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.

To search for alternative electrostrictive polymers and to understand the underlying mechanism, the structure‐ferroelectric/electrostrictive property relationship for nylon‐12‐based poly(ether‐b‐amide) multiblock copolymers (PEBAX) is investigated. Two PEBAX samples are studied, namely, P6333 and P7033 with 37 and 25 mol.% of soft poly(tetramethylene oxide) (PTMO) blocks, respectively. In both samples, poorly hydrogen‐bonded mesophase facilitates electric field‐induced ferroelectric switching. Meanwhile, the longitudinal electrostrictive strain (S1)–electric field (E) loops are obtained at 2 Hz. Different from conventional poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)]‐based terpolymers, uniaxially stretched nylon‐12‐based PEBAX samples exhibit negative S1, that is, shrinking rather than elongation in the longitudinal direction. This is attributed to the unique conformation transformation of nylon‐12 crystals during ferroelectric switching. Namely, at a zero electric field, crystalline nylon‐12 chains adopt a more or less antiparallel arrangement of amide groups. Upon high‐field poling, ferroelectric domains are enforced with more twisted chains adopting a parallel arrangement of amide groups. Meanwhile, extensional S1 is observed for P6333 at electric fields above 150 MV m−1. This is attributed to the elongation of the amorphous phases (i.e., amorphous nylon‐12 and PTMO). Therefore, competition between shrinking S1 from mesomorphic nylon‐12 crystals (i.e., nanoactuation) and elongational S1 from amorphous phases determines the ultimate electrostriction behavior in stretched PEBAX films.

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