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Abstract Despite extensive research on piezoelectric polymers since the discovery of piezoelectric poly(vinylidene fluoride) (PVDF) in 1969, the fundamental physics of polymer piezoelectricity has remained elusive. Based on the classic principle of piezoelectricity, polymer piezoelectricity should originate from the polar crystalline phase. Surprisingly, the crystal contribution to the piezoelectric strain coefficientd31is determined to be less than 10%, primarily owing to the difficulty in changing the molecular bond lengths and bond angles. Instead, >85% contribution is from Poisson's ratio, which is closely related to the oriented amorphous fraction (OAF) in uniaxially stretched films of semicrystalline ferroelectric (FE) polymers. In this perspective, the semicrystalline structure–piezoelectric property relationship is revealed using PVDF‐based FE polymers as a model system. In melt‐processed FE polymers, the OAF is often present and links the crystalline lamellae to the isotropic amorphous fraction. Molecular dynamics simulations demonstrate that the electrostrictive conformation transformation of the OAF chains induces a polarization change upon the application of either a stress (the direct piezoelectric effect) or an electric field (the converse piezoelectric effect). Meanwhile, relaxor‐like secondary crystals in OAF (SCOAF), which are favored to grow in the extended‐chain crystal (ECC) structure, can further enhance the piezoelectricity. However, the ECC structure is difficult to achieve in PVDF homopolymers without high‐pressure crystallization. We have discovered that high‐power ultrasonication can effectively induce SCOAFin PVDF homopolymers to improve its piezoelectric performance. Finally, we envision that the electrostrictive OAF mechanism should also be applicable for other FE polymers such as odd‐numbered nylons and piezoelectric biopolymers.
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A Close Look at the Local Structure of Functional Polymers: The Example of Poly(Vinylidene Fluoride)
Abstract Functional macromolecular materials promise to enable important improvements in many aspects of everyday life, including for energy applications, novel electronics systems, or wearable health care products. In contrast to widely‐used commodity plastics such as polyolefins, polyamides, or polyesters, it, however, remains challenging to advance detailed insights into the interrelation of structure and performance for functional polymers, limiting progress. The reason is that the macroscopic properties of such polymers often depend on local chain arrangements rather than long‐range order only. Here, it is demonstrated on the example of the well‐investigated ferroelectric poly(vinylidene fluoride) (PVDF) that modern nuclear magnetic resonance (NMR) spectroscopy enables thequantitativeanalysis of the complex solid‐state structure of this polymer, providing unprecedented insight. The precise fractions of chain segments of different conformations are revealed, as well as their spatial distributions with respect to each other. Thereby, a significant population of short‐range ordered chain segments is identified, a large fraction of which in close proximity to defects and disordered segments. Unsurprisingly, different environments lead to different structural dynamics — collectively showing that characterizing local order/disorder and their dynamics is imperative to accurately describe the properties of functional polymers such as PVDF.
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- Award ID(s):
- 2324190
- PAR ID:
- 10610877
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 35
- Issue:
- 24
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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