Surface modification techniques toward controlling the dispersion stability and particle-assembled structures of slurries

Special Article The 71st CerSJ Awards for Advancements in Ceramic Science and Technology: Review Surface modification techniques toward controlling the dispersion stability and particle-assembled structures of slurries Motoyuki IIJIMA ³ Graduate School of Environment and Information Sciences, Yokohama National University, 79 7 Tokiwadai, Hodogayaku, Yokohama 240 8501, Japan Controlling the dispersion stability of functional fine/nano-particles without forming strong uncontrollable aggregates and controlling their assembled structure during material processing is a powerful method to improve the properties of composite materials, such as ceramics and polymer nanocomposites. In this article, surface modification techniques for functional fine/ nano-particles toward their homogeneous dispersion in solvents will be briefly reviewed. A suitable combination of surfaceengineering protocols and the selection of surface modifiers comprising segments that effectively attach to the particle surface and possess high affinities toward solvents is one of the keys to achieve homogeneous dispersion of fine/nano-particles. Then, a concept using a series of polyethyleneimine (PEI) and fatty acid complexes as surface modifiers to control the dispersion stability of multicomponent slurries will be reviewed. The dispersion stability of slurries can be tuned by simply controlling the structures of the PEI-fatty acid complex. Furthermore, processing methods to control the particle-assembly structures using the PEI-fatty acid complex will also be introduced. 2017 The Ceramic Society of Japan. All rights reserved. Key-words : Surface modification, Dispersion, Slurry, Surfactants, Polymer dispersants, Silane coupling agents [Received April 24, 2017; Accepted June 5, 2017] 1. Introduction To date, various composite materials processed from functional fine particles and/or nanoparticles (typically size below c.a. 100 nm, depending on research areas) have been widely applied as advanced ceramics, polymer composites, batteries, etc., 1) 3) and various attempts have been made to improve and/or to add new properties to create new substances and to design material processing routes. Among these attempts, controlling the dispersion stability of fine reagent particles without forming strong, uncontrollable aggregates and controlling their assembled structures during material processing will be a powerful tool to tune and improve the properties of composite materials. For instance, Si 3 N 4 is a promising ceramic material with an excellent mechanical strength and a high resistance to fracture and thermal shock from ambient to relatively high temperatures; therefore, it is widely applied to bearings, motor shafts, burner nozzles, and heat exchangers. 4),5) These Si 3 N 4 ceramics are typically processed from mixtures of fine particles, including Si 3 N 4 and sintering aids (e.g., Al 2 O 3, MgO, and rare oxides), dispersed in a non-aqueous solvent to form a slurry, and then transferred to the shaping process and sintering process. It is very important to strategically control the microstructures of green compacts and particleassembled structures of fine/nano-particles in the slurry because the properties of Si 3 N 4 ceramics are critically affected by the microstructures of final sintered products, such as size distributions and shapes of grains, composition, amount and shape of the glass phase, and existence of large pores and grains. Similar requirements exist for processing polymer nanocomposites, which functional nanoparticles were dispersed into polymers to gain benefits both from the functions of nanoparticles and polymers. A complete dispersion of nanoparticles throughout processing (i.e., ³ Corresponding author: M. Iijima; E-mail: iijima@ynu.ac.jp dispersion in solvents and monomers, polymerization, casting, and drying) is required to design transparent polymer composites, 6) while a controlled nanoparticle network structure will be necessary for conductive devices. 7) Herein, we shortly review our surface modification process for functional fine/nano-particles toward their homogeneous dispersion in solvents; we then introduce some examples for designing surface modifiers that can be realized in a simple process and are applicable to various types of particles. Applications toward controlling the assembly structures of surface-modified particles will also be presented. 2. Surface modification of fine/nano-particles for their uniform dispersion The surface of functional nanoparticles can be modified to improve their dispersion stability in solvents. This requires a suitable selection of surface-engineering methods that do not generate strong aggregates and provide an appropriate design/ choice for the surface modifier (i.e., with segments that effectively adsorb/react on/with the desired particle surface and organic chains that improve the wettability of the solvent). Mostly, the reagent particles are likely in the form of aqueous colloids stabilized by electrostatic repulsive interactions, non-aqueous colloids stabilized by organic capping agents, and dry powder manufactured by dry processing. For modifying the surface of aqueous colloids, we have found that a process of diluting the colloids with alcohols followed by careful addition of the surface modifier to achieve saturated adsorption, washing and collecting the surfacemodified particles with the assistance of centrifugation, and drying the collected wet particle cake provides surface-modified nanoparticles that are redispersible in organic solvents. 8) 10) For example, hydrophilic TiO 2 aqueous colloids which the particle size were c.a. 7 nm analyzed by dynamic light scattering (DLS) method were surface-modified using oleyl phosphate 8) or decyltrimethoxysilane 9) using this process. The surface-modified nano- 2017 The Ceramic Society of Japan DOI http://doi.org/10.2109/jcersj2.17093 603

JCS-Japan Iijima: Surface modification techniques toward controlling the dispersion stability and particle-assembled structures of slurries particles became redispersible in non-polar solvents, such as toluene and hexane in an aggregated size below 50 nm. In addition, the surface hydrophobicity could be successfully tuned using mixed silane alkoxides, such as a combination of decyltrimethoxysilane and 3-aminopropyltrimethoxysilane, 10) which caused homogeneous redispersion of surface-modified particles in various organic solvents, depending on the ratio of the hydrophobic decyl group and hydrophilic 3-aminopropyl group. The relationship between the hydrophobic/hydrophilic group ratio on the solvents with redispersed nanoparticles was mapped in our previous report 10) and this mapping will be a useful platform to design the nanoparticle surface to achieve homogeneous dispersion in a desired solvent. Instead of tuning the hydrophobic/ hydrophilic group ratio on the particle surface, a design of anionic surfactants with functional chain branched into polyethylene glycol-based hydrophilic chain and alkyl-based hydrophobic chain near the anionic head group will be another powerful selection. 11) 13) We reported that TiO 2 nanoparticles (c.a. 7 nm) modified with functional branched surfactants could be uniformly dispersed in a wide range of organic solvents in a size of several tens of nanometers, including ethanol, methyl methacrylate, ethyl acetate, and toluene, using one particular surface design without tuning the hydrophilic/hydrophobic ratios. 11) This surface modification will be useful to maintain the stability of nanoparticles in a process having a situation of solvent polarity changes such as addition of different solvents, drying/condensation of mixed solvents, and monomer polymerization. These concepts for the design/selection of surface modifiers can be expanded to dry powders and non-aqueous colloids with combinations of suitable surface-engineering protocols. A combination of bead-milling processes to pulverize the necked structure for nanoparticles with surface modifications, such as the use of silane coupling agents mentioned above, was effective to modify the surface of dry powder manufactured by dry processing. 13),14) For instance, SiO 2 /TiO 2 composite nanoparticles prepared using a dry process were bead-milled in N-methylpyrrolidone with phenyltrimethoxysilane and achieved SiO 2 / TiO 2 slurries when the nanoparticles were dispersed at a size near to their primary particle size. 14) A ligand exchange process conducted by adding excess amounts of the surface modifier with heating is useful to modify the surface of non-aqueous colloids stabilized by organic capping agents. 12),15) Nanoparticles can be uniformly dispersed in a desired solvent using a suitable combination of surface-engineering protocols and selection of surface modifiers mentioned above. However, there remain issues that need to be solved. For example, the adsorption ratio of the surface modifier is often different for different particle species. Therefore, a suitable surface modifier must be selected for each particle species, which severely increases the difficulty to control the stability of multi-component slurries. Development of a simple protocol to tune the structures of the surface modifiers without a careful chemical synthesis is also in high demand to provide rapid feedback on the surface structure design toward controlling the particle dispersion stabilities. Some of our recent progress to overcome these issues will be introduced in the following section. 3. Polyethyleneimine-fatty acid complex as a surface modifier to improve the dispersion stability of multi-component slurries Recently, we found that a complex of polyethyleneimine (PEI) with fatty acids is useful for controlling the dispersion stability of non-aqueous slurries comprised of various species of fine Fig. 1. FT-IR spectra of toluene, OA, PEI (Mw = 1800), and PEI-OA with various amounts of OA additives. (Reprinted with permission from Ref. 16. Copyright 2015 American Chemical Society.) particles in a simple manner. 16),17) The PEI-fatty acid complex can be simply prepared by mixing PEI with a fatty acid in a lowpolar solvent, such as toluene or -terpineol. PEI itself is not soluble in low-polar solvents because it is a hydrophilic cationic polymer; however, PEI complexed with a fatty acid can be visibly dissolved in low-polar solvents. One of the major benefits of using a PEI-fatty acid complex as a surface modifier is the possibility to design various series of PEI complexes by selecting a combination of the molecular weights of PEI, species of fatty acids, and PEI/fatty acid mixing ratios. Fourier transform infrared (FT-IR) spectra were recorded for a PEI-oleic acid complex (PEI-OA)/toluene solution (Fig. 1) to understand the structure of the PEI-fatty acid complex. The FT-IR spectra of toluene, original PEI, and oleic acid are also shown in the same figure. A strong signal is observed for the stretching vibration of hydrogen-bonded C=O of COOH (1710 cm ¹1 ) for the original oleic acid; this peak disappears and new peaks related to symmetric (1543 cm ¹1 ) and as-symmetric vibrations (1407 cm ¹1 )of COO ¹ (carboxylates) appear together with signals of PEI and toluene in the FT-IR spectra of the PEI-OA/toluene solution. The hydrogen-bonded C=O of COOH at 1710 cm ¹1 relates to the free oleic acid, and it strongly indicates that oleic acid formed a complex with the amine groups of PEI through COO ¹ groups. Similar tendencies were also found for PEI complexes with isostearic acid and stearic acid. Another major benefit of using a PEI-fatty acid complex as a surface modifier is their effective adsorption properties on various species of particles. Figure 2 shows the relation between the additive content of PEI-OA and their adsorbed content. Though various species of particles, including nitrides, oxides, and metals, have been tested, almost all added PEI-OA adsorbs onto the particle surface at lower additive conditions and then reaches saturated adsorbed conditions as the additive PEI-OA content for each particle increases. These adsorption properties are quite favorable toward controlling the dispersion stabilities of multicomponent slurries. In order to present the effect of the PEI-fatty acid complex on the slurry stabilities, Fig. 3(a) shows the flow curves of PEI-OAstabilized TiO 2 nanoparticle (c.a. 35 nm, provided as dry powder)/toluene slurries with different solid concentrations. The 604

JCS-Japan additive content of PEI-OA was controlled to be 1.4 mg/m 2, which was a condition to achieve saturated adsorption on TiO 2 nanoparticles. While the TiO 2 /toluene slurry without PEI-OA addition strongly aggregated and solidified, saturated adsorption of PEI-OA on the TiO 2 nanoparticles created a flowable slurry, and the flow curves do not have hysteresis properties, even at elevated solid concentrations. The dispersion stability of TiO 2 nanoparticles is effectively improved by PEI-OA adsorption. Tuning the structures of the PEI-fatty acid complex by changing the fatty acid species and molecular weights of PEI is a useful technique for controlling the flow behavior of dense, nonaqueous slurries. For example, Fig. 3(b) shows the flow curves of a PEI-isostearic acid (ISA) complex stabilized by TiO 2 /toluene slurries. The additive content of PEI-ISA was controlled to be 1.4 mg/m 2, which was the condition to achieve saturated adsorption of PEI-ISA. The data for the 25 and 30 vol % slurries are not shown because the flow curve could not be measured because of the high viscosity. Compared to the PEI-OA system shown in Fig. 3(a), the viscosity for the PEI-ISA-stabilized slurry was higher at the same solid concentration, and it possessed slight hysteresis properties for flow curves measured at elevated solid concentrations. We expected that PEI-ISA would possess smaller repulsive forces than PEI-OA, and therefore a slightly increased tendency of agglomeration because of the branched structure and relatively shorter alkyl chain length. Figure 4 presents another example for tuning the slurry viscosity, measured at 300 s ¹1,by changing the molecular weights of PEI used for PEI-OA. The slurry viscosity increased as the particle concentration increased, and this tendency was more effective for PEI with high molecular weights. 4. Controlling particle-assembled structures using the PEI-fatty acid complex as a surface modifier The PEI-fatty acid complex was not only useful to improve the stability of fine/nano-particles in solvents but also favorable to control the particle-assembled structures in multi-component slurries. We found that particle surfaces modified by PEI-OA effectively adsorb onto bare particles in toluene through simple mixing. 18) Figure 5 shows an example for attaching PEI-OAmodified SiO 2 nanoparticles onto Ni fine particles. Ni fine particles were simply added to a toluene suspension of PEI-OAmodified SiO 2 nanoparticles, and then Ni particles attached to Fig. 2. Effect of the additive content on the adsorbed amount of the PEI (Mw = 1800)-OA complex on various particle surfaces in toluene. Fig. 4. Effect of particle concentration and molecular weight of PEI on the viscosity of a Si 3 N 4 /toluene slurry stabilized by 1.8 mg/m 2 of PEI- OA. (Reprinted with permission from Ref. 16. Copyright 2015 American Chemical Society.) Fig. 3. Flow curves of TiO 2 /toluene slurries with various particle concentrations treated with 1.4 mg/m 2 of (a) PEI-OA and (b) PEI-ISA. Molecular weight of PEI was 1800. (Reprinted with permission from Ref. 17. Copyright 2016 The Ceramic Society of Japan and the Korean Ceramic Society.) 605

JCS-Japan Iijima: Surface modification techniques toward controlling the dispersion stability and particle-assembled structures of slurries techniques using effective adsorption of surface-modified nanoparticles on bare core particles can be applied to various combinations, such as Ag nanoparticles (c.a. 6 nm) attached to aramid nanofibers. 19) Therefore, it should be a powerful tool to simultaneously achieve dispersion stability improvement of fine/nanoparticles and particle assembly structure control toward designing the microstructures of composite materials. Fig. 5. Field-emission scanning electron microscopy images of (a) raw Ni and (b f ) modified Ni/SiO 2 composite particles with SiO 2 nanoparticle additive amounts of: (b) 0.50 wt %, (c) 1.5 wt %, (d) 5.0 wt %, (e) 10 wt %, and (f ) 20 wt % based on Ni particle content. (Reprinted with permission from Ref. 18. Copyright 2016 The Society of Powder Technology Japan.) 5. Conclusion In this article, we briefly reviewed our surface modification process for functional nanoparticles toward their homogeneous dispersion in solvents using a suitable combination of surfaceengineering protocols and a selection of surface modifiers, depending on the nanoparticle species and solvent species. Then, a concept for designing and using PEI-fatty acid complexes as surface modifiers to improve the dispersion stability of multicomponent dispersions and to control the assembled structures of particles in solvents was introduced. We strongly believe that these surface modifications and dispersion techniques should play an important role in controlling the microstructures of composite materials and designing enhanced properties. Acknowledgements The author gratefully acknowledges Prof. Hidehiro Kamiya (Tokyo University of Agriculture and Technology) and Prof. Junichi Tatami (Yokohama National University) for their continuous support. The author also thanks all co-workers and students for their contributions to the original work. Fig. 6. Effect of the additive content of PEI-OA-modified SiO 2 nanoparticles on the sedimentation behavior of Ni/toluene suspensions. The additive amounts of PEI-OA-modified SiO 2 are (a) 0 wt % (raw), (b) 5.0 wt %, (c) 10 wt %, and (d) 20 wt % based on Ni particle content. (Reprinted with permission from Ref. 18. Copyright 2016 The Society of Powder Technology Japan.) SiO 2 nanoparticles were magnetically collected. Note that free SiO 2 nanoparticles that are not attached to Ni particles will remain in the solution during magnetic collection. Compared to raw Ni fine particles, SiO 2 nanoparticles effectively attached to Ni fine particles as the SiO 2 additive content increased. Figure 6 shows the sedimentation behavior of Ni and Ni/SiO 2 particles in toluene. While raw Ni particles rapidly sedimented because of agglomerate formation, the particle stabilities improved after attaching PEI-OA-modified SiO 2 nanoparticles to the Ni particles. The PEI-OA-modified SiO 2 nanoparticles likely act as a surface modifier for the Ni particles. Nanoparticle assembly References 1) N. P. Padture, Adv. Mater., 21, 1767 1770 (2009). 2) R. D. Farahani, M. Dube and D. Therriault, Adv. Mater., 28, 5794 5821 (2016). 3) S. Srivastava, J. L. Schaefer, Z. Yang, Z. Tu and L. A. Archer, Adv. Mater., 26, 201 234 (2014). 4) H. Klemm, J. Am. Ceram. Soc., 93, 1501 1522 (2010). 5) F. L. Rilay, J. Am. Ceram. Soc., 83, 245 265 (2000). 6) M. Iijima, S. Omori, K. Hirano and H. Kamiya, Adv. Powder Technol., 24, 625 631 (2013). 7) H.-L. Gao, L. Xu, F. Long, Z. Pan, Y.-X. Du, Y. Lu, J. Ge and S.-H. Yu, Angew. Chem., Int. Edit., 53, 4561 4566 (2014). 8) M. Iijima, S. Tajima, M. Yamazaki and H. Kamiya, J. Soc. Powder Technol., Jpn., 49, 108 115 (2012). 9) M. Iijima, M. Kobayakawa and H. Kamiya, J. Colloid Interf. Sci., 337, 61 65 (2009). 10) M. Iijima, S. Takenouchi, I. W. Lenggoro and H. Kamiya, Adv. Powder Technol., 22, 663 668 (2011). 11) M. Iijima, M. Kobayakawa, M. Yamazaki, Y. Ohta and H. Kamiya, J. Am. Chem. Soc., 131, 16342 16343 (2009). 12) M. Iijima and H. Kamiya, Langmuir, 26, 17943 17948 (2010). 13) M. Iijima, M. Yamazaki, Y. Nomura and H. Kamiya, Chem. Eng. Sci., 85, 30 37 (2013). 14) K. Takebayashi, S. Sasabe, M. Iijima and H. Kamiya, J. Soc. Powder Technol., Jpn., 47, 310 316 (2010). 15) M. Iijima, A. Kurumiya, J. Esashi, H. Miyazaki and H. Kamiya, Proc. SPIE, 9176, 91760I (2014). 16) M. Iijima, N. Okamura and J. Tatami, Ind. Eng. Chem. Res., 54, 12847 12854 (2015). 17) M. Iijima, Y. Kawaharada and J. Tatami, J. Asian Ceram. Soc., 4, 277 281 (2016). 18) S. Morita, M. Iijima and J. Tatami, Adv. Powder Technol., 28, 30 36 (2017). 19) M. Iijima and H. Kamiya, Colloid. Surface. A, 482, 195 202 (2015). 606

JCS-Japan Motoyuki Iijima graduated from Tokyo University of Agriculture and Technology (TUAT) in 2004, and from the graduate school of TUAT in 2005. He received his Ph.D. degree of Engineering from TUAT in 2007. He then became a Research Fellow of the Japan Society for the Promotion of Science in 2007, and an Assistant Professor of TUAT in 2008. He joined Yokohama National University in 2013 as a Lecturer, and currently Associate Professor since 2017. Areas of his research interests includes surface modification of fine/nanoparticles, dispersion control, and wet powder processing. 607