Chapter 1: Functionality of Surface-modified Magnetite Nanoparticles with Controlled Sizes and Shapes Free

As a unique type of intelligent fluid, magnetorheological (MR) fluids (MRFs) with varying rheological properties were discovered in the late 1940s.¹  In general, MRFs are a type of field-responsive colloidal suspension composed of micron-sized magnetic particles in dispersing carrier oils.^2,3  In the absence of an external magnetic field, MRFs behave as a viscous suspension liquid. By the application of an external magnetic field, the magnetic particles acquire magnetic polarization, attract each other due to the effect of magnetic dipolar−dipolar interactions, and aggregate with anisotropic alignments along the magnetic field direction.^4,5  Thus, MRFs can transit from liquid-like fluids to solid-like states in a fraction of a millisecond and form chain-like structures upon application of an external magnetic field.⁶  However, the reverse structural transition occurs in MRFs when the external magnetic field is removed. Hence, controlling the external magnetic field can effectively modulate and adjust the MR properties, including viscosity, shear stress, and yield stress.⁷  Shear stress is the stress component parallel to the cross-sectional area of the material, while yield stress is the amount of stress or force required to cause a material to undergo plastic deformation or permanent deformation. The promising field-dependent rheological properties of MRFs have attracted significant attention over the past few decades for use in various engineering fields, such as in the production of dampers,⁸  absorbers,⁹  and brakes.¹⁰  More recently, biocompatible MRFs, concerning their applications in biotechnology, have been intensively researched,¹¹  including haptic devices¹²  and soft robotic grippers.¹³ 

The enhancement of MR performance is an important subject for the above-mentioned applications of MRFs. Many factors have significant influences on the rheological properties of MRFs, including magnetic field strengths,¹⁴  additives,¹⁵  particle sizes,¹⁶  particle shapes,¹⁷  and surface modification.¹⁸  Hence, the effects of size, shape, surface modification, and polymer molecular weight are mainly discussed in this chapter. Table 1.1 summarizes some studies on the effect of particle sizes, particle shapes, surface modification, and polymer molecular weights.

Particle sizes have a significant influence on magnetic properties, formation of chain structures, and dipolar−dipolar interactions in MRFs. In general, MRFs composed of large magnetic particles are supposed to exhibit highly magnetizable properties and high MR performance. Wu et al. investigated the effect of size on MRFs through experimental and simulation analyses.¹⁹  They found that MRFs composed of large magnetic particles show better rheological performance than those composed of small magnetic particles owing to the stronger chain structures and larger dipolar−dipolar interactions. In addition, Molazemi et al. also confirmed that the viscosity and magnetization of MRFs increase with increasing particle sizes, resulting in a significant improvement in mechanical properties.²¹ 

On the other hand, the rheological properties of MRFs are also affected by particle shapes. Shilan et al. compared the rheological effects of plate-like particles and spherical particles on MRFs.²⁶  They found that plate-like particles show larger contact surface and stronger frictional force than spherical particles, leading to an increase in sedimentation stability. More importantly, rheological measurements reveal that the yield shear stress of MRFs composed of plate-like particles is also significantly improved compared with those composed of spherical particles. In addition, Upadhyay et al. also observed the enhanced storage modulus and elastic modulus in flake-based MRFs compared with sphere-based MRFs.²³  Furthermore, by adding anisotropic rod-like particles to MRFs, Arief and Mukhopadhyay found that MRFs with more rod-like particles show stronger Payne effect and better MR response due to the enhanced field-induced structuration in contrast to MRFs with more spherical particles.²⁴ 

The poor long-term stability of MRFs restrains their applications in industrial fields although they exhibit substantial progress in research and commercialization.^34,35  To improve the MR performance, increasing the sedimentation stability and dispersion stability of magnetic particles to restrain their agglomeration has become an important issue.³⁶  Surface modification of magnetic particles with organic layers or polymers is regarded as a simple and efficient way to enhance the dispersion stability and maintain a homogeneous magnetic field.^37,38  By using silane coupling agents as organic modifiers, Wang et al. reported that the prepared MRFs exhibit good dispersion and typical MR performance.²⁷  Ronzova et al. also proved that silane coating improves the dispersion stability of magnetic particles in MRFs.³⁰  On the other hand, Cvek et al. used poly(glycidyl methacrylate) (PGMA) to modify magnetic particles in MR systems.³³  They found that MRFs composed of PGMA-modified particles exhibit promising stabilities and rheological properties in contrast to those composed of unmodified particles. However, magnetic particles modified by high-molecular-weight PGMA show few signs of increasing the rheological performance of MRFs compared with magnetic particles modified by low-molecular-weight PGMA. Further expanding the gap in polymer molecular weight might cause different effects. However, the investigation of the effect of polymer molecular weight is rarely reported. Hence, controlling the polymer molecular weight is also an important research focus in this chapter.

This chapter is organized as follows: we aim to explore the recent research about the influence of particle sizes, particle shapes, and surface modification with organic layers in Section 1.2 and investigate the effect of polymer molecular weight on the rheological properties of MRFs in Section 1.3. Finally, conclusions and prospects are outlined in Section 1.4.

In this section, we described the preparation of silica-coated Fe₃O₄ (Fe₃O₄@SiO₂) nanoparticles using a combination of “gel−sol method”, silica coating, and reduction process.³⁹  To compare the effect of size on the rheological performance of MRFs, we prepared two types of Fe₃O₄@SiO₂ nanospindles (MSn, n = 1, 2) with different sizes. To compare the effect of shape, two types of Fe₃O₄@SiO₂ nanocubes (MSn, n = 3, 4) with different sizes are also prepared. The corresponding transmission electron microscopy (TEM) images are shown in Figure 1.1a–d. In addition, to investigate the effect of surface modification on MRFs, dodecyltrimethoxysilane (DTM) is coated on the surfaces of MSn by a silane coupling reaction to make MSn form stable dispersions in silicon oil. The silane coupling reaction ensures that the coordinated organic layers of DTM are unlikely to fall off the MSn surface because of the formation of a covalent bond between the MSn surface and the organic layers of DTM. TEM images of DTM-modified MSn (DTM/MSn, n = 1–4) are shown in Figure 1.1e–h. Table 1.2 shows the information about particle sizes and particle volumes of MSn. The precise adjustment of particle volumes ensures the comparability of the effect of shape on MRFs.

To investigate the microstructures of magnetic particles upon application of an external magnetic field, we observed the TEM grids of MSn and DTM/MSn treated with an external magnetic field. As shown in Figure 1.2a–d, unmodified MSn with significant aggregations only show limited movement even under the influence of an external magnetic field. However, all DTM/MSn tend to form a chain-like structure under the action of an external magnetic field, as shown in Figure 1.2e–h. In addition, both DTM/MS1 and DTM/MS2 are oriented along the magnetic field direction and the long-axis direction of nanospindles. The oriented mechanism of nanospindles will be investigated in the following research.

As shown in Figure 1.3, the TEM image and electron diffraction pattern of MS1 are characterized to investigate the crystal growth direction of nanospindles. The electron diffraction pattern shows that the (111) direction is assigned as the major axis direction of MS1, which is the same as the easy magnetic axis of Fe₃O₄. Hence, it makes nanospindles orient along the long-axis direction upon application of an external magnetic field.

Magnetic properties of particles are important factors influencing the rheological performance of MRFs. Here, we investigated magnetization curves of DTM/MSn at room temperature as a function of the applied magnetic field (H) and mass magnetization (M). Figure 1.4 shows the determined saturation magnetization (Ms) values of DTM/MSn. The results indicate that the Ms values present an increasing tendency from DTM/MS1 to DTM/MS4. In addition, we can also find that large particles show a larger Ms value than small particles even though their shapes are the same. Furthermore, an inset image in Figure 1.4 suggests that the magnetization values of DTM/MSn approach the saturation values at 1 kOe of magnetic field strength. Hence, 1 kOe is regarded as a suitable strength of the applied magnetic field to compare the rheological performance of MRFs.

After being treated with a silane coupling reaction, DTM/MSn are mixed with silicone oil to form paste-like MRFs with a particle concentration of 7 vol%. Then, we studied the rheological properties of MRFs using a rotational rheometer (parallel gap: 0.20 mm) with the applied magnetic field ranging from 0 to 1 kOe. Here, we select a function of the shear stress (1 s⁻¹) and the magnetic field strengths to evaluate the related rheological properties of MRFs based on different magnetic particles.

First, we investigated the effect of size on the MR performance by comparing the shear stress curves of DTM/MS1 and DTM/MS2 in MRFs under different magnetic field strengths, as shown in Figure 1.5. The results indicate that the rate of shear stress increase of DTM/MS1 is lower than that of DTM/MS2, and the shear stress values of DTM/MS1 are exceeded by those of DTM/MS2 when applying a stronger magnetic field (over 0.5 kOe). As for the exact reason, the higher Ms value of DTM/MS2 has an impact on the rheological performance, and the larger size of DTM/MS2 promotes the formation of a powerful chain-like structure.

Then, we studied the effect of shape on the rheological performance by introducing DTM/MS1 and DTM/MS3 into MRFs. As mentioned earlier, MS1 and MS3 have similar particle volumes, and their particle volumes are almost unchanged even with the surface modification of DTM. In addition, the differences in Ms values in DTM/MS1 and DTM/MS3 are extremely quite low below 1 kOe of magnetic field strength. Hence, Figure 1.6 depicts an appropriate evaluation for the comparison of particle shape. The results of the measurement demonstrate that both shear stress values (1 s⁻¹) and growth rate of DTM/MS1 are higher than those of DTM/MS3 with the applied magnetic field ranging from 0 to 1 kOe. The electron diffraction analysis suggests that the aligned long-axis direction of MS1 is consistent with the easy magnetic axis direction. Considering the lower demagnetizing field coefficient of MS1, MS1 are easily magnetized along the magnetic field direction compared with MS3. The above-mentioned mechanisms make MS1 have a powerful strength in the form of a chain-like structure. Consequently, introducing anisotropic nanospindles MS1 into MRFs can enhance the MR performance.

Finally, we used a similar approach to investigate the effect of surface modification on the MR performance by comparing MRFs composed of DTM-modified particles (DTM/MS2) and those with unmodified particles (MS2), as shown in Figure 1.7. The dependence of the magnetic field on the shear stress (1 s⁻¹) indicates that the rheological performance of MRFs is reinforced after carrying out surface modification for the unmodified particles MS2. In previous TEM images of the particle orientation, we found that DTM/MS2 tend to form a powerful chain-like structure, while MS2 form a nonuniformly aggregated structure. Hence, the introduction of DTM effectively inhibits particle aggregation and reinforces the chain-like structure strength and rheological performance of MRFs.

Furthermore, to provide an adequate fitting model for the above-mentioned rheological behaviours, the Casson model, a conventional rheological model, is selected to evaluate the yield stress of MRFs. The corresponding fitting equation is given as follows (eqn (1.1)):

where $τ$ (Pa) is the shear stress, $τ0$ (Pa) is the yield stress associated with the applied magnetic field, $η∞$ is the shear viscosity at the infinite shear rate, and $γ$ is the shear rate (s⁻¹). Figure 1.8 summarizes the applied magnetic field dependence of the yield stress (⁠$τ0$⁠) based on the Casson model. The results suggest that the Casson model well reflected the previous rheological behaviours, as shown in Figures 1.5-7. Hence, the fitting model reveals the rationality of the rheological comparison in MRFs.

In this section, we describe the synthesis of Fe₃O₄ nanoplates (M) by the following steps: hydrothermal synthesis of α-Fe₂O₃ nanoplates and wet chemical reduction of the obtained α-Fe₂O₃ nanoplates.⁴⁰  TEM images of α-Fe₂O₃ nanoplates and M are shown in Figure 1.9a and b. By comparing the two TEM images, we can find that the wet chemical reduction has almost no influence on the particle shape and size of nanoplates. In addition, Figure 1.9b shows the anisotropic shape of nanoplates with the information on average size (176.5 ± 18.7 nm) and thickness (23.3 ± 2.9 nm). It also determines the corresponding aspect ratio, namely 7.7. Hence, the particle volume of individual M is calculated as 4.7 × 10⁵ nm³.

Afterwards, M are modified with poly(methyl methacrylate) (PMMA) chains via surface-initiated atom transfer radical polymerization (SI-ATRP). The existence of PMMA chains ensures the dispersion stability of M in organic solvents. By controlling the polymerization time of SI-ATRP, two types of PMMA-modified M (MPn, n = 1, 2) with different molecular weights of PMMA are prepared. The corresponding molecular weights of PMMA chains on MP1 and MP2 are determined by size exclusion chromatography (SEC). Table 1.3 shows the SEC results, including the number average molecular weight (M_n), weight average molecular weight (M_w), and molecular weight distribution (M_w/M_n).

MPn were dispersed in toluene and then cast on TEM grids. Representative TEM images of MPn are shown in Figure 1.10. In Figure 1.10a and b, we can observe that MPn could not easily overlap with each other due to the existence of PMMA chains on the surface of M. In addition, the particle distance between MPn obviously increases with increasing molecular weights of PMMA chains. Hence, the introduction of long PMMA chains can effectively inhibit the aggregation of MPn.

To study the responsive alignments of nanoplates upon the application of an external magnetic field, a drop of particle solution (M, MP1, or MP2) was cast on a TEM grid while applying a parallel or a vertical magnetic field using a magnet. As shown in Figure 1.11, we can observe a change in particle alignments responding to an external magnetic field. Different from the particle alignments of M and MP1, as shown in Figure 1.11a and b, MP2 show a nematic columnar structure along the parallel magnetic field direction, as depicted in Figure 1.11c. Upon application of a vertical magnetic field, M tend to arrange vertically along the magnetic field direction, showing a stacked structure, as shown in Figure 1.11d. By observing the particle alignment of MP1 treated with a vertical magnetic field, as shown in Figure 1.11e, we find that MP1 show a stacked structure and a columnar structure simultaneously. However, MP2 move towards the columnar structure under the action of a vertical magnetic field, as shown in Figure 1.11f. Hence, it is easier for MP2 modified with high-molecular-weight PMMA to form the special columnar structure than the unmodified M and MP2 modified with low-molecular-weight PMMA upon the application of an external magnetic field.

To explore the formation mechanism of the above-mentioned alignment, high-resolution TEM (HR-TEM), scanning TEM (STEM), and nanobeam diffraction (NBD) techniques were performed on the unmodified M. Figure 1.12a shows the HR-TEM of the front of M, in which the selected square area is further analysed by STEM and NBD. By observing and analysing the STEM image in Figure 1.12b, we find that the lattice spacing of the front of M is approximately 0.30 nm, which corresponds to the (220) plane. The corresponding NBD image (Figure 1.12b, inset) shows that six-fold diffraction spots can be designated to the equivalent (220) planes, and all six equivalent planes are parallel to the [111] direction. Hence, the growing interface of crystal in the front of M is along the [111] direction. Similarly, we observed and analysed the HR-TEM, STEM, and NBD images of the side of M, as shown in Figure 1.12c and d. The analysis results reveal that the lattice spacing of the side of M is approximately 0.49 nm, and it can be indexed to the (111) plane, which is consistent with the growing direction of the front of M. Based on the above-mentioned analysis, the growth axis of M can be determined, which is along the [111] direction.

Single Fe₃O₄ particles have an easy magnetic axis ([111] direction) and a relatively difficult magnetic axis ([110] direction). Thus, unmodified M are supposed to align vertically with the magnetic field direction, as shown in Figure 1.11d. However, PMMA-modified M tend to align with a nematic columnar structure along the magnetic field direction, especially high-molecular-weight PMMA-modified M (MP2). Two mechanisms are regarded as the main reasons for inducing the special alignment. The first one is excluded volume effects, which exist between parallel plate particles. The force (per unit area) between two parallel nanoplates exponentially decreases with an increasing distance between the two parallel nanoplates under an electromagnetic field. It means that two parallel nanoplates, kept away from each other, can block the electromagnetic effects due to the decreasing force (per unit area) between them. The other one is stronger dipolar fields. The agglomeration of multiple nanoplates may produce stronger dipolar fields than the individual nanoplates. It is relatively difficult for an external magnetic field to penetrate the axial direction of dipolar fields, while [111] plane is parallel to the axial direction. The above-mentioned two effects inevitably block the exposure of the [111] plane and provide opportunities for the exposure of the [110] plane to the external magnetic field. Hence, the existence of PMMA chains on the surface of M enlarges the particle distance of parallel M, resulting in the nematic columnar structure of MP2 along the [110] direction (a relatively difficult magnetic axis). The above-mentioned analysis also suggests that the controllable interparticle interaction has an influence on the PMMA-modified M even with an applied magnetic field.

To investigate the plausible structure of MPn, MPn were mixed with ionic liquids (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, IL) at different weight ratios. Then, MPn/IL were examined by ultrasmall-angle synchrotron X-ray scattering (USAXS) measurements on the beamline BL03XU with a PILATUS 1M detector (Dectris^®) at SPring-8. Interparticle distance (I_D) is an important factor for the USAXS scattering peaks, which can be determined by USAXS curves (intensity-q patterns). As shown in Figure 1.13a, we find that I_D values of MP1/IL marked on the USAXS curves increase with increasing weight ratios of IL. Specifically, I_D values increase from 173 to 219 nm over a wide range of weight ratios from 3/1 to 1/3 (MP1/IL). A similar tendency is also observed in the USAXS curves of MP2/IL. In addition, by comparing the I_D values at the same weight ratios in MP1/IL and MP1/IL, we also find that the molecular weight of PMMA chains can raise the I_D values. The USAXS results are consistent with the previous TEM observations. The above-mentioned results demonstrate that changing the weight ratios of IL or changing the molecular weights of PMMA chains are effective ways to control the I_D values.

In addition, to study the responsive self-organized structure in the external magnetic field, MP2/IL (weight ratio: 1/3) were analysed by the USAXS measurements with a custom-made 2-axis magnetic field equipment. Scattering USAXS images of MP2/IL (weight ratio: 1/3) without applying an external magnetic field and with applying an external magnetic field (320 kOe) are shown in Figure 1.13c and d, respectively. Owing to the fast response of liquid crystals to the external magnetic field, an isotropic scattering image turns into an anisotropic scattering image in a split second. The magnetic field direction is parallel to the long-axis direction but is vertical to the short-axis direction. Afterwards, the anisotropic scattering image is converted into an intensity-q pattern (Figure 1.13e) with a scanning range of 30°. As shown in Figure 1.13e, two broad peaks appear on the long-axis direction, while no peaks appear on the short-axis direction. It means that MP2/IL show orientation only in the long-axis direction. In addition, the two broad peaks are assigned to (001) and (002) planes of the columnar structure. Hence, the above-mentioned analysis indicates that MP2/IL (weight ratio: 1/3) show a uniaxial nematic columnar structure under the external magnetic field. Similarly, MP1/IL (weight ratio: 1/3) were also analysed by USAXS measurements. Under the action of the external magnetic field, I_D values of MP1/IL (weight ratio: 1/3) and MP2/IL (weight ratio: 1/3) are determined as 180 and 195 nm, respectively. By comparing the data at a weight ratio of 1/3, as shown in Figure 1.13a and b, we find that the I_D values are decreased. The most possible reason is that the soft PMMA chains on the surface of the particles are squeezed when applying an external magnetic field. Consequently, controlling the external magnetic field is also a feasible method to change I_D values.

Magnetization curves, as a function of magnetization and applied magnetic field ranging from −55 to 55 kOe, were measured at room temperature and are shown in Figure 1.14. The saturation magnetization (Ms) values of MP1 and MP2 are 70 and 45 emu g⁻¹, respectively. We find that the molecular weight of PMMA chains on the surface of M affects their magnetic properties. In our previous research,⁴⁰  we quantified the molecular weight of PMMA chains. The weight fraction of PMMA chains on the surface of MP1 and MP2 is 11% and 44%, respectively. It also means that the high weight fraction of PMMA chains can reduce Ms values. The enlarged image indicates that both MP1 and MP2 show an obvious magnetic response and approach to the saturation state at 2 kOe. Hence, during the rheological measurements, we can control the magnetic field strength from 0 to 2 kOe.

To investigate the effect of molecular weights of PMMA on the MR performance, both MP1/IL and MP2/IL (weight ratio: 1/4; volume concentration: 16.3%) were adjusted to form MRFs to evaluate their rheological properties using a rotational rheometer (parallel gap: 0.10 mm). Here, we studied the shear stress (shear rate: 1 s⁻¹ and 10 s⁻¹) of MRFs as a function of the applied magnetic field strengths by comparing MP1/IL and MP2/IL. As shown in Figure 1.15a, we find that the growth rate of MP1/IL is faster than that of MP2/IL, and the shear stress values of MP1/IL exceed those of MP2/IL when applying a stronger magnetic field (over 1 kOe) at 1 s⁻¹ of shear rate. However, MP2/IL show a higher shear stress than that of MP1/IL at the same magnetic field strength at 10 s⁻¹ of shear rate, as shown in Figure 1.15b. The above-mentioned results suggest that the molecular weight of PMMA chains determines the viscosity of MRFs, which also affects the shear stress. At the low shear rate (1 s⁻¹), both MP1/IL and MP2/IL show a high viscosity without applying an external magnetic field, whereas Ms values play a decisive role in affecting the viscosity and shear stress when applying an external magnetic field. At the high shear rate (10 s⁻¹), the high molecular weight of PMMA chains increases the viscosity and shear stress of MP2/IL under various magnetic field strengths even though MP1/IL have a higher Ms value. In addition, the strong chain structures (special columnar alignment) of MP2/IL might be attributed to the increase in shear stress at a high shear rate. Consequently, the above-mentioned results reveal that the high molecular weight of PMMA chains on the surface of M can enhance the rheological performance of MRFs.

Magnetic nanoparticles with controlled sizes, shapes, surface modifications, and polymer molecular weights have been prepared and introduced into MRFs. The increase in particle sizes, the introduction of anisotropic shapes, the surface modification with organic layers, and the high molecular weight of PMMA all play positive roles in stimulating Fe₃O₄ nanoparticles to organize strong chain structures under an external magnetic field to reinforce the MR performance. This study also highlights the importance of controlling the anisotropic alignments of magnetic nanoparticles in MRFs. The special columnar alignments of PMMA-modified Fe₃O₄ nanoplates might have further research values in other fields. Introducing other types of polymers on the surface of Fe₃O₄ nanoplates is also a feasible research proposal. To summarize, this chapter provides valuable and comprehensive research effects for further investigating the improvement of MR systems, including MRFs and MR elastomers.

This work was financially supported by the Japan Society for the Promotion of Science (JSPS), Scientific Research Account No. 19H00845 (K. Kanie), and the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Management Expenses Grants for the National University Corporation (K. Kanie).