Pseudoboehmite nanorod–polymethylsilsesquioxane monoliths formed by colloidal gelation

ABSTRACT The addition of a trifunctional silicon alkoxide methyltrimethoxysilane (MTMS) to aluminum oxide hydroxide pseudoboehmite nanorod (PBNR) aqueous dispersions resulted in adhesion between the PBNR colloids to form macroporous monoliths. The use of higher amounts of MTMS led to coarsening of the skeletons and strengthening of the skeletal structures, giving the monoliths water resistance. When a dispersion of zirconium oxide nanoparticles and MTMS was used as the starting material, a macroporous monolith was also obtained by the same simple process. Colloidal gelation occurs because the silanol moiety is more likely to react with the colloid surface of ceramic materials than with other silanols derived from MTMS and their oligomer. With the development of materials chemistry, colloidal dispersions with various shapes and compositions are becoming available as products. The present mechanism is expected to make fabrication of various porous monoliths with characteristic morphologies and properties feasible depending on the colloid made as the starting material.


Introduction
The ongoing development of materials chemistry has enabled preparation of various types of micro-and nanoparticle-based colloidal dispersions. Colloidal dispersions of ceramic and metallic particles with various shapes and sizes, such as spherical particles, rods, fibers, and plates, are now commercially available. To maintain the dispersed state, additives such as surfactants and polymers are often used [1]. Particles can also be dispersed by charge repulsion under aqueous conditions without coating the dispersoid by appropriately adjusting the pH [2]. Such colloids are known to aggregate at pH changes approaching the isoelectric point, which can be used for processing of dense ceramic materials [3]. In some cases, monolithic porous materials (gels) can also be obtained through successful control over the process. Although this phenomenon of gel formation by colloids is well known and extensively studied [4][5][6], the process has attracted limited attention for fabrication of porous monoliths, because they generally have poor mechanical strength and collapse easily under ambient drying conditions. The preparation method using a colloidal dispersion as a starting material has not clearly demonstrated advantages over other production methods.
In 2008, Schmidt et al. reported that macroporous monoliths could be fabricated through adherence of dispersed ceramic or polymer particles using melamine-formaldehyde resin as a "glue." [7] We have also succeeded in producing polymethylsilsesquioxane (PMSQ) gels with boehmite nanofiber cores by coating and bonding the nanofibers with trifunctional silicon alkoxide methyltrimethoxysilane (MTMS) [8]. In these materials, although the proportion of the colloid (nanofibers) in the skeletal framework was not large, it seems that the formation mechanism was similar to that described by Schmidt et al. The boehmite nanofiber-PMSQ gels had a characteristic fiber-like skeleton derived from the nanofibers and exhibited relatively high strength. Furthermore, they displayed good thermal insulation properties under low vacuum conditions. It is known that the properties of porous monoliths can be controlled by their microstructure morphology [9][10][11]. We have reported, for example, that thermal conductivity can be controlled by changing the pore structure of PMSQ macroporous monoliths [12]. When macroporous monoliths are prepared through monomer reaction via a sol-gel process, the microstructures obtained by solid-liquid phase separation are somewhat similar regardless of the composition, which can be predicted by calculation [13]. If a colloid with an anisotropic shape is used as a starting material, however, a complicated structure can be formed [14]. The ability to fabricate monoliths from various forms of colloidal dispersion broadens the range and composition of available microstructures and permits the development of new materials with novel applications. In this study, fabrication of macroporous monoliths using pseudoboehmite nanorods (PBNRs) as the colloids and PMSQ derived from the precursor MTMS as the glue was investigated.
CONTACT Gen Hayase gen@aerogel.jp Supplemental data for this article can be accessed here.

Preparation of monoliths
To prepare the PBNR-PMSQ monoliths, x mL of MTMS was first added to 10 mL of the PBNR dispersion A2, and the mixture was stirred for 5 min. After degassing, the mixture was poured into a sealed container and left to stand at 80°C. The gelation time was approximately 1 h. After 6 h, the obtained gel was washed with ethanol, and the solvent was exchanged for hexane. To prepare the aerogel samples, supercritical CO 2 drying was performed at 40°C and 10 MPa. A schematic image of the process is shown in Figure 1. To prepare the xerogel samples, evaporative drying was performed at RT over 2 d at a controlled evaporation rate. The samples obtained are referred to hereinafter as PPx, where x denotes the volume (mL) of MTMS added to the PBNR dispersion. To prepare the ZrO 2 -PMSQ aerogel monoliths, the procedure described above was repeated using 2 mL of MTMS and 10 mL of the ZrO 2 dispersion.

Characterization
The bulk density was calculated based on the measured weight and volume. The error was within approximately 5%. The microstructure was examined using scanning electron microscopy (SEM; S-5200, Hitachi High-Technologies Corp., Japan) and transmission electron microscopy (TEM; H-7650, Hitachi High-Technologies Corp., Japan). Uniaxial compression tests were performed using a universal/tensile tester (EZ-SX, Shimadzu Corp., Japan) and a 100 N pressure gauge. Samples were cut to dimensions of approximately 10 × 10 × 5 mm 3 at a head speed of 1 mm min −1 . The Young's modulus was calculated for values of compressive stress in the range of 0.2-0.4 MPa. The Fourier transform infrared (FTIR) spectra were recorded with FT/IR-6100TY (JASCO, Japan) using an attenuated total reflection (ATR) attachment (ATR PRO470-H). A total of 100 scans of samples dried at 90°C for 12 h in advance were recorded at a resolution of 4 cm −1 . The visible light transmittance was measured using a spectrophotometer (HSU-100H, Asahi Spectra Co., Ltd., Japan) equipped with a halogen light source (HL-20, Asahi Spectra Co., Ltd., Japan) and an integrating sphere (HSU-O-DTR, Asahi Spectra Co., Ltd., Japan). The direct-hemispherical transmittance was recorded, and the transmittance data obtained at 550 nm were normalized to a thickness of 10 mm using the Lambert-Beer equation. Thermogravimetric-differential thermal analysis (TG-DTA) was conducted using a Thermo plus EVO 2 instrument (TG-DTA 8122, Rigaku Corp., Japan). The thermal conductivity was measured at 25°C using a heat flow meter (HFM 436 Lambda, Netzsch GmbH, Germany) for PP5 xerogel samples with a thickness of approximately 10 mm. Although an attempt was made to perform contact angle measurements by dropping 5 μL droplets of water onto samples of PP5, a large error was observed owing to the surface conditions, and reproducible results could not be obtained. The particle size distribution of the pseudoboehmite nanorods and ZrO 2 particles was evaluated using dynamic light scattering (DLS; Zetasizer Nano-ZS, Malvern Instruments Ltd., UK).

Properties of PBNR-PMSQ macroporous monoliths
The addition of various amounts of MTMS to samples of the PBNR dispersion and maintain the temperature at 80°C for 6 h resulted in the formation of translucent wet monolithic gels. The gel opacity increased with increases in the amount of MTMS added as was more apparent after supercritical CO 2 drying ( Figure 2). Cracking of the samples sometimes occurred during the subsequent solvent exchange to hexane, although the degree of damage decreased with increases in the amount of MTMS added. The skeletal structure was found to gain strength with increases in the amount of PMSQ added to serve as a binder between the PBNRs. After supercritical CO 2 drying, the gels shrank by several to ten-odd percent in length. Table 1 and Figure 3 show the physical properties and SEM images, respectively, of aerogels obtained using various amounts of MTMS. The bulk density of the monoliths increased with increases in the amount of MTMS added, and their skeletal structures became coarser. The skeleton diameter in PP1 was less than 10 nm, whereas it had almost doubled to 15-20 nm in PP5. Transmission electron microscopic observation revealed that the PBNRs underwent structural changes during the reaction (Figure 4). Scanning electron microscopic images suggest, however, that the PBNR underwent a partial change to needlelike crystals. (Since the contrast difference between the original PBNR and PMSQ is small, only needlelike crystals are clearly shown in the TEM images.) As the skeletal structure became coarser, the visible light transmittance of the monolith also decreased significantly. Samples PP1, PP2, and PP3 were translucent bulk bodies that were slightly transparent at thicknesses of several millimeters. By contrast, the coarse skeletal structures of PP4 and PP5 increased the Mie scattering of visible light, resulting in white monoliths that were virtually opaque to visible light (Table 1 and Figure 5). Although boehmite is hydrophilic, PMSQ is hydrophobic, and the water resistance of the obtained samples increased with the amount of MTMS added as a result. Whereas PP1 instantly absorbed water resulting in the collapse of the monolith, for example, PP5 underwent no noticeable change, even after floating in water for 1 month.
Uniaxial compression tests revealed that PP4 exhibited the highest Young's modulus of the obtained aerogel samples. A higher proportion of PMSQ relative to the PBNRs resulted in thickening of the necks between   PBNRs and a stronger microstructure against stresses. Increasing the proportion of PMSQ, which is more flexible than pseudoboehmite crystal, beyond a certain amount, however, caused the Young's modulus and durability against stresses to decrease again with coarsening of the structure. Of the samples obtained, PP5 exhibited the highest yield strength, since cracks were least likely to occur, and this sample returned to its original shape after applying 50% uniaxial compression (Figure 6(a)). Although shrinkage occurred upon drying, evaporative drying of PP5 enabled 10 cm square panels to be obtained. This xerogel also exhibited water resistance like the corresponding aerogel ( Figure 6(b)). Since it is known that some porous monoliths based on PMSQ are good thermal insulators, the thermal conductivity of this composite material was measured with the expectation of similar physical properties. The thermal conductivity of the PP5 xerogel was 53.9 mW m −1 K −1 , however, which was equivalent to fiberglass wool and perlite and higher than our previous PMSQ macroporous materials [8,12,15,16]. The samples prepared in this study required a high bulk density to obtain a certain strength, and the requisite pore diameter was also large. Also, pseudoboehmite has much higher thermal conductivity compared to PMSQ. It is considered that the thermal conductivity of the solid phase or gas phase increased than that of pure PMSQ materials. On the other hand, the heat resistance was similar to that of other PMSQ materials. Upon heating of the PP5 aerogel, gradual water loss occurred, followed by loss of the methyl group (1275 cm −1 of FTIR spectra), and the PMSQ was converted to silica at temperatures exceeding 450°C (Figure 7) [17].

Formation mechanism of a PBNR-PMSQ structure
When preparing PMSQ monoliths using MTMS as a precursor, cage-type and cyclic siloxane oligomers are easily formed in the presence of an acid catalyst and the product precipitates as a resin [18]. To obtain a homogeneous monolithic gel, it is generally necessary to perform a two-step acid-base reaction while suppressing phase separation via the use of a surfactant or an appropriate solvent composition [19][20][21][22]. When an acid is used in the reaction, it must be a strong acid such as nitric acid [23]. The PBNR-PMSQ system described in this report was obtained without increasing the pH with respect to basic conditions during the reaction, however, and the entire sol content was converted uniformly to a gel during a one-step reaction under acidic conditions. This PMSQ formation reaction is different from those previously reported. The gelation of a PBNR-PMSQ system is due to a reaction of silanol moieties of hydrolyzed MTMS with the hydroxyl groups on the PBNR surface under acidic conditions to form Si-O-Al bonds, thereby increasing the acidity of the Si atom and increasing its reactivity to the next silanol of another MTMSderived unit [24]. As the reaction progresses faster on the surface of a pseudoboehmite dispersed in an aqueous sol, PMSQ grows on the surface of a pseudoboehmite without precipitating as a resin, thereby forming the microstructure. Comparison of the hydrophobicities of PP1 and PP5 indicated that the skeletal structure was hydrophilic at the initial stage of gelation, and that subsequent aging increased the hydrophobicity owing to the inclusion of greater amounts of PMSQ. This can also be confirmed from the FTIR spectrum, where enhancement of the methyl group peak at 1275 cm −1 is seen with the growth of the siloxane network shown at 1025 cm −1 and 1100 cm −1 by with increases in the MTMS in the starting composition ( Figure 8) [17]. Macroporous monoliths can be obtained using the same process as for several other types of pseudoboehmite particle dispersions (10A, Kawaken Fine Chemicals Co., Ltd., Japan; and AS-200 and AS-520-A, Nissan Chemical Corp., Ltd., Japan). Although an attempt was made to use a higher concentration of PBNR dispersion solution by evaporating the solvent of A2, it was difficult to prepare a homogeneous sol. It is known that the physical properties of nanorod dispersions change with the concentration [4], and materials with different properties may be prepared as preparation of higher dispersion concentrations is enabled. Various methods for preparing aluminum oxide hydroxide nanocrystals which can be dispersed in aqueous conditions have been reported [25][26][27]. It is expected that PMSQ-composite porous monoliths with various structures and properties can be prepared using the synthesis method reported in this paper.
Using metal oxide and hydroxide colloids with an element with lower electronegativity than silicon was expected to facilitate to preparation of various porous monoliths using PMSQ as a glue. Monoliths were indeed formed upon adding MTMS to a ZrO 2 nanoparticle dispersion (Figure 9), but considerable syneresis occurred during aging, and many cracks appeared. It appears that the morphology of colloidal particles exerts an influence on syneresis. The use of spherical particles seems to lead to a higher probability of cracks and shrinkage and a lower yield than the use of rod-and fiber-like colloids.  Further understanding of these phenomena will require investigation of the detailed mechanisms and conditions of monolith formation using colloids with a wider variety of shapes.
As concerns use of tetramethoxysilane (i.e. a tetrafunctional silicon alkoxide) instead of MTMS, the tetramethoxysilane underwent a reaction with itself to form a network of silica like those of aerogels [28]. Consequently, the PBNRs remained dispersed in the silica gel without assembling a network ( Figure 10). By contrast, the use of vinyltrimethoxysilane successfully produced a crack-free monolith. It therefore appears that the function of serving as a glue for pseudoboehmite colloids is a peculiar feature of trifunctional silicon alkoxides.

Conclusions
The addition of the trifunctional silicon alkoxide MTMS to PBNR dispersions resulted in adhesion between the colloidal particles to form macroporous monoliths. The use of greater amounts of MTMS led to coarsening of the skeleton and strengthening of the skeletal structure. Since the hydrophilic pseudoboehmite was coated with hydrophobic PMSQ, the water resistance of the obtained materials was greatly improved. Colloidal gelation occurs when MTMS is used as a precursor, because the silanol moiety is more likely to react with the pseudoboehmite surface than with other silanols. It is considered feasible to fabricate various porous monoliths based on this mechanism with characteristic morphologies and properties using dispersion colloids.