Tailoring the Solvent Polarity Enables PVDF to Be High Sensitivity for Phonic and Respiratory Recognition

The polar
$\beta $
-phase of polyvinylidene fluoride (PVDF) is the characteristic crystal phase of ferroelectric, pyroelectric, and piezoelectric, and it is also the strongest crystal dipole moment perpendicular to the chain direction. A single high-polarity solvent (dielectric constant >15) can increase the dipole arrangement of PVDF molecular chains, but its internal hydrogen atoms bind other atoms firmly, which makes it difficult for PVDF to obtain more active phases. Herein, adopting the low-polarity solvents as transition solvents by the “nucleophilic substitution” can easily promote the formation of dipole–dipole bonds between the “transition state” PVDF and polar solvents. Meanwhile, the continuous vacuum crystallization method is used to weaken the rapid dispersion of solvent due to heating, and a controlled process of forming a fixed crystalline phase structure from the “transition state” is achieved. The pressure sensor possesses excellent charge density (4.375 nC/cm2), sensitivity (
${S}$
= 0.175 V/N), and stability due to high piezoelectric property (
$\text{d}_{{33}} \sim $
35 pC/N) at the volume ratio (acetone: DMF = 8:2). The sensor is potential in detecting the mechanical motion, body signals (pronounce, breathing state, and finger tapping), and the applicability of dyspnea diseases.