Superior elastomeric nanocomposites with electrospun nanofibers and nanoparticles of CoFe2O4 for magnetorheological applications

Abstract

We show for the first time the greatly enhanced magnetic and magnetorheological (MR) properties of CoFe₂O₄–silicone elastomer (cross-linked polydimethylsiloxane) composites containing electrospun CoFe₂O₄ nanofibers. The effects of CoFe₂O₄ particle shape (spherical to fibrous), concentration and degree of alignment in the elastomeric matrix are explored. Low nanofiber loadings (5–10 wt%) give MR properties comparable to those obtained by much higher microparticle additions (60–80 wt%) for the same polymer matrix. Compared to spherical nanoparticles, the nanofiber composites have 100–400% improvement in MR properties that are vital in tunable rheology applications such as vibration damping, variable stiffness devices, actuators, etc. The nanofibers effectively formed an interconnected network within the polymer matrix but nanoparticles exhibited large gaps between two disconnected short particle chains at lower filler loading. Spherical particles with 69.46 emu g⁻¹ saturation magnetization were synthesized by ultrasonic assisted co-precipitation method using a novel precipitating solvent (i.e. propylamine) followed by calcination. The nanofibers were prepared by electrospinning a precursor of CoFe₂O₄ in a sacrificial polymer solution of polyvinylpyrrolydone (PVP) followed by calcination of electrospun fibers. Crystal structure and magnetic properties of the CoFe₂O₄ nanofillers were evaluated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometery (VSM), which showed the generation of larger crystallite size with greater coercivity and remnant magnetization in the nanofibers compared to nanoparticles. Thin films of the nanocomposites containing nanoparticles and nanofibers were prepared with 5 and 10 wt% of CoFe₂O₄. Anisotropic characteristics of the nanocomposites were generated by applying an in situ magnetic field of 0.4 T during solution casting of thin composite films. The magnetorheological study revealed that at 1.023 T, the isotropic nanocomposites with 10 wt% nanofibers showed absolute and relative MR effects of 4.4 kPa and 3.21%, respectively, compared to the corresponding nanoparticle containing counterpart's values of 2.0 kPa and 0.64%.