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Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design
Advanced Powder Technology xxx (xxxx) xxx
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design Seitaro Morita a, Motoyuki Iijima a,b,⇑, Junichi Tatami a,b a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Institute of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 14 October 2019 Received in revised form 15 November 2019 Accepted 19 November 2019 Available online xxxx Keywords: Hetero-assembly Non-aqueous suspension Colloidal processing Surface modification Polymer dispersant
a b s t r a c t Although many colloidal assembling systems have been reported, most systems suffer from severe aggregation under high solid concentrations, which can often be observed in typical hetero-aggregation system. In this study, we created a hetero-assembly system using concentrated (~50 vol%) suspensions by mixing large SiO2 particles modified with polyacrylic acid partially complexed with oleylamine (PAAOAm) and small SiO2 particles modified with polyethyleneimine partially complexed with oleic acid (PEI-OA) in a non-aqueous solvent. We demonstrated that hetero-assembly is driven by the interactions between the uncomplexed carboxyl/amine groups of the PAA/PEI present on the particles, while severe aggregation is simultaneously prevented by the steric repulsions of the aliphatic oleyl chains. Comparison of the cross sections of the in-situ solidified hetero-assembled suspensions with those of ideally assembled structures which were reproduced by a simulation considering the statistical distribution of particles strongly supported successful particle assembling via the proposed approach. The results revealed that the OA content in the PEI-OA complex was the dominant factor that controlled the dispersion and assembling state of the binary particles. The significance of this study is that our findings will provide a class of colloidal dispersion state which binary particles were assembled in a high solid content suspension without forming strong aggregates. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Colloidal assembly in multicomponent suspensions is an important technique used to design composite materials with unique structures [1–7] and functions [8–14] that are derived from their controlled microstructures. Therefore, these techniques are widely applied in various fields such as ceramics [7,8], polymer-based composites [12,13], and carbon-based composites [14–16]. The properties of these materials, which are manufactured via colloidal processing, are tuned by the local arrangement of each particle. Thus, colloidal hetero-assembly, in which small particles are assembled onto large particles, has attracted a wide range of research. From the 1970 s, many hetero-assembly systems have been reported and applied to material processing [6–9,11–30] in both aqueous and non-aqueous media. Aqueous hetero-assembly systems utilize electrostatic attractions based on pH control ⇑ Corresponding author at: Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. E-mail address: [email protected] (M. Iijima).
[17–19] and layer-by-layer surface modifications [11], while nonaqueous hetero-assembly systems employ other attraction forces such as covalent bonds [13,21,22], acid-base interactions [23], hydrogen bonds [24–26], and pi–pi stacking [16,27]. Although these systems successfully achieve particle assembling, most of these reported systems comprise dilute suspensions (<10 vol%) and suffers from a marked increase in viscosity [28] when processed under high solid concentrations, owing to the aggregation induced by small-particle bridges between the large particles [29,30]. Because numerous industrial applications require colloidal processing under concentrated conditions, the realization of particle assembling in dense and flowable suspensions would provide new options for material processing. Herein, we introduce a strategy to realize a class of colloidal dispersion systems in which binary spherical particles are hetero-assembled under highly concentrated conditions in flowable non-aqueous suspensions. In our previous studies, we designed a series of polymer dispersants comprising a cationic polymer partially complexed with fatty acids. We reported that the designed complex improved the stability of various particles in non-aqueous solvents [31,32]. The key concept in this work
https://doi.org/10.1016/j.apt.2019.11.029 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Nomenclature Ap DL DS N X Y Z xs
random dense parking limit [–] diameters of the large SiO2 particles [lm] diameters of the small SiO2 particles [lm] maximum number of small particles that surround a large particle [–] length of a rectangular frame for simulation [lm] width of a rectangular frame for simulation [lm] height of a rectangular frame for simulation [lm] volume fraction of small particles based on the total solid content in suspensions [%]
was the design of an anionic polymer partially complexed with an aliphatic amine and the subsequent mixing of two dense colloidal dispersions stabilized by a cationic polymer partially complexed with a fatty acid and an anionic polymer partially complexed with an aliphatic amine (Fig. 1). We expected that the interactions between the remaining cationic and anionic functional groups would drive the hetero-assembly process, while the aliphatic chain would prevent severe aggregation by the steric repulsions generated by the improved affinity between the aliphatic chain and non-aqueous solvent. To proof this concept, a cationic complex of polyethyleneimine and oleic acid (PEI-OA) [31,32] and an anionic complex of polyacrylic acid and oleylamine (PAA-OAm) were designed as functional dispersants for small (217 nm) and large (2.24 lm) spherical SiO2 particles, respectively. The toluene suspension of the respective PAA-OAm- and PEI-OA-stabilized large and small SiO2 particles were mixed under various volume ratios of small-to-large particles and concentrated conditions (50 vol%) to achieve particle assembly in an flowable suspension state. The degree of hetero-assembly in the concentrated suspensions was confirmed by comparing the particle arrangement structures of the in situ solidified specimens to those of ideally assembled structures reproduced by computer simulation considering the statistical distribution of particles. To optimize the interactions between the small/large particles and the repulsive interactions between the resulting hetero-assembled particles, the chemical structures on the small-particle surfaces were varied by tuning the OA content in PEI-OA, and their effect on the particle assembling/dispersion behavior was investigated. 2. Materials and methods 2.1. Materials Polyacrylic acid (PAA, Mw 5000), oleylamine (OAm), polyethyleneimine (PEI, Mw 1800), oleic acid (OA), toluene, and ethanol were purchased from FUJIFILM Wako Pure Chemical Co. Ltd., Japan.
d h / /L
mean inter-particle surface-to-surface distance of the large particles [lm] degree of surface coverage of small particles on a large particle [%] total solid content in suspension [vol%] volume fraction of the large particles in suspension [vol %]
Poly(ethylene glycol) diacrylate (PEGDA, average polymerization degree 250) was purchased from Sigma-Aldrich Co. LLC. 2,2-Dime thoxy-2-phenylacetophenone (DPA) was purchased from Tokyo Chemical Industry Co. Ltd., Japan. The spherical large SiO2 powder (2.24 lm, calculated from BET specific surface area: 1.22 m2/g) and spherical small SiO2 powder (217 nm, calculated from BET specific surface area: 12.6 m2/g) were obtained from Nippon Shokubai Co. Ltd., Japan. All the materials were used as received without further purification. 2.2. Cationic and anionic polymer dispersants Solution of PEI–OA complexes of different OA contents (15–50 mol% OA based on the number of amino groups in PEI) were prepared using the method described in our previous report [31]. Solution of PAA-OAm (70 mol% OAm based on the number of carboxyl group in PAA) were prepared by mixing 1.5 g of 16.7 wt% PAA/ethanol solution and 8.5 g of 7.64 wt% OAm/toluene solution and then treated in an ultrasonic bath for 5 min. (See Supplementary material and Figs. S1–S4 for detailed characterization of PAA-OAm and their ability to improve the large SiO2/toluene suspensions). 2.3. Hetero-assembled binary SiO2 suspensions First, small SiO2/toluene dispersions stabilized by PEI-OA having different OA contents (15, 30, 50 mol%) were prepared by mixing SiO2 particles (50 vol%) in toluene which the respective PEI-OA were dissolved. The PEI-OA contents were controlled to values of 1.3, 1.4, and 1.6 mg/m2 (based on the total SiO2 surface area in the suspension) for the samples with 15, 30, and 50 mol% OA ratios, respectively, to reach saturated adsorption. Next, the large SiO2/toluene dispersions stabilized by PAA-OAm were prepared as follows; 3.00 g of large SiO2 powder was added to the PAA-OAm solution (1.5 mg/m2) and the suspension was stirred for 24 h. The suspensions were centrifuged for 5 min at 2000 g
Fig. 1. Schematic illustration of the hetero-assembly of binary particles using PAA-OAm and PEI-OA dispersants to improve the dispersion state in concentrated suspensions.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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and the collected wet cake was washed with toluene and dried at 60 °C under vacuum. The PAA-OAm-modified large SiO2 powders were resuspended in toluene to be 50 vol% using a planetary mixer (ARE-250, THINKY Co., Ltd., rotation speed 800 rpm, revolution speed 2000 rpm, 3 min.). Then, the 50 vol% hetero-assembled suspensions were prepared by mixing the 50 vol% toluene dispersions of PEI-OA-stabilized small SiO2 particles and PAA-OAm stabilized large SiO2 particles under various small-particle mixing fractions (xs) using a planetary mixer (rotation speed 800 rpm, revolution speed 2000 rpm, 3 min.). Here, xs was defined as volume fraction of small particles based on the total solid content in the suspensions. Diluted binary SiO2 suspensions were also prepared similarly by mixing 0.5 vol% PAA-OAm- and PEI-OA-stabilized SiO2/toluene suspensions. 2.4. Non-assembled binary SiO2 suspensions Large SiO2/toluene dispersions stabilized by PEI-OA having different OA contents (15, 30, 50 mol%) were prepared similarly to the case for small SiO2 particles instead that the PEI-OA contents were controlled to values of 2.1, 1.6, and 1.8 mg/m2 for the samples with 15, 30, and 50 mol% OA ratios, respectively, to reach saturated adsorption. The 50 vol% toluene dispersions of the small and large SiO2 particles, which were both stabilized with PEI-OA, were mixed together under various small/large particle ratios using a planetary mixer (rotation speed 800 rpm, revolution speed 2000 rpm, 3 min). 2.5. Characterizations Viscosity curves of the hetero-assembled and non-assembled binary 50 vol% suspensions were measured using a rheometer (TA Instruments, AR-G2) by increasing the shear rate from 0 to 80 s1 in 90 sec. and then decreasing it from 80 s1 to 0 s1 in 90 sec. The particle arrangements in dilute suspensions were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi High-Technologies, SU8010) of the particles dried from the 0.5 vol% binary SiO2 suspensions on aluminum SEM stages. The particle arrangements in the dense suspensions were also characterized by the modified in situ solidification methods as follows: first, a photo-curable acrylate resin was prepared by dissolving DPA as a photo-radical polymerization initiator in PEGDA. Next, as prepared acrylate resin was mixed with 50 vol% binary SiO2/toluene suspensions using a planetary mixer to attain 40 vol %. Samples (1.0 g) of the obtained suspensions were poured into Teflon molds and cured by irradiating 365 nm UV light for 1 min. The in situ solidified suspensions were dried at 60 °C under vacuum and the cross-sectional planes were processed using a cross section polisher (JEOL, SM-9010). The particle arrangements of the in situ solidified bodies were characterized by FE-SEM. For all the sample images, the inter-surface distances between all the small particles and large particles were calculated and their shortest inter-surface distance distributions were determined. The number of small particles used to calculate the shortest inter-surface distances was 250. 2.6. Simulation of the hetero- and non-assembled systems First, the maximum number of small (0.217 lm) spherical particles that could be attached on a large (2.240 lm) spherical particle was estimated by a simulation based on the random sequential adsorption model, under the assumptions that fixed small particles do not deform and move on large particles. A single large particle was settled on the center of the spherical coordinate system and the polar angle and azimuthal angle were generated randomly. The radial distance was set to 1.2285 lm, which is the core-to-core distance of the small and large particles. The central
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position of a small particle was then located at the coordinate having randomly generated angles and the radial distance of 1.2285 lm. The coordinates of the central position of the newly located small particle were compared to those of the alreadyexisting small particles. The newly located small particles were fixed on a large particle if the core-to-core distance between the central position of the newly added small particles and of the already existing particles were not less than 0.217 lm (i.e. no volumetric overlapping with the already existing small particles); otherwise, the small particle was discarded. These small particle setting trials were conducted for 108 calculation steps. The entire calculation was performed sixfold to estimate the maximum number of small particles that could be settled on a large particle through a random process. Next, the particle arrangements of the hetero- and nonassembled systems were reproduced by the following protocol: a rectangular frame (X = 21.31 lm, Y = 14.20 lm, and Z = 9.47 lm) was generated and 150 large spherical particles and 47,500 small particles were inserted in the frame using the following rules: The solid content in the frame was 40 vol%, the same particle fraction as that of the in situ solidified suspensions before curing and drying. To reproduce the hetero-assembled system, 150 large spherical particles were randomly placed in the rectangular frame. The small particles were then randomly arranged on each large particle, using the abovementioned method, for 106 steps. During the attachment of the small particles to the large particles, the small particles that were discarded due to volumetric overlapping with the already existing small particles were collected. After randomly attaching the small particles on all the large particles, the collected and remaining small particles were randomly placed in a vacant space of the frame. To reproduce the non-assembled system, 150 large particles were randomly placed in the frame. Next, 47,500 small particles were randomly placed in the vacant space. The distributions of the surface distances between the small particles and their nearest neighboring large particles were analyzed using the three-dimensional coordination data. Crosssectional two-dimensional images of both simulated suspensions were also obtained based on the coordination of the particles in the frame at the typical X–Y plane. The frames were trimmed (X width, 12.68 lm and Y width, 8.82 lm) to compare the distributions of the apparent surface distances between the small particle surfaces and the large particle surface nearest to them obtained from the experimental results. The surface distributions of the simulated suspensions were estimated from nine slices of the cross-sectional planes at various Z coordinates (Z = 2.8–6.8 lm).
3. Results To demonstrate the proposed hetero-assembly system (Fig. 1), toluene suspensions of the PAA-OAm- and PEI-OA-stabilized large SiO2 particles and PEI-OA-stabilized small SiO2 particles were mixed under dilute conditions (0.5 vol%) and different large-tosmall particle mixing ratios for the first step. When the PAAOAm-stabilized large particles and PEI-OA-stabilized small particles were mixed together, almost all the small particles were adsorbed onto the large particles without forming severe agglomerates, regardless of the OA content in PEI-OA (Fig. 2A–C). Based on a qualitative visual comparison, we also confirmed that the coverage of small particles on the large particles increased with increasing xs [(I) and (II) in Fig. 2A–C and Supplementary material, Figs. S5A–C]. Conversely, when PEI-OA dispersants were used for both the small and large particles, no small particles were fixed onto the large particles, regardless of the OA contents and xs (Fig. 2D–F). The lower magnification images revealed the presence of many small particles between the large particles, suggesting
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 2. Scanning electron micrographs of the large and small SiO2 particles dried from 0.5 vol% binary SiO2 suspensions. Hetero-assembled system using PAA-OAm-modified large particles and small particles modified with PEI-OA having (A) 15, (B) 30, and (C) 50 mol% OA contents. Non-assembled system using large and small particles both modified with PEI-OA having (D) 15, (E) 30, and (F) 50 mol% OA contents. (I) xs = 7.9% and (II) xs = 23%; (scale bar: 1 lm).
small attractive interactions between the large and small particles. Thus, we concluded that the small particles were localized between the large particles, via capillary forces, during the drying process (Supplementary material, Figs. S5D–F). These results clearly revealed that hetero-assembly of the binary SiO2 particles was achieved when the PAA-OAm- and PEI-OA-stabilized SiO2 suspensions were mixed together. As the mixing of PEI-OA-stabilized suspensions did not form assembled structures, the complexing interactions between the amino groups of PEI-OA on small SiO2 particles and the carboxyl groups of PAA-OAm on large SiO2 particles are expected to be the major factor in the formation of the assembled structures. To confirm the applicability of the proposed hetero-assembling process to dense suspensions with flowable state, 50 vol% binary hetero-assembled suspensions were next prepared and their microstructures and flowing properties were subsequently characterized. The particle arrangements in the concentrated binary suspensions of the hetero- and non-assembled systems were characterized by SEM analysis of the in situ solidified specimens (Fig. 3A–F, xs = 23%). Fig. 3G display histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the cross section images. Notably, the solid content of the suspension used for in situ solidification was diluted to 40 vol% as a result of monomer addition. Furthermore, shrinkage of the in situ solidified specimen was observed during curing/drying. Therefore, the respective actual solid contents of the in situ solidified specimen for the non– and hetero-assembled systems were 49 and 43 vol%, which slightly differed from those of the original suspensions. Fig. 3A–C reveal that many small particles were present on the large particles for the dense suspensions comprising PAA-OAm-stabilized large SiO2 suspensions mixed with PEI-OA-stabilized small SiO2 suspensions (hetero-assembled system). The histogram in Fig. 3 reveal that 71% of the small particles were located within 100 nm of the large-particle surface for the hetero-assembled samples comprising a 15 mol% OA content in PEI-OA. On increasing the OA content in PEI-OA (30 and 50 mol%) in the hetero-assembled system, the number of small particles gathering around the large particles was slightly reduced; however, 60% of the small particles were still located within 100 nm of the large-particle surface. Conversely, the small particles were distributed among the large particles when both the large and small particles were stabilized by PEI-OA (non-assembled system). Only 40% of the small particles were located within 100 nm of the large-particle surface in all the specimens of the non-assembled system (Fig. 3D–F). To compare these results with the ideal hetero-assembled and randomly dispersed states, the particle arrangements in the cross-sectional planes of the hetero- and non-assembled systems
were simulated considering statistical distribution of particles. Typical snap shots of the cross-sectional planes and the histograms for the distributions of the surface distances between the small particles and the nearest large particles are displayed in Fig. 4. The simulation of the hetero-assembled system was divided into three steps: (i) random arrangement of the large particles in a three-dimensional frame box, (ii) random arrangement of the small particles on the large particles until full coverage was attained, and (iii) random placement of the residual small particles in a vacant space of the frame box. On the other hand, the calculations for the non-assembled system were simply conducted by randomly placing the small particles after settling the large particles. Solid contents and xs were 40 vol% and 23%, respectively, for both cases. For the non-assembled system, the calculated histograms and particle localizations (Fig. 4B) were different from those attained from the experimental results. Conversely, the hetero-assembled system (Fig. 4A) exhibited similar trends. Focusing to the non-assembled system, the frequency of the small particles located near the large-particle surface in the experimental results was larger than that observed in the simulated images. Additionally, some small-particle domains were observed in the experimental specimens that were not present in the calculated images. These results suggested the presence of some attractive interactions between the PEI-OA-modified surfaces under static conditions, indicating that the non-assembled system was not in a completely randomly-dispersed state. The hydrogen bonds in the non-aqueous solvent, between the remaining PEI-OA amino groups, may contribute towards these attractive interactions. For the hetero-assembled system, the experimental frequency of the small particles fixed onto the large particles were in good agreement with the calculated results, strongly indicating that the small particles effectively assembled onto the large particles by our proposed process. Interestingly, together with the isolated free small particles, some aggregated small particles were observed in the cross-sectional view of the simulated results that were not present in the simulated images of the non-assembled system (images for other cross-sectional planes at different Z coordinates are shown in Supplementary material, Figs. S6 and S7). These small-particle domains were also present in the in situ solidified specimen of the hetero-assembled system. The series of simulated crosssectional planes, attained by gradually changing the Z coordinates (Fig. 5), strongly suggest that these small-particle aggregates originate from the cross sections of the small particles assembled on the edge of large particles. On the other hand, the isolated small particles were free particles that could not interact with the fully covered large particles. These results indicated that more smallparticle domains would appear in the two-dimensional crosssectional views on increasing the particle concentration of the
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 3. (A-F) Field emission scanning electron micrographs of the cross-sectional planes of the in situ solidified concentrated suspensions. Hetero-assembled system using PAA-OAm-stabilized large particles and small particles modified with PEI-OA having (A) 15, (B) 30, and (C) 50 mol% OA contents. Non-assembled system using large and small particles both modified with PEI-OA having (D) 15, (E) 30, and (F) 50 mol% OA contents. 23% of xs in all the samples; (scale bar: 1 lm). (G) Histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the cross section images of (A)–(F).
Fig. 4. Typical cross-sectional planes of (A) hetero-assembled- and (B) non-assembled suspensions reproduced by the computer simulation. 23% of xs in both samples; (scale bar: 1 lm). (C) Histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the images of (A) and (B).
hetero-assembled suspension. Moreover, the presence of these small-particle domains in the two-dimensional cross-sectional view would reduce the apparent degree of particle assembling. Thus, at high particle concentrations of the hetero-assembled suspension, the distribution of the distances between the small particles and the large particles closest to them in the crosssectional observation would be greater than the actual distance distribution estimated from the three-dimensional coordinates (Supplementary material, Fig. S8). Once the formation of the hetero-assembled structure was confirmed, the flow curves of the 50 vol% binary SiO2 suspensions [50 vol% PAA-OAm-stabilized large SiO2 suspensions and 50 vol% PEI-OA-stabilized small SiO2 suspensions] were measured using a rheometer (Fig. 6A). xs and the OA contents in PEI-OA fixed on
the small SiO2 particles were varied. Flow curves of the binary suspensions prepared by mixing the two suspensions, both stabilized by PEI-OA, are also displayed in Fig. 6B. Moreover, to clarify the effect of the SiO2 surface structure and xs on the flowing properties of the binary suspensions, the apparent viscosity at the shear rate 10 s1 for the 50 vol% binary suspensions with various xs are summarized in Fig. 6C. In this figure, suspensions with various xs were classified into three states based on the visual aspect and viscosity curves: the symbols d, ▲, and j denote stable (highly flowable state without hysteresis in the viscosity curve, 0.08–4 Pas), flowable (moderate flowable state with hysteresis in the viscosity curve, 8–10 Pas), and ‘‘sandy paste” (low fluid state that appears like wet sand with significant hysteresis in the viscosity curve, 10–200 Pas), respectively. The 50 vol% PAA-OAm-stabilized large
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 5. Cross-sectional planes of the reproduced hetero-assembled suspensions at different coordinates with different heights from Z in the range 6.10–6.50 lm; (scale bar: 1 lm).
SiO2 monomodal suspension presented a flow curve without any hysteresis character during elevating and descending the shear rate. However, the presence of small content (xs = 4.1–7.9%) of small particles modified with PEI-OA with using 15 mol% OA content resulted in binary 50 vol% suspension having flow curves with significant hysteresis and higher apparent viscosities [Fig. 6 A(I) and C(I)]. As increasing xs (7.9% to 23.0%), the viscosity increase were subsequently suppressed although the hysteresis character in the flow cures were still remained. The suspension became stable at xs > 30%. On the other hand, the binary suspensions in which the large and small SiO2 particles were stabilized by PEI-OA with a 15 mol% OA content displayed stable dispersion (flow curves without hysteresis and relatively low viscosities) at all xs [Fig. 6 B(I) and C(I)]. A slight increase in the viscosity was measured when xs was increased. This was attributed to the frictional resistance between the polymer layers on the particles with shorter mean inter-particle distances at elevated xs. Notably, no significant changes in the apparent viscosity between the heteroand non-assembled suspensions were observed at xs > 45.7% [Fig. 6C(I)]. A similar trend was observed for the binary suspensions comprising PEI-OA having 30 and 50 mol% OA contents, with some exceptions. The hetero-assembled suspension began to stabilize with fewer amounts of small particles (~11.4%) compared to the suspension with PEI-OA having 15 mol% OA (Fig. 6C). Furthermore, the hetero-assembled suspension using PEI-OA with 50 mol % OA surprisingly presented a curve without a marked increase in the viscosity and suppressed hysteresis properties at all xs [Fig. 6 A (III) and C(III)]. The particle assembling structure illustrated in Fig. 3C confirms that a colloidal dispersion system in which the binary spherical particles are hetero-assembled under highly concentrated conditions in flowable non-aqueous suspension was successfully designed. 4. Discussion The relationship between the OA content in PEI-OA on the small SiO2 particles, xs, particle assembled structures, and dispersion states of the binary dense suspensions can be discussed based on the afforded results. Fig. 7A displays the diagram of the effects of the OA content in PEI-OA and xs on the stability of the binary
suspensions. The diagram was classified into stable and unstable region based on apparent viscosity and presence of hysteresis in the viscosity curves. In the stable region (blue areas), the suspension flow curves exhibit no hysteresis and low viscosities (<4 Pas at the shear rate 10 s1). The other suspensions in unstable region (gray areas) exhibit higher viscosities (>8 Pas at the shear rate 10 s1) and hysteresis. In this figure, the degree of surface coverage (h) of small-particles on the large particles was considered. Defining the degree of surface coverage as a ratio of the number of small particles fixed on a large particle to the maximum number of small particle that can be surrounded on a large particle, the surface coverage is estimated as:
h¼
3 xS DL N ð 1 xS Þ D S
ð1Þ
where DS and DL are the diameters of the small and large particles, respectively; and N is the maximum number of small particles that surround a large particle, estimated from the simulation results presented in Fig. 7B. In this figure, the small particles were randomly distributed on a large-particle surface. At each step, the newly introduced small particles were only considered when their volume did not overlap with the already existing small particles. The calculated results revealed that the small particles that were randomly arranged on a large particle were saturated to N = 279 ± 4 at > 106 calculation steps with good reproducibility. Assuming that the arrangement of the small particles adsorbed onto the large-particle surface can be approximated by the random sequential adsorption (random dense parking) model of circles on a surface, N can also be expressed as [33]:
"
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi#1 1 þ 2DS =DL N ¼ 2Ap 1 1 þ DS =DL
ð2Þ
where Ap is the random dense parking limit. Many studies have reported the value Ap = 0.55 in both the experiment [34] and simulation [35]. Applying Ap = 0.55 in Eq (2) leads to N = 281 in our simulation system, which strongly supports the validity of our simulation. The mean inter-particle surface-to-surface distance of the large particles (d) was also considered in Fig. 7A. Assuming that all the
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 6. Viscosity curves of the 50 vol% binary SiO2 suspensions of (A) hetero- and (B) non-assembled systems using PEI-OA with (I) 15, (II) 30, and (III) 50 mol% OA contents; xs were varied from 0 to 100% in all the samples. Data collected from the upward and downward process were shown in darker and lighter color, respectively. (C) Effect of xs on the apparent viscosity at the shear rate 10 s1 during upward process. The dispersion states were classified into three states: d, stable state without hysteresis in the viscosity curve (0.08–4 Pas); ▲, moderate flowable state with hysteresis (8–10 Pas); and j, sandy pastes with significant hysteresis (10–200 Pas).
smaller particles behave as a viscous continuous phase (as part of the solvent) [36] and the large particles are randomly distributed in the suspension, then d can be expressed as [37]:
d¼
1=3 0:63 DL DL /L
ð3Þ
where /L is the volume fraction of the large particles. Assuming all the small particles are in a continuous phase in the suspensions, /L can be expressed as:
/L ¼ ð1 xs Þ/
ð4Þ
where / is the total solid content. The mean inter-particle distance in the 50 vol% suspensions was estimated from Eqs. (3) and (4) as a function of xs (Supplementary material, Fig. S9).
From the viewpoint of xs, the OA content in PEI-OA on the small particles, degree of surface coverage on the large particles, and mean inter-particle distance between the large particles, the diagram in Fig. 7A can be further classified into five regions. In region (I), the degree of surface coverage on the large particles greatly exceeded 100%. Stable dispersions were obtained since the main surface interactions in the suspension were the repulsive interactions between the PEI-OA molecules on the small particles (located on the large particles and/or existing as free particles) [Fig. 7C(I)]. Notably, when xs decreased so that the mean inter-particle distance between the large particles decreased to d < 2Ds, both the stable [region (II)] and unstable [region (III)] states were dependent of the OA content in PEI-OA, despite the small-particle coverage being >100%. Under these conditions, owing to the small gaps between the large particles (d < 2Ds), the small particles that were
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 7. (A) Phase diagram of the dispersion states of the hetero-assembled binary SiO2 dense suspensions. The dispersion states were classified into three states: d, stable state without hysteresis in the viscosity curve; ▲, flowable state with hysteresis; and j, sandy pastes with significant hysteresis. (B) Convergence trend of the number of randomly arranged small particles on a large particle via simulations considering statistical distribution of particles. (C) Schematic illustrations of the expected state in the concentrated suspensions for the five regions in the diagram.
fixed on the large particles could also come into contact with other large particles. Thus, considering the interactions between the PEI-OA and PAA-OAm on the large particles, the chances for repulsive interactions between the small/large SiO2 particles increased [Fig. 7C(II)] at higher OA contents in PEI-OA; this resulted in a stable dispersion [Region (II)]. On the other hand, the chances of attractive interactions increase when the OA content in PEI-OA is small [Region (III)]. This occurs because of the numerous free amino groups present in PEI-OA, which play a dominant role in possessing the interactions with the PAA-OAm-modified large SiO2 particles [Fig. 7C(III)]. These attractive interactions decrease the stability of the suspension in region (III). With the decrease in xs (<100% degree of surface coverage), the obtained suspensions became more unstable [region (IV) in Fig. 7A]. Since lower coverage and smaller mean inter-particle distances lead to a higher contact probability between a small particle assembled on a large particle and another large particle, the probability of forming an aggregated structure via small SiO2 particles increases [Fig. 7C(IV)]. SEM analysis of the binary particles dried from the respective 50 vol% hetero-assembled binary suspensions reveal the presence of aggregated structures (Supplementary material, Figs. S10A and B). The minor coverage of the large SiO2 particles and the presence of strong attractive force between the PEI-OA-modified small SiO2 particles and PAA-OAmmodified large SiO2 particle would contribute towards a large irregular network formation. The noisy flow curves with hysteresis character [Fig. 6A(I) and (II)] can be explained by competition of this network formation and disagglomeration by induced shear. Conversely, when the OA contents of PEI-OA was increased, the hetero-assembled binary suspension became stable even under low surface coverage and small mean inter-particle distance
conditions [region (V) in Fig. 7A and Supporting Information, Fig. S10C]. Although suppression of the interactions between PEI-OA and PAA-OAm resulted in a very slight reduction in the degree of particle assembly (Fig. 3C), the increase in the OA content of PEI-OA significantly improved the stability of the heteroassembled binary SiO2 suspensions, owing to an increase in the repulsive interactions between the assembled particles [Fig. 7C (V)]. We strongly believe that our proposed system, which involves the mixing of PEI-OA-and PAA-OAm-stabilized particles in nonaqueous solvents, is a powerful tool to achieve a class of colloidal dispersion states comprising a flowable concentrated dispersion of hetero-assembled particles. Using this system, we have demonstrated that the tuning of attractive/repulsive interactions via the design of the OA content in PEI-OA is a simple and useful protocol to control the microstructures and flowing properties of these suspensions.
5. Conclusion We have successfully designed a colloidal dispersion system in which small spherical particles were hetero-assembled on large spherical particles in concentrated and flowable suspensions. The simple mixing of a small spherical silica dispersion stabilized by a cationic polymer-fatty acid complex and large spherical silica stabilized by an anionic polymer-aliphatic amine complex compatibly achieved particle assembly and suspension flowability in non-aqueous media, even at 50 vol% particle loading. Comparison of the cross-sectional images of the experimentaland simulated suspensions strongly supported hetero-assembly in the concentrated suspensions through our proposed process.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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The dispersion stability/instability of the binary colloidal suspensions were discussed based on the control of the attractive/repulsive interactions via the OA content in PEI-OA, degree of surface coverage on the large particles, and average surface-to-surface distance between the large particles. We believe that our proposed hetero-assembly system is a powerful tool to achieve a flowable dispersion of hetero-assembled particles based on the tuning of attractive/repulsive interactions by a simple protocol and adjustment of the mixing ratio of commercially available reagents. Therefore, this system should open a new window toward various industrial applications of material designing through colloidal processing. Acknowledgments This work was supported by JSPS KAKENHI Grant Numbers JP18H01704 and JP17J11031. The authors also gratefully thanks the support from Instrumental Analysis Center of Yokohama National University. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.11.029. References [1] Y. Wang, Y. Wang, D.R. Breed, V.N. Manoharan, L. Feng, A.D. Hollingsworth, M. Weck, D.J. Pine, Colloids with valence and specific directional bonding, Nature 491 (2012) 51–61. [2] R.M. Erb, H.S. Son, B. Samanta, V.M. Rotello, B.B. Yellen, Magnetic assembly of colloidal superstructures with multipole symmetry, Nature 457 (2009) 999–1002. [3] F.M. Hecht, A.R. Bausch, Kinetically guided colloidal structure formation, Proc. Natl. Acad. Sci. USA 113 (2016) 8577–8582. [4] A.V. Tkachenko, Generic phase diagram of binary superlattices, Proc. Natl. Acad. Sci. USA 113 (2016) 10269–10274. [5] K. Jabłczyn´ska, J.M. Gac, T.R. Sosnowski, Self-organization of colloidal particles during drying of a droplet: modeling and experimental study, Adv. Powder Technol. 29 (2018) 3542–3551. [6] A.L. Costa, B. Ballarin, A. Spegni, F. Casoli, D. Gardini, Synthesis of nanostructured magnetic photocatalyst by colloidal approach and spraydrying technique, J. Colloid Interface Sci. 388 (2012) 31–39. [7] F. Tang, H. Fudouzi, Y. Sakka, Fabrication of macroporous alumina with tailored porosity, J. Am. Ceram. Soc. 86 (2003) 2050–2054. [8] J.J. Rosário, P.N. Dyachenko, R. Kubrin, R.M. Pasquarelli, A.Yu. Petrov, M. Eich, G. A. Schneider, Facile deposition of YSZ-inverse photonic glass films, ACS Appl. Mater. Interfaces 6 (2014) 12335–12345. [9] C.-P. Hsu, S.N. Ramakrishna, M. Zanini, N.D. Spencer, L. Isaa, Roughnessdependent tribology effects on discontinuous shear thickening, Proc. Natl. Acad. Sci. USA 115 (2018) 5117–5122. [10] F. Iskandar, H. Chang, K. Okuyama, Preparation of microencapsulated powders by an aerosol spray method and their optical properties, Adv. Powder Technol. 14 (2003) 349–367. [11] X. Li, J. He, In situ assembly of raspberry- and mulberry-like silica nanospheres toward antireflective and antifogging coatings, ACS Appl. Mater. Interfaces 4 (2012) 2204–2211. [12] K. Sato, Y. Tominaga, Y. Hotta, H. Shibuya, M. Sugie, T. Saruyama, Cellulose nanofiber/nanodiamond composite films: thermal conductivity enhancement achieved by a tuned nanostructure, Adv. Powder Technol. 29 (2018) 972–976.
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Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Original Research Paper
Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design Seitaro Morita a, Motoyuki Iijima a,b,⇑, Junichi Tatami a,b a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Institute of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 14 October 2019 Received in revised form 15 November 2019 Accepted 19 November 2019 Available online xxxx Keywords: Hetero-assembly Non-aqueous suspension Colloidal processing Surface modification Polymer dispersant
a b s t r a c t Although many colloidal assembling systems have been reported, most systems suffer from severe aggregation under high solid concentrations, which can often be observed in typical hetero-aggregation system. In this study, we created a hetero-assembly system using concentrated (~50 vol%) suspensions by mixing large SiO2 particles modified with polyacrylic acid partially complexed with oleylamine (PAAOAm) and small SiO2 particles modified with polyethyleneimine partially complexed with oleic acid (PEI-OA) in a non-aqueous solvent. We demonstrated that hetero-assembly is driven by the interactions between the uncomplexed carboxyl/amine groups of the PAA/PEI present on the particles, while severe aggregation is simultaneously prevented by the steric repulsions of the aliphatic oleyl chains. Comparison of the cross sections of the in-situ solidified hetero-assembled suspensions with those of ideally assembled structures which were reproduced by a simulation considering the statistical distribution of particles strongly supported successful particle assembling via the proposed approach. The results revealed that the OA content in the PEI-OA complex was the dominant factor that controlled the dispersion and assembling state of the binary particles. The significance of this study is that our findings will provide a class of colloidal dispersion state which binary particles were assembled in a high solid content suspension without forming strong aggregates. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Colloidal assembly in multicomponent suspensions is an important technique used to design composite materials with unique structures [1–7] and functions [8–14] that are derived from their controlled microstructures. Therefore, these techniques are widely applied in various fields such as ceramics [7,8], polymer-based composites [12,13], and carbon-based composites [14–16]. The properties of these materials, which are manufactured via colloidal processing, are tuned by the local arrangement of each particle. Thus, colloidal hetero-assembly, in which small particles are assembled onto large particles, has attracted a wide range of research. From the 1970 s, many hetero-assembly systems have been reported and applied to material processing [6–9,11–30] in both aqueous and non-aqueous media. Aqueous hetero-assembly systems utilize electrostatic attractions based on pH control ⇑ Corresponding author at: Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. E-mail address: [email protected] (M. Iijima).
[17–19] and layer-by-layer surface modifications [11], while nonaqueous hetero-assembly systems employ other attraction forces such as covalent bonds [13,21,22], acid-base interactions [23], hydrogen bonds [24–26], and pi–pi stacking [16,27]. Although these systems successfully achieve particle assembling, most of these reported systems comprise dilute suspensions (<10 vol%) and suffers from a marked increase in viscosity [28] when processed under high solid concentrations, owing to the aggregation induced by small-particle bridges between the large particles [29,30]. Because numerous industrial applications require colloidal processing under concentrated conditions, the realization of particle assembling in dense and flowable suspensions would provide new options for material processing. Herein, we introduce a strategy to realize a class of colloidal dispersion systems in which binary spherical particles are hetero-assembled under highly concentrated conditions in flowable non-aqueous suspensions. In our previous studies, we designed a series of polymer dispersants comprising a cationic polymer partially complexed with fatty acids. We reported that the designed complex improved the stability of various particles in non-aqueous solvents [31,32]. The key concept in this work
https://doi.org/10.1016/j.apt.2019.11.029 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Nomenclature Ap DL DS N X Y Z xs
random dense parking limit [–] diameters of the large SiO2 particles [lm] diameters of the small SiO2 particles [lm] maximum number of small particles that surround a large particle [–] length of a rectangular frame for simulation [lm] width of a rectangular frame for simulation [lm] height of a rectangular frame for simulation [lm] volume fraction of small particles based on the total solid content in suspensions [%]
was the design of an anionic polymer partially complexed with an aliphatic amine and the subsequent mixing of two dense colloidal dispersions stabilized by a cationic polymer partially complexed with a fatty acid and an anionic polymer partially complexed with an aliphatic amine (Fig. 1). We expected that the interactions between the remaining cationic and anionic functional groups would drive the hetero-assembly process, while the aliphatic chain would prevent severe aggregation by the steric repulsions generated by the improved affinity between the aliphatic chain and non-aqueous solvent. To proof this concept, a cationic complex of polyethyleneimine and oleic acid (PEI-OA) [31,32] and an anionic complex of polyacrylic acid and oleylamine (PAA-OAm) were designed as functional dispersants for small (217 nm) and large (2.24 lm) spherical SiO2 particles, respectively. The toluene suspension of the respective PAA-OAm- and PEI-OA-stabilized large and small SiO2 particles were mixed under various volume ratios of small-to-large particles and concentrated conditions (50 vol%) to achieve particle assembly in an flowable suspension state. The degree of hetero-assembly in the concentrated suspensions was confirmed by comparing the particle arrangement structures of the in situ solidified specimens to those of ideally assembled structures reproduced by computer simulation considering the statistical distribution of particles. To optimize the interactions between the small/large particles and the repulsive interactions between the resulting hetero-assembled particles, the chemical structures on the small-particle surfaces were varied by tuning the OA content in PEI-OA, and their effect on the particle assembling/dispersion behavior was investigated. 2. Materials and methods 2.1. Materials Polyacrylic acid (PAA, Mw 5000), oleylamine (OAm), polyethyleneimine (PEI, Mw 1800), oleic acid (OA), toluene, and ethanol were purchased from FUJIFILM Wako Pure Chemical Co. Ltd., Japan.
d h / /L
mean inter-particle surface-to-surface distance of the large particles [lm] degree of surface coverage of small particles on a large particle [%] total solid content in suspension [vol%] volume fraction of the large particles in suspension [vol %]
Poly(ethylene glycol) diacrylate (PEGDA, average polymerization degree 250) was purchased from Sigma-Aldrich Co. LLC. 2,2-Dime thoxy-2-phenylacetophenone (DPA) was purchased from Tokyo Chemical Industry Co. Ltd., Japan. The spherical large SiO2 powder (2.24 lm, calculated from BET specific surface area: 1.22 m2/g) and spherical small SiO2 powder (217 nm, calculated from BET specific surface area: 12.6 m2/g) were obtained from Nippon Shokubai Co. Ltd., Japan. All the materials were used as received without further purification. 2.2. Cationic and anionic polymer dispersants Solution of PEI–OA complexes of different OA contents (15–50 mol% OA based on the number of amino groups in PEI) were prepared using the method described in our previous report [31]. Solution of PAA-OAm (70 mol% OAm based on the number of carboxyl group in PAA) were prepared by mixing 1.5 g of 16.7 wt% PAA/ethanol solution and 8.5 g of 7.64 wt% OAm/toluene solution and then treated in an ultrasonic bath for 5 min. (See Supplementary material and Figs. S1–S4 for detailed characterization of PAA-OAm and their ability to improve the large SiO2/toluene suspensions). 2.3. Hetero-assembled binary SiO2 suspensions First, small SiO2/toluene dispersions stabilized by PEI-OA having different OA contents (15, 30, 50 mol%) were prepared by mixing SiO2 particles (50 vol%) in toluene which the respective PEI-OA were dissolved. The PEI-OA contents were controlled to values of 1.3, 1.4, and 1.6 mg/m2 (based on the total SiO2 surface area in the suspension) for the samples with 15, 30, and 50 mol% OA ratios, respectively, to reach saturated adsorption. Next, the large SiO2/toluene dispersions stabilized by PAA-OAm were prepared as follows; 3.00 g of large SiO2 powder was added to the PAA-OAm solution (1.5 mg/m2) and the suspension was stirred for 24 h. The suspensions were centrifuged for 5 min at 2000 g
Fig. 1. Schematic illustration of the hetero-assembly of binary particles using PAA-OAm and PEI-OA dispersants to improve the dispersion state in concentrated suspensions.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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and the collected wet cake was washed with toluene and dried at 60 °C under vacuum. The PAA-OAm-modified large SiO2 powders were resuspended in toluene to be 50 vol% using a planetary mixer (ARE-250, THINKY Co., Ltd., rotation speed 800 rpm, revolution speed 2000 rpm, 3 min.). Then, the 50 vol% hetero-assembled suspensions were prepared by mixing the 50 vol% toluene dispersions of PEI-OA-stabilized small SiO2 particles and PAA-OAm stabilized large SiO2 particles under various small-particle mixing fractions (xs) using a planetary mixer (rotation speed 800 rpm, revolution speed 2000 rpm, 3 min.). Here, xs was defined as volume fraction of small particles based on the total solid content in the suspensions. Diluted binary SiO2 suspensions were also prepared similarly by mixing 0.5 vol% PAA-OAm- and PEI-OA-stabilized SiO2/toluene suspensions. 2.4. Non-assembled binary SiO2 suspensions Large SiO2/toluene dispersions stabilized by PEI-OA having different OA contents (15, 30, 50 mol%) were prepared similarly to the case for small SiO2 particles instead that the PEI-OA contents were controlled to values of 2.1, 1.6, and 1.8 mg/m2 for the samples with 15, 30, and 50 mol% OA ratios, respectively, to reach saturated adsorption. The 50 vol% toluene dispersions of the small and large SiO2 particles, which were both stabilized with PEI-OA, were mixed together under various small/large particle ratios using a planetary mixer (rotation speed 800 rpm, revolution speed 2000 rpm, 3 min). 2.5. Characterizations Viscosity curves of the hetero-assembled and non-assembled binary 50 vol% suspensions were measured using a rheometer (TA Instruments, AR-G2) by increasing the shear rate from 0 to 80 s1 in 90 sec. and then decreasing it from 80 s1 to 0 s1 in 90 sec. The particle arrangements in dilute suspensions were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi High-Technologies, SU8010) of the particles dried from the 0.5 vol% binary SiO2 suspensions on aluminum SEM stages. The particle arrangements in the dense suspensions were also characterized by the modified in situ solidification methods as follows: first, a photo-curable acrylate resin was prepared by dissolving DPA as a photo-radical polymerization initiator in PEGDA. Next, as prepared acrylate resin was mixed with 50 vol% binary SiO2/toluene suspensions using a planetary mixer to attain 40 vol %. Samples (1.0 g) of the obtained suspensions were poured into Teflon molds and cured by irradiating 365 nm UV light for 1 min. The in situ solidified suspensions were dried at 60 °C under vacuum and the cross-sectional planes were processed using a cross section polisher (JEOL, SM-9010). The particle arrangements of the in situ solidified bodies were characterized by FE-SEM. For all the sample images, the inter-surface distances between all the small particles and large particles were calculated and their shortest inter-surface distance distributions were determined. The number of small particles used to calculate the shortest inter-surface distances was 250. 2.6. Simulation of the hetero- and non-assembled systems First, the maximum number of small (0.217 lm) spherical particles that could be attached on a large (2.240 lm) spherical particle was estimated by a simulation based on the random sequential adsorption model, under the assumptions that fixed small particles do not deform and move on large particles. A single large particle was settled on the center of the spherical coordinate system and the polar angle and azimuthal angle were generated randomly. The radial distance was set to 1.2285 lm, which is the core-to-core distance of the small and large particles. The central
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position of a small particle was then located at the coordinate having randomly generated angles and the radial distance of 1.2285 lm. The coordinates of the central position of the newly located small particle were compared to those of the alreadyexisting small particles. The newly located small particles were fixed on a large particle if the core-to-core distance between the central position of the newly added small particles and of the already existing particles were not less than 0.217 lm (i.e. no volumetric overlapping with the already existing small particles); otherwise, the small particle was discarded. These small particle setting trials were conducted for 108 calculation steps. The entire calculation was performed sixfold to estimate the maximum number of small particles that could be settled on a large particle through a random process. Next, the particle arrangements of the hetero- and nonassembled systems were reproduced by the following protocol: a rectangular frame (X = 21.31 lm, Y = 14.20 lm, and Z = 9.47 lm) was generated and 150 large spherical particles and 47,500 small particles were inserted in the frame using the following rules: The solid content in the frame was 40 vol%, the same particle fraction as that of the in situ solidified suspensions before curing and drying. To reproduce the hetero-assembled system, 150 large spherical particles were randomly placed in the rectangular frame. The small particles were then randomly arranged on each large particle, using the abovementioned method, for 106 steps. During the attachment of the small particles to the large particles, the small particles that were discarded due to volumetric overlapping with the already existing small particles were collected. After randomly attaching the small particles on all the large particles, the collected and remaining small particles were randomly placed in a vacant space of the frame. To reproduce the non-assembled system, 150 large particles were randomly placed in the frame. Next, 47,500 small particles were randomly placed in the vacant space. The distributions of the surface distances between the small particles and their nearest neighboring large particles were analyzed using the three-dimensional coordination data. Crosssectional two-dimensional images of both simulated suspensions were also obtained based on the coordination of the particles in the frame at the typical X–Y plane. The frames were trimmed (X width, 12.68 lm and Y width, 8.82 lm) to compare the distributions of the apparent surface distances between the small particle surfaces and the large particle surface nearest to them obtained from the experimental results. The surface distributions of the simulated suspensions were estimated from nine slices of the cross-sectional planes at various Z coordinates (Z = 2.8–6.8 lm).
3. Results To demonstrate the proposed hetero-assembly system (Fig. 1), toluene suspensions of the PAA-OAm- and PEI-OA-stabilized large SiO2 particles and PEI-OA-stabilized small SiO2 particles were mixed under dilute conditions (0.5 vol%) and different large-tosmall particle mixing ratios for the first step. When the PAAOAm-stabilized large particles and PEI-OA-stabilized small particles were mixed together, almost all the small particles were adsorbed onto the large particles without forming severe agglomerates, regardless of the OA content in PEI-OA (Fig. 2A–C). Based on a qualitative visual comparison, we also confirmed that the coverage of small particles on the large particles increased with increasing xs [(I) and (II) in Fig. 2A–C and Supplementary material, Figs. S5A–C]. Conversely, when PEI-OA dispersants were used for both the small and large particles, no small particles were fixed onto the large particles, regardless of the OA contents and xs (Fig. 2D–F). The lower magnification images revealed the presence of many small particles between the large particles, suggesting
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 2. Scanning electron micrographs of the large and small SiO2 particles dried from 0.5 vol% binary SiO2 suspensions. Hetero-assembled system using PAA-OAm-modified large particles and small particles modified with PEI-OA having (A) 15, (B) 30, and (C) 50 mol% OA contents. Non-assembled system using large and small particles both modified with PEI-OA having (D) 15, (E) 30, and (F) 50 mol% OA contents. (I) xs = 7.9% and (II) xs = 23%; (scale bar: 1 lm).
small attractive interactions between the large and small particles. Thus, we concluded that the small particles were localized between the large particles, via capillary forces, during the drying process (Supplementary material, Figs. S5D–F). These results clearly revealed that hetero-assembly of the binary SiO2 particles was achieved when the PAA-OAm- and PEI-OA-stabilized SiO2 suspensions were mixed together. As the mixing of PEI-OA-stabilized suspensions did not form assembled structures, the complexing interactions between the amino groups of PEI-OA on small SiO2 particles and the carboxyl groups of PAA-OAm on large SiO2 particles are expected to be the major factor in the formation of the assembled structures. To confirm the applicability of the proposed hetero-assembling process to dense suspensions with flowable state, 50 vol% binary hetero-assembled suspensions were next prepared and their microstructures and flowing properties were subsequently characterized. The particle arrangements in the concentrated binary suspensions of the hetero- and non-assembled systems were characterized by SEM analysis of the in situ solidified specimens (Fig. 3A–F, xs = 23%). Fig. 3G display histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the cross section images. Notably, the solid content of the suspension used for in situ solidification was diluted to 40 vol% as a result of monomer addition. Furthermore, shrinkage of the in situ solidified specimen was observed during curing/drying. Therefore, the respective actual solid contents of the in situ solidified specimen for the non– and hetero-assembled systems were 49 and 43 vol%, which slightly differed from those of the original suspensions. Fig. 3A–C reveal that many small particles were present on the large particles for the dense suspensions comprising PAA-OAm-stabilized large SiO2 suspensions mixed with PEI-OA-stabilized small SiO2 suspensions (hetero-assembled system). The histogram in Fig. 3 reveal that 71% of the small particles were located within 100 nm of the large-particle surface for the hetero-assembled samples comprising a 15 mol% OA content in PEI-OA. On increasing the OA content in PEI-OA (30 and 50 mol%) in the hetero-assembled system, the number of small particles gathering around the large particles was slightly reduced; however, 60% of the small particles were still located within 100 nm of the large-particle surface. Conversely, the small particles were distributed among the large particles when both the large and small particles were stabilized by PEI-OA (non-assembled system). Only 40% of the small particles were located within 100 nm of the large-particle surface in all the specimens of the non-assembled system (Fig. 3D–F). To compare these results with the ideal hetero-assembled and randomly dispersed states, the particle arrangements in the cross-sectional planes of the hetero- and non-assembled systems
were simulated considering statistical distribution of particles. Typical snap shots of the cross-sectional planes and the histograms for the distributions of the surface distances between the small particles and the nearest large particles are displayed in Fig. 4. The simulation of the hetero-assembled system was divided into three steps: (i) random arrangement of the large particles in a three-dimensional frame box, (ii) random arrangement of the small particles on the large particles until full coverage was attained, and (iii) random placement of the residual small particles in a vacant space of the frame box. On the other hand, the calculations for the non-assembled system were simply conducted by randomly placing the small particles after settling the large particles. Solid contents and xs were 40 vol% and 23%, respectively, for both cases. For the non-assembled system, the calculated histograms and particle localizations (Fig. 4B) were different from those attained from the experimental results. Conversely, the hetero-assembled system (Fig. 4A) exhibited similar trends. Focusing to the non-assembled system, the frequency of the small particles located near the large-particle surface in the experimental results was larger than that observed in the simulated images. Additionally, some small-particle domains were observed in the experimental specimens that were not present in the calculated images. These results suggested the presence of some attractive interactions between the PEI-OA-modified surfaces under static conditions, indicating that the non-assembled system was not in a completely randomly-dispersed state. The hydrogen bonds in the non-aqueous solvent, between the remaining PEI-OA amino groups, may contribute towards these attractive interactions. For the hetero-assembled system, the experimental frequency of the small particles fixed onto the large particles were in good agreement with the calculated results, strongly indicating that the small particles effectively assembled onto the large particles by our proposed process. Interestingly, together with the isolated free small particles, some aggregated small particles were observed in the cross-sectional view of the simulated results that were not present in the simulated images of the non-assembled system (images for other cross-sectional planes at different Z coordinates are shown in Supplementary material, Figs. S6 and S7). These small-particle domains were also present in the in situ solidified specimen of the hetero-assembled system. The series of simulated crosssectional planes, attained by gradually changing the Z coordinates (Fig. 5), strongly suggest that these small-particle aggregates originate from the cross sections of the small particles assembled on the edge of large particles. On the other hand, the isolated small particles were free particles that could not interact with the fully covered large particles. These results indicated that more smallparticle domains would appear in the two-dimensional crosssectional views on increasing the particle concentration of the
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Fig. 3. (A-F) Field emission scanning electron micrographs of the cross-sectional planes of the in situ solidified concentrated suspensions. Hetero-assembled system using PAA-OAm-stabilized large particles and small particles modified with PEI-OA having (A) 15, (B) 30, and (C) 50 mol% OA contents. Non-assembled system using large and small particles both modified with PEI-OA having (D) 15, (E) 30, and (F) 50 mol% OA contents. 23% of xs in all the samples; (scale bar: 1 lm). (G) Histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the cross section images of (A)–(F).
Fig. 4. Typical cross-sectional planes of (A) hetero-assembled- and (B) non-assembled suspensions reproduced by the computer simulation. 23% of xs in both samples; (scale bar: 1 lm). (C) Histograms for the distributions of the surface distances between the small particles and the large particles closest to them, estimated from the images of (A) and (B).
hetero-assembled suspension. Moreover, the presence of these small-particle domains in the two-dimensional cross-sectional view would reduce the apparent degree of particle assembling. Thus, at high particle concentrations of the hetero-assembled suspension, the distribution of the distances between the small particles and the large particles closest to them in the crosssectional observation would be greater than the actual distance distribution estimated from the three-dimensional coordinates (Supplementary material, Fig. S8). Once the formation of the hetero-assembled structure was confirmed, the flow curves of the 50 vol% binary SiO2 suspensions [50 vol% PAA-OAm-stabilized large SiO2 suspensions and 50 vol% PEI-OA-stabilized small SiO2 suspensions] were measured using a rheometer (Fig. 6A). xs and the OA contents in PEI-OA fixed on
the small SiO2 particles were varied. Flow curves of the binary suspensions prepared by mixing the two suspensions, both stabilized by PEI-OA, are also displayed in Fig. 6B. Moreover, to clarify the effect of the SiO2 surface structure and xs on the flowing properties of the binary suspensions, the apparent viscosity at the shear rate 10 s1 for the 50 vol% binary suspensions with various xs are summarized in Fig. 6C. In this figure, suspensions with various xs were classified into three states based on the visual aspect and viscosity curves: the symbols d, ▲, and j denote stable (highly flowable state without hysteresis in the viscosity curve, 0.08–4 Pas), flowable (moderate flowable state with hysteresis in the viscosity curve, 8–10 Pas), and ‘‘sandy paste” (low fluid state that appears like wet sand with significant hysteresis in the viscosity curve, 10–200 Pas), respectively. The 50 vol% PAA-OAm-stabilized large
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 5. Cross-sectional planes of the reproduced hetero-assembled suspensions at different coordinates with different heights from Z in the range 6.10–6.50 lm; (scale bar: 1 lm).
SiO2 monomodal suspension presented a flow curve without any hysteresis character during elevating and descending the shear rate. However, the presence of small content (xs = 4.1–7.9%) of small particles modified with PEI-OA with using 15 mol% OA content resulted in binary 50 vol% suspension having flow curves with significant hysteresis and higher apparent viscosities [Fig. 6 A(I) and C(I)]. As increasing xs (7.9% to 23.0%), the viscosity increase were subsequently suppressed although the hysteresis character in the flow cures were still remained. The suspension became stable at xs > 30%. On the other hand, the binary suspensions in which the large and small SiO2 particles were stabilized by PEI-OA with a 15 mol% OA content displayed stable dispersion (flow curves without hysteresis and relatively low viscosities) at all xs [Fig. 6 B(I) and C(I)]. A slight increase in the viscosity was measured when xs was increased. This was attributed to the frictional resistance between the polymer layers on the particles with shorter mean inter-particle distances at elevated xs. Notably, no significant changes in the apparent viscosity between the heteroand non-assembled suspensions were observed at xs > 45.7% [Fig. 6C(I)]. A similar trend was observed for the binary suspensions comprising PEI-OA having 30 and 50 mol% OA contents, with some exceptions. The hetero-assembled suspension began to stabilize with fewer amounts of small particles (~11.4%) compared to the suspension with PEI-OA having 15 mol% OA (Fig. 6C). Furthermore, the hetero-assembled suspension using PEI-OA with 50 mol % OA surprisingly presented a curve without a marked increase in the viscosity and suppressed hysteresis properties at all xs [Fig. 6 A (III) and C(III)]. The particle assembling structure illustrated in Fig. 3C confirms that a colloidal dispersion system in which the binary spherical particles are hetero-assembled under highly concentrated conditions in flowable non-aqueous suspension was successfully designed. 4. Discussion The relationship between the OA content in PEI-OA on the small SiO2 particles, xs, particle assembled structures, and dispersion states of the binary dense suspensions can be discussed based on the afforded results. Fig. 7A displays the diagram of the effects of the OA content in PEI-OA and xs on the stability of the binary
suspensions. The diagram was classified into stable and unstable region based on apparent viscosity and presence of hysteresis in the viscosity curves. In the stable region (blue areas), the suspension flow curves exhibit no hysteresis and low viscosities (<4 Pas at the shear rate 10 s1). The other suspensions in unstable region (gray areas) exhibit higher viscosities (>8 Pas at the shear rate 10 s1) and hysteresis. In this figure, the degree of surface coverage (h) of small-particles on the large particles was considered. Defining the degree of surface coverage as a ratio of the number of small particles fixed on a large particle to the maximum number of small particle that can be surrounded on a large particle, the surface coverage is estimated as:
h¼
3 xS DL N ð 1 xS Þ D S
ð1Þ
where DS and DL are the diameters of the small and large particles, respectively; and N is the maximum number of small particles that surround a large particle, estimated from the simulation results presented in Fig. 7B. In this figure, the small particles were randomly distributed on a large-particle surface. At each step, the newly introduced small particles were only considered when their volume did not overlap with the already existing small particles. The calculated results revealed that the small particles that were randomly arranged on a large particle were saturated to N = 279 ± 4 at > 106 calculation steps with good reproducibility. Assuming that the arrangement of the small particles adsorbed onto the large-particle surface can be approximated by the random sequential adsorption (random dense parking) model of circles on a surface, N can also be expressed as [33]:
"
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi#1 1 þ 2DS =DL N ¼ 2Ap 1 1 þ DS =DL
ð2Þ
where Ap is the random dense parking limit. Many studies have reported the value Ap = 0.55 in both the experiment [34] and simulation [35]. Applying Ap = 0.55 in Eq (2) leads to N = 281 in our simulation system, which strongly supports the validity of our simulation. The mean inter-particle surface-to-surface distance of the large particles (d) was also considered in Fig. 7A. Assuming that all the
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 6. Viscosity curves of the 50 vol% binary SiO2 suspensions of (A) hetero- and (B) non-assembled systems using PEI-OA with (I) 15, (II) 30, and (III) 50 mol% OA contents; xs were varied from 0 to 100% in all the samples. Data collected from the upward and downward process were shown in darker and lighter color, respectively. (C) Effect of xs on the apparent viscosity at the shear rate 10 s1 during upward process. The dispersion states were classified into three states: d, stable state without hysteresis in the viscosity curve (0.08–4 Pas); ▲, moderate flowable state with hysteresis (8–10 Pas); and j, sandy pastes with significant hysteresis (10–200 Pas).
smaller particles behave as a viscous continuous phase (as part of the solvent) [36] and the large particles are randomly distributed in the suspension, then d can be expressed as [37]:
d¼
1=3 0:63 DL DL /L
ð3Þ
where /L is the volume fraction of the large particles. Assuming all the small particles are in a continuous phase in the suspensions, /L can be expressed as:
/L ¼ ð1 xs Þ/
ð4Þ
where / is the total solid content. The mean inter-particle distance in the 50 vol% suspensions was estimated from Eqs. (3) and (4) as a function of xs (Supplementary material, Fig. S9).
From the viewpoint of xs, the OA content in PEI-OA on the small particles, degree of surface coverage on the large particles, and mean inter-particle distance between the large particles, the diagram in Fig. 7A can be further classified into five regions. In region (I), the degree of surface coverage on the large particles greatly exceeded 100%. Stable dispersions were obtained since the main surface interactions in the suspension were the repulsive interactions between the PEI-OA molecules on the small particles (located on the large particles and/or existing as free particles) [Fig. 7C(I)]. Notably, when xs decreased so that the mean inter-particle distance between the large particles decreased to d < 2Ds, both the stable [region (II)] and unstable [region (III)] states were dependent of the OA content in PEI-OA, despite the small-particle coverage being >100%. Under these conditions, owing to the small gaps between the large particles (d < 2Ds), the small particles that were
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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Fig. 7. (A) Phase diagram of the dispersion states of the hetero-assembled binary SiO2 dense suspensions. The dispersion states were classified into three states: d, stable state without hysteresis in the viscosity curve; ▲, flowable state with hysteresis; and j, sandy pastes with significant hysteresis. (B) Convergence trend of the number of randomly arranged small particles on a large particle via simulations considering statistical distribution of particles. (C) Schematic illustrations of the expected state in the concentrated suspensions for the five regions in the diagram.
fixed on the large particles could also come into contact with other large particles. Thus, considering the interactions between the PEI-OA and PAA-OAm on the large particles, the chances for repulsive interactions between the small/large SiO2 particles increased [Fig. 7C(II)] at higher OA contents in PEI-OA; this resulted in a stable dispersion [Region (II)]. On the other hand, the chances of attractive interactions increase when the OA content in PEI-OA is small [Region (III)]. This occurs because of the numerous free amino groups present in PEI-OA, which play a dominant role in possessing the interactions with the PAA-OAm-modified large SiO2 particles [Fig. 7C(III)]. These attractive interactions decrease the stability of the suspension in region (III). With the decrease in xs (<100% degree of surface coverage), the obtained suspensions became more unstable [region (IV) in Fig. 7A]. Since lower coverage and smaller mean inter-particle distances lead to a higher contact probability between a small particle assembled on a large particle and another large particle, the probability of forming an aggregated structure via small SiO2 particles increases [Fig. 7C(IV)]. SEM analysis of the binary particles dried from the respective 50 vol% hetero-assembled binary suspensions reveal the presence of aggregated structures (Supplementary material, Figs. S10A and B). The minor coverage of the large SiO2 particles and the presence of strong attractive force between the PEI-OA-modified small SiO2 particles and PAA-OAmmodified large SiO2 particle would contribute towards a large irregular network formation. The noisy flow curves with hysteresis character [Fig. 6A(I) and (II)] can be explained by competition of this network formation and disagglomeration by induced shear. Conversely, when the OA contents of PEI-OA was increased, the hetero-assembled binary suspension became stable even under low surface coverage and small mean inter-particle distance
conditions [region (V) in Fig. 7A and Supporting Information, Fig. S10C]. Although suppression of the interactions between PEI-OA and PAA-OAm resulted in a very slight reduction in the degree of particle assembly (Fig. 3C), the increase in the OA content of PEI-OA significantly improved the stability of the heteroassembled binary SiO2 suspensions, owing to an increase in the repulsive interactions between the assembled particles [Fig. 7C (V)]. We strongly believe that our proposed system, which involves the mixing of PEI-OA-and PAA-OAm-stabilized particles in nonaqueous solvents, is a powerful tool to achieve a class of colloidal dispersion states comprising a flowable concentrated dispersion of hetero-assembled particles. Using this system, we have demonstrated that the tuning of attractive/repulsive interactions via the design of the OA content in PEI-OA is a simple and useful protocol to control the microstructures and flowing properties of these suspensions.
5. Conclusion We have successfully designed a colloidal dispersion system in which small spherical particles were hetero-assembled on large spherical particles in concentrated and flowable suspensions. The simple mixing of a small spherical silica dispersion stabilized by a cationic polymer-fatty acid complex and large spherical silica stabilized by an anionic polymer-aliphatic amine complex compatibly achieved particle assembly and suspension flowability in non-aqueous media, even at 50 vol% particle loading. Comparison of the cross-sectional images of the experimentaland simulated suspensions strongly supported hetero-assembly in the concentrated suspensions through our proposed process.
Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029
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The dispersion stability/instability of the binary colloidal suspensions were discussed based on the control of the attractive/repulsive interactions via the OA content in PEI-OA, degree of surface coverage on the large particles, and average surface-to-surface distance between the large particles. We believe that our proposed hetero-assembly system is a powerful tool to achieve a flowable dispersion of hetero-assembled particles based on the tuning of attractive/repulsive interactions by a simple protocol and adjustment of the mixing ratio of commercially available reagents. Therefore, this system should open a new window toward various industrial applications of material designing through colloidal processing. Acknowledgments This work was supported by JSPS KAKENHI Grant Numbers JP18H01704 and JP17J11031. The authors also gratefully thanks the support from Instrumental Analysis Center of Yokohama National University. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.11.029. References [1] Y. Wang, Y. Wang, D.R. Breed, V.N. Manoharan, L. Feng, A.D. Hollingsworth, M. Weck, D.J. Pine, Colloids with valence and specific directional bonding, Nature 491 (2012) 51–61. [2] R.M. Erb, H.S. Son, B. Samanta, V.M. Rotello, B.B. Yellen, Magnetic assembly of colloidal superstructures with multipole symmetry, Nature 457 (2009) 999–1002. [3] F.M. Hecht, A.R. Bausch, Kinetically guided colloidal structure formation, Proc. Natl. Acad. Sci. USA 113 (2016) 8577–8582. [4] A.V. Tkachenko, Generic phase diagram of binary superlattices, Proc. Natl. Acad. Sci. USA 113 (2016) 10269–10274. [5] K. Jabłczyn´ska, J.M. Gac, T.R. Sosnowski, Self-organization of colloidal particles during drying of a droplet: modeling and experimental study, Adv. Powder Technol. 29 (2018) 3542–3551. [6] A.L. Costa, B. Ballarin, A. Spegni, F. Casoli, D. Gardini, Synthesis of nanostructured magnetic photocatalyst by colloidal approach and spraydrying technique, J. Colloid Interface Sci. 388 (2012) 31–39. [7] F. Tang, H. Fudouzi, Y. Sakka, Fabrication of macroporous alumina with tailored porosity, J. Am. Ceram. Soc. 86 (2003) 2050–2054. [8] J.J. Rosário, P.N. Dyachenko, R. Kubrin, R.M. Pasquarelli, A.Yu. Petrov, M. Eich, G. A. Schneider, Facile deposition of YSZ-inverse photonic glass films, ACS Appl. Mater. Interfaces 6 (2014) 12335–12345. [9] C.-P. Hsu, S.N. Ramakrishna, M. Zanini, N.D. Spencer, L. Isaa, Roughnessdependent tribology effects on discontinuous shear thickening, Proc. Natl. Acad. Sci. USA 115 (2018) 5117–5122. [10] F. Iskandar, H. Chang, K. Okuyama, Preparation of microencapsulated powders by an aerosol spray method and their optical properties, Adv. Powder Technol. 14 (2003) 349–367. [11] X. Li, J. He, In situ assembly of raspberry- and mulberry-like silica nanospheres toward antireflective and antifogging coatings, ACS Appl. Mater. Interfaces 4 (2012) 2204–2211. [12] K. Sato, Y. Tominaga, Y. Hotta, H. Shibuya, M. Sugie, T. Saruyama, Cellulose nanofiber/nanodiamond composite films: thermal conductivity enhancement achieved by a tuned nanostructure, Adv. Powder Technol. 29 (2018) 972–976.
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Please cite this article as: S. Morita, M. Iijima and J. Tatami, Hetero-assembly of colloidal particles in concentrated non-aqueous suspensions by polymer dispersant design, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.029