No abstract

The extremely low room-temperature ionic conductivity of solid-state polymer electrolytes (SPEs) ranging from 10-7 to 10-5 S cm-1 seriously restricts their practical application in solid-state lithium metal batteries (LMBs). Herein,...

Piezoelectric polymers hold great potential for various electromechanical applications, but only show low performance, with |d33 | < 30 pC/N. We prepare a highly piezoelectric polymer (d33 = −62 pC/N) based on a biaxially oriented poly(vinylidene fluoride) (BOPVDF, crystallinity = 0.52). After unidirectional poling, macroscopically aligned samples with pure β crystals are achieved, which show a high spontaneous polarization (Ps) of 140 mC/m2. Given the theoretical limit of Ps,β = 188 mC/m2 for the neat β crystal, the high Ps cannot be explained by the crystalline-amorphous two-phase model (i.e., Ps,β = 270 mC/m2). Instead, we deduce that a significant amount (at least 0.25) of an oriented amorphous fraction (OAF) must be present between these two phases. Experimental data suggest that the mobile OAF resulted in the negative and high d33 for the poled BOPVDF. The plausibility of this conclusion is supported by molecular dynamics simulations.

Oriented amorphous fraction in biaxially oriented poly(vinylidene fluoride) gives it significantly enhanced dielectric and ferroelectric properties.

No abstract

Although electrostriction is ubiquitous for dielectric polymers, giant electrostriction has not been observed until relaxor ferroelectric (RFE) poly­(vinylidene fluoride) (PVDF)-based polymers are achieved. However, the exact origin for giant electrostriction in these polymers has not been fully understood. By studying the electrostriction in the uniaxially stretched films of a ferroelectric poly­(VDF-co-trifluoroethylene) [P­(VDF-TrFE)] random copolymer and an RFE poly­(VDF-co-TrFE-co-chlorotrifluoroethylene) [P­(VDF-TrFE-CTFE)] random terpolymer in this work, we confirmed that ferroelectric switching with large hysteresis, such as in the case of P­(VDF-TrFE), was not genuine electrostriction. By decreasing large ferroelectric domains to the nanometer scale (i.e., 2–3 nm), such as in the case of the P­(VDF-TrFE-CTFE) terpolymer, electrostriction with low hysteresis could be achieved. Two origins of the large electrostriction in these polymers were identified. The first was the mechano-electrostriction due to the poling field-induced conformation transformation of oriented polymer chains. The second was the electric repulsion of electrically aligned nanodomains. These effects could occur in both crystals and the oriented amorphous fraction, which links between the nanocrystals and the isotropic amorphous fraction. When the poling field was relatively low (e.g., <40 MV/m), the mechano-electrostriction was the major contribution and the electric repulsion effect was a minor contribution to electrostriction. Meanwhile, a strong temperature dependence of the low-field electrostriction coefficient was observed. Finally, we found an empirical inverse relationship between the electrostriction coefficient and the product of Young’s modulus and dielectric constant. The knowledge obtained from this study provides an insightful understanding of the electrostriction mechanism in PVDF-based electroactive polymers, which will find potential applications in sensors and actuators for wearable electronics and soft robotics.

In response to the stringent requirements for future DC-link capacitors in electric vehicles (EVs), it is desirable to develop dielectric polymer films with high-temperature tolerance (at least 105 °C) and low loss (dissipation factor, tan δ < 0.003). Although the biaxially oriented poly(ethylene terephthalate) (BOPET) film has an alleged temperature rating of 120 °C, its dielectric performance in terms of breakdown strength and lifetime cannot satisfy the stringent requirements for power electronics in EVs. In this work, we carried out a structure−electrical insulation property relationship study to understand the working mechanism for various PET films, including a commercial BOPET film, an amorphous PET (AmPET) film, and two annealed PET films (AnPET, i.e., cold-crystallized from AmPET). Structural analyses revealed a uniform edge-on crystalline orientation in BOPET with the a* axis in the film normal direction. Meanwhile, a high content of the rigid amorphous fraction (RAF) was identified for BOPET, which resulted from biaxial stretching during processing. On the contrary, AnPET films had a random crystal orientation with lower RAF contents. From dielectric breakdown and lifetime studies, the high-crystallinity AnPET film exhibited better electrical insulation than BOPET, and AmPET had the worst electrical insulation. Electrical conductivity results revealed that the high RAF content in BOPET led to reasonably high breakdown strength and long lifetime only at low temperatures (<100 °C). Meanwhile, PET crystals were more insulating than the amorphous phase, whether mobile, rigid, or glassy. In particular, the flat-on lamellae in the AnPET film were more effective than the edge-on lamellae in BOPET in blocking the conduction of charge carriers (electrons and impurity ions). This understanding will help us design high-temperature semicrystalline polymer films for DC-link capacitors in EVs.

The low properties of recycled polymers associated with high cost of recycling hinder development of the thermoplastic recycling industry. Dynamic cross-linking of recycled thermoplastics with the formation of vitrimers enables superior mechanical properties, good reprocessability, and superior chemical and environmental resistance. Herein, a poly(ethylene-vinyl acetate) (EVA) vitrimer was formed by cross-linking with triethyl borate with the catalyst (bis-(acetylacetonato)dioxomolybdenum(VI)). The resultant EVA vitrimer shows enhanced thermal stability and mechanical properties with up to two times improvement in Young' modulus and storage modulus by comparison with the thermoplastic EVA. Moreover, 90% of Young's modulus was maintained in the EVA vitrimer after five times of recycling, whereas only 72% can be maintained for recycling the thermoplastic EVA. This dynamic cross-linked EVA also exhibits superior UV and solvent resistance, which helps extend the service time of the recycled material. This work introduces a facile and efficient method to recycle and reuse EVA with low property loss. It has the potential to enable the production of highperformance EVA from EVA waste for different applications.