Recently, Poly(vinylidene fluoride) (PVDF) based magnetoelectric (ME) composites have been studied with strong interests for feasible applications using high flexibility, non-brittle, low-temperature processing and good price competitiveness. In order to improve ME characteristics, polymer based ME composites were supposed to be developed with an effective transition of β phase exhibiting electrically asymmetric structure in the piezoelectric matrix. Therefore, various approaches were addressed for high β phase transition via inorganic filler addition, thermal annealing, uniaxial stretching, melting-quenching, electrical poling and solvent casting. Even though most of above approaches were simultaneously used during ME sample preparation, any research group has never been scrutinized exact roles of nanofillers on PVDF phase transition at each processing step.
In this study, the effects of two-type magnetostrictive nanofillers on PVDF phase transition were investigated by FT-IR analysis at each processing step of solvent casting, annealing, stretching and electrical poling, respectively. As magnetostrictive nanofillers CoFe2O4 were synthesized with two different structures of nanoparticles and nanorods by a hydrothermal method. Then neat PVDF (S1), PVDF/nanoparticles (S2) and PVDF/nanorods (S3) films were fabricated by solvent casting. Those films were found to exhibit β phase fraction of S1=77.1%, S2=77.4% and S3=78.6%. After thermal annealing of 210oC for 10min, the β phase fraction was decreased to S1=29.5%, S2=29.9% and S3=35.8%. From two processes of solvent casting and thermal annealing, it is described that nanorods can serve confinement effect preventing α-phase transition from β-phase via mechanically hindering of PVDF chain shrinkage. After stretching process with stretch ratios 5, the β phase fraction was increased up to S1=72.5%, S2=78.5% and S3=76.7%. That is because two critical influences of stress concentration effect and active area occurred. The S2 and S3 films exhibited higher β-phase through stress concentration given by nanofiller structures in PVDF matrix. As well, the S2 films showed maximum β-phase since nanoparticles have large surface area compared with nanorods, which can be an active area expressing dominant concentration effect. After poling process following thermal annealing, the β phase fraction was increased up to S1=40.4%, S2=46.5% and S3=37.3%. During the poling process, nanorods served as high electrical leakage source in the S3 films. On the other hand, the S2 films have less leakage current with maximum transition, which might be induced by an effective surface charge. We believe that this study about confinement effect, stress concentration effect, active area and leakage current in terms of magnetostrictive filler’s roles can be a meaningful reference to optimize magnetoelectric composites.