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Relaxor ferroelectric polymers exhibiting a giant electrocaloric effect (ECE) can potentially be used to create next‐generation solid‐state coolers. Under an electric field, poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) terpolymer goes through a large dipolar entropy change producing a high adiabatic temperature change (ΔT_ECE). This work resolves the molecular origins of the large entropy change behind the electric field‐induced dipole switching. A Fourier transform infrared spectroscopy equipped with a high voltage source is used to operandoly observe the characteristic molecular vibrational modes. A short‐range trans (T) conformation of the CF₂‐CH₂dyads interrupted by a gauche (G) conformation, e.g., TTTG in the terpolymer chain, undergoes a dynamic transformation that leads to a corresponding ΔT_ECEwhenever an electric field is applied. The molecular dynamics simulation also proves that the energy barrier that the transformation from TTTGs into a long T sequence overcomes is smaller than that for all other conformations. A mixed solvent system is used to obtain T3G‐enriched terpolymer films exhibiting a 4.02 K ΔT_ECEat 60 MV m⁻¹and these films are employed to manufacture a 2‐layer‐cascaded cooling device that achieves a 6.7 K temperature lift, the highest reported value for a 2‐layer cascaded device made of fluoropolymers.

 

Poly(vinylidene fluoride) (PVDF)‐based polymers demonstrate great potential for applications in flexible and wearable electronics but show low piezoelectric coefficients (e.g., −d₃₃< 30 pC N⁻¹). The effective improvement for the piezoelectricity of PVDF is achieved by manipulating its semicrystalline structures. However, there is still a debate about which component is the primary contributor to piezoelectricity. Therefore, current methods to improve the piezoelectricity of PVDF can be classified into modulations of the amorphous phase, the crystalline region, and the crystalline–amorphous interface. Here, the basic principles and measurements of piezoelectric coefficients for soft polymers are first discussed. Then, three different categories of structural modulations are reviewed. In each category, the physical understanding and strategies to improve the piezoelectric performance of PVDF are discussed. In particular, the crucial role of the oriented amorphous fraction at the crystalline–amorphous interface in determining the piezoelectricity of PVDF is emphasized. At last, the future development of high performance piezoelectric polymers is outlooked.

 

Among all ferroelectric polymers, poly(vinylidene fluoride) (PVDF)-based polymers exhibit the best piezoelectric properties and thus are promising for sensors, actuators, and energy harvesters in flexible/wearable electronics and soft robotics. Despite decades of research effort, the structure-property relationship is still unclear for ferroelectric polymers, and their piezoelectric performance is often limited to ~30 pC/N. In this study, we report the effects of chemical defects [i.e., the head-to-head and tail-to-tail (HHTT) sequence] and high-power ultrasonication on the piezoelectric performance of PVDF. Two PVDF homopolymers with different HHTT contents were studied. The PVDF with a lower HHTT content (4.3%) exhibited a higher melting temperature (Tm, denoted as HMT), whereas that with a higher HHTT content (5.9%) exhibited a lower Tm (denoted as LMT). In addition to the primary crystals (PCs) and the isotropic amorphous fraction, wide-angle X-ray diffraction also suggested the presence of the oriented amorphous fraction (OAF) and secondary crystals (SCs), which are important in enhancing the piezoelectricity for PVDF. Intriguingly, the LMT PVDF exhibited higher piezoelectric performance than the HMT PVDF, because it had a higher OAF/SC content. In addition, high-power ultrasonication was shown to effectively break relaxor-like SCs off from the PCs, further enhancing the piezoelectric performance. That is, the inverse piezoelectric coefficient d31 reached as high as 76.2 pm/V at 65 °C for the ultrasonicated LMT PVDF. The insight from this study will enable us to design better piezoelectric PVDF polymers for practical electromechanical applications. 

Although high piezoelectric coefficients have recently been observed in poly(vinylidene fluoride- co -trifluoroethylene) [P(VDF-TrFE)] random copolymers, they have low Curie temperatures, which makes their piezoelectricity thermally unstable. It has been challenging to achieve high piezoelectric performance from the more thermally stable PVDF homopolymer. In this report, we describe how high-power ultrasonic processing was used to induce a hard-to-soft piezoelectric transition and improve the piezoelectric coefficient d 31 in neat PVDF. After high-power ultrasonication for 20 min, a uniaxially stretched and poled PVDF film exhibited a high d 31 of 50.2 ± 1.7 pm V −1 at room temperature. Upon heating to 65 °C, the d 31 increased to a maximum value of 76.2 ± 1.2 pm V −1 , and the high piezoelectric performance persisted up to 110 °C. The enhanced piezoelectricity was attributed to the relaxor-like secondary crystals in the oriented amorphous fraction, broken off from the primary crystals by ultrasonication, as suggested by differential scanning calorimetry and broadband dielectric spectroscopy studies. 

Whether intentionally generating acoustic waves or attempting to mitigate unwanted noise, sound control is an area of challenge and opportunity. This study investigates traditional fabrics as emitters and suppressors of sound. When attached to a single strand of a piezoelectric fiber actuator, a silk fabric emits up to 70 dB of sound. Despite the complex fabric structure, vibrometer measurements reveal behavior reminiscent of a classical thin plate. Fabric pore size relative to the viscous boundary layer thickness is found—through comparative fabric analysis—to influence acoustic‐emission efficiency. Sound suppression is demonstrated using two distinct mechanisms. In the first, direct acoustic interference is shown to reduce sound by up to 37 dB. The second relies on pacifying the fabric vibrations by the piezoelectric fiber, reducing the amplitude of vibration waves by 95% and attenuating the transmitted sound by up to 75%. Interestingly, this vibration‐mediated suppression in principle reduces sound in an unlimited volume. It also allows the acoustic reflectivity of the fabric to be dynamically controlled, increasing by up to 68%. The sound emission and suppression efficiency of a 130 µm silk fabric presents opportunities for sound control in a variety of applications ranging from apparel to transportation to architecture.