Self-assembly of core-shell magnetic bottlebrush poly(ionic liquids): morphologies vs magnetic property

Magnetic polymers show great potential in biomedical, electronic devices and magnetic responsive materials. The structure and property relationship are very important in design of organic magnetic polymers. In this work, core-shell magnetic bottlebrush polymers (PS-b-QPDMA[FeCl4]) with polystyrene as core block and magnetic poly(ionic liquid)s (QPDMA[FeCl4]) as shell block is prepared by ring-opening metathesis polymerization and post-modification. The self-assembly of core-shell PS-b-QPDMA[FeCl4] is studied in core and shell selecting solvent, respectively. It self-assembles into single macromolecular core-shell nanorod in shell selecting solvent while it self-assembles into core-shell nanoparticles in core selecting solvent. The magnetic property of PS-b-QPDMA[FeCl4] are studied in different morphologies via magnetic field and temperature dependent magnetization, respectively. Compared with the paramagnetic linear block copolymer, the bottlebrush polymer without self-assembly shows ferromagnetic interaction at low temperature with the bifurcation temperature (Tb) of the zero field cooling and field cooling curves of 3.2 K. The nanoparticle self-assemblies show the similar Tb at 3.8 K, while the single macromolecular nanorod self-assemblies show a high Tb of 9.8 K, demonstrating the much stronger ferromagnetic interaction for the nanorod self-assemblies. The increasing of Tb is probably due to the confinement of glassy PS core in the nanorod self-assemblies, which decreases the Fe(III)-Fe(III) distance to enhanced the ferromagnetic interaction of FeCl4- units.