Article pubs.acs.org/Langmuir
Transparent Ethenylene-Bridged Polymethylsiloxane Aerogels: Mechanical Flexibility and Strength and Availability for Addition Reaction Taiyo Shimizu,† Kazuyoshi Kanamori,*,† Ayaka Maeno,‡ Hironori Kaji,‡ Cara M. Doherty,§ and Kazuki Nakanishi† †
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan § CSIRO Manufacturing, Private Bag 10, Clayton South, Victoria 3169, Australia ‡
S Supporting Information *
ABSTRACT: Transparent, low-density ethenylene-bridged polymethylsiloxane [Ethe-BPMS, O2/2(CH3)Si−CHCH− Si(CH3)O2/2] aerogels from 1,2-bis(methyldiethoxysilyl)ethene have successfully been synthesized via a sol−gel process. A two-step sol−gel process composed of hydrolysis under acidic conditions and polycondensation under basic conditions in a liquid surfactant produces a homogeneous pore structure based on cross-linked nanosized colloidal particles. Visible-light transmittance of the aerogels varies with the concentration of the base catalyst and reaches as high as 87% (at a wavelength of 550 nm for a 10 mm thick sample). Gelation and aging temperature strongly affect the deformation behavior of the resultant aerogels against uniaxial compression, and the obtained aerogels prepared at 80 °C show high elasticity after being unloaded. This highly resilient behavior is primarily derived from the rigidity of ethenylene groups, which is confirmed by a comparison with other aerogels with similar molecular structures, ethylene-bridged polymethylsiloxane and polymethylsilsesquioxane. Applicability of the addition reaction using a Diels−Alder reaction of benzocyclobutene has also been investigated, revealing that a successful addition takes place on the ethenylene linkings, which is verified using Raman and solid-state NMR spectroscopies. Insights into the effect of molecular structure on mechanical properties and the availability of surface functionalization provided in this study are important for realizing transparent aerogels with the desired functionality.
1. INTRODUCTION Since its invention in 1931,1 silica aerogels have been attracting considerable attention from scientists and engineers in various fields. High porosity, high surface area, low dielectric constant, and so forth are preferable properties for applications such as low-k films2 and catalyst supports.3 In particular, silica aerogels show high transparency for visible light, which makes them distinguishable from other porous materials. This characteristic transparency is derived from their fine pore structure, namely, pores of approximately 50 nm in size and solid skeletons composed of colloidal nanoparticles with the size of approximately 10 nm.4 The fine porous structure contributes to suppressing the Mie scattering and forms bluish, transparent monoliths, the tinted color of which is derived from the Rayleigh scattering.5 The small pore size is also advantageous for suppressing the heat transfer by the gas phase inside of the pore space, resulting in a very low total thermal conductivity.6 The combination of transparency and a low thermal conductivity is also profitable for high-performance thermal insulating windows.7 However, in addition to the native fragility of the silica gel network, the aforementioned structural features lead to a low mechanical © XXXX American Chemical Society
strength, which makes processing and handling of silica aerogels difficult. To reduce the native fragility of silica, organic−inorganic hybridization is one of the most promising ways. Because most of the unique properties of silica aerogels, such as transparency, are derived from their pore structure, aerogels comprising more flexible cross-linked networks will display enhanced flexibility while preserving other characteristic properties of silica aerogels, if a similar pore structure can be fabricated. In our recent works, transparent polymethylsilsesquioxane (PMSQ, CH3SiO3/2) aerogels have been successfully obtained from a sole precursor methyltrimethoxysilane (MTMS) by carefully designing the synthesis conditions.8−10 In most cases, using only an organotrialkoxysilane as the precursor results in failure in the formation of a monolithic gel because of steric hindrance and hydrophobicity derived from the organic moiety.11 The synthesis conditions for monolithic gel formations are limited even in the PMSQ system, which contains the simplest organic Received: February 7, 2017 Revised: April 15, 2017 Published: April 17, 2017 A
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir moiety, methyl.12 Even when monolithic gels are formed, the resultant PMSQ gels exhibit a coarsened pore structure on the micrometer scale, originating from the phase separation between the solvent and PMSQ condensate phases, and their appearance is opaque.13 Through acid-catalyzed hydrolysis and base-catalyzed polycondensation in the presence of a surfactant, transparent PMSQ aerogels have been synthesized for the first time. The obtained transparent PMSQ aerogels are more flexible and resilient against compressive deformations than silica, which can also induce “spring-back”14,15 in the course of ambient pressure drying. Other polysilsesquioxane aerogels with visiblelight transparency, such as polyvinylsilsesquioxane, also show flexible and unique properties.16 These polyorganosiloxane materials have great potential for realizing flexible and transparent aerogels for practical applications. However, studies on the mechanical properties of transparent aerogels containing a relatively large amount of organic constituents are strictly limited, even if aerogels with visible-light transparency from purely organic polymers are taken into account.17−19 In the present circumstances, effects of molecular structure on the mechanical properties of transparent aerogels are unclear, and there is no guide for achieving desirable mechanical properties. To consider the effect of molecular structure on the mechanical properties, our group has recently synthesized transparent ethylene-bridged polymethylsiloxane (Ethy-BPMS) aerogels.20 The Ethy-BPMS aerogel is composed of a network similar to that of PMSQ, except a partial substitution of the ethylene bridge for the oxygen bridge in the siloxane bond. The obtained Ethy-BPMS aerogels show more flexible and viscoelastic deformation behaviors than PMSQ aerogels, primarily resulting from the introduced ethylene linkings into the siloxane networks. This result suggests that the molecular structure of the cross-linked network actually affects the macroscopic mechanical properties of the transparent aerogels. In this paper, we report on the synthesis and properties of transparent ethenylene-bridged polymethylsiloxane (EtheBPMS) aerogels, which have ethenylene (−CHCH−) bridges instead of ethylene (−CH2−CH2−) in Ethy-BPMS, mainly focusing on light transmittance, bulk density, and mechanical properties. Transparent Ethe-BPMS aerogels have successfully been obtained for the first time in the monolithic form by using the same procedure as that for transparent EthyBPMS aerogels, which uses a strong acid and a strong base as catalysts for the sol−gel reactions and a liquid surfactant as the main component of the solvent system. The influence of the organic bridging groups on the mechanical properties of the transparent Ethe-BPMS, Ethy-BPMS, and PMSQ aerogels is discussed. In particular, superior flexibility and strength have been found in the Ethe-BPMS aerogel compared with other polymethylsiloxane aerogels listed above. Moreover, the applicability of a Diels−Alder reaction onto the ethenylenebridging parts has also been investigated, aiming for opening the possibility of chemical modification of the colloidal network toward new functionality and enhanced mechanical properties.
Chemical Co., Ltd. (Japan). Distilled water and urea were purchased from Hayashi Pure Chemical Ind., Ltd. (Japan). Polyoxyethylene 2ethylhexylether (Nonion EH-208) was kindly donated by NOF Corporation (Japan). Aqueous tetramethylammonium hydroxide (TMAOH) (approximately 25%), n-hexadecyltrimethylammonium bromide (CTAB), and benzocyclobutene (BCB) were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). All chemicals were used without further purification. 2.2. Preparation of Ethe-BPMS Aerogels. In the present study, three synthesis conditions were varied for the preparation of EtheBPMS aerogels: the concentration of TMAOH aq (from 0.050 to 1.5 M), the volume ratio of BMDEethe to each starting component (5 mM nitric acid, EH-208, and TMAOH aq, from 5:5 to 5:15), and gelation and aging temperature (60 or 80 °C). The volume fractions of 5 mM nitric acid, EH-208, and TMAOH aq were fixed. The synthesis procedure for an Ethe-BPMS aerogel 0.60M-5:5-80 (that is, the concentration of TMAOH is 0.60 M, the volume ratio of BMDEethe to each of nitric acid, EH-208, and TMAOH aq is 5:5, and the gelation and aging temperature is 80 °C) is as follows: first, a mixture of BMDEethe (0.50 mL, 1.57 mmol) and EH-208 (0.50 mL) in a glass vial sealed with a screw cap was stirred until it became a homogeneous solution, and 5 mM nitric acid (0.50 mL) was then added under vigorous stirring to promote acid-catalyzed hydrolysis of BMDEethe. After 10 min of stirring, 0.60 M TMAOH (0.50 mL) was added to the mixture and stirred for 30 s. The glass vial was subsequently transferred to an oven at 80 °C, followed by gelation and aging for 4 days. The obtained gel was then soaked in water at 60 °C for 1 day, allowing further aging. The aged gel was washed with methanol and 2propanol by soaking the gel for at least 8 h, and this process was repeated three times. The obtained wet gel containing 2-propanol as pore liquid is referred to as the alcogel. The alcogel was dried in a custom-built supercritical drying autoclave (Mitsubishi Materials Techno Co., Japan). The pore liquid 2-propanol was replaced with supercritical CO2 by keeping the alcogel in the autoclave for 10 h at 14 MPa and 80 °C, under a flow of CO2 at 20 mL min−1. 2.3. Preparation of Ethy-BPMS Aerogels. Transparent EthyBPMS aerogels were synthesized by the procedure reported earlier.20 First, a mixture of BMDEetha (4.0 mL, 12.5 mmol) and 5 mM nitric acid (4.0 mL) was stirred for 30 min for hydrolysis, and then, EH-208 (4.0 mL) was added into the mixture. After 3 min of stirring, 0.60 M TMAOH (4.0 mL) was added to the mixture and further stirred for 30 s. The mixture was then placed in an oven at 80 °C, followed by gelation and aging for 4 days. The subsequent procedure is identical to that for Ethe-BPMS aerogels. 2.4. Preparation of PMSQ Aerogels. The synthesis procedure of PMSQ aerogels is identical to that in the previous report.10 Urea (3.0 g, 50.0 mmol) and CTAB (0.40 g, 1.10 mmol) were dissolved in 5 mM acetic acid (10 mL), and MTMS (5.0 mL, 34.9 mmol) was then added to the mixture and stirred for 30 min for hydrolysis. The mixture was then left to stand in an oven at 60 °C for 4 days for gelation and aging. The subsequent procedure is identical to that for Ethe-BPMS aerogels. 2.5. Diels−Alder Reaction on Ethenylene-Bridging Groups. Ethe-BPMS alcogel (0.60M-5:10-80, the volume of gel is approximately 3.50 cm3) was immersed in 1,3,5-trimethylbenzene at 60 °C for at least 8 h to replace the pore liquid with 1,3,5-trimethylbenzene. After three repetitions of this exchange process, the obtained gel was transferred into a Teflon-lined autoclave with 1,3,5-trimethylbenzene (20 mL) and BCB (0.170 mL, 1.57 mmol, 1.0 equiv) and solvothermally treated at 200 °C for 4 days. After the solvothermal treatment, the obtained gel was washed with 2-propanol and supercritically dried in the aforementioned manner. 2.6. Visible-Light Transparency. Transparency of aerogels was evaluated using a V-670 UV−vis−NIR spectrophotometer (JASCO Corporation, Japan) equipped with an integrating sphere. Directhemispherical transmittance was recorded, and the obtained transmittance data at the wavelength of 550 nm were converted into the values corresponding to 10 mm-thick samples according to the Lambert−Beer equation. 2.7. Electron Microscope Observation. To observe the pore structure of the aerogels on the nanometer scale, a field emission
2. EXPERIMENTAL SECTION 2.1. Materials. 1,2-Bis(methyldiethoxysilyl)ethene (BMDEethe, cis- and trans-isomers mixture, primarily trans21) and 1,2-bis(methyldiethoxysilyl)ethane (BMDEetha) were purchased from Gelest Inc. (USA). MTMS was purchased from Shin-Etsu Chemical Co. (Japan). Aqueous nitric acid (60%), acetic acid (99.7%), methanol, 2propanol, and 1,3,5-trimethylbenzene were purchased from Kishida B
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir scanning electron microscope (FE-SEM) JSM-6700F (JEOL Ltd., Japan) was used. The 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). To reduce the effect of surface charge during observation, the sample stage with aerogels was coated with Pt for 40 s with a 20 mA electric current using an ion sputtering device JFC-1600 (JEOL Ltd., Japan). Sample observation was carried out under an accelerating voltage of 1.20 kV. 2.8. Mechanical Properties. The mechanical properties of the aerogels were examined using a material tester EZGraph (Shimadzu Corporation, Japan). In compression−decompression tests, the samples were reshaped with a razor into square cuboids (typically of 10 × 10 × 3 mm3) or cylinders (typically of 7.5 mm in diameter and 10 mm in height) with parallel top and bottom surfaces. The crosshead speed was fixed at 0.50 mm min−1 in both compression and decompression. For three-point bending tests, cylindrical samples with approximately 7.5 mm diameter were used. The samples were set onto two supports separated by 20 mm and were bent at the cross-head speed of 0.25 mm min−1. 2.9. Molecular Structure Investigation. The molecular structures of the aerogels were examined using Raman and solidstate NMR spectroscopies. A Raman microscope XploRA (HORIBA, Ltd., Japan) with incident laser at the wavelength of 532 nm was used. To obtain information on chemical environments, solid-state 13C and 29 Si cross-polarization magic angle spinning (CP/MAS) NMR spectroscopy was used. All solid-state NMR spectra were obtained by using a Bruker Avance III 800US Plus NMR system operating under a static magnetic field of 18.8 T. A 4 mm probe was used with a MAS frequency at 15 kHz. The contact times for cross-polarization were fixed at 4.5 ms for 13C and 5.5 ms for 29Si. Hexamethylbenzene and hexamethylcyclotrisiloxane were used as external reference materials for 13C and 29Si referring to the signal at 132.07 and −9.66 ppm, respectively. 2.10. Thermogravimetric Analysis. To estimate the yield of the addition reaction, thermogravimetric analysis under an atmospheric condition was performed using Thermo plus EVO TG 8120 analyzer (Rigaku Corporation, Japan). The heating rate and the flow rate of air were fixed at 5 °C min−1 and 100 mL min−1, respectively. 2.11. Gas Adsorption Measurement. Nitrogen adsorption− desorption isotherms were obtained using BELSORP-max (MicrotracBEL Corp., Japan). Samples (approximately 0.020 g) were degassed in a sample cell under vacuum at 80 °C for at least 24 h before measurement. The adsorption−desorption measurement was conducted at 77 K. 2.12. Positron Annihilation Lifetime Spectroscopy (PALS). The average pore diameter and the relative number of pores within the aerogel samples were 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 on either side of the positron source and placed under vacuum. The lifetime spectra of each file was collected for 4.5 × 106 integrated counts, and at least 5 files were averaged per sample. The spectra were analyzed with LT-v9 software and fitted to four components (the first two components accounting for the free and the para-positronium formation) including 2 ortho-positronium (o-Ps) long-lifetime components, which indicates the presence of a bimodal pore distribution. A source component for 22 NaCl was also subtracted (1.511 ns and 3.486%). The o-Ps lifetimes less than 5 ns were converted to pore diameters using the Tao−Eldrup model, and longer lifetimes were calculated using the rectangular Tao−Eldrup (RTE) model.22,23
the obtained Ethe-BPMS aerogels are represented as XM-5:5(60 or 80). Figure 1a shows the appearance of Ethe-BPMS aerogels obtained by using different concentrations of TMAOH, gelled
Figure 1. (a) Appearance of Ethe-BPMS aerogels (XM-5:5-80) prepared with different concentrations of TMAOH; relationship between the concentration of the base catalyst and (b) light transmittance (corresponding values to those of 10 mm-thick samples) and (c) bulk density. Cracks in the gel bodies shown in (a) were caused when gels were taken out from glass vials after gelation and aging.
at 80 °C. There is a clear tendency that the transparency of aerogels first increases and then decreases with the increasing concentration. Similar trends are also observed in polyvinylsilsesquioxane16 and Ethy-BPMS systems.20 The inhomogeneous appearance of aerogels prepared using 1.2 and 1.5 M TMAOH implies that the dissolution−reprecipitation of the colloidal network24,25 took place on a macroscopic scale under highly basic and high-temperature conditions. The electron-withdrawing character of ethenylene bridges increases the positive partial charge on Si atoms, resulting in enhanced susceptibility toward attacking of OH− ions to Si and hydrolysis of siloxane bonds. Figure 1b shows the transition of light transmittance of aerogels prepared at 60 and 80 °C. Below 0.90 M, the transmittance of both systems shows a similar tendency with the increasing concentration of the base. Aerogels from both gelation temperatures show the highest transmittance at 0.60 M, and the value reaches as high as 87% (for 60 °C) and 83% (80 °C). Above 0.90 M, the light transmittance of those prepared at 60 °C gradually decreases with the increasing concentration, whereas that of 80 °C sharply decreases because of the aforementioned dissolution−reprecipitation of the network. Such a macroscopic inhomogeneity cannot be observed in the aerogels prepared at 60 °C (Figure S1). Figure 1c displays the transition of bulk density. In the case of Ethy-BPMS systems,20 the degree of shrinkage (particularly shrinkage during supercritical drying) of aerogels is largely different, depending on the gelation temperature. Consequently, the transparency of wet gels prepared at 60 °C decreased after supercritical drying because of a disturbed pore structure caused by the shrinkage. By contrast, bulk densities of the Ethe-BPMS aerogels shown in Figure 1c are almost
3. RESULTS AND DISCUSSION 3.1. Effect of the Concentration of the Base Catalyst. As reported in recent studies,16,20 the concentration of the base catalyst plays a substantial role in determining the properties of resultant aerogels prepared by the strong-acid strong-base sol− gel reactions. Thus, the effect of the concentration of the base catalyst on the properties is addressed first. In this section, all of C
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
aerogels show relatively high compressive stress and high resilience (degree of re-expansion after the removal of load) as compared with previously reported transparent polysilsesquioxanes,10,16 which can be attributed to the rigidity of the network on a molecular scale, as discussed later. 3.2. Effect of the Volume Fraction of the Precursor. Effects of volume fraction of the precursor were subsequently investigated to find out the range of the precursor fraction where transparent aerogels can be obtained. In this section, all of the obtained aerogels are denoted as 0.60M-5:X-Y, where X is 5, 7, 10, or 15, and Y is 60 or 80. The appearance of the obtained aerogels is shown in Figure 3. In the case of the gels prepared at 60 °C, shrinkage during
constant in the range of our investigation. An apparent change in the gel size during supercritical drying cannot be observed on all of the obtained gels. This difference between Ethy-BPMS and Ethe-BPMS may be related to differences in the mechanical properties. Concerning the microstructure, FE-SEM images of the three aerogels shown in Figure 1a, 0.050 M (0.050M-5:5-80), 0.60 M (0.60M-5:5-80), and 1.5 M (1.5M-5:5-80), are shown in Figure 2a−c, respectively. The pore structure of the almost opaque
Figure 3. Appearance of aerogels 0.60M-5:X-Y, where X = 5, 7, 10, or 15 and Y = 60 or 80.
supercritical drying becomes more intensive with a decreasing volume fraction of the precursor, resulting in collapsed aerogels. On the other hand, all of the gels prepared at 80 °C display almost no difference in size and monolithicity. Even the aerogel 0.60M-5:15-80 shows only small shrinkage, and its bulk density reaches 0.0525 g cm−3. This difference in shrinkage is due to the higher rigidity of solid skeletons originating from higher degree of condensation of siloxane networks prepared at 80 °C. Figure S3 shows the stress−strain curves obtained from uniaxial compression−decompression on 0.60M-5:5-60 and 0.60M-5:5-80. Although both aerogels show a similar bulk density (0.150 g cm−3 for 0.60M-5:5-60 and 0.140 g cm−3 for 0.60M-5:5-80) and light transmittance (87 and 83%), compressive behaviors are largely different. The aerogel 0.60M-5:5-80 exhibits slightly higher compressive stress and much higher resilience after unloading than 0.60M-5:5-60. Such difference in the compressive behavior was also observed in the Ethy-BPMS system, and the behavior can be explained by the enhanced polycondensation of Si-OH or Si-OR groups at a higher temperature.20 This explanation should also be true in the present study. Enhanced polycondensation in the network decreases the shrinkage during supercritical drying, and thus, the highly porous aerogel 0.60M-5:15-80 is obtained. In terms of transparency, however, appearance of 0.60M-5:15-80 is completely opaque because of a coarsened pore structure developed with too low a concentration of the precursor. The coarsened pore structure with large pores (approximately 100 nm), as observed in Figure S4, strongly scatters visible light and results in the opaque appearance. 3.3. Comparison of Mechanical Properties and Microstructures with Other Polymethylsiloxane Aerogels. To investigate the effects of the organic parts incorporated in the polysiloxane networks, we subsequently conducted the comparison of mechanical properties and microstructures of Ethe-BPMS aerogels with those of other polymethylsiloxane
Figure 2. (a−c) FE-SEM images and (d) stress−strain curves on uniaxial compression−decompression tests on the aerogels obtained from using 0.050 M (0.050M-5:5-80), 0.60 M (0.60M-5:5-80), and 1.5 M TMAOH (1.5M-5:5-80), the appearance of which is shown in Figure 1a. Dotted circles in (c) clarify the relatively large pores.
aerogel 0.050M-5:5-80 is homogeneous but somewhat coarsened and displays relatively large pores (approximately 100 nm) and skeletons (20 nm), as shown in Figure 2a. Pore structures greater than 100 nm cause strong visible-light scattering, resulting in the opaque appearance of the aerogel. The transparent aerogel 0.60M-5:5-80 displays a more homogeneous and finer pore structure with small pores (approximately 20 nm) and skeletons (10 nm), as shown in Figure 2b, which can effectively suppress light scattering by the Mie mode. In the case of opaque aerogel 1.5M-5:5-80, a relatively fine but inhomogeneous pore structure is revealed, as shown in Figure 2c. As highlighted with circles, larger pores (100−150 nm) exist inside of the relatively fine pore structure. The decrease in the transmittance can be attributed to this inhomogeneity in the pore structure. These large pores exist over a broad region of pore structure, which can be confirmed from the FE-SEM image in a lower magnification (Figure S2). To investigate the differences in the mechanical properties derived from the pore structure, we subsequently carried out uniaxial compression−decompression tests on these three aerogels. Aerogels were compressed until 50% strain and then decompressed until the detected force became less than 0.1 N. The obtained stress−strain curves are displayed in Figure 2d. These three aerogels exhibit similar behaviors during compressive deformation, irrespective of the microstructures. The thinnest parts of solid skeletons of these three aerogels, which should be responsible for the behavior during compression, are almost the same in size, and thus, these aerogels are expected to behave in a similar manner. All of these D
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
flexural moduli of Ethe-BPMS and Ethy-BPMS show a difference, the bending strengths of both aerogels are similar. On the other hand, the PMSQ aerogel shows smaller flexural moduli and can endure larger bending strain until they break, whereas their bending strength is smaller than these bridged polymethylsiloxanes. Figure 5c displays the stress−strain curves obtained from uniaxial compression−decompression tests. It can be clearly observed that the values of the compressive stress at 50% compression, which is related to stiffness against deformation, are on the order of Ethe-BPMS (1.3 MPa), EthyBPMS (0.97 MPa), and PMSQ (0.76 MPa). As observed also in the three-point bending tests, these two bridged polymethylsiloxanes show more rigid behaviors than PMSQ, and Ethe-BPMS shows higher rigidity than Ethy-BPMS. In terms of resilience, however, the order is changed to Ethe-BPMS (100% − 13% = 87%), PMSQ (67%), and Ethy-BPMS (64%). This difference between compressive stress and resilience is presumably related to stress relaxation, as discussed below. Figure 5d shows the stress relaxation behaviors while keeping 50% compressive strain on the aerogels. Aerogels were compressed to 50% strain at which the compressive stress is σ0, and subsequent decreases in the stress σ were recorded for 3600 s. Obviously, there are clear differences between the relaxation behaviors of the three aerogels. The degree of stress relaxation on Ethy-BPMS is higher than that of Ethe-BPMS (by 1.50 times) and PMSQ (1.38 times). This large stress relaxation of Ethy-BPMS profoundly contributes to the small resilience after compression−decompression. The applied load on EthyBPMS during deformation is largely dissipated, and thus, the large irreversible strain remains. In comparison to Ethy-BPMS, the Ethe-BPMS aerogel produced under almost the same conditions show a much smaller stress relaxation. This difference in the deformation behavior can be ascribed to the rigidity of the networks. More rigid ethenylene bridges contribute to higher resilience against compression compared with the ethylene bridges.26 Other examples of the observation on stress-relaxation in compression on aerogels are highly limited;27,28 however, these observations may provide useful information on resilience and other mechanical properties, which is an important factor for producing transparent and lowdensity dried gels without supercritical drying.8,9 To obtain information on microporosity derived from molecular structures, we performed PALS measurements. This characterization technique is effective for detecting the size and relative number of pores on the (sub)nanometer range, existing both outside (open) and inside (closed pores, i.e., voids) of the gel skeletons. In fact, PALS elucidated the difference in micropores/voids in polyorganosilsesquioxane networks derived from the alkoxysilanes with different organic substituent groups in our previous report.16 As shown in Table 1, there are two components in the lifetime of orthopositronium; τ3 is relevant to the micropores/voids, and τ4 is relevant to the mesopores. The size (D3) and relative number (I3) of micropores/voids of Ethe-BPMS are larger than those of Ethy-BPMS. The PALS result is consistent with the results obtained from the computational calculations on ethylene, ethenylene, and ethynylene-bridged polysilsesquioxane membranes for gas separations and water desalination.29 The increased rigidity of bridging parts hinders the dense packing of the network, resulting in a more open structure on the subnanometer scale. The Brunauer−Emmett−Teller (BET) specific surface area obtained from nitrogen adsorption isotherms shows the highest value in Ethe-BPMS, presumably
aerogels reported previously, that is, Ethy-BPMS and PMSQ aerogels. We first conducted the comparison of mechanical properties. The molecular structures of these polymethylsiloxane aerogels are simply depicted in Figure 4. These three aerogels
Figure 4. Schematic of the networks of polymethylsiloxane aerogels for comparison of mechanical properties.
are different only in the blue circles within the network, and thus, their mechanical properties will reflect the difference in the structure on the molecular scale. In particular, transparent Ethy-BPMS aerogels can be obtained in almost identical conditions (volume fractions of starting materials, gelation temperature, etc.), and hence, comparison of Ethe-BPMS and Ethy-BPMS should purely reflect the difference derived from the organic linking, ethenylene, and ethylene bridges. In accordance with the previous reports,10,20 Ethy-BPMS and PMSQ aerogels with a similar transparency and bulk density were synthesized. The appearance and properties of these aerogels are shown in Figure 5a and Table 1, respectively. To evaluate their mechanical properties, three types of tests were performed. Figure 5b shows the stress−strain curves obtained from three-point bending tests on the aerogels. Although
Figure 5. (a) Appearance; stress−strain curves obtained from (b) three-point bending tests, (n = 2) and (c) uniaxial compression− decompression tests on Ethe-BPMS (0.60M-5:5-80), Ethy-BPMS, and PMSQ aerogels; and (d) stress relaxation behavior while keeping 50% compressive strain on the three aerogels. E
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Table 1. Properties of Polymethylsiloxane Aerogels Used for Comparison of Mechanical Properties and Microstructures aerogels Ethe-BPMS (0.60M-5:5-80) Ethy-BPMS PMSQ
T550nma (%)
ρbulkb (g cm−3)
SBETc (m2 g−1)
τ3d (ns)
τ4d (ns)
I3e (%)
I4e (%)
D3f (nm)
D4f (nm)
83
0.140
946
2.677 ± 0.081
81.9 ± 3.4
2.40 ± 0.06
8.67 ± 0.35
0.681 ± 0.012
8.2 ± 0.7
74 87
0.150 0.141
722 599
2.322 ± 0.195 3.042 ± 0.093g
90.2 ± 8.0 105.3 ± 4.1g
1.35 ± 0.05 3.09 ± 0.12g
5.04 ± 0.46 13.68 ± 0.53g
0.626 ± 0.032 0.732 ± 0.012g
10.2 ± 2.3 16.1 ± 2.3g
a
Light transmittance at 550 nm, normalized into the value of 10 mm-thick sample using the Lambert−Beer equation. bBulk density. cBET specific surface area obtained from nitrogen adsorption isotherms. dLifetime of ortho-positronium. eRelative intensity of ortho-positronium. fAverage diameter of pore/voids. gReported values for the sample prepared under identical synthesis conditions in ref 16.
reflecting the high relative number I3 and the small mesopore size (D4). 3.4. Availability of Diels−Alder Reaction. We also attempted an addition reaction on the ethenylene bridges, using a Diels−Alder reaction. Because the Ethe-BPMS network contains double-bond carbons, it should be profitable for aerogels if chemical functionality can be imparted by the addition of other functional molecules. Concerning periodic mesoporous organosilicas (PMOs) with ethenylene-bridged polysilsesquioxane (O3/2Si−CHCH−SiO3/2, not polymethylsiloxane) networks in the form of fine powder, it has been reported that the Diels−Alder reaction using BCB is applicable to the preimmobilized ethenylene cross-links.30,31 To investigate the efficiency of the reaction on a monolithic material with a nanometer-sized pore structure, we performed the addition reaction, using BCB, on Ethe-BPMS gels using a similar method. A Diels−Alder reaction of BCB on a dienophile is depicted in Scheme 1. BCB is isomerized by heat into o-
organo-bridged polymethylsiloxane aerogels under atmospheric conditions results in conversion into a silica gel.35 In the case of the present Ethe-BPMS, no further loss of weight can be observed above 700 °C, and thus, the conversion into silica has been probably completed at that point. On the basis of the weight loss at 800 °C (30 and 20% for BCB and pristine, respectively), the ratio of the ethenylene bridges that underwent the Diels−Alder reaction can be estimated as approximately 21%, assuming complete conversion into pure SiO2. Note that, because this value does not take into account other factors affecting the weight loss, such as oxidation of ethenylene groups during the solvothermal treatment, the actual value of the yield may be slightly lower than 21%. We also conducted solid-state NMR to confirm whether BCB is chemically bonded to the ethenylene moieties. Figure 6d,e shows the 13C and 29Si CP/MAS NMR spectra of both aerogels, respectively. After the Diels−Alder reaction, the intensity of the peak at 150 ppm in the 13C spectra corresponding to ethenylene carbons decreases, and the peaks corresponding to tetralin moieties appear, as shown in Figure 6d. The spectra of 29Si NMR also display significant changes in their shapes. The pristine aerogel shows only one intense peak at −16 ppm, which is attributed to the Si nuclei of D2 unit ((SiO)2(CH3)Si−CH) in Figure 6e (note that a small contribution from D1 is overlapped). On the other hand, the solvothermally treated aerogel displays another distinct peak at −3 ppm in the spectrum, which corresponds to the Si nuclei bonded to the reacted ethenylene group. From these results, we can conclude that BCB is chemically bonded to the ethenylene groups. In terms of compressive behaviors of pristine and treated aerogels, only a slight increase in compressive stress can be observed in Figure S5. This addition reaction does not contribute to an increase in the cross-linking density of the network, in contrast to the reports on reinforced aerogels using radical polymerization of pendent vinyl groups16 or styrene as a cross-linker onto vinyl groups.36 Thus, in this case, only a slight increase in compressive stress derived from the increase in the bulk density can be observed, and no improvement in resilience is found. However, the availability of additional reactions is highly beneficial for applications such as catalysts, as reported in the aforementioned PMO systems,30,31 and possesses potential for realizing mechanical improvements in a well-designed manner.
Scheme 1. Diels−Alder Reaction of BCB on a Dienophile
quinodimethane, and then, it reacts with a dienophile.32 The Ethe-BPMS gels are solvothermally treated at 200 °C with BCB in the present study. Figure 6a shows the appearance of the pristine Ethe-BPMS (0.60M-5:10-80) aerogel and one solvothermally treated aerogel with BCB. As compared with the pristine aerogel, the aerogel solvothermally treated with BCB shows a small increase in the bulk density (from 0.081 to 0.10 g cm−3), whereas no apparent shrinkage can be observed. Moreover, light transmittance also slightly decreases (from 32 to 19%), whereas homogeneous appearance on a macroscopic scale is retained. Considering that the bulk density and the transmittance of an aerogel solvothermally treated without BCB (not shown) are 0.086 g cm−3 and 28%, respectively, these changes are mainly derived from the addition of BCB, not from aging effects by the solvothermal treatment. Figure 6b displays the Raman spectra of both aerogels. A substantial decrease in the intensity of the peaks at 1296 cm−1, which can be assigned to the in-plane C−H deformation, and 1573 cm−1, the CC stretching vibration, can be clearly observed.33 In addition, a strong peak emerged in the treated sample at 677 cm−1, which is presumably derived from the bending vibration of the benzene ring.34 To estimate the degree of addition, thermogravimetric analysis was conducted, and the obtained results are shown in Figure 6c. It has been reported that the heat treatment on
4. CONCLUSIONS We have for the first time synthesized transparent polymethylsiloxane aerogels with ethenylene bridging parts. The obtained ethenylene-bridged polymethylsiloxane (Ethe-BPMS) aerogels are composed of connected nanosized colloidal particles and exhibit high visible-light transparency, the value F
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 6. (a) Appearance, (b) Raman spectra, (c) thermogravimetric analysis curves, (d) solid-state 13C, and (e) 29Si CP/MAS NMR spectra of pristine Ethe-BPMS (0.60M-5:10-80) and solvothermally treated aerogels with BCB. Asterisks in (b) denote spike noises. The intensities of spectra shown in (d,e) are normalized, referring to the peak at 0 and −16 ppm, respectively.
of which reaches as high as 87%/10 mm at 550 nm. The gelation and aging temperature significantly affect the compressive behavior of the resultant aerogels, and the aerogels prepared at 80 °C show a good resilience against 50% compression and excellent flexibility against three-point bending, which is considered to reflect the rigidity of ethenylene linkings. We further investigated the mechanical properties by comparing with other polymethylsiloxane aerogels such as Ethy-BPMS and PMSQ. In particular, because both EtheBPMS and Ethy-BPMS aerogels were prepared under almost the same synthesis conditions, the comparison of both aerogels can elucidate the effect of the molecular structure. Concerning the compression behavior, these two showed a significant difference, which reflects the difference in rigidity between ethenylene- and ethylene-linkings. Effects of the difference in rigidity is also observed in the microstructures, as confirmed using PALS. We also found that the addition reaction utilizing a Diels− Alder reaction of BCB onto ethenylene moieties is applicable, which is confirmed using Raman and solid-state NMR spectroscopies. Although this modification did not improve the mechanical properties in the present study, the availability of surface modification is highly profitable for applications such as catalysts. The profound understanding of the effects of molecular structure on the macroscopic mechanical properties and the availability of the addition reaction revealed in this study are an important step for realizing transparent aerogels with desired functionalities.
■
■
Sample photograph, FE-SEM images, and stress−strain curves (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kazuyoshi Kanamori: 0000-0001-5087-9808 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The present study has been performed under financial support from Advanced Low Carbon Technology Research and Development Program (ALCA, JST Japan) and JSPS KAKENHI grant number JP26288106. The NMR measurements were carried out in the JURC at the Institute for Chemical Research, Kyoto University. Prof. Toyoshi Shimada is acknowledged for molecular structural analysis. C.M.D. is supported by the Australian Research Council (DE40101359). Dr. Kristina Konstas is acknowledged for her help in the PALS measurements.
■
REFERENCES
(1) Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741. (2) Hyun, S. H.; Kim, J. J.; Park, H. H. Synthesis and Characterization of Low-Dielectric Silica Aerogel Films. J. Am. Ceram. Soc. 2000, 83, 533−540. (3) Dunn, B. C.; Cole, P.; Covington, D.; Webster, M. C.; Pugmire, R. J.; Ernst, R. D.; Eyring, E. M.; Shah, N.; Huffman, G. P. Silica Aerogel Supported Catalysts for Fischer−Tropsch Synthesis. Appl. Catal., A 2005, 278, 233−238. (4) Kanamori, K. Organic−Inorganic Hybrid Aerogels with High Mechanical Properties via Organotrialkoxysilane-Derived Sol−Gel Process. J. Ceram. Soc. Jpn. 2011, 119, 16−22.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00434. G
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (5) Cao, W.; Hunt, A. J. Improving the Visible Transparency of Silica Aerogels. J. Non-Cryst. Solids 1994, 176, 18−25. (6) Baetens, R.; Jelle, B. P.; Gustavsen, A. Aerogel Insulation for Building Applications: A State-of-the-Art Review. Energ. Build. 2011, 43, 761−769. (7) Jensen, K. I.; Schultz, J. M.; Kristiansen, F. H. Development of Windows Based on Highly Insulating Aerogel Glazings. J. Non-Cryst. Solids 2004, 350, 351−357. (8) Kanamori, K.; Aizawa, M.; Nakanishi, K.; Hanada, T. New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties. Adv. Mater. 2007, 19, 1589−1593. (9) Kanamori, K.; Aizawa, M.; Nakanishi, K.; Hanada, T. Elastic Organic−Inorganic Hybrid Aerogels and Xerogels. J. Sol-Gel Sci. Technol. 2008, 48, 172−181. (10) Kanamori, K.; Nakanishi, K.; Hanada, T. Sol−Gel Synthesis, Porous Structure, and Mechanical Property of Polymethylsilsesquioxane Aerogels. J. Ceram. Soc. Jpn. 2009, 117, 1333−1338. (11) Loy, D. A.; Baugher, B. M.; Baugher, C. R.; Schneider, D. A.; Rahimian, K. Substituent Effects on the Sol−Gel Chemistry of Organotrialkoxysilanes. Chem. Mater. 2000, 12, 3624−3632. (12) Dong, H.; Brook, M. A.; Brennan, J. D. A New Route to Monolithic Methylsilsesquioxanes: Gelation Behavior of Methyltrimethoxysilane and Morphology of Resulting Methylsilsesquioxanes under One-Step and Two-Step Processing. Chem. Mater. 2005, 17, 2807− 2816. (13) Rao, A. V.; Bhagat, S. D.; Hirashima, H.; Pajonk, G. M. Synthesis of Flexible Silica Aerogels Using Methyltrimethoxysilane (MTMS) Precursor. J. Colloid Interface Sci. 2006, 300, 279−285. (14) Smith, D. M.; Stein, D.; Anderson, J. M.; Ackerman, W. Preparation of Low-Density Xerogels at Ambient Pressure. J. NonCryst. Solids 1995, 186, 104−112. (15) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage. Nature 1995, 374, 439−443. (16) Shimizu, T.; Kanamori, K.; Maeno, A.; Kaji, H.; Doherty, C. M.; Falcaro, P.; Nakanishi, K. Transparent, Highly Insulating Polyethyland Polyvinylsilsesquioxane Aerogels: Mechanical Improvements by Vulcanization for Ambient Pressure Drying. Chem. Mater. 2016, 28, 6860−6868. (17) Kobayashi, Y.; Saito, T.; Isogai, A. Aerogels with 3D Ordered Nanofiber Skeletons of Liquid-Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators. Angew. Chem., Int. Ed. 2014, 53, 10394−10397. (18) Takeshita, S.; Yoda, S. Chitosan Aerogels: Transparent, Flexible Thermal Insulators. Chem. Mater. 2015, 27, 7569−7572. (19) Mi, Q.-y.; Ma, S.-r.; Yu, J.; He, J.-s.; Zhang, J. Flexible and Transparent Cellulose Aerogels with Uniform Nanoporous Structure by a Controlled Regeneration Process. ACS Sustainable Chem. Eng. 2016, 4, 656−660. (20) Shimizu, T.; Kanamori, K.; Maeno, A.; Kaji, H.; Nakanishi, K. Transparent Ethylene-Bridged Polymethylsiloxane Aerogels and Xerogels with Improved Bending Flexibility. Langmuir 2016, 32, 13427−13434. (21) We found a bottle of the reagent “1,2-bis(methyldiethoxysilyl) ethene” from Gelest contains a considerable amount of a structural isomer, 1,1-bis(methyldiethoxysilyl)ethene. See Supporting Information for details. (22) Tao, S. J. Positronium Annihilation in Molecular Substances. J. Chem. Phys. 1972, 56, 5499−5510. (23) Eldrup, M.; Lightbody, D.; Sherwood, J. N. The Temperature Dependence of Positron Lifetimes in Solid Pivalic Acid. Chem. Phys. 1981, 63, 51−58. (24) Hæreid, S.; Anderson, J.; Einarsrud, M. A.; Hua, D. W.; Smith, D. M. Thermal and Temporal Aging of TMOS-Based Aerogel Precursors in Water. J. Non-Cryst. Solids 1995, 185, 221−226. (25) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; Wiley Interscience: New York, 1979; Chapter 3.
(26) Shea, K. J.; Loy, D. A. Bridged Polysilsesquioxanes. MolecularEngineered Hybrid Organic−Inorganic Materials. Chem. Mater. 2001, 13, 3306−3319. (27) de la Rosa-Fox, N.; Toledo-Fernández, J. A.; Morales-Flórez, V.; Piñero, M.; Esquivias, L. Creep and Stress Relaxation of Hybrid Organic−Inorganic Aerogels. Key Eng. Mater. 2010, 423, 167−172. (28) Yang, X.; Sun, Y.; Shi, D.; Liu, J. Experimental Investigation on Mechanical Properties of a Fiber-Reinforced Silica Aerogel Composite. Mater. Sci. Eng., A 2011, 528, 4830−4836. (29) Xu, R.; Ibrahim, S. M.; Kanezashi, M.; Yoshioka, T.; Ito, K.; Ohshita, J.; Tsuru, T. New Insights into the Microstructure-Separation Properties of Organosilica Membranes with Ethane, Ethylene, and Acetylene Bridges. ACS Appl. Mater. Interfaces 2014, 6, 9357−9364. (30) Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N. A Stable and Highly Active Hybrid Mesoporous Solid Acid Catalyst. Adv. Mater. 2005, 17, 1839−1842. (31) Esquivel, D.; De Canck, E.; Jiménez-Sanchidrián, C.; Van Der Voort, P.; Romero-Salguero, F. J. Formation and Functionalization of Surface Diels−Alder Adducts on Ethenylene-Bridged Periodic Mesoporous Organosilica. J. Mater. Chem. 2011, 21, 10990−10998. (32) Meador, M. A. Recent Advances in the Development of Processable High-Temperature Polymers. Annu. Rev. Mater. Sci. 1998, 28, 599−630. (33) Vercaemst, C.; Ide, M.; Wiper, P. V.; Jones, J. T. A.; Khimyak, Y. Z.; Verpoort, F.; Van Der Voort, P. Ethenylene-Bridged Periodic Mesoporous Organosilicas: From E to Z. Chem. Mater. 2009, 21, 5792−5800. (34) Autrey, D.; Yang, J.; Laane, J. Vibrational Spectra and DFT Calculations of Tetralin and 1,4-Benzodioxan. J. Mol. Struct. 2003, 661−662, 23−32. (35) Loy, D. A.; Jamison, G. M.; Baugher, B. M.; Myers, S. A.; Assink, R. A.; Shea, K. J. Sol−Gel Synthesis of Hybrid Organic−Inorganic Materials. Hexylene- and Phenylene-Bridged Polysiloxanes. Chem. Mater. 1996, 8, 656−663. (36) Nguyen, B. N.; Meador, M. A. B.; Tousley, M. E.; Shonkwiler, B.; McCorkle, L.; Scheiman, D. A.; Palczer, A. Tailoring Elastic Properties of Silica Aerogels Cross-Linked with Polystyrene. ACS Appl. Mater. Interfaces 2009, 1, 621−630.
H
DOI: 10.1021/acs.langmuir.7b00434 Langmuir XXXX, XXX, XXX−XXX
