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Transparent, Highly Insulating Polyethyl- and Polyvinylsilsesquioxane Aerogels: Mechanical Strengthening by Vulcanization for Ambient Pressure Drying Taiyo Shimizu, Kazuyoshi Kanamori, Ayaka Maeno, Hironori Kaji, Cara M. Doherty, Paolo Falcaro, and Kazuki Nakanishi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01936 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016
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Chemistry of Materials
Transparent, Highly Insulating Polyethyl- and Polyvinylsilsesquioxane Aerogels: Mechanical Improvements by Vulcanization for Ambient Pressure Drying Taiyo Shimizu,† Kazuyoshi Kanamori,*,† Ayaka Maeno,‡ Hironori Kaji,‡ Cara M. Doherty,§ Paolo Falcaro,¶ Kazuki Nakanishi† †
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 6068502, Japan
‡
Institute for Chemical Research, Kyoto University, Gokasho, Uji-city, Kyoto 611-0011, Japan
§
CSIRO Manufacturing, Private Bag 10, Clayton South, VIC 3169, Australia
¶
Graz University of Technology, Institute of Physical and Theoretical Chemistry, Stremayrgasse 9, 8010 Graz, Austria
ABSTRACT: Silica aerogels are unique porous materials possessing high visible-light transparency and low thermal conductivity. However, the practical applications are limited due to the native fragility of silica, and a lot of research focuses on the improvement of mechanical properties by organic-inorganic hybridization, etc. Here, the first synthesis of polyethylsilsesquioxane (PESQ; CH3CH2SiO1.5) and polyvinylsilsesquioxane (PVSQ; CH2=CHSiO1.5) aerogels is reported. The resultant PESQ and PVSQ aerogels obtained through a two-step acid-base sol-gel reaction in a surfactant-based solution exhibit visible-light transmittance and flexibility against compression without collapsing. The microstructural variations of these aerogels are systematically investigated by positron annihilation lifetime spectroscopy (PALS) in order to clarify the differences in properties derived from substituent groups. Furthermore, a post cure on the PVSQ wet gel using a radical initiator induces polymerization of vinyl groups in the solid network, resulting in mechanically reinforced aerogels with higher compressive modulus and resilience. This chemical modification, similar to vulcanization in silicone rubber materials, helps to produce xerogels with comparable properties to those of aerogels via ambient pressure drying. Since the resultant xerogel obtained from the vulcanization of PVSQ shows sufficiently low thermal conductivity of 15.3 mW m−1 K−1, these novel polysilsesquioxane materials are promising for transparent aerogels/xerogels superinsulators.
1. INTRODUCTION Highly porous, lightweight materials obtained by extracting pore liquid from a wet gel using a supercritical drying method are generally called as “aerogel”. Typical aerogels show unique properties such as low density, low thermal conductivity, and high surface area.1 Silica aerogels, in particular, can be easily obtained in a highly transparent monolithic form, while most other porous materials are opaque due to visible light scattering caused by the presence of pores in sub-micron or micrometer scales. Due to their low thermal conductivity and transparency, silica aerogels have been considered as potential thermal superinsulators for energy-saving such as transparent insulating windows.2,3 However, applications of silica aerogels reported so far are limited to specialized areas (e.g. high energy physics,4 aerospace science,5 etc.) since the invention of aerogels in 1931,6 because of their low mechanical strength, which makes handling and pro-
cessing of aerogels difficult. Although there are a lot of reports on the improvement of mechanical properties by modification of aging steps,7,8 the friable nature of silica is still an unavoidable barrier to achieve mechanically strong, ubiquitously usable aerogels. Organic-inorganic hybridization is a promising way to improve mechanical properties of aerogels.9,10 Generally, there are three types of hybridization strategies used for the modification of aerogels; (1) forming composite with polymers or structural supports, (2) surface modification, and (3) employment of organoalkoxysilanes as a precursor. Cai et al. reported a synthesis of cellulose-silica composite aerogels, which showed flexibility against bending and could be tied like a ribbon without any collapse despite its relatively high bulk density (≥ 0.32 g cm−3).11 Leventis et al. prepared isocyanate-crosslinked silica aerogels by soaking wet silica gels into isocyanate solutions and allowing covalent bonding formation between surface
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silanols and isocyanate groups.12 The resultant hybrid aerogels were able to endure a higher loading than that of pure silica aerogels in the three point bending test. Rao et al. reported polymethylsilsesquioxane (PMSQ, CH3SiO1.5) aerogels by using methytrimethoxysilane (MTMS) as the single precursor.13 The aerogels obtained by a one-step base process displayed high flexibility against both bending and compression. While these three examples of hybridization successfully reduced the native mechanical friability of silica aerogels, the resulting hybrid aerogels were opaque. Thus, high visible light transparency, one of the most unique properties of silica aerogels, was lost by these strategies. Therefore, the combination of high transparency and high mechanical strength remained an open challenge. Recently, our group has reported the synthesis of transparent PMSQ aerogels.14 The synthesis of organicinorganic hybrid aerogels only from organotrialkoxysilane is difficult due to hydrophobicity of the condensates and steric effect by the organic moiety.15 In particular, the hydrophobicity of siloxane condensates increases with the progress of polycondensation,16 and phase separation between condensates and polar solvent takes place before gelation in most cases. Thus, only opaque polysilsesquioxane aerogels have been obtained from organotrialkoxysilane as the single precursor. In order to obtain transparent aerogels, a two-step acid-base process has been employed in the presence of a surfactant. The acid catalyst promotes homogeneous hydrolysis of alkoxy groups and the base catalyst accelerates polycondensation of silanol groups, leading to homogeneous gelation. The surfactant helps the hydrophobic silsesquioxane condensates and the polar solvent to remain miscible in the course of gelation. With optimized starting compositions, obtained PMSQ aerogels show high visible light transmittance comparable to that of silica aerogels,17,18 and high resilience (nearly 100 %) after uniaxial compression.14 This flexibility enables the formation of aerogellike xerogels not by supercritical drying but by ambient pressure drying, which highlights the potential use of such silsesquioxane materials for realizing aerogels/xerogels with high transparency and mechanical strength. The properties of the other polysilsesquioxane aerogels have not been explored due to the difficulty of synthesizing transparent monolithic gels, despite such polysilsesquioxane aerogels realizing even better mechanical properties. In this paper, we report on the synthesis and properties of polyethylsilsesquioxane (PESQ: CH3CH2SiO1.5) and polyvinylsilsesquioxane (PVSQ: CH2=CHSiO1.5) aerogels for the first time. To the best of our knowledge, PMSQ has been the only chemical composition with which we can obtain transparent polysilsesquioxane aerogels so far, except bridged polysilsesquioxanes.19 The new two-step sol-gel reaction successfully formed PESQ and PVSQ aerogels that exhibit visible-light transparency. Moreover, a post treatment with a radical initiator polymerizes the vinyl groups in the PVSQ network, resulting in a drastic improvement of mechanical properties, which realizes
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xerogels having comparable properties with the corresponding aerogels through ambient pressure drying. 2. EXPERIMENTAL SECTION Materials. Ethyltrimethoxysilane (ETMS), vinyltrimethoxysilane (VTMS), methyltrimethoxysilane (MTMS) and tetramethoxysilane (TMOS) were purchased from Shin-Etsu Chemical Co. (Japan). Aqueous nitric acid (60 %), acetic acid (99.7 %), methanol, 2-propanol and nhexane were purchased from Kishida Chemical Co., Ltd. (Japan). Distilled water and urea were purchased from Hayashi Pure Chemical Ind., Ltd. (Japan). Surfactant Nonion EH-208 (polyoxyethylene 2-ethylhexyl ether) was kindly supplied from NOF Corporation (Japan). Aqueous tetraethylammonium hydroxide (TEAOH) (35 wt%) was purchased from Sigma-Aldrich Japan. Aqueous tetramethylammonium hydroxide (TMAOH) (ca. 25 %), nhexadecyltrimethylammonium bromide (CTAB) and 2,2'azobisisobutyronitrile (AIBN) were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). All the chemicals were used without further purification. Preparation of PESQ alcogels. The synthetic procedure of a typical sample E-e1.5-60 is as follows: ETMS (5.0 mL, 31.6 mmol) and 5 mM nitric acid (2.0 mL) were mixed under vigorous stirring in a glass vial sealed with a screw cap. After 6 min of stirring, surfactant EH-208 (4.0 mL) was added to the mixture and stirred for 3 min. Then 1.5 M TEAOH aq. (2.0 mL) was added to the reaction solution, which was further stirred for 3 min. The solution was subsequently transferred to a polystyrene container, which was then stood in an oven at 60 °C. Gelation took place in 1 h, followed by aging for 4 d. The obtained gel was then soaked into distilled water at 60 °C and further aged for 1 d. Gels were subsequently washed in methanol and 2-propanol for 8 h (each for three times) at 60 °C to remove byproducts and unreacted species. Obtained gels containing 2-propanol as pore liquid are referred to as alcogels. Preparation of PVSQ alcogels. A similar process to that of the PESQ system was adopted. The synthetic procedure of a typical sample V-m0.6-4 is as follows: VTMS (5.0 mL, 32.6 mmol) and 5 mM nitric acid (5.0 mL) were mixed under vigorous stirring for 6 min in a glass vial sealed with a screw cap. Surfactant EH-208 (6.0 mL) was then added to the mixture and the glass vial was transferred into an ice bath. After stirring for 3 min, 0.60 M TMAOH aq. (2.0 mL) was added to the reaction solution and further stirred for 3 min. The solution was subsequently transferred to a polystyrene container, which was then stood in a refrigerator at 4 °C. Gelation took place in 3-4 h, followed by aging for 4 d at 40 °C. The obtained gel was then soaked in distilled water at 60 °C and further aged for 1 d. Gels were subsequently soaked in methanol and 2-propanol for 8 h (each for three times) at 60 °C to remove byproducts and unreacted species. Preparation of silica and PMSQ alcogels. In order to investigate microstructural differences caused by the substituent groups, silica and PMSQ gels17 were also prepared
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by two-step acid-base sol-gel reactions. The starting compositions are as follows; TMOS (5.0 ml, 33.7 mmol), 1.0 M acetic acid (10 mL) and urea (1.0 g, 16.7 mmol) for the silica system, and MTMS (5.0 mL, 34.9 mmol), 5 mM acetic acid (10 mL), urea (3.0 g, 50.0 mmol) and CTAB (0.40 g, 1.10 mmol) for the PMSQ system. In both systems, water-soluble compounds (urea, or urea and CTAB) were dissolved in aqueous acetic acid, and then alkoxysilane (TMOS or MTMS) was added to the solution and stirred for 15 min (silica system), or 30 min (PMSQ) in order to promote hydrolysis. The mixture was then transferred into a polystyrene container, which was then stood in an oven at 60 °C. Gelation took place within 2-3 h under increasing pH, and the obtained gel was aged at 60 °C for 4 d, followed by further aging in water at 60 °C for 1 d. The resultant gels were washed in the same manner to obtain alcogels. Post-modification of PVSQ gels. Radically-modified PVSQ gels were obtained in the following procedure. VR8 is employed as an example. An alcogel V-m0.6-4 was soaked into the mixture of 2-propanol (100 mL) and AIBN (0.80 g, 4.87 mmol), followed by Ar bubbling for 5 min in order to exclude the dissolved air. The mixture was then sealed and stood for 24 h at room temperature in order for homogeneous diffusion of the radical initiator in the whole pore liquid. The solution was subsequently transferred into an oven at 60 °C to initiate radical polymerization. After 24 h, the obtained gel was soaked in fresh 2propanol at 60 °C for 8 h (3 times), resulting in a radically modified (vulcanized) PVSQ alcogel. Supercritical drying. Obtained alcogels were dried in a custom-built supercritical drying autoclave (Mitsubishi Materials Techno Co., Japan). Pore liquid of 2-propanol was completely replaced with supercritical CO2 by keeping alcogel in the autoclave for 10 h at 14 MPa and 80 °C, under a flow of CO2 at 20 mL min−1. Obtained dried gels in this process are referred to as aerogels. Evaporative drying. Alcogels were soaked in fresh nhexane at 50 °C for 8 h (3 times) in order to replace 2propanol. Obtained gels were then slowly dried at ambient pressure and room temperature. Complete drying required 3-4 d. Obtained dried gels in this process are referred to as xerogels. For details, refer to the Experimental Section and Figure S1 in Supporting Information. Visible-light transparency. Transparency of aerogels/xerogels was evaluated by using a V-670 UV-Vis-NIR spectrophotometer (JASCO, Japan) equipped with an integrating sphere. Direct-hemispherical transmittance was recorded, and obtained transmittance data at the wavelength of 550 nm were normalized into those corresponding to 10 mm-thick samples according to the LambertBeer equation. Electron microscope observation. In order to observe the microstructure of the aerogels, a field emission scanning electron microscope (FE-SEM) JSM-6700F (JEOL Ltd., Japan) was utilized. Before observation, aerogels were gently crushed into small pieces with tweezers and scattered onto a brass sample stage with a conductive
silver paste (DOTITE, Fujikura Kasei Co., Ltd., Japan). In order to decrease surface charge and obtain clear images, the sample stage with aerogels was coated with Pt for 3040 s with 20 mA electric current using an ion sputtering device JFC-1600 (JEOL Ltd., Japan). Sample observation was carried out under the accelerating voltage of 1.20 kV. Gas adsorption measurement. Nitrogen adsorption isotherms were obtained by using BELSORP-max (MicrotracBEL Corp., Japan). Before measurement, samples (typically ca. 0.020 g) were degassed in a sample cell under vacuum at 80 °C for 24 h. Adsorption-desorption measurement was carried out at 77 K. Uniaxial compression test. Stress-strain curves on uniaxial compression-decompression were obtained by using a materials tester (EZ Graph, Shimadzu Corporation, Japan). Aerogels were re-shaped with a razor into square cuboids of typically 10 × 10 × 5 mm3. Compression tests were conducted at 0.50 mm min−1 crosshead speed until strain reached at 50 %, and then the crosshead was raised at the same rate until detected force became less than 0.1 N. Solid-state NMR spectroscopy. In order to obtain the information on the local structure, solid-state 29Si single pulse magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was utilized. All solid-state NMR spectra were obtained by using a Bruker Avance III 800US Plus NMR system operating at 158.96 MHz for 29Si. A 4 mm probe was used with MAS frequency at 15 kHz. Hexamethylcyclotrisiloxane was used as an external reference material. Thermal conductivity. Thermal conductivity of obtained samples was determined by using Heat Flow Meter HFM 436 Lambda (NETZSCH Japan K.K., Japan). Samples in size of ca. 10 × 10 × 1 cm3 were interposed between hot and cold plates, average temperature and increment temperature of which were 25 °C and 20 °C, respectively. Positron annihilation lifetime spectroscopy (PALS). The average pore diameter and relative number of pores within the aerogel samples was determined using EG&G Ortec fast-fast coincidence spectrometers and a 1.1 MBq 22NaCl positron source sealed in a Mylar envelope (2.54 µm thick). Each sample was cut into 2 pieces approximately 10 × 10 × 2 mm3 and placed either side of the positron source. The lifetime spectra for each file was collected for 1 × 106 integrated counts and at least 5 files were averaged per sample. The spectra were analyzed with LT-v9 software and fitted to 4 components (the first two components accounting for the free and the parapositronium formation) including 2 ortho-positronium (o-Ps) long-lifetime components which indicates the presence of a bimodal pore distribution. A source component for the 22NaCl was also subtracted (1.609 ns and 4.002 %). The o-Ps lifetimes were converted to pore diameters using the Tao-Eldrup model.20,21 3. RESULTS AND DISCUSSION
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The outline of the synthetic procedure in both PESQ and PVSQ systems is depicted in Figure 1. In both systems, it is confirmed that there are a lot of synthetic parameters that affect the transparency of resultant gels, such as water/alkoxysilane molar ratio, volume fraction of surfactant, base catalyst concentration for gelation, etc. However, transparent aerogels have only been obtained when relatively high concentrations of base catalyst are employed. We thus presumed that the concentration of the base catalyst is the key variable to obtain transparent aerogels, and other starting compositional parameters (volume of alkoxysilane, nitric acid, surfactant and base catalyst, concentration of nitric acid) were fixed in the present study for simplicity. Obtained aerogels were investigated in terms of bulk density, light transmittance (at 550 nm through an equivalent thickness of 10 mm), microstructure and the compressive deformation behavior. Properties of PESQ aerogels. The porous structure and properties of the obtained PESQ aerogels are shown in Figure 2. Figure 2a shows the appearance of two PESQ aerogels, denoted as E-e0.1-40 (this notation means an aerogel of; E = PESQ system, e0.1 = 0.10 M tetra"ethyl"ammonium hydroxide (TEAOH), 40 = gelled at 40 °C) and E-e1.5-60, respectively. While these two samples have an almost identical starting composition except for the concentration of base catalyst and gelation temperature (see Table 1), transparency is obviously different, which can be attributed to the difference in their microstructures. Figure 2b and 2c show field emission scanning electron microscope (FE-SEM) images of the samples Ee0.1-40 and E-e1.5-60, respectively. The microstructure of aerogel E-e0.1-40 is composed of relatively thick gel skeletons with pores of larger than 100 nm. The presence of relatively coarsened porous structure causes the strong visible light scattering, i.e. the Mie scattering,22 resulting in the opaque aerogel. On the other hand, the microstructure of E-e1.5-60, shown in Figure 2c, is composed of finer skeletons with smaller pores. Hence, the contribution of Mie scattering is suppressed while that of the Rayleigh mode23,24 becomes dominant, which makes E-e1.5-60 a transparent, bluish solid. These microstructural differences should be attributed to the different concentrations of base catalyst used, the effect of which on the structural formation is discussed later. Obtaining low-density transparent PESQ aerogels was found to require a deliberate tuning of synthetic conditions as explained below. Figure 2d shows high shrinkage of E-e1.5-40 after supercritical drying. Although the wet gel of E-e1.5-40 prepared at the lower temperature was transparent, the resulting aerogel after supercritical drying became opaque and denser due to linear shrinkage as high as ca. 30 % (in the case of E-e1.5-60, ca. 2.6 %), which can be attributed to loose networks that undergo irreversible condensation reaction between residual silanol groups during drying at 80 °C.25 Under this highly basic condition, the hydrolyzed alkoxysilane undergoes not only condensation of silanols but also the reverse reaction of condensation, i.e., hydrolysis/alcoholysis of the siloxane bonds. At the lower gelation temperature in E-e1.5-
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40, the hydrolysis/alcoholysis of siloxane bonds maintains the concentration of the silanol/alkoxy groups constant in the network, while at the higher drying temperature, the condensation reaction leading to the irreversible shrinkage (syneresis) takes place during drying, resulting in the denser gels. These results suggest that it is crucial to optimize the synthetic conditions such as concentration of base catalyst and gelation temperature, in order to obtain transparent, low-density PESQ aerogels. The obtained PESQ aerogels show maximum light transmittance of 26 % (normalized to the sample thickness of 10 mm by the Lambert-Beer law) at 550 nm. This value is lower than that of previously reported PMSQ aerogels and xerogels;14,17,18 however, an aerogel that shows transparency from a different organotrialkoxysilane is successfully obtained for the first time. Mechanical properties of the obtained PESQ aerogels were also investigated. Figure 2e shows stress-strain curves on uniaxial compression-decompression of the two samples shown in Figure 2a. Both aerogels demonstrate a flexible behavior on uniaxial compression up to 50 % without collapse. Although bulk densities of both gels are almost the same, E-e0.1-40 (red line) shows higher resilience of ca. 92 % after unloaded, while resilience of E-e1.560 (blue line) is ca. 55 %. This difference can be attributed to their microstructures. As described above, the gel skeletons of E-e0.1-40 are thicker than those of E-e1.5-60. Since the thick and strong skeletons can endure large deformation without local fracture, E-e0.1-40 shows higher resilience. Properties of PVSQ aerogels. We have also obtained PVSQ aerogels by using a similar synthetic method to that of the PESQ system. Figure 3 shows the pore structure and properties of the obtained PVSQ gels. As in the PESQ system, there is a similar tendency for the concentration of base catalyst to clearly affect the appearance of the PVSQ aerogels. Three samples denoted as V-m0.1-4, V-m0.6-4 and V-m1.0-4, (m means tetra"methyl"ammonium hydroxide as the base catalyst), are shown in Figure 3a from left to right. The transparency of the aerogels varies from opaque to transparent and less transparent with the increasing concentration of base catalyst. This tendency can also be, as in the PESQ system, attributed to the difference of their microstructures. Figure 3b-3d show FE-SEM images of these aerogels. The gel skeletons of V-m0.1-4 comprise relatively large aggregations and thin connecting walls (ca. 10-20 nm) with large pores. These large structural constituents contribute to the scattering of visible light, resulting in the opaque appearance. On the contrary, V-m0.6-4 exhibits a finer and more homogeneous microstructure composed of ca. 10 nm-thick skeletons with a few tens of nm pores (Figure 3c). Since these small structural constituents allow high visible light transmittance, the resultant aerogel shows high transparency. In the case where an even higher concentration of base catalyst was used, the obtained aerogel (V-m1.0-4) exhibits less transmittance than that of V-m0.6-4. It can be confirmed by FE-SEM (Figure 3d) that the microstructure of V-m1.0-4 possesses a simi-
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lar size of pores and skeletons to those of V-m0.6-4, while some larger pores exist, showing inhomogeneity of the porous structure. This is presumably because the vinyl groups withdraw more electronic charge from the silicon and thus stability of the PVSQ network against hydrolysis/alcoholysis of siloxane bond is not sufficient at such high pH, which disturbs the homogeneity during the aging step. The formed gel skeletons should partially dissolve into the pore liquid and possibly undergo reprecipitation,26 resulting in the less uniform structure with coarsened pores, which cause more enhanced light scattering.
condensates should be weaker than above surfactants, which comprise a few tens of EO and PO repeating units. In addition, when using simple alcohols instead of EH208, a translucent gel is obtained with the less polar 1butanol, while opaque gels are obtained with more polar alcohols such as methanol and ethanol (Figure S2). The pore size of the resultant aerogels also decreases with the size of alkyl chain of the alcohols (Figure S3). The above consideration and result suggest that EH-208 acts mainly as a solvent that reduces the net polarity of the solvent system and increases the compatibility with silsesquioxane condensates, rather than as a surfactant.
In order to investigate effects of the microstructural difference on the mechanical properties, uniaxial compression tests were also performed on these three samples shown in Figure 3a. In the PESQ system, the opaque aerogel (E-e0.1-40) shows higher resilience than that of the transparent one (E-e1.5-60). However, for the PVSQ aerogels, the deformation behaviors in these three samples during uniaxial compression-decompression are similar (Figure 3e). This result can be explained as follows: Despite the microstructures in different size scales, the framework is composed of thin (ca. 10 nm) fiber-like skeletons in all these samples. When the load is applied, stress should be concentrated on the thin interconnections, which makes the resultant mechanical response in the macroscopic scale similar in these three samples.
Next, the effects of the catalyst for hydrolysis and polycondensation are discussed. In the previously reported PMSQ systems,14,17,18,27 a weak acid (acetic acid) and a weak base (ammonia, generated by hydrolysis of urea) act as the catalyst for hydrolysis and polycondensation, respectively. Dilute nitric acid and tetraalkylammonium hydroxides employed in this work are deduced to act in a similar manner. The strong base is added after the fast hydrolysis of alkoxysilane, and the concentration of strong base affects the properties of the resultant gels. In the PMSQ system,28 it is reported that a hydrothermal post-treatment using aqueous TEAOH on a hierarchically PMSQ monolith promotes the additional condensation and rearrangement of colloidal skeletons and decreases micro/mesoporosity. The strong base catalyst in the present work also influences the formation of the microstructure. Figure S4 shows the dependence of gelation time on the concentration of TEAOH solution in the PESQ system. As is well known in silica sol-gel systems,29 it is clearly observed here that as the concentration of TEAOH aq. increases, gelation time first decreases and shows the minimum at 0.20 M, then increases, because the equilibrium shifts toward the hydrolysis/alcoholysis of polysiloxane networks. Figure S5 shows the appearance of resultant PESQ wet gels, gelation times of which are plotted in Figure S4. There appears to be a clear tendency that more transparent wet gels are obtained with higher concentration of TEAOH. In particular, transparency of the gels drastically changes around 0.20 M from opaque to translucent, and then transparent. For concentrations lower than 0.20 M TEAOH, the hydrolysis/alcoholysis of siloxane bonds is negligible and the reaction rate of polycondensation is low, thus the particle growth in this case has a strong character of reaction-limited monomer-cluster growth.29 Since monomers surrounded by a larger number of reactive sites are more likely to form bonding, PESQ condensates gradually grow in size while keeping the spherical shape. In parallel with the growth of condensates, phase separation due to the hydrophobicity of the condensates occurs to form the porous structure with aggregated particles (30-40 nm) as can be seen in Figure 2b. Under higher pH, the hydrolysis/alcoholysis of formed siloxane bonds becomes significant, which reduces the crosslinking of the network. Hence, the phase separation tendency becomes lower and the condensates of around 10-20 nm form the fine porous structure with the relatively uniform skeleton size (Figure 2c). In addition,
Consideration on the role of surfactant and catalyst. Although there are small differences in detailed synthetic tactics, both PESQ and PVSQ aerogels can be obtained in a transparent monolithic form by a similar synthetic procedure utilizing two-step strong-acid strongbase reactions and a liquid surfactant. Here, a brief consideration on the role of these newly developed synthetic techniques is discussed. From the viewpoint of extending the chemical composition of aerogels, it is important to consider how these techniques contribute to obtaining transparent gels. First, the role of surfactant EH-208 is addressed. In the previously reported PMSQ systems, several types of surfactants, such as nonionic triblock copolymer Pluronic F127,14,27 P105,27 and cationic ammonium salt n-hexadecyltrimethylammonium bromide (CTAB),14 and n-hexadecyltrimethylammonium chloride,17,18 are effective to obtain transparent aerogels. In both the PESQ and PVSQ systems, however, transparent gels could not be obtained by using these surfactants. Hydrophobicity and steric hindrance of PESQ or PVSQ condensates are supposed to prevent effective interaction between condensates and the surfactant, and impede the miscibility of condensates with solvent. According to the reported paper using Pluronic-type surfactants,27 the predominant interaction between silsesquioxane condensates and surfactant changes from hydrogen bonding between silanols and ether oxygens in the ethylene oxide (EO) unit (at an early stage of polycondensation) to hydrophobic interaction between condensates and the propylene oxide (PO) unit (at a later stage). Since EH-208 is composed of small hydrophilic (EO units) and hydrophobic (2-ethylhexyl) parts, the interaction between EH-208 and silsesquioxane
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the Ostwald-type ripening26 through dissolutionreprecipitation possibly occurs and leads to the fiber-like skeletons with smooth surfaces. However, the high tendency of hydrolysis/alcoholysis of siloxane bonds leaves a lot of silanol/alkoxy groups even after the aging step, resulting in large volume shrinkage during the drying step as seen in Figure 2d. The above discussion is also applicable to the PVSQ systems. Dependence of gelation time on the concentration of base catalyst shows a similar tendency (Figure S6) to that of PESQ. As discussed earlier, since the vinyl groups have more electron-withdrawing character than ethyl, which decreases the stability of PVSQ network at high pH. Structural rearrangements and coarsening by the Ostwald ripening are thus easier to occur. In addition, polycondensation under basic conditions is relatively high, which may lead to a broadened distribution of molecular weight resulting in higher inhomogeneity, which can be clearly seen in the PVSQ aerogel obtained by using ethanol as the solvent instead of EH-208 (Figure S3a and S3b). This morphology derived from the phase separation is composed of coarsened particles and a fine fiber-like reticular structure, reflecting the broad size distribution of the PVSQ polymers. Since the phase separation is effectively suppressed in the samples (such as Vm0.1-4) prepared using EH-208 as the solvent, the macroscopic inhomogeneity is absent. Properties of vulcanized PVSQ aerogels. In addition to the successful synthesis of the PESQ and PVSQ aerogels with visible-light transparency, we found that the PVSQ aerogels can be strengthened by a post treatment through radical polymerization in the solid phase. A simple scheme is depicted in Figure 4a. Immersing a wet gel in a 2-propanol (IPA) solution of a radical initiator 2,2’azobisisobutyronitrile (AIBN) at 60 °C can modify the mechanical properties of the resultant aerogels. We conducted this modification on the wet gel of V-m0.6-4 with the highest transparency, using AIBN solutions at different concentrations. Sample names (VR-X; X = concentration of AIBN in IPA in the unit of g L−1), modification conditions and properties of the obtained modified-PVSQ aerogels are listed in Table 2. Bulk density and light transmittance show virtually no changes irrespective of the modification conditions. However, the postmodification gives a significant effect on the deformation behavior of aerogels during uniaxial compression, as shown in Figure 4b. Apparently, the modified aerogels demonstrate higher compression stress and lower irreversible strain after the release of compressive load with increasing concentration of AIBN. In particular, the VR-8 aerogel shows the highest compression stress (ca. 2.6 MPa at 50 % uniaxial compression) and the least irreversible strain (ca. 5 %) with retaining light transmittance higher than 55 % and bulk density of ca. 0.15 g cm−3 after the modification. The stress value is superior to that of the previously reported PMSQ aerogel (compression stress of ca. 0.6 MPa at 50 % uniaxial compression for a sample with bulk density of 0.14 g cm−3).17 These drastic mechanical improvements including hardening and increasing
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elasticity can be attributed to the radical polymerization of vinyl groups in the network, similar to the crosslinking process of silicone oils (“vulcanization”).30 According to the literature,31 the molecular-level connectivity between Si atoms (Si-X-Si, X = O or CH2-CH2) has substantial effects on their bulk moduli in hybrid glass systems. Similarly, the molecular-level connectivity increased by radical polymerization should also contribute to their improved bulk properties in the present PVSQ aerogel system. Figure 4c shows solid-state 29Si dipolar decoupled magic angle spinning (DDMAS) NMR spectra of the unmodified and vulcanized PVSQ aerogels. A new peak around −65 ppm emerges and the peak intensity increases as the AIBN concentration increases. Since the T3 peak of the Si nucleus (RSi(OSi)3, crosslinked with other Si atoms with three siloxane bonds) attached to a saturated hydrocarbon group is located ca. −65 ppm,15 this new peak should correspond to the Si species bonded to the polymerized vinyl groups. The NMR spectrum of pristine PVSQ aerogel predominantly shows a single resonance peak around −82 ppm, which corresponds to the T3 Si species. Hence, we can estimate the ratio of radically reacted and unreacted vinyl groups by comparing the integrated intensities around −65 ppm and −82 ppm. The aerogel treated in the highest concentration of AIBN, VR-8, is found to contain ca. 44 % of reacted vinyl groups, which value is notably high in spite of the radical reaction within the preimmobilized solid matrix. Concerning PVSQ films, there are some reports on the improvement of mechanical properties via polymerization of vinyl groups by UV irradiation.32,33 Both the ratio of reacted vinyl groups (44 %) and improvement of elastic moduli (max. 3.7 times higher than that of the pristine PVSQ) are higher as compared to those reports (ca. 30 % reaction ratio estimated from IR spectra32 and ca. 3 times elastic modulus,33 respectively). From these results, the radical reaction using a conventional radical initiator is an effective way to significantly improve the mechanical properties by polymerizing vinyl groups in a preformed gel matrix at least for the materials with an open porous structure, where radical initiators can diffuse over the whole material. As shown above, the PVSQ aerogels vulcanized with a high concentration of AIBN, such as VR-8, demonstrate high resilient behavior after compression. This elastic behavior is a preferable property not only for handling but for obtaining transparent and low-density dried xerogels without using the supercritical drying technique. Wet gels under evaporative drying are forced to shrink by the capillary force introduced by liquid-vapor interfaces accompanied by solvent evaporation.34 Thus, obtaining highly porous xerogels requires this kind of reversible compression-reexpansion, so-called "spring-back" phenomena.35,36 Figure 5 shows the appearance of aerogels and xerogels of V-m0.6-4 and VR-8 prepared in the same dimensions (5 × 5 cm2). The xerogels were prepared by ambient pressure drying of wet gels containing n-hexane, which is a conventional organic solvent and can reduce irreversible volume shrinkage after the drying, due to its low surface tension.37 It can be easily confirmed that the
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Chemistry of Materials
VR-8 xerogel retains almost the same dimensions and transparency compared to the corresponding aerogel, while the pristine PVSQ xerogel exhibits large irreversible shrinkage after ambient pressure drying. The irreversible shrinkage after drying becomes less with increasing concentration of AIBN (with increasing X). In the case of pristine PVSQ, bulk density largely increased from 0.14 g cm−3 (aerogel) to 0.81 g cm−3 (xerogel); however, the vulcanized VR-8 aerogel and xerogel possess almost the same values in bulk density (0.16 g cm−3 for aerogel and 0.17 g cm−3 for xerogel) and transmittance (59.5 % and 63.5 %). In terms of microstructure, there is no obvious change observed in the FE-SEM images (Figure S8) and N2 adsorption/desorption isotherms (Figure S9). There is also no visible difference in the molecular structure from FTIR spectra (Figure S10). Further, the VR-8 xerogel shows low thermal conductivity (15.3 mW m−1 K−1), which value is comparable to those of typical silica (17-21 mW m−1 K−1)1 and PMSQ aerogels (15-20 mW m−1 K−1).38 Radical vulcanization on PVSQ gels is thus an effective way to obtain aerogel-like xerogels with very low thermal conductivity and high visible light transparency, which extends the potential applications of aerogel-type materials toward practical uses as, for example, superinsulating windows. Microstructural comparison of silsesquioxane and silica aerogels. In order to investigate the structural difference caused by the size of substituent group, we also performed positron annihilation lifetime spectroscopy (PALS). This characterization technique can probe not only the open accessible pores but also the closed pores (voids) at the subnanometer size range within the gel skeletons.39,40 Results derived by presuming a bimodal distribution of the pores/voids in subnano- and nanometer scale are listed in Table 3. It is clear that the size (D3) as well as the relative number (I3) of the micropores/voids increases in the order of silica, PMSQ, PVSQ (V-m0.6-4) and PESQ (E-e1.5-60). The size D4 lies in the mesopore region and could be related with the open mesopores in the aerogel structures. The above tendency in D3 can be attributed to the size of substituent groups in the network. Since silica possesses no organic substituent group and each Si atom can form up to four siloxane bonds, the size of pores/voids involved in the network is relatively small. In the silsesquioxane compositions, however, every Si atom can form up to only three siloxane bonds and one substituent group remains in the network. The increase in the size of substituent group discourages the dense packing of the network with the aid of higher flexibility of the polysiloxane network, resulting in the larger pores/voids. The vinyl groups dangling on the relatively loose PVSQ network show some mobility, and are reacted by AIBN to form the aliphatic hydrocarbon chains. The structural change from this reaction would render the network tighter and thus smaller D3 micropores. It is also confirmed that the size of resultant pores/voids in the network decreases with increasing concentration of AIBN, which reflects the enhanced elongation of aliphatic chains densifying the network. The nitrogen adsorption-
desorption isotherms of these aerogels (Figure S11) and pore size distributions derived from these isotherms using the Saito-Foley method (Figure S12), which assumes a cylindrical model as the pore shape,41 are consistent with this tendency. Adsorption uptake and the peak of pore size shifts to the higher relative pressure and larger size, respectively, in the order of silica, PMSQ, PVSQ (V-m0.64) and PESQ (E-e1.5-60). Since smaller pores adsorb N2 molecules stronger,42 adsorption in the lower relative pressure region corresponds to smaller micropores (
