pure P(VDF-TrFE). Additionally, the d33 value of the composite is 80% higher than the pure polymer at 140°C, with a value of -34 pC/N as compared to -19 pC/N. These materials, which are lead free, nontoxic and solution processable, are composed of Buckminsterfullerene (C60) nanoparticles functionalized with diamine (N-R-N) crosslinking chains composited into a P(VDF-TrFE) polymer matrix. The blended thermoplastic was spin-coated into films and thermally cured to covalently attach the amine-functionalized C60 onto polymer backbones to form crosslinking "bridges."X-ray diffraction spectra confirmed the presence of a crosslinked network, and differential scanning calorimetry thermographs show the disappearance of the ferroelectric-paraelectric (F-P) transition peak at 106°C, indicating retention of ferroelectricity until the material melts at 152°C. The loss of this transition was confirmed through P-E hysteresis testing at 140°C, where the pure polymer showed paraelectric behavior and the composite remained ferroelectric. In addition, the crosslinked composites showed significant increases in remnant and maximum polarization above the pure polymer. Piezoresponse force microscopy (PFM) was used to measure local piezoelectric and hysteretic effects of crosslinked P(VDF-TrFE)/C60 composites and demonstrated improvement over neat P(VDF-TrFE) samples. These materials show promise toward the design of sensors for extreme environments such as structural sensors, monitoring sensors in automobiles, vibrational sensors for machinery supporting oil, gas, mining and manufacturing operations, and for harvesting human-based or environmental energy.