Effect of Dipole Mobility in Secondary Crystals on Piezoelectricity of a Poly(vinylidene fluoride-co-trifluoroethylene) 52/48 mol % Random Copolymer with an Extended-Chain Crystal Structure

• Published in: Macromolecules 2021-11, Vol.54 (21), p.9879-9887
• Main Authors: Zhu, Zhiwen, Rui, Guanchun, Li, Ruipeng, He, Hezhi, Zhu, Lei
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.1c01655

High-performance piezoelectric polymers are promising for a broad range of practical applications, such as sensors, actuators, and energy generators in medical devices, wearable electronics, and soft robotics. Recently, high piezoelectric performance was reported for poly­(vinylidene fluoride-co-trifluoroethylene) [P­(VDF-TrFE)] copolymers with the composition around 50/50 mol %, which was attributed to the relaxor-like secondary crystals (SCs) in the oriented amorphous fraction (SCOAF). However, it was still unclear how the dipole mobility in the SCOAF affected the piezoelectricity of ferroelectric polymers. In this work, we compared two P­(VDF-TrFE) 52/48 mol % samples with an extended-chain crystal structure. The first sample was hot pressed, quenched, stretched, annealed at 130 °C, and unidirectionally poled at 100 MV/m (denoted as coP-52/48QSAP). After additional uniaxial stretching of coP-52/48QSAP at 6%, followed by electric poling, the coP-52/48QSAPSP film was obtained. Intriguingly, coP-52/48QSAPSP exhibited much lower piezoelectric performance than coP-52/48QSAP at room temperature. Based on a broadband dielectric spectroscopy study, it was found that the dipole mobility in coP-52/48QSAPSP was significantly decreased as compared to that in coP-52/48QSAP. It was the lower dipole mobility that decreased the piezoelectric performance for coP-52/48QSAPSP. However, upon heating toward the melting temperature (∼58 °C) of the SCOAF, the dipole mobility was regained, and high piezoelectric performance was achieved at 50 °C: the piezoelectric strain constant d 31 = 62.5 ± 7.2 pm/V and the electromechanical coupling factor k 31 = 0.123. This understanding of the dipole mobility in SCOAF provides a guide to achieving high piezoelectric performance in other ferroelectric polymers such as PVDF homopolymers.