Self-assembly of core-shell magnetic bottlebrush poly(ionic liquid)s: morphologies vs. magnetic properties†

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