Surface Modification and Reinforcement of Silica Aerogels Using

Oct 9, 2012 - Journal of Chemical Education · Journal of Chemical Information and Modeling ..... Tetraethoxysilane (TEOS) was the primary silane to fo...
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Surface Modification and Reinforcement of Silica Aerogels Using Polyhedral Oligomeric Silsesquioxanes Yannan Duan,†,§ Sadhan C. Jana,*,† Anna M. Reinsel,‡ Bimala Lama,‡ and Matthew P. Espe‡ †

Department of Polymer Engineering, The University of Akron, Akron, Ohio, 44325-0301 United States Department of Chemistry, The University of Akron, Akron, Ohio, 44325-3601 United States

‡

S Supporting Information *

ABSTRACT: This study evaluated polyhedral oligomeric silsesquioxane (POSS) molecules as useful, multifunctional reinforcing agents of silica aerogels. Silica aerogels have lowdensity and high surface area, although their durability is often compromised by the inherent fragility and strong moisture absorption behavior of the silica networks. POSS molecules carrying phenyl, iso-butyl, and cyclohexyl organic side groups, and several Si−OH functionalities were incorporated into silica networks via reactions between Si−OH functionalities in POSS molecules and silanes. Solid state 13C and 29Si NMR spectra established that greater than 90% of POSS molecules grafted onto silica networks and led to an increase in fractal dimensions. An almost 6-fold increase in compressive modulus was achieved with less than 5 wt % trisilanol phenyl POSS, and a 50-fold decrease in polarity with negligible changes in density were seen in aerogels modified with less than 5 wt % trisilanol isobutyl POSS. of silane precursors with flexible molecular structures and amine functionality and cross-linking urethane and epoxy molecules. A brief summary of prior work on reinforcement of silica networks is presented below before introducing the scope of reinforcement by polyhedral oligomeric silsesquioxanes (POSS). Kramer et al.32 reinforced tetraethoxysilane (TEOS)-based silica aerogels via reactions with hydroxyl-terminated polydimethylsiloxane (PDMS) in acid and two-step acid/base catalyzed processes. These organically modified aerogels offered optical transparency, surface area of up to 1200 m2/g, and elongation at break exceeding 5%. Several authors used crosslinking reactions with isocyanates and epoxies for improvement of mechanical properties of aerogels. Leventis and coworkers31,33 used reactions between isocyanates and residual Si−OH groups on the surfaces of silica particles and obtained a factor of ∼300 increase of breaking force and a factor of ∼3 increase of density compared to unmodified aerogels. Capadona et al.34 and Katti et al.35 incorporated aminopropyltriethoxysilane (APTES) in silane formulations and exploited faster reactions between isocyanates and amines compared to hydroxyls36 to obtain polyurea-cross-linked aerogels. These aerogels offered compressive Young’s modulus at room temperature of 129 ± 8 MPa. Meador et al.37 used di-, tri-, and tetra-functional epoxies to cross-link the aminemodified mesoporous silica surfaces and observed more than 2

1. INTRODUCTION The most common, extensively studied, and widely used aerogels are silica aerogels with porosity greater than 95%, density of the order of 3−350 mg/cm3,1 large surface area, 500−1200 m2/g, low thermal conductivity ranging from 0.004 to 0.03 W/m K, low dielectric constant (1.1−2.2),2−4 and low index of refraction (∼1.05).5 They are synthesized in different morphological forms, such as monoliths or powders, and show different optical properties6 such as transparent,7,8 opaque, or translucent.1,9−12 Monolithic silica aerogels find usage in manufacturing of transparent and superinsulating double windows13−15 and in promising space science applications.16 Additional applications of silica aerogels are found as adsorbents of oils and organic liquids, as sensors,17−19 catalysts,20 storage media,21 as templates,21 in biocatalysis,21,22 for detection of viral particles by immobilized bacteria,14,23−26 as host for biomaterials or drugs,27 and as antifouling materials.28 A recent article lists several other attributes of silica aerogels along with means of tailoring mechanical properties.29 The inherent fragility of the silica networks and ability to rapidly absorb moisture often limit more widespread applications. A native silica aerogel derived from tetramethoxysilane (TMOS) with a density of about 120 mg/cm3 can be easily destroyed under a stress of 31 kPa.30 This failure at relatively low stress is a ramification of the “pearl necklace” structure and is attributed to spherical secondary silica particles joined to each other by only a few Si−O−Si bonds31 (Figure S1). Randall et al.29 presented several means for tailoring of mechanical properties of silica aerogels. These include the use © 2012 American Chemical Society

Received: July 20, 2012 Revised: October 5, 2012 Published: October 9, 2012 15362

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Figure 1. Illustration showing reactions leading to silica network formation. Path 1 shows formation of unmodified silica networks, and path 2 shows modification of silica networks by POSS molecules. Illustration shows how a tri-POSS molecule can covalently bond to the silica network.

(MTMS) is a popular coprecursor in conjunction with tetramethoxysilane (TMOS),41,48,49 whereas methyltriethoxysilane (MTES) was used together with tetraethoxy-silane (TEOS).43 Kanamori et al.50 reported transparent monolithic aerogels and xerogels with improved mechanical properties, derived from MTMS using a cationic surfactant cetyltrimethylammonium bromide (CTAB) or a nonionic surfactant, a triblock copolymer, poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), to promote mixing. Urea was used to accelerate polymerization of MTMS. Rao et al.51 investigated the synthesis of flexible and superhydrophobic silica aerogels using methyltrimethoxysilane (MTMS) precusor. The effects of various sol−gel parameters on the flexibility of the aerogels were investigated. It was observed that Young’s modulus of the aerogels decreased from 14.11 × 104 to 3.43 × 104 N/m2 and density reduced from 100 to 40 kg/m3. Budunoglu et al.52 reported preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica aerogel(ORMOSIL) thin films from colloidal dispersions at ambient conditions. ORMOSIL films exhibited and retained superhydrophobic behavior up to 500 °C. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of 90% of the POSS was retained in the aerogel even when the mass % of POSS was increased from 1 to 5 or 10%. These results show that the system was not saturated with POSS at the mass % levels used in this study.

Figure 2. Solid-state 13C NMR spectra of (a) unmodified aerogel and (b) T-tri-POSS-5 aerogel.

ethoxy groups in TEOS did not participate in the hydrolysis and condensation reactions. The 13C SSNMR spectrum of Ttri-POSS-5 aerogel, Figure 2b, contains peaks from the unreacted TEOS and peaks between 125 and 140 ppm from the phenyl groups of tri-POSS. The fraction of unreacted TEOS in the aerogel was determined to be ∼5% by taking into account the amount of POSS present in the sample and the relative peak intensities. This calculation was based on the fact that almost all of the POSS added in the starting formulation

Table 2. Compositions of Aerogels Reinforced with POSS Molecules sample

percentage of added POSS in the wash

percentage of added POSS remaining in aerogels

amount of POSS (over total silane) remaining in aerogels

T-i-butyl-POSS-1 T-i-butyl-POSS-3 T-i-butyl-POSS-5 T-i-butyl-POSS-10 T-tetra-POSS-1 T-tetra-POSS-3 T-tetra-POSS-5 T-tri-POSS-1 T-tri-POSS-3 T-tri-POSS-5 T-cyclo-POSS-3 T-cyclo-POSS-5

0.0% 6.4% 5.4% 7.6% 6.3% 9.7% 8.2% 2.4% 4.7% 5.2% 5.7% 2.1%

100.0% 93.6% 94.6% 92.4% 93.7% 90.3% 91.8% 97.6% 95.3% 94.8% 94.3% 97.9%

1.0% 2.8% 4.7% 9.2% 0.9% 2.7% 4.6% 1.0% 2.9% 4.7% 2.8% 4.9%

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The reactivity and product composition of the aerogels was followed using 29Si SSNMR. In the 29Si NMR spectrum from silica materials it is typical to observe peaks that arise from the Si sites with structures of Si(−OSi) 2 (−OH) 2 , Si(−OSi)3(−OH) and Si(−OSi)4.56 These species are labeled Q2, Q3, and Q4, respectively, with the peaks occurring more upfield with larger Q value. The 29Si NMR spectrum from the TEOS aerogel, Figure 3b, show the expected resonances in the

Figure 4. Solid-state 13C NMR spectra of (a) tetra-POSS powder, (b) T-tetra-POSS-2 aerogel, and (c) T-tetra-POSS-5 aerogel.

that about half of the reacted tri-POSS molecules formed bonds via one Si−OH groups and the other half reacted via two Si− OH groups. The close proximity of the three Si−OH sites in tri-POSS and the presence of the phenyl rings must have provided steric restriction to reactions of all three Si−OH groups in tri-POSS. The aerogels containing i-butyl- and cyclo-POSS were also characterized by SSNMR. Similar to the other two systems, these aerogels contained a small fraction of unreacted TEOS. The 29Si NMR results also showed that nearly all of the Q3 (Si−OH) sites of these two POSS derivatives reacted with the silica. Although i-butyl and cyclo-POSS share a framework structure with tri-POSS, where the Si−OH sites are on one face, the tri-POSS is less reactive toward the silica. The large, bulky phenyl groups in tri-POSS and the larger number of degrees of freedom of the substituents in i-butyl and cyclo-POSS must have contributed to the differences in reactivity. In view of the data presented in Table 2 and the data in Figure 4, we can infer the following: (1) Amounts of POSS remaining in aerogel specimens (>90% of the initial amount) are almost independent of the POSS type. (2) Only 50% of the three Si−OH groups in tri-POSS participated in condensation reactions, whereas almost all of the four Si−OH groups in tetraPOSS underwent condensation. Also, much larger fractions of Si−OH groups in i-butyl-POSS and cyclo-POSS participated in condensation than in tri-POSS. (3) The data in Figure 4, however, do not indicate if all reacted Si−OH groups of POSS formed covalent bonds with silica networks. It is noted that POSS−POSS condensation reactions are also possible. This point will be useful in analyzing the data on surface polarity of aerogels. 3.4. Morphology and Surface Properties. The morphology of silica aerogels reinforced with POSS molecules is presented in Figure 5 and Figure S3. The primary silica particles (∼1−3 nm) or POSS molecules, due to their small sizes, cannot be identified in the SEM images. It is apparent that the pearl-necklace structure of native silica aerogels is evident in the presence of POSS molecules and that the average particle size of the pearl-necklace structures are in the range of 80−120 nm, which are much larger than the secondary particles (∼5 nm). An intriguing question arises about the participation of POSS molecules in the formation of primary and secondary silica particles. It is noted that the silanes derived from hydrolysis of TEOS molecules may undergo relatively faster condensation reactions due to smaller size and less steric hindrance than POSS molecules. Consequently, it is conceivable that the Si

Figure 3. Solid-state 29Si NMR spectra of (a) tri-POSS, (b) TEOS, and (c) T-tri-POSS-5 aerogel.

silica region of the spectrum, with peaks at −98 ppm (Q2), −101 ppm (Q3), and −110 ppm (Q4). The larger population of Q3 sites relative to Q4 sites results from the very high surface area of the material and the large number of surface hydroxyl groups present. The 29Si spectrum from tri-POSS, Figure 3a, consists of only two peaks from the presence of Q3 and Q4 sites. Also, the peak intensity for the Q4 Si sites is slightly larger than the peak from Si in the Q3 structure. The presence of only Q3 and Q4 sites and the ratio of peak intensities are consistent with the structure of the compounds, as shown in Figure 1. The peaks from the POSS species are shifted downfield relative to that observed for silica, as the Si in POSS is directly bonded to a carbon.57,58 The 29Si NMR spectrum from T-tri-POSS-5 is shown in Figure 3c and is simply a composition of the spectra from the aerogel and tri-POSS. The peaks from the silica in this sample are little changed from the spectrum of the unmodified aerogel, as is expected from the fact that the POSS is only a small fraction of the sample and its interaction with the silica is expected to cause only a small change in the spectrum. The spectrum from the POSS in the aerogel has a larger Q4/Q3 peak ratio than that observed for the neat POSS, indicating that a portion (∼50%) of the available POSS Si−OH groups have reacted with the silica. The material composition of the aerogels containing tetraPOSS was also studied using 13C and 29Si NMR techniques. The 13C solid state NMR spectra from the aerogel containing tetra-POSS showed the expected peaks for tetra-POSS and, again, showed that a small portion (∼5%) of the TEOS had not reacted during the aerogel synthesis (data not shown). The 29Si SSNMR spectrum from tetra-POSS and two aerogels containing tetra-POSS at two different levels are shown in Figure 4. The spitting in the peaks from tetra-POSS, Figure 4a, results from different packing environments in the solid-state. Most interesting is the fact that the peak from the Q3 sites of tetra-POSS, at −67 ppm, is larger than the peak from the Q4 sites (−80 ppm) in the neat material, but the ratio is reversed in the aerogel. Comparison of the peak intensities indicates that nearly all of the four Si−OH groups in tetra-POSS have reacted. This is in contrast to tri-POSS where ∼50% of the three Si− OH sites have undergone a condensation reaction, indicating 15366

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is more effective than other POSS molecules included in the study in reducing the surface energy and rendering the aerogel hydrophobic. The surface polarity of compressed discs of aerogel T-i-butyl-POSS-5 and T-i-butyl-POSS-10 is almost zero. The dependence of water contact angle on POSS type in aerogel specimens T-tetra-POSS-5, T-tri-POSS-5, T-cycloPOSS-5, and T-i-butyl-POSS-5 can be attributed to three factors: (1) the number of hydrophobic alkyl and aryl groups in the molecular structures of POSS molecules, (2) the amount of POSS molecules retained in the aerogel structure, and (3) the coverage of the surfaces of secondary silica particles by POSS molecules. The data presented in Table 2 reflect that the aerogel specimens T-i-butyl-POSS-5, T-tri-POSS-5, T-tetraPOSS-5, and T-cyclo-POSS-5 contained similar amounts of POSS, e.g., 4.7, 4.7, 4.6, and 4.9 wt %, respectively. These are due primarily to POSS molecules reacted directly onto the silica structures. Recall that silica gels were repeatedly washed with ethanol to remove physically adsorbed POSS molecules. The NMR data discussed in section 3.2 revealed that about 50% of Si−OH in tri-POSS and almost all Si−OH in tetraPOSS, i-butyl-POSS, and cyclohexyl-POSS participated in condensation, although it is not known if all such reactions also led to covalent bonding with silica networks. In this context, several scenarios can be considered. If a tetra-POSS molecule reacts with a silica particle via only one Si−OH group and with other tetra-POSS molecules via the other three Si− OH groups, it is bonded to the network and is retained in the aerogel, but it cannot fully remove the polar Si−OH groups from the silica surfaces. Consequently, the resultant aerogel exhibits higher surface energy due to a higher concentration of residual Si−OH groups. On the other hand, POSS-silica reactions via multiple Si−OH groups in the POSS molecule are more effective in reducing the surface energy. Among the POSS-modified aerogels, the lowest values of water contact angle (28−45°) and the highest values of surface energy (60−68 dyn/cm) are seen in specimens of T-tetraPOSS aerogel. This indicates the possibility that tetra-POSS is much less effective in reacting with the residual Si−OH groups on silica surfaces. It is quite possible that only one or two of the four reactive sites in tetra-POSS participated in chemical bond formation with the silica network. In this context, cyclo-POSS altered the polarity of silica aerogels more effectively than triPOSS and tetra-POSS quite possibly due to more POSS−silica covalent bond formation. The interactions between water and silica aerogels also present visual evidence of polarity changes in the presence of POSS, as shown in Figure 6. The uncompressed TEOS-based aerogel absorbed the drop of water readily due to capillarity. The capillary stress thus developed also collapsed the aerogel structure in contact with water droplet (Figure 6a). On the other hand, the TEOS-based aerogel specimen T-i-butyl-POSS-

Figure 5. SEM images of aerogels without/with POSS (a) TEOSbased aerogel without POSS reinforcement and (b) TEOS-based aerogel with 5 wt % tri-POSS.

atoms inside the secondary particles in the aerogel structure originated mainly from the hydrolyzed TEOS species. It is already seen from NMR data presented in previous section that Si−OH functionalities in tri-POSS, cyclo-POSS, tetra-POSS, and i-butyl-POSS molecules reacted with the residual Si−OH groups on the surfaces of silica particles and the data in Table 2 presented evidence that more than 90% of POSS molecules in initial formulations remained in the aerogels. Such reactions led to populations of POSS molecules stationed on the surfaces of secondary particles. The surface energy of TEOS-based aerogels with and without modification by POSS molecules was obtained from the values of contact angles of deionized water and diiodomethane on compressed discs of aerogel specimens. It was anticipated that the pore structures in aerogel specimens and associated capillary forces would influence the values of contact angle. In view of this, the aerogel specimens were compressed into solid discs under high pressure. The surfaces of the compressed discs were examined by optical microscope and AFM to determine residual surface roughness. The AFM height image indicated relatively uniform surfaces. The surface roughness was seen to be less than 10 nm in an area of 3 × 3 μm2. Therefore, the values of contact angle captured only the effects of surface chemistry of the silica particles. The values of contact angle of water and diiodomethane were used to obtain components of surface energy and polarity. Detailed data for various specimens are presented in Supporting Information in Table S1. Note that these results reflect only the influence of chemical makeup of the surfaces of silica particles. The effects of surface roughness were negligible. The contact angle of water increased significantly in the presence of POSS and the values strongly depended on the type of POSS used. The water contact angle increased from 23° for a compressed disk of unmodified silica aerogel to respectively 45°, 68°, 77°, and 99° for compressed discs of aerogel specimens T-tetra-POSS-5, T-tri-POSS-5, T-cycloPOSS-5, and T-i-butyl-POSS-5. It is evident that i-butyl-POSS

Figure 6. Images of aerogels with and without i-butyl POSS when exposed to deionized water. (a) Deionized water absorbed by unmodified aerogel, (b) deionized water droplet sitting on top of T-i-butyl POSS-10 aerogel, and (c) a piece of T-i-butyl POSS-10 aerogel floats on top of deionized water. 15367

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10 reinforced with 10 wt % i-butyl-POSS did not collapse and allowed the water droplet to sit on top of the specimen (Figure 6b). The same specimen is seen floating in Figure 6c on top of water. We now turn to mesoscopic structures of the aerogel specimens. The adsorption/desorption isotherms of T-i-butylPOSS-10 specimen are shown in Figure 7. These adsorption

Table 3. Surface Area, Average Pore Size, Dominant Pore Size, Porod Slope, and Fractal Dimension of Aerogel Samples sample T-0 T-i-butylPOSS-1 T-i-butylPOSS-3 T-i-butylPOSS-5 T-i-butylPOSS10 T-tetraPOSS-1 T-tetraPOSS-3 T-tetraPOSS-5 T-triPOSS-1 T-triPOSS-3 T-triPOSS-5 T-cycloPOSS-3 T-cycloPOSS-5

Figure 7. Adsorption and desorption isotherms of aerogels.

and desorption isotherms are typical for a combination of type II and type IV isotherms. The micropores inside the networks are responsible for the initial increase of adsorption. The hysteresis loop at P/P0 > 0.5 is typical for slit-shaped pores in contrast to more common spherical pores also suggesting the existence of mesopores. The isotherms for other specimens are similar to those presented in Figure 7. The values of surface area and the average pore size are listed in the first two columns in Table 3. It is seen that the average pore diameter was in the range of 11−18 nm. It is also seen that specimens containing POSS had slightly higher average pore diameters (13−18 nm) and about 14−26% lower BET surface areas compared to those of unmodified aerogels. The aerogel specimens T-i-butyl-POSS-5, T-tetra-POSS-5, T-tri-POSS-5, and T-cyclo-POSS-5 all have similar (22−24%) reduction of surface area. The reduction of surface area may be attributed to POSS molecules covering the surfaces of secondary silica particles due to POSS−silica reactions discussed earlier. The pore size distribution of aerogels reinforced with triPOSS, tetra-POSS, i-butyl-POSS, and cyclo-POSS was determined by BJH method. The unmodified aerogel showed one broad peak at a predominant pore diameter of 22 nm (Figure S4). Interestingly, POSS-modified aerogels with 1 and sometimes 3 wt % POSS show two broad peaks. For example, T-tri-POSS-1 aerogel show two very broad peaks, one at 3 nm, and the other at 21 nm while one peak is observed for T-triPOSS-3 and T-tri-POSS-5 aerogels at 23 and 32 nm respectively. The following peak locations were noted for various aerogels: T-tetra-POSS-1 (6 and 31 nm), T-tetra-POSS3 (3 and 31 nm), T-i-butyl-POSS-1 (4 and 33 nm), T-i-butylPOSS-3 (3 and 33 nm), T-i-butyl-POSS-5 (22 nm), T-i-butylPOSS-10 (33 nm), T-cyclo-POSS-1 (3 and 33 nm), and T-cycloPOSS-5 (31 nm). The appearance of the peak at around 3 nm for aerogels containing 1 and 3 wt % POSS can be attributed to the interstitial space formed by POSS molecules grafted onto the secondary particle surfaces. The surfaces of secondary

BET surface area (m2/g)

average pore diameter (nm)

dominant pore diameter (nm)

Porod slope

Ds

805 681

11 13

22 32, 4

−3.41 NA

2.59 NA

689

16

33, 3

−3.34

2.66

612

18

22

−3.16

2.74

598

15

33

−3.16

2.84

666

18

31, 6

NA

NA

641

18

31, 3

−3.42

2.58

597

18

32

−3.28

2.72

695

14

21, 3

NA

NA

651

13

23

−3.44

2.56

624

14

32

−3.18

2.82

659

16

33, 3

−3.33

2.67

624

14

31

−3.26

2.74

particles were more heavily grafted by the POSS molecules at a higher concentration of POSS, e.g., 5 and 10 wt %, and the interstitial space disappeared. 3.5. Fractal Dimension from SAXS. The fractal dimension of silica aerogels containing POSS molecules was studied by SAXS to determine if the presence of POSS molecules interfered with the fractal dimensions of the original silica particle networks. The SAXS intensity I(q) for aerogels reinforced with POSS decreased rapidly with increasing the scattering vector q and the absence of distinct peaks indicates disordered structure in the aerogel network (Figure S5). The fractal dimensions of the aerogel networks were determined from log I(q) vs log q plot. The intensity followed a power law behavior in the interval 0.06≤ q ≤ 0.18 Ȧ −1; corresponding fractal dimension values Ds are listed in Table 3. The power law is not valid for q ≥ 0.18 Ȧ −1 as the scattering process in this regime relates to the structures of the size of individual atoms and consequently the two-phase approximation is no longer valid.59 The slopes of log I(q) vs log q plots in the Porod’s range were found to be between −4 and −3, indicating rough surfaces of the particle aggregates.59 The data in Table 3 indicate that the fractal dimensions of the silica networks increased with the introduction of POSS molecules. It is apparent that POSS molecules grafted onto silica particles increased the surface roughness and led to an increase of fractal dimension. In this context, the steric hindrance precludes grafting of two POSS molecules onto adjacent sites. Therefore, grafting by POSS molecules may have occurred uniformly on the silica particles. Of these, the POSS molecules bonded to the neck regions are responsible for mechanical reinforcement of the networks. 3.6. Reinforcement Mechanism. The large organic side groups on Si atoms in POSS molecules used in this study are 15368

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in compressive modulus and i-butyl-POSS provided the highest increase of hydrophobicity and the smallest change in density.

thought to have slowed down the condensation reactions of POSS molecules with Si−OH groups of silanes derived from TEOS. Thus, the Si atoms from POSS molecules possibly remained at the surfaces of primary particles resulting in a continuous or discontinuous coating layer on secondary particles. This in turn rendered the aerogels more hydrophobic as was inferred from the contact angle data. It is seen that i-butyl POSS modified the surface more effectively than other silanol POSS molecules, probably due to less steric hindrance.60,61 tri-POSS and cyclo-POSS show similar effects in terms of mechanical reinforcement and surface modification. However, a slightly higher contact angle is observed for cyclo-POSS which might be due to its less rigid side groups compared to phenyl side groups in tri-POSS and tetra-POSS. The increase of compressive modulus was smallest for tetra-POSS possibly due to a lesser number of POSS−silica covalent attachments by tetra-POSS molecules. This is also supported by the smaller increase of the contact angle values. The compressive modulus shows almost a 6-fold increase with tri-POSS compared to unmodified silica aerogels. The data in Table 1 showed that compressive modulus of tri-POSS reinforced aerogels varied with density as ∼ρ6.4, i-butyl-POSS reinforced aerogels as ∼ρ13.4, tetra-POSS reinforced aerogels as ∼ρ4.2, and cyclo-POSS reinforced aerogels as ∼ρ4.1. Previous efforts on reinforcement of aerogels using polymer cross-linkers at high concentration (50 wt % or higher) led to a 2 orders of magnitude increase in modulus33 but with large reduction of surface area and large increase of density. Reinforcements at lower loading level (5 wt %) of carbon nanofibers62 with no penalty of density and porosity showed tripling of compressive modulus compared to unreinforced silica aerogels. In the present case, however, the pregelation modification process using tri-POSS molecules led to significant reinforcement of silica aerogels (almost 6-fold increase in compressive modulus) with small increase of density and small sacrifice of surface area by effectively eliminating the diffusion step encountered in most polymer modification processes. Furthermore, the organic side groups of POSS molecules rendered the silica aerogels hydrophilic. More importantly, the method of making the aerogels was greatly simplified by eliminating the two washing steps before cross-linking and the diffusion step encountered in most polymer modification methods. This also reduced the amount of solvent needed for preparation of aerogels by at least half.63

■

ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables and a description of characterization methods along with relevant references. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Currently at PolyOne Corporation.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to express special thanks to Dr. Robert A. Weiss and Dr. Stephen Z. D. Cheng for allowing us to use the contact angle goniometer and SAXS equipment. This work was partially funded by the National Science Foundation under Award No. CMMI-1200484.

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4. CONCLUSIONS The study established that POSS molecules bearing Si−OH groups are good reinforcing agents of silica aerogels. The data presented in this paper indicates that the pearl-necklace structure of silica networks was preserved in the presence of POSS molecules. In addition, POSS molecules grafted onto silica networks via Si−OH groups as established from 13C and 29 Si NMR data and increased the fractal dimension of the silica networks and, in some cases, gave birth to additional micropores. More than 90% of POSS molecules initially added to silane precursors reacted onto silica networks. The aerogels exhibited small increases in density but significant increases in compressive modulus. The presence of organic side groups in POSS also significantly improved the hydrophobicity of treated aerogels as the water contact angle of the compressed aerogel disk changed from 27° for unmodified aerogels to 119° for T-i-butyl-POSS-10 aerogel. Of the three POSS molecules, tri-POSS presented the highest improvements 15369

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