Mesophase Structure-Enabled Electrostrictive Property in Nylon-12-Based Poly(ether-block-amide) Copolymers

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 (S-1)-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 S-1, 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 S-1 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 S-1 from mesomorphic nylon-12 crystals (i.e., nanoactuation) and elongational S-1 from amorphous phases determines the ultimate electrostriction behavior in stretched PEBAX films.