Magnetoresponsive, anisotropic composite particles reversibly changing their chain lengths by a combined external field

Abstract

Magnetoresponsive, anisotropic composite particles were prepared to explore a new type of building blocks reversibly changing their chain lengths by switching on an external magnetic field. The composite particles were synthesized with three-step polymerization comprising (i) polymerization to coat magnetoresponsive silica particles with crosslinked poly(methyl methacrylate) (PMMA), (ii) polymerization to form a polystyrene (PSt) lobe on the PMMA-coated particles and (iii) polymerization to form another PSt lobe on the opposite side of the former lobe. The structure of the composite particles was analyzed with scanning transmission electron microscopy showing rod-like polymer particles incorporating a magnetoresponsive particle in the middle of a rod-like particle. The composite particles suspended in aqueous solution of polyvinylpyrrolidone used as a viscosity enhancer were observed by optical microscopy under applied external fields. Application of an alternating electric field at a high frequency of 2 MHz oriented the rod-like particles parallel to the electric field and assembled them to form pearl-chain structures of the composite particles. The chain lengths of the oriented rod-like particles were extended during the application of the electric field. While applying the electric field, an additional application of magnetic field with a field strength of 100 mT changed the chain structure so as to allow the magnetoresponsive parts to come close to each other. A combined application in which the magnetic field was switched on and off intermittently under a fixed electric field could reversibly compress and extend the particle chains and control their chain lengths.