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

Abstract

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.