Phase structure deciphering for pure polymers with a giant piezoelectric response

Piezoelectric polymers hold great promise in flexible electromechanical conversion devices. The conventional view is that the piezoelectric phase of these polymers is dominated by a polar crystal phase. Guided by this understanding, enormous effort has been dedicated to enhancing piezoelectric performance via mediating the proportion or orientation of polar crystal. However, theoretical and experimental results indicate that the piezoelectric response of a pure polymer cannot be doubled, and the piezoelectric constant (|d|) can hardly reach 60 pm/V, greatly hindering the future progress of piezoelectric polymers. Recent evidence suggests that the structure distortions within the polar crystal phase as well as the paracrystal between the polar crystal and amorphous fraction are closely connected with piezoelectricity. With this new understanding, pure polymers with a giant piezoelectric response (featuring a |d| above 60 pm/V) can be readily achieved. Numerous recent studies have demonstrated the great potential of this new understanding in obtaining high-performance piezoelectric polymers. Herein, this review highlights the newly discovered piezoelectric phase structures, including structure distortion (within polar crystal) and interphase paracrystal, via analyzing the structure features and their piezoelectric contributions. Inspired by the newly evolved phase structure, the possibility of obtaining a giant piezoelectric response is expected in renewable and biodegradable piezoelectric polymers due to the similar phase configuration. Furthermore, possible theoretical developments, including new insight into the giant piezoelectric response and the dynamics at piezoelectric polymer/liquid interface are discussed. The feasibility and great promise of these developments have been demonstrated via the emerging applications in piezoelectric sensor/nanogenerator/actuator, self-display sensing, air filtration, droplet hydraulic generator, solar interfacial vapor, battery with liquid electrolyte, water treatment and electrical stimulation therapy.