Stretchy ferroelectrics: a) A new ferroelectric material combines elasticity with relatively high crystallinity. b) The unstrained material. c) The elastic ferroelectric under 70% strain. (Courtesy: NIMTE)

Although polymers are usually flexible, polymer-based ferroelectric materials tend to be rigid. Adding a small amount of crosslinking material can change that, however, and researchers at China’s Ningbo Institute of Materials Technology and Engineering (NIMTE) say that their new “elastic ferroelectrics” are resilient and flexible enough for use in wearable electronics and implantable medical devices.

Ferroelectricity is a material’s ability to change its electrical properties in response to an applied electric field. It was discovered just over a century ago in certain naturally-occurring crystals and is now exploited in a wide range of technologies, including digital information storage, sensing, optoelectronics and neuromorphic computing.

Conventional ferroelectrics can be made from either ceramics or polymers, but even polymer-based ferroelectrics are not very elastic. This is because they contain crystalline regions that are rigid.

Crosslinked polymer network

Researchers led by Run-Wei Li have now solved this problem by adding a cross-linking chemical, soft-long-chain polyethylene oxide, to the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene).

“Crosslinking is a general way to endow resilience to plastic polymers in which the crosslinking density range is 1-10% (that is, one to ten repeat units crosslinked in each one hundred repeat units in polymer chains),” explains study team member Ben-Lin Hu.

At the higher end of this range, however, Hu adds that the crystallinity of the mixture decreases dramatically, weakening the material’s ferroelectric response. The crosslinking density needed to make elastic ferroelectrics is therefore much lower, leading the researchers to call it “slight crosslinking”.

When the NIMTE researchers limited the density of the crosslinker to just 1-2%, they found that a beta-phase crystalline structure was uniformly dispersed in the crosslinked polymer network. This new crosslinked polymer network can evenly distribute and bear external forces, say the researchers, mitigating damage to the crystalline regions and creating a new ferroelectric material that combines elasticity with relatively high crystallinity. Indeed, the cross-linked film retains its ferroelectricity even under strains of 70% thanks to its improved elasticity.

The new elastic ferroelectric could be used in wearable/implantable electronics, such as sensors and smart healthcare, as well as in information storage and energy transduction, Hu says. Elastic ferroelectrics also have some exotic properties that might be useful for structures such as elastomers with a giant (>1000) dielectric constant, spin valves with a large magnetoelectric coupling effect and dielectric capacitors that have energy densities on a par with lithium-ion batteries, but with charging and discharging times on the order of just microseconds.

The researchers say they now plan to optimize the properties of their elastic ferroelectrics and will focus mainly on materials with high dielectric and high piezoelectric constants. “These could be used in energy storage and transduction and information sensing and memory,” Hu tells Physics World.

They detail their present work in Science.

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