Magnetic field induced uniaxial alignment of the lyotropic liquid-crystalline PMMA-grafted Fe3O4 nanoplates with controllable interparticle interaction

Magnetite (Fe3O4) nanoplates with a hexagonal platelet shape were synthesized by two steps: hydrothermal synthesis of iron(iii) oxide (α-Fe2O3) nanoplates followed by wet chemical reduction of the α-Fe2O3 nanoplates. Then, poly(methyl methacrylate) (PMMA) chains were grafted onto the surface of the hexagonal Fe3O4 nanoplates (F) via surface-initiated atom transfer radical polymerization (SI-ATRP), which ensures dispersion stability in organic solvents and ionic liquids. After mixing with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim+][NTf2−]), a representative ionic liquid, the resulting PMMA-modified F were found to show good lyotropic liquid-crystalline (LC) behaviour in [Emim+][NTf2−] and to exhibit a fast response to the application of an external magnetic field. Ultrasmall-angle synchrotron X-ray scattering (USAXS) measurements verified that the PMMA chain length, the weight ratio of the ionic liquid and the external magnetic field could significantly influence the interparticle distance (ID) of the PMMA-modified F in [Emim+][NTf2−]. In particular, the lyotropic LC phase could be assigned as a nematic phase with a columnar alignment. In addition, the PMMA-modified F maintained a uniaxially aligned nematic columnar structure along the magnetic field direction. Our study also determined the mechanism for the special alignment of the PMMA-modified F under an external magnetic field by analysing the growth axis, the easy magnetic axes, and the interparticle distance of F. The results suggested that the special alignment of the PMMA-modified F was affected by the interparticle interaction caused by the PMMA long chains on F under the magnetic field. Furthermore, the present study revealed that PMMA-modified F exhibited a new magnetic field responsive behaviour that led not only to the formation of a uniaxial alignment structure but also to control of ID with the help of the PMMA soft corona under the application of a magnetic field. These features could prove to be a promising advance towards novel applications of magnetic nanoparticles (NPs), such as functional magnetic fluids, rewritable magnetic switching devices, and smart magneto-electrochemical nanosensors.


Determination of modification densities on Fe 3 O 4 1.1 Calculation of modification densities of amine groups and SI-ATRP initiators on Fe 3 O 4
Modification amounts of amine groups and SI-ATRP initiators on F were determined by TA. The TA measurements were carried out under Ar gas, and the heating ratio was fixed to 10 ºC/min. Here, aminegroups modified F and SI-ATRP initiator-modified F were abbreviated by FN and F*, respectively. Weight losses of F, FN, and F* by TA measurements were abbreviated as L F (%), L FN (%), and L F* (%), respectively. The modification density of amine groups on F was calculated by the following equation (eq. S1). Here, N A is Avogadro constant and SA ES is the estimated specific surface area of F. The measurement result of SA ES is 14 m 2 /g. MW a means a molecular weight of organic moieties on F. In this case, we subtracted molecular weight of trimethoxysilyl group from molecular weight of N- [3-(trimethoxysilyl)propyl]aniline to calculate MW a (MW a = 134). eq. S1 Next, SI-ATRP initiator modification densities (D I ) on F were calculated by the following equation (eq. S2 and S3). Here, x means reaction rate of amine groups in the initiator modification and y means amine groups modification amount for 1 g of F. MW b means a molecular weight of organic moieties on F which subtracted molecular weight of trimethoxysilyl group after reacting with BBI (MW b = 401).

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Electronic Supplementary Material (ESI) for Nanoscale Advances. This journal is © The Royal Society of Chemistry 2020 S1 shows TA profiles of F, amine-groups modified F, and SI-ATRP initiator-modified F.

PMMA modification densities
Modification amounts of PMMA were also determined by TA. Weight of PMMA for 1 g of F was marked as W FP . Here, L FP % is weight loss of FPm (m = 1, 2, 3). eq. S4

Calculation results
The D I values, D P values and molecular weight distribution were listed in Table. S1. The weight fractions (Fw) and volume fractions (Fv) mean the proportion of PMMA chains' weight and volume in overall FPm, respectively.

POM
POM observation was utilized to observe the lyotropic LC phases of FPm in ionic liquids. As is shown in Fig. S4a, only black images were observed because of the color of F and the not long enough PMMA chains. Fig. S4b

Sketches of dripping FP3/toluene solution under magnetic field
A drop of FP3/toluene solution (concentration: 0.1 g/L) was dripped on a TEM grid after applying an external magnetic field that was vertical or parallel to the TEM grid.

TEM image of F under a vertical magnetic field
A drop of F/toluene solution (concentration: 0.05 g/L) was dripped on a TEM grid after applying an external magnetic field that was vertical to the TEM grid. Most of F lay vertically along the magnetic field direction as is shown in Fig. S6.

Interparticle distance of FPm under an external magnetic field.
The interparticle distance results of FP1, FP2, and FP3 under an external magnetic field (320 Oe), obtained from the USAXS curves, were listed in the Table. S2.