High piezoelectric responses in P(VDF-HFP) copolymers for sensors and transducers

Piezopolymers with high piezoelectric responses are highly desirable for a broad range of sensor, actuator and transducer applications. Defects modification concept applied on PVDF yields a high transverse piezoelectric response with both high piezoelectric d₃₁ (d₃₁=43.1pm/V) and electromechanical coupling k₃₁ coefficients (k ₃₁=0.187) in poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] 10wt% copolymers under quasi-static condition, much higher than these in the piezoelectric poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers. High frequency electromechanical resonance measurement was carried out under both LE mode and TE mode. Frequency dispersion of the piezoelectric response results in a smaller d₃₁ (=20.5pm/V) at 5OkHz. Thickness resonance measurement gives k_t=0.14 at 60MHz, which demonstrates it can be used as high frequency (>50MHz) ultrasonic transducers. Furthermore, the copolymers also display a higher d₃₁ coefficient compared to the d₃₃ coefficient, which seems to be unusual compared with most other piezopolymers. The experimental data suggests that the origin of the unusual piezoelectric response in these P(VDF-HFP) copolymers originates from a reversible change between a poled a-like structure and ß-like structure.