Key Takeaways & Executive Findings
- •• Fluorine-free disulfonyl polymers achieve high dipole moment (9 D) comparable to rod-like molecules, enabling ferroelectricity without fluoropolymers. • Simple R-group substitution (H to CH3) tunes normal ferroelectricity to relaxor ferroelectricity, demonstrating tunable FE properties. • Relaxor ferroelectric polymer exhibits exceptional electroactuation strain (-4%) and electrocaloric effect (ΔS = 14.8 J·kg⁻¹·K⁻¹) under low electric fields. • Performance matches state-of-the-art PVDF-based tetrapolymers, offering a sustainable alternative for flexible electronics and thermal management.
Abstract
Ferroelectrics (FEs) have shown great potential in sensors, actuators, and electrocaloric cooling due to their direct cross-couplings between electric polarization and mechanical, thermal, and dielectric properties. Compared with oxide FEs, polymer FEs possess good flexibility and shape adaptability, making them promising candidates for flexible electronics and biocompatible devices. Despite decades of research, the number of FE polymers remains limited, with poly(vinylidene fluoride) (PVDF) being the most prominent due to its well-defined Curie transition and large spontaneous polarization. However, chemically modifying the semicrystalline structures of FE fluoropolymers is not only complex and costly, but also raises environmental and health concerns, as these materials are considered as "forever chemicals" due to their persistence. In pursuing fluorine-free FE polymers, the use of strong dipolar molecules to induce FE ordering is regarded as a promising strategy. This approach, initially proposed by Bohr a century ago and later validated in FE nematic and smectic A liquid crystals, relies on the alignment of rod-shaped molecules with large dipole moments (μ, μ = qd, where q is the partial charge and d is the molecular length). However, the long rod length in this system results in significant losses and heat generation during FE switching. Therefore, enhancing the partial charge (q) of compact dipolar molecules while maintaining high dipole moment density and tunable FE properties remains an ongoing challenge. Now, writing in Science, Zhu et al. tackled these problems through a unique design strategy that accommodates small, strongly dipolar disulfonyl fluorine-free polymers, –SO2CH2CHRCH2SO2– (R = –H or –CH3), which endows a high dipole moment (9 D) comparable to that of the long rod-like molecules (~10 D). The most striking feature of these polymers used in their case is that by manipulating the R group from R = –H to R = –CH3, the normal ferroelectricity (FE-2SO2P) can be simply tailored to relaxor ferroelectricity (RFE-2SO2P). Both experiments and simulations confirm that this ferroelectric order arises from the strong dipole–dipole interactions between adjacent disulfonyl groups. Remarkably, RFE-2SO2P displays exceptional electroactuation and electrocaloric performance, with an excellent electroactuation strain (–4%) and a significant electrocaloric effect (ΔS of 14.8 J·kg–1·K–1) under low electric fields, matching state-of-the-art PVDF-based tetrapolymers, highlighting its potential for advanced thermal management applications.
1. Introduction
Ferroelectrics (FEs) have shown great potential in sensors, actuators, and electrocaloric cooling due to their direct cross-couplings between electric polarization and mechanical, thermal, and dielectric properties. Compared with oxide FEs, polymer FEs possess good flexibility and shape adaptability, making them promising candidates for flexible electronics and biocompatible devices. Despite decades of research, the number of FE polymers remains limited, with poly(vinylidene fluoride) (PVDF) being the most prominent due to its well-defined Curie transition and large spontaneous polarization. However, chemically modifying the semicrystalline structures of FE fluoropolymers is not only complex and costly, but also raises environmental and health concerns, as these materials are considered as "forever chemicals" due to their persistence.
In pursuing fluorine-free FE polymers, the use of strong dipolar molecules to induce FE ordering is regarded as a promising strategy. This approach, initially proposed by Bohr a century ago and later validated in FE nematic and smectic A liquid crystals, relies on the alignment of rod-shaped molecules with large dipole moments (μ, μ = qd, where q is the partial charge and d is the molecular length). However, the long rod length in this system results in significant losses and heat generation during FE switching. Therefore, enhancing the partial charge (q) of compact dipolar molecules while maintaining high dipole moment density and tunable FE properties remains an ongoing challenge.
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Wentao Yao, Mingli Liang, Sasa Wang, Qiang Zhao (2025). Fluorine-free polymers set a new benchmark for ferroelectrics. SinoTechIntel Verified Research. https://doi.org/10.1088/1674-4926/25080021
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Frequently Asked Questions
What are fluorine-free ferroelectric polymers?
Fluorine-free ferroelectric polymers are materials that exhibit ferroelectricity without containing fluorine atoms, addressing environmental and health concerns associated with fluoropolymers like PVDF. They rely on strong dipolar interactions, such as those in disulfonyl groups, to achieve polarization switching.
How do disulfonyl polymers achieve high dipole moments?
Disulfonyl polymers incorporate compact, strongly dipolar sulfonyl groups (–SO2–) that contribute to a high dipole moment of about 9 Debye, comparable to longer rod-like molecules. The strong dipole–dipole interactions between adjacent groups induce ferroelectric ordering.
What is the significance of tuning R groups in these polymers?
By changing the R group from hydrogen (–H) to methyl (–CH3), the polymer transitions from normal ferroelectricity (FE-2SO2P) to relaxor ferroelectricity (RFE-2SO2P). This simple modification allows tailoring of ferroelectric properties, enabling optimization for specific applications.
What performance do these fluorine-free polymers achieve?
The relaxor ferroelectric polymer (RFE-2SO2P) exhibits an electroactuation strain of –4% and a significant electrocaloric effect (ΔS = 14.8 J·kg⁻¹·K⁻¹) under low electric fields, matching state-of-the-art PVDF-based tetrapolymers. This highlights its potential for advanced thermal management and flexible electronic devices.
Why are fluorine-free alternatives important?
Fluoropolymers like PVDF are persistent environmental pollutants, often termed 'forever chemicals,' and their synthesis is complex and costly. Fluorine-free alternatives offer a more sustainable and environmentally friendly option while maintaining comparable ferroelectric performance.
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