Moisture-Resistant Polyimide Aerogels Containing Propylene Oxide

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Moisture-Resistant Polyimide Aerogels Containing Propylene Oxide Links in the Backbone Mary Ann B. Meador,* Marika Agnello, Linda McCorkle, Stephanie L. Vivod, and Nathan Wilmoth NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States ABSTRACT: Polyimide aerogels made using anhydridecapped oligomers from 4,4′-oxydianiline (ODA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) crosslinked with 1,3,5-tri(aminophenoxy)benzene (TAB) have been reported with very good mechanical properties but poor resistance to moisture. Replacing 50 mol % of the ODA with poly(propylene glycol)bis(2-aminopropyl ether) (PPG) with an average molecular weight of 230 g/mol in the oligomer backbone gives aerogels with water contact angles of 80°. The aerogels also absorb very little moisture on soaking in water. The aerogels also shrink less with increasing PPG concentration and therefore have significantly lower density and higher porosity than those made without PPG. Mechanical properties of the aerogels increased with increasing density, regardless of the polymer backbone. Brunauer−Emmett−Teller (BET) surface area of the aerogels studied ranged from 300 to 400 m2/g, depending mainly on PPG concentration. The high moisture resistance makes them promising materials for substrates for lightweight antennas as well as insulation for a variety of applications. KEYWORDS: aerogel, polyimide, mesoporous, cross-linked, insulation, hydrophobic

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cross-links18 to fabricate the aerogels. For the most part, properties of the polyimide aerogels are dominated by the backbone chemistry of the oligomers and not the cross-linker. For example, use of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) in combination with 2,2′-dimethylbenzidine (DMBZ) in the oligomer backbone provides aerogels with a higher modulus at lower density due to the stiffness of the backbone, while 4,4′-oxydianiline (ODA) used with BPDA in the backbone provides a lower modulus material which results in more flexible thin films. A combination of 50 mol % DMBZ and 50 mol % ODA used as diamine in a backbone with BPDA provides moisture resistance and still affords enough flexibility in the backbone to make foldable, thin films. Moisture resistance is needed in the aerogels because the porous structure will typically not survive intact once it is wet and then redried. In this paper, we examine the effect of using more flexible poly(propylene glycol) (PPG) units in the polyimide backbone to provide both greater pliability to the aerogel and improved moisture resistance. For this purpose, the PPG chosen for the study is poly(propylene glycol)bis(2-aminopropyl ether) with an average molecular weight of 230 g/mol (approximately 2.68 propylene glycol repeat units per molecule). PPG has been used previously to alter properties of dense polyimides in this

INTRODUCTION Aerogels are low density solids with typically higher surface area and small pore sizes, leading to interesting properties1,2 which make them suitable for a variety of aerospace applications such as insulation for astronaut suits and habitats for Mars missions,3,4 launch vehicles,5 inflatable decelerators for entry, descent, and landing operations,6 and low dielectric substrates for lightweight antennas.7 Polymer aerogels have similar low densities, low dielectric constants, and low thermal conductivities compared to silica aerogels, but typically mechanical properties are far superior. For this reason, there is increasing interest in polymer aerogels for many applications such as thermal insulation, low dielectric substrates for lightweight antennas, and absorbents for environmental cleanup. Polymer aerogels have been fabricated with many different backbone chemistries, including polyurea,8,9 polystyrene,10 polyamide,11 and polyimide. Of these, the most versatile may be the polyimide (PI) aerogels fabricated by cross-linking either amine-terminated or anhydride-terminated oligomers, since these can be fabricated as flexible, foldable thin films or as stiff, thicker substrates. Anhydride-terminated oligomers have been cross-linked with polyamines such as 1,3,5-triaminophenylbenzene (TAB), 1 2 octa(aminophenyl)silsesquioxane (OAPS),13,14 2,4,6-tris(4-aminophenyl)pyridine (TAPP),15 and 1,3,5-tris(aminophenyl)benzene (TAPB)16 to form gels which were dried into aerogels. More recently, amine-capped oligomers have been demonstrated with 1,3,5-benzenetricarbonyl trichloride (BTC)17 or polymaleic anhydride © XXXX American Chemical Society

Received: August 15, 2016 Accepted: October 6, 2016

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DOI: 10.1021/acsami.6b10248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Cross-Linked Polyimide Aerogels Using a Mixture of Aliphatic (PPG) and Aromatic (ODA) as the Diamine

Table 1. Properties of the Different Polyamide Aerogels Investigated in This Study sample

n

PPG (mol %)

polymer conc (wt %)

density (g/cm3)

shrinkage (%)

porosity (%)

surface area (m2/g)

modulus (MPa)

stress at 10% strain (MPa)

water uptake (wt %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

20 10 10 30 10 30 20 20 20 30 10 30 10 20 30 20 20 20 20 30 10 30 10

50 25 25 37.5 50 50 25 37.5 37.5 25 37.5 50 50 37.5 25 37.5 37.5 37.5 0 0 0 0 0

8.5 10 7 8.5 10 7 8.5 7 10 10 8.5 10 7 8.5 7 8.5 8.5 8.5 8.5 10 10 7 7

0.071 0.119 0.098 0.087 0.089 0.067 0.113 0.078 0.104 0.136 0.088 0.112 0.056 0.082 0.098 0.096 0.087 0.088 0.153 0.190 0.226 0.143 0.145

7.4 11.8 14.3 9.4 7.0 7.4 15.3 9.2 9.4 16.2 9.7 8.1 7.4 9.0 16.1 9.8 10.0 9.5 17.2 19.5 23.9 20.2 20.9

95.6 92.5 93.5 94.2 93.6 95.5 92.5 94.9 92.8 90.6 93.9 92.2 96.2 94.4 93.9 93.4 94.0 93.9 89.4 86.7 84.2 89.8 89.9

345 405 396 377 302 341 359 389 363 381 352 338 354 364 379 369 382 374 340 327 337 372 362

5.6 62.0 4.5 8.4 24.2 a 33.1 5.7 11.1 29.8 a 12.4 2.6 6.3 17.2 6.2 11.7 8.0 20. 7 36.1 36.2 14. 8 13.2

0.16 0.52 0.27 0.24 0.31 a 0.41 0.19 0.41 0.63 a 0.31 0.11 0.22 0.30 0.26 0.26 0.27 0.84 1.31 1.79 0.60 0.63

21 673 748 1071 18 33 657 820 850 660 1005 a 46 1045 980 1076 1093 1066 a a a a a

a

Not measured.

way.19 Fabrication of the aerogels was carried out as shown in Scheme 1 by replacing up to 50% of the ODA in the backbone

with PPG and cross-linking with TAB. The number of repeat units, n, and the total polymer concentration in solution were B

DOI: 10.1021/acsami.6b10248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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polymer concentrations from 7 to 10 wt % and oligomers with repeat units, n, varying from 10 to 30, so that the effect of these variables on the measured properties can be assessed. Variables used to make each aerogel formulation in the study and the measured properties are shown in Table 1. This data was used in statistical analysis to create empirical models for each response as a function of the variables tested. All of the aerogels in the study were made as cylindrical monoliths using 20 mm diameter syringe molds. The monoliths were rigid and yellow in color. Solid NMR spectra of two representative formulations (sample 5 made using 50 mol % PPG and sample 2 using 25 mol % PPG) are shown in Figure 1.

also varied to study these effects on moisture uptake, mechanical properties, and pore structure of the aerogels.

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EXPERIMENTAL SECTION

Materials. Anhydrous N-methylpyrrolidinone (NMP), poly(propylene glycol)bis(2-aminopropyl ether) with an average molecular weight of 230 g/mol (PPG), acetic anhydride, and triethylamine were purchased from Sigma-Aldrich (St. Louis, MO). 4,4′-Oxydianiline (ODA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) were obtained from Chriskev, Inc. (Lenexa, KS). 1,3,5-Triaminophenoxybenzene (TAB) was obtained from Triton Systems (Chelmsford, MA). All reagents were used without further purification. General. Thermogravimetric analysis (TGA) was run on a TA Instruments Q500 with a ramp rate of 5 °C/min under nitrogen. Helium pycnometry was carried out on a Micrometrics Accupyc 1340 gas pycnometer. Samples were imaged by scanning electron microscopy (SEM) on a Hitachi S-4700 field emission microscope after sputter-coating the samples with platinum. Solid state 13C NMR was performed on a Bruker Avance-300 spectrometer with crosspolarization and magic angle spinning (11 kHz). The resulting spectra were externally referenced to the carbonyl of glycine (176.1 ppm relative to tetramethylsilane). Surface areas were measured with an ASAP 2000 surface area/pore distribution analyzer (Micrometrics Corp.) after the samples were degassed at 80 °C under vacuum for 8 h. FT-IR spectra were obtained on a Nicolet Nexus 470 FT-IR spectrometer. Compression tests were run on the aerogels as previously described12 according to ASTM D695-10 using a model 4505 Instron load frame. Contact angle measurements were made using a Rame goniometer with Drop Image, version 1.5.04, after sanding the aerogel surface smooth with 400-grit sandpaper. The effects of the n, PPG concentration, and concentration of polymer in solution on the resulting characteristics of the aerogel materials were analyzed using Design Expert Version 8.1 (Stat-Ease Inc., Minneapolis, MN). Multiple linear regression analysis was employed to develop empirical models that describe the effects of these variables on properties. Preparation of PI Aerogels. Aerogels were fabricated similar to previous studies of TAB cross-linked aerogels with some modifications as shown in Scheme 1. Anhydride-terminated oligomers were fabricated by combining n diamine with (n + 1) dianhydride in NMP solution, where n is the formulated number of repeat units in the oligomer backbone. As shown in Table 1, up to 50 mol % of PPG was used in combination with ODA (100 − PPG mol %) as the diamine in the oligomer which was cross-linked with TAB. As an example, the synthesis of sample 1 with 20 repeat units, 50 mol % PPG, and a polymer concentration of 8.5 wt %, according to Table 1, is as follows: To a solution of 1.55 g (6.4 mmol) of PPG and 1.278 g (6.4 mmol) of ODA in 56 mL of NMP was added 3.943 g (13.4 mmol) of BPDA. The mixture was stirred for 15 min until dissolved to form polyamic acid oligomer terminated with anhydride groups. The TAB cross-linker (0.170 g, 0.425 mmol) was separately dissolved in 10.56 mL of NMP, added to the polyamic acid oligomer solution, and stirred until a homogeneous solution was attained. Before gelation, 10.13 mL of acetic anhydride and 1.87 mL of triethylamine were added to promote imidization. The resulting solution was poured into syringe molds and allowed to gel. Typically the gels which form in 60 min were allowed to age for 24 h, after which the NMP reaction solvent in the gel was replaced with acetone, by washing first with 25% acetone and 75% NMP, followed by 75% acetone and 25% NMP and then four consecutive washes of pure acetone. Drying through supercritical fluid extraction using liquid CO2 followed by vacuum drying at 100 °C for 24 h produced the aerogel with density of 0.071 g/cm3. FTIR (neat, cm−1): 3315 (br), 1651, 1607, 1537, 1486, 1417, 1306, 1246, 1195, 1167, 1087. Solid 13C NMR (ppm): 16.1, 46.5, 74.7, 125.3, 130.4, 143.9, 154.1, 166.3.

Figure 1. 13C CP-MAS spectra of representative aerogels from the study, including sample 5 made using 50 mol % PPG (top) and sample 2 made using 25 mol % PPG (bottom).

As expected, the aerogels all contained peaks at 166 ppm (imide carbonyl), at 154 ppm (aromatic ether carbons of ODA and TAB), at 143.9 ppm (aromatic carbons with nitrogen attached) as well as broad peaks at 130 and 125 ppm (all other aromatic carbons). In addition, those made using PPG also contained peaks at 74.7 ppm (methylene adjacent to nitrogen), 46.5 ppm (methylene adjacent to oxygen), and 16 ppm (methyl) all from the glycol links. As seen in Figure 1, the peaks from PPG are smaller in the bottom spectrum as expected, since this aerogel contained half as much PPG. Figure 2 shows FTIR spectra of two representative formulations from the study made using 50, 37.5, and 25 mol % PPG. In both spectra, peaks at 1375, 1772, and 1715 cm−1 indicate the presence of imide rings and the broad peak at 1100−1120 cm−1 is from the propylene oxide links, while the peak at 1504 cm−1 is characteristic of the para-substituted

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RESULTS AND DISCUSSION Polyimide aerogels with diamine fractions ranging from 0 to 50 mol % PPG were fabricated according to Scheme 1, using

Figure 2. FT-IR spectra of representative samples from the study. C

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Figure 3. Graphs of empirical models for (a) density, (b) shrinkage, and (c) porosity.

Figure 4. Scanning electron micrographs of n = 30 polyimide aerogels made using 7 wt % polymer with (a) no PPG, (b) 25 mol % PPG, and (c) 50 mol % PPG and 10 wt % polymer with (d) no PPG, (e) 25 mol % PPG, and (f) 50 mol % PPG.

aerogels containing PPG in the backbone also had an onset of decomposition occurring between 425 and 450 °C, which is somewhat lower than aerogels made using only ODA in the backbone (about 550 °C). Densities of all of the formulations of aerogel made in the study ranged from 0.07 to 0.23 g/cm3, depending mostly on the amount of PPG and the polymer concentration used to fabricate the aerogels. An empirical model for density (standard error = 0.009 g/cm3, R2 = 0.96) is shown in Figure 3a. As expected, density increases as polymer concentration in

phenyl rings of ODA. No major peaks from unreacted anhydride (1850 cm−1) amide (1535 cm−1), acid (1660 cm−1), or isoimide (1810 and 980 cm−1) are present, indicating that imidization is complete. Thermogravimetric analysis (TGA) of the PPG containing aerogels all contained a small amount of weight loss (1−2%) at 100−150 °C, indicating a small amount of retained solvent, despite vacuum drying for 24 h after supercritical fluid extraction. Evidently, solvents interact more strongly with the PPG units, making them harder to remove completely. The D

DOI: 10.1021/acsami.6b10248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Results of nitrogen sorption tests, including (a) empirical model for surface area graphed vs PPG fraction and polymer concentration and (b) pore size distribution of select samples from the study.

Figure 6. Results of compression tests of the aerogels in the study, including (a) stress−strain curves for aerogels made using 10 wt % polymer and n = 30 at three PPG concentrations, (b) modulus graphed vs PPG fraction and n, (c) stress at 10% strain graphed vs PPG fraction and polymer concentration, and (d) log−log plots of modulus and stress at 10% strain vs density.

too close, thereby reducing shrinkage during processing. It is seen in Figure 3c in the empirical model for porosity (standard error = 0.82, R2 = 0.94) that porosity decreases with increasing polymer concentration and increases with increasing PPG concentration in opposition to density and shrinkage. This is as expected since increasing polymer concentration increases the amount of solid phase, while the reduction in shrinkage due to PPG concentration increases the overall size of the as fabricated aerogel. The number of repeat units, n, has a small though significant synergistic effect on density and porosity. At 0 mol % PPG, density slightly decreases and porosity slightly increases with increasing n, while the opposite is true at 50 mol % PPG concentration.

solution is increased. However, PPG concentration had the largest effect on density. In fact, at each level of polymer concentration, it is seen that density is decreased by about half going from no PPG in the oligomer backbone to 50 mol % PPG. This decrease in density is due to the large effect of PPG concentration on shrinkage occurring during aerogel processing as seen in the graph of the empirical model for shrinkage (standard error = 1.49%, R2 = 0.93) shown in Figure 3b. The presence of 50 mol % PPG in the oligomer backbone decreased shrinkage to only 7−8% compared to about 20% for those aerogels containing only ODA. It is possible that the methyl groups on the PPG units prevent polymer chains from packing E

DOI: 10.1021/acsami.6b10248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. Comparison of water uptake between aerogels with different amounts of PPG in the backbone, including (a) graph of the empirical model for water uptake and (b) a bar graph comparing three concentrations of PPG.

modulus typically scales with density in a family of aerogels and density increased with increasing polymer concentration and decreased with increasing PPG concentration as well. It might be expected that increasing PPG concentration would decrease modulus more since the aliphatic PPG units might be more flexible than ODA in the polymer backbone. However, the log− log plot of modulus vs density shown in Figure 6d demonstrates that modulus is strongly dependent on density for all of the aerogels in the study regardless of backbone chemistry or n. The empirical model for compressive strength taken as the stress at 10% strain (log standard error = 0.43, R2 = 0.98) is shown in Figure 6c. This plot shows that only PPG fraction and polymer concentration have a significant effect on compressive strength. As before with modulus, this is largely arising from the effect of these variables on density as shown in the log−log plot of stress at 10% strain vs density (Figure 6d). Moisture resistance is an important property for aerogels whether used as a low dielectric substrate or as insulation, since absorption of liquid into the pores can cause them to collapse, especially on redrying. Aerogels in the study made using 50 mol % PPG have water contact angles of 80°−90° due to the greater hydrophobicity of the poly(propylene oxide) groups present in higher concentration. All other aerogels in the study absorbed the water droplet over a matter of minutes. Previously studied aerogels with 100 mol % ODA (0 mol % PPG) as the amine in the backbone readily absorb moisture and shrivel when dried.14 To quantify the moisture resistance of aerogels made using 25 to 50 mol % PPG, they were soaked for 24 h in water at room temperature, and the amount of water absorbed was measured by weighing the samples before and after soaking. The empirical model (log standard error = 0.29, R2 = 0.81) shown in Figure 7a reveals that only PPG concentration and polymer concentration have a significant effect on water uptake. There is a small though significant effect of polymer concentration that is probably due to the fact that the porosity is higher for these aerogels. As seen in bar graphs comparing water uptake for aerogels made using 10 wt % polymer (Figure 7b), water uptake for aerogels made using 50 mol % PPG absorb less than 20% w/w water, which accounts for only 1.5% of the porosity, while aerogels with 25 mol % PPG absorb over 600% w/w water, which accounts for over 90% of the porosity. This must be due to the increasing concentration of hydrophobic poly(propylene oxide) groups in the polymer backbone. Interestingly, water absorption is slightly higher for aerogels made using 37.5 mol % PPG, presumably because

Scanning electron micrographs (SEM) of representative aerogels from the study are shown in Figure 4. The morphology is similar to that seen for other polyimide aerogels with the polymer appearing as an assembly of fibrous strands. Figures 4a and 4d are of aerogels made using only ODA in the backbone and appear to have much thinner strands than those made using 25 mol % PPG (Figures 4b and 4e) and those made using 50 mol % PPG (Figures 4c and 4f). Typically, a hierarchical pore structure is seen when two different diamines such as ODA and DMBZ are allowed to react in a random fashion to fabricate the aerogels, indicating that there is macrophase separation occurring. Interestingly, there is no evidence of macrophase separation here, suggesting that the PPG and ODA moieties are more compatible. SEMs in the top row are for polymers made using 7 wt % solutions, while the bottom row are made using 10 wt % solutions and appear to be less porous, as expected. Nitrogen sorption analysis was run on all of the formulations in the study and analyzed using the Brunauer−Emmett−Teller (BET) method.20 Surface areas only vary from 300 to 400 m2/g across the whole study. Figure 5a shows an empirical model for the BET surface area (standard error = 14.18, R2 = 0.70). The number of repeat units had no significant effect on the surface area above and beyond random error, while the polymer concentration had a small though significant effect. The PPG concentration again had a large second order effect on surface area, with the surface being highest when PPG concentration is at 25 mol %. Figure 5b shows pore diameter graphed versus pore volume for aerogels made using 10 wt % polymer concentration and n = 30 to compare those made with different PPG concentrations. At 25 mol % PPG concentration, the pore size distribution is slightly broader than for those aerogels made using 0 mol % PPG. At 50 mol % the pore size distribution is broader and the average pore diameter is larger. The curve also begins to resemble a bimodal distribution. Pore size distributions made using other n and lower polymer concentration follow these same trends. Compression tests were carried out on all of the formulations in the study. Typical stress−strain curves are shown in Figure 6a for three different levels of PPG concentration. Young’s modulus is taken as the initial slope of the stress−strain curves and the compressive strength is taken as the stress at 10% strain. The empirical model for modulus (log standard error = 0.19, R2 = 0.76) is shown in Figure 6b. As shown, modulus increases with increasing polymer concentration and decreases with increasing PPG concentration. This is expected since F

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ator Systems Technology Conference and Seminar; 23−26 May 2011; Dublin; Ireland; American Inst. of Aeronautics and Astronautics: Reston, VA, 2011; 2011-01-2051. (7) Meador, M. A. B.; Wright, S.; Sandberg, A.; Nguyen, B. N.; Van Keuls, F. W.; Mueller, C. H.; Rodriquez-Solis, R.; Miranda, F. A. Low Dielectric Polyimide Aerogels as Substrates for Lightweight Patch Antennas. ACS Appl. Mater. Interfaces 2012, 4, 6346−6353. (8) Leventis, N.; Sotiriou-Leventis, C.; Chandrasekaran, N.; Mulik, S.; Larimore, Z. J.; Lu, H.; Churu, G.; Mang, J. T. Multifunctional Polyurea Aerogels from Isocyanates and Water. A Structure−Property Case Study. Chem. Mater. 2010, 22, 6692−6710. (9) Shinko, A.; Jana, S. C.; Meador, M. A. B. Crosslinked Polyurea Aerogels with Controlled Porosity. RSC Adv. 2015, 5, 105329− 105338. (10) Daniel, C.; Sannino, D.; Guerra, G. Syndiotactic Polystyrene Aerogels: Adsorption in Amorphous Pores and Absorption in Crystalline Cavities. Chem. Mater. 2008, 20, 577−582. (11) Williams, J. C.; Meador, M. A. B.; McCorkle, L.; Mueller, C.; Wilmoth, N. Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides. Chem. Mater. 2014, 26, 4163−417. (12) Meador, M. A. B.; Malow, E. J.; Silva, R.; Wright, S.; Quade, D.; Vivod, S. L.; Guo, H.; Guo, J.; Cakmak, M. Mechanically Strong, Flexible Polyimide Aerogels Cross-Linked with Aromatic Triamine. ACS Appl. Mater. Interfaces 2012, 4, 536−544. (13) Guo, H.; Meador, M. A. B.; McCorkle, L.; Quade, D. J.; Guo, J.; Hamilton, B.; Cakmak, M.; Sprowl, G. Polyimide Aerogels CrossLinked through Amine Functionalized Polyoligomeric Silsesquioxane. ACS Appl. Mater. Interfaces 2011, 3, 546−552. (14) Guo, H.; Meador, M. A. B.; McCorkle, L.; Quade, D. J.; Guo, J.; Hamilton, B.; Cakmak, M. Tailoring Properties of Cross-linked Polyimide Aerogels for Better Moisture Resistance. ACS Appl. Mater. Interfaces 2012, 4, 5422−5429. (15) Shen, D.; Liu, J.; Yang, H.; Yang, S. Highly Thermally Resistant and Flexible Polyimide Aerogels Containing Rigid-Rod Biphenyl, Benzimidazole and Triphenylpyridine Moieties: Synthesis and Characterization. Chem. Lett. 2013, 42, 1545−1547. (16) Kawagishi, K.; Saito, H.; Furukawa, H.; Horie, K. Superior Nanoporous Polyimides via Supercritical CO2 Drying of Jungle Gym Type Polyimide Gels. Macromol. Rapid Commun. 2007, 28, 96−100. (17) Meador, M. A. B.; Alemán, C. R.; Hanson, K.; Ramirez, N.; Vivod, S. L.; Wilmoth, N.; McCorkle, L. Polyimide Aerogels with Amide Cross-Links: A Low Cost Alternative for Mechanically Strong Polymer Aerogels. ACS Appl. Mater. Interfaces 2015, 7, 1240−1249. (18) Guo, H.; Meador, M. A. B.; McCorkle, L.; Scheiman, D.; McCrone, J. D.; Wilkewitz, B. Poly(maleic anhydride) cross-linked polyimide aerogels: synthesis and properties. RSC Adv. 2016, 6, 26055−26065. (19) Baldwin, A. F.; Ma, R.; Wang, C.; Ramprasad, R.; Sotzing, G. A. Structure−Property Relationship of Polyimides Based on Pyromellitic Dianhydride and Short-Chain Aliphatic Diamines for Dielectric Material Applications. J. Appl. Polym. Sci. 2013, 130 (2), 1276−1280. (20) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309−319.

these are higher in porosity and have larger average pore sizes than those made with 25 mol % PPG. Although the aerogels made using 50 mol % PPG are still higher in porosity and have larger pores than those made using 37.5 mol % PPG, that increase in PPG makes the backbone hydrophobic enough to resist moisture uptake. The small amount of water absorption in the aerogels made using 50 mol % PPG is also not enough to cause pore collapse as aerogels exhibit no change before and after soaking and after redrying.

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CONCLUSION Polyimide aerogels made using anhydride-capped oligomers from ODA and BPDA cross-linked with 1,3,5-tri(aminophenoxy)benzene (TAB) have been reported with very good mechanical properties but have poor resistance to moisture. Replacing 50 mol % of ODA with PPG with an average molecular weight of 230 g/mol in the oligomer backbone gives aerogels with water contact angles of around 80° and very low moisture absorption when soaked in water. The aerogels do not collapse during soaking or subsequent drying, making them good candidates for use as substrates for lightweight antennas as well as insulation for a variety of applications. The aerogels also shrink less with increasing PPG concentration and therefore have significantly lower density and higher porosity than those made without PPG. The BET surface area of the aerogels studied ranged from 300 to 400 m2/g, depending mainly on PPG concentration. Mechanical properties of the aerogels increased with increasing density, regardless of the amount of PPG in the polymer backbone.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.B.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for support from the National Aeronautic and Space Administration’s Space Technology Mission Directorate Game Changing Development Program. We also thank Daniel Scheiman (Ohio Aerospace Institute) for thermal analysis and FT-IR and Baochau Nguyen (Ohio Aerospace Institute) for solid NMR and nitrogen sorption measurements.

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REFERENCES

(1) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243−4265. (2) Du, A.; Zhou, B.; Zhang, Z.; Shen, J. A Special Material or a New State of Matter: A Review and Reconsideration of the Aerogel. Materials 2013, 6, 941−968. (3) Paul, H. L.; Diller, K. R. Comparison of Thermal Insulation Performance of Fibrous Materials for the Advanced Space Suit. J. Biomech. Eng. 2003, 125, 639−647. (4) Tang, H. H.; Orndoff, E. S.; Trevino, L. A. Thermal Performance for Space Suit Elements with Aerogel Insulation for Moon and Mars Exploration; 36th International Conference on Environmental Systems, Norfolk, VA, July 17−20, 2006; American Institute of Aeronautics and Astronautics: Reston, VA, 2006; 2006-01-2235. (5) Fesmire, J. E. Aerogel insulation Systems for Space Launch Applications. Cryogenics 2006, 46, 111−117. (6) DelCorso, J. A.; Cheatwood, F. M.; Bruce, W. E., III; Hughes, S. J.; Calomino, A. M. Advanced High-Temperature Flexible TPS for Inflatable Aerodynamic Decelerators; 21st AIAA Aerodynamic DecelerG

DOI: 10.1021/acsami.6b10248 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX