Magnetic Field‐Directed Self‐Assembly of Magnetic Nanoparticle Chains in Bulk Polymers

Abstract

Self-assembly is the thermodynamically guided organization of disordered systems into more highly ordered structures. The process of life exemplifi es the power and diversity of selfassembly, and understanding and controlling self-assembly is crucial for engineering advanced materials from molecular and nanoscale precursors. Nature has preceded researchers' efforts in magnetic fi eld-directed self-assembly (MFDSA) in magnetotactic bacteria, which navigate in the earth's magnetic fi eld using magnetically coupled chains of iron oxide nanoparticles (NPs). [ 1 ] Here, we report the MFDSA of magnetic NPs into threedimensional (3D) arrays of chains embedded within bulk poly mers, where no volatile solvent is present during the polymerization process. Application of uniform magnetic fi elds drives formation of magnetic NP chains that repel each other, resulting in their assembly into an array with quasiperiodic ordering. Removing the solvent from a 3D structure would cause collapse due to capillary forces. Therefore, special measures are required to preserve the structure [ 2 ] after removing the fi eld, such as embedding within a polymer. Polymer composites and thin fi lms containing NPs, including magnetic NPs, are well known, [3][4][5][6][7][8][9][10][11][12][13][14][15][16] as are microgels [ 17 ] and microcapsules. [ 18 ] There is signifi cant interest in embedding NP chains within polymers for their potentially anisotropic mechanical, [ 19 ] electrical, optical, magnetic, and thermal properties. Diverse applications are envisioned, including actuators, [ 20,21 ] artifi cial cilia, [ 22,23 ] ferrogels, [24][25][26] sensors, [ 27 ] polymer solar cells, [ 28,29 ] electromagnetic interference (EMI) shielding, [ 30,31 ] and drug delivery. [ 32 ] Several methods have been employed to form NP chains, often utilizing polymer templates to direct the assembly of NPs. [ 33 ] For example, there have been many studies of NP selfassembly at interfaces within [34][35][36][37][38][39][40][41][42][43][44][45][46] and on the surfaces of block copolymers, [47][48][49][50] but these systems are limited by the morphologies of block copolymers. Here, our focus is on template-free self-assembly of magnetic NPs, where MFDSA has potential to provide control and tunability over the self-assembly process without the use of templates.Stellacci and co-workers [ 51 ] have assembled magnetic NP chains in zero fi eld by crosslinking the ligand shells, [ 52 ] but most examples of magnetic NP chaining have utilized magnetic interactions between the NPs: Magnetotactic bacteria synthesize iron oxide NPs that form chains through magnetic interactions. Cryogenic TEM measurements of solutions of iron oxide NPs by Philipse and co-workers [ 53,54 ] showed that magnetic NPs spontaneously form disordered chains in solution in zero applied fi eld, if the NP diameter exceeds a minimum size. Pyun and co-workers [ 55,56 ] have also shown that strongly interacting Co NPs spontaneously form chains, and application of magnetic fi elds causes the chains to straig...

Generic model for tunable colloidal aggregation in multidirectional fields

“…The aggregated clusters can be dispersed in liquid, while the percolated networks can be embedded in a polymer or gel medium. 54 The key to the fabrication of such novel classes of materials containing particle clusters and networks is the control of the process parameters to obtain the desired interconnectivity, density and structure. Against this background, the focus of our theoretical study was to understand the formation of transient, aggregated structures appearing at low-temperatures.…”

Tuning the Colloidal Crystal Structure of Magnetic Particles by External Field

“…Using microradian X-ray scattering technique we have investigated the different crystal structures exhibited by self-assembly of core-shell magnetite/silica nanoparticles. [8][9][10][11][12][13][14][15] This bottom up approach utilizes the fast, anisotropic, and reversible nature of the magnetic dipole-dipole interaction, which can either be attractive or repulsive in nature depending on the angle between the magnetic field and the line connecting the dipoles. We find that the equilibrium structure in absence of the field is random hexagonal close-packed (RHCP) one.…”

“…They have been widely used in the material fabrication and design of functional devices in many fields, including imaging, [1] catalysis, [2] medicine, [3][4][5] and DNA separation. [8][9][10][11][12][13][14][15] This bottom up approach utilizes the fast, anisotropic, and reversible nature of the magnetic dipole-dipole interaction, which can either be attractive or repulsive in nature depending on the angle between the magnetic field and the line connecting the dipoles. For instance, magnetic and electric field guided nanoparticles are being used to create superstructures, ranging from string fluids to crystalline phases.…”

“…For instance, magnetic and electric field guided nanoparticles are being used to create superstructures, ranging from string fluids to crystalline phases. [8][9][10][11][12][13][14][15] This bottom up approach utilizes the fast, anisotropic, and reversible nature of the magnetic dipole-dipole interaction, which can either be attractive or repulsive in nature depending on the angle between the magnetic field and the line connecting the dipoles. The mere fact that a colloidal self-assembly can be tuned externally by a magnetic field has provided enormous flexibility as well as a wider scope for experimental design in diverse fields, for example, photonics, [16,17] drug delivery, [18,19] patterning, [20][21][22][23][24] and magnetic levitation.…”

Surface degradation and nanoparticle release of a commercial nanosilica/polyurethane coating under UV exposure

“…Flocculation behavior of nanoparticles dispersed in polymer systems has been reported in previous studies (28–31). Various strategies for controlling the dispersion of nanoparticles in polymers are employed such as directed self-assembly techniques (32) or including additives or surfactants, which may have been used here. The role of dispersion on the degradation and nanoparticle release of nanocoatings and nanocomposites is a subject of great interest for both performance and EHS perspective.…”