Surface-initiated mechano-ATRP as a convenient tool for tuning of bidisperse magnetorheological suspensions toward extreme kinetic stability

A concept initially intended for mechanically controlled atom transfer radical polymerization (hereinafter referred to as “mechano-ATRP”) was extended in order to modify magnetic nanoparticles (NPs) with poly(methyl acrylate) (PMA) grafts, resulting in NPs@PMA hybrids which served as efficient additives in magnetorheological (MR) suspensions. In a novel procedure, magnetic NPs with an ATRP initiator anchored on their surfaces were added into a mechano-ATRP mixture, bringing about surface-initiated growth of the PMA chains after exposure to ultrasonication (35 kHz, 45 °C). The reaction proceeded with low concentration (at hundreds of “ppm”) of the CuBr2 catalyst, thereby providing the PMA chains with low dispersity of molar masses and high monomer conversions. Investigation was conducted as to the effect of hexagonal micro-ZnO and cubic-phase BaTiO3 piezoelectric transducers on the feasibility of the process. The presence of PMA on the surfaces of NPs@PMA hybrids was proven by infra-red spectroscopy, as well as thermal and magnetization analyses. The NPs@PMA hybrids were subsequently applied as additives to fabricate a bidisperse MR suspension, in which they unexpectedly enhanced accessible shear stress and yield stress values by up to ∼840 Pa under a magnetic field (of up to 432 kA m−1). Notably, the MR suspension supplemented with the NPs@PMA hybrids synthesized by mechano-ATRP exhibited no sign of sedimentation when left undisturbed for 2 days, although sedimentation of the reference sample occurred within a few hours. Such enhancements were attributed to the effects of friction, formation of a 3D gel-like network and the reduced density of the NPs@PMA hybrids.