(255t) Ferromagnetic Ink: Covalent Attachment of Charged Polymers on Magnetic Nanoparticles Enables Magnetic Deinking

Deinking of printed paper is within the most expensive steps in paper recycling. Application of a highly magnetic ink can simplify this step and might face current issues, as printed (magnetic) and non-printed (non-magnetic) paper parts can be distinguished. Screening and sorting of magnetic from non-magnetic wastes is already a routine procedure in large scale waste processing. Via previous screening step, the use of magnetic ink can minimize the paper amount which has to be fed into the expensive deinking process. Furthermore the deinking process itself can be facilitated using magnetic inks as magnetic pigments can be simply separated from paper fibers by application of a magnetic field¹.

An appropriate magnetic ink affords high saturation magnetization and dispersionstability in an environmental friendly solvent (e.g. water). Magnetic nanoparticle dispersions are usually made from superparamagnetic materials since in the absence of an external magnetic field, the lack of magnetic particle/particle attraction allows the creation of stable dispersions². Common attempts to prepare stable dispersions of ferromagnetic particles (much higher saturation magnetization) failed since the strong magnetic particle/particle interaction can overcome repulsive effects from surfactants or steric stabilizers (typically polymers).

In the present work graphene protected metallic nanomagnets were modified with charged poly-(3-(methacryloylamino)propyl]trimethylammonium chloride (polyMAPTAC), in order to create highly stable dispersions of ferromagnetic nanoparticles. The covalently attached charged polymer brushes implement both steric and electrostatic repulsion forces which can counteract the magnetic attraction. It is demonstrated how the direct, covalent attachment of highly charged polymers on the graphene layers can circumvent stabilizer detachment³.

More specifically, graphene-coated metal nanoparticles, prepared by flame spray synthesis, are covalently functionalized with initiator moieties for atom transfer radical polymerization (ATRP) via diazonium chemistry. Using surface initiated ATRP the ionic polymer brushes (polyMAPTAC) could be successfully grafted from the particle surface. As the charged polymer grow from the surface, the created brushes cannot collapse due to the mutual electrostatic repulsion of the charged polymer side chains. Thus, the polymer brushes are outstretched and evolve a significant steric repulsion force besides the intrinsic electrostatic repulsion of the charged polymers. Particle size distributions with an average diameter of 24 nm provide a ferromagnetic fluid with unprecedented stability in water over months. Due to the covalent character of the connection between the ion-containing polymer brushes and the particle surface, detachment of the hydrophilic polymer brushes is not observed. As prepared fluids can be printed by common ink-jet devices and act as ferromagnetic water based ink¹.

[1] M. Zeltner, L. M. Toedtli, N. Hild, R. Fuhrer, M. Rossier, L. C. Gerber, R. A. Raso, R. N. Grass and W. J. Stark, Langmuir, 2013, 29, 5093.

[2] E. Kita, T. Oda, T. Kayano, S. Sato, M. Minagawa, H. Yanagihara, M. Kishimoto, C. Mitsumata, S. Hashimoto, K. Yamada and N. Ohkohchi, J. Phys. D. Appl. Phys., 2010, 43, 47401.

[3] M. Zeltner, R.N. Grass, A. Schaetz, S.B. Bubenhofer, N.A. Luechinger, W.J. Stark, J. Mater. Chem., 2012, 22, 12064.