Highly Porous, Rigid-Rod Polyamide Aerogels with Superior

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Highly Porous, Rigid-Rod Polyamide Aerogels with Superior Mechanical Properties and Unusually High Thermal Conductivity Jarrod C. Williams,† Baochau N. Nguyen,‡ Linda McCorkle,‡ Daniel Scheiman,‡ Justin S. Griffin,§ Stephen A. Steiner, III,§ and Mary Ann B. Meador*,† †

NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142, United States § Aerogel Technologies, LLC, 270 Dorchester Avenue, Boston, Massachusetts 02127, United States ‡

ABSTRACT: We report here the fabrication of polyamide aerogels composed of poly-p-phenylene-terephthalamide, the same backbone chemistry as DuPont’s Kevlar. The all-para-substituted polymers gel without the use of cross-linker and maintain their shape during processingan improvement over the meta-substituted cross-linked polyamide aerogels reported previously. Solutions containing calcium chloride (CaCl2) and para-phenylenediamine (pPDA) in N-methylpyrrolidinone (NMP) at low temperature are reacted with terephthaloyl chloride (TPC). Polymerization proceeds over the course of 5 min resulting in gelation. Removal of the reaction solvent via solvent exchange followed by extraction with supercritical carbon dioxide provides aerogels with densities ranging from 0.1 to 0.3 g/cm3, depending on the concentration of calcium chloride, the formulated number of repeat units, n, and the concentration of polymer in the reaction mixture. These variables were assessed in a statistical experimental study to understand their effects on the properties of the aerogels. Aerogels made using at least 30 wt % CaCl2 had the best strength when compared to aerogels of similar density. Furthermore, aerogels made using 30 wt % CaCl2 exhibited the lowest shrinkage when aged at elevated temperatures. Notably, whereas most aerogel materials are highly insulating (thermal conductivities of 10−30 mW/m K), the polyamide aerogels produced here exhibit remarkably high thermal conductivities (50− 80 mW/(m K)) at the same densities as other inorganic and polymer aerogels. These high thermal conductivities are attributed to efficient phonon transport by the rigid-rod polymer backbone. In conjunction with their low cost, ease of fabrication with respect to other polymer aerogels, low densities, and high mass-normalized strength and stiffness properties, these aerogels are uniquely valuable for applications such as lightweighting in consumer electronics, automobiles, and aerospace where weight reduction is desirable but trapping of heat may be undesirableapplications where other polymer aerogels have to date otherwise been unsuitablecreating new opportunities for commercialization of aerogels. KEYWORDS: polyamide, porous polymers, aerogels, mesoporous, thermal conductivity

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mine.6,7 Similar to inorganic alumina and silica aerogels, these phenolic aerogels possess high surface areas, low densities, and low thermal conductivity but, like most aerogels of the time, are brittle and have poor mechanical properties. In the past decade, polymer aerogels of many types have been synthesized by adapting the sol−gel processes used to make aerogels to a variety of polymer backbones. Gels formed

INTRODUCTION Aerogels are unique lightweight, high-surface-area materials with great utility as thermal superinsulators, low-dielectricconstant substrates, low-density core materials for sandwich structures, and highly active and functional chemical separation media.1−3 Aerogels were first produced by Kistler who coined the name after using supercritical fluid extraction to remove the liquid from a variety of gels, including silica, gelatin, agar, cellulose, and nitrocellulose and replacing it with air.4,5 Pekala and others created the first synthetic polymer aerogels based on polycondensation of formaldehyde with resorcinol or mela© XXXX American Chemical Society

Received: October 14, 2016 Accepted: December 19, 2016

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DOI: 10.1021/acsami.6b13100 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

backbone identical to DuPont’s Kevlar, a high-stiffness fiber used in lightweight fiber/resin composites and bullet-proof vests, which might be expected to be even higher in modulus.27 However, using the combination pPDA and TPC under the conditions previously described resulted in precipitation of short oligomers prior to gelation. It is known that reacting TPC with pPDA in the presence of calcium chloride or lithium chloride provides a method for keeping the polymer in solution long enough to obtain higher molecular weight polymers.28−30 In this study, we utilized this same technique of incorporating calcium chloride in solution with pPDA and TPC to fabricate polyamide aerogels. Gelation occurs too rapidly when crosslinkers are added; hence these aerogels are physically bound rather than covalently cross-linked, as shown in Scheme 1. We

by crystallization of linear polymers in solution are typically prepared by cooling hot solutions of polymers such as polyvinylidene fluoride (PVDF),8 poly(4-methyl-pentene-1) (i-P4MP1),9 and syndiotactic polystyrene (s-PS).10−12 Factors such as solvent choice and cooling rate dictate the types of crystalline morphologies and the amount of fibrous, amorphous regions that are present. Because of the controlled combination of crystalline and amorphous regions and the high porosity, syndiotactic polystyrene (s-PS) aerogels are valued as absorbents for volatile organic compounds. Aerogels of linear polymers, however, typically exhibit low mechanical strength and are generally fragile, limiting their utility for many commercial applications. Interest in improving the mechanical stability of aerogels has led to the development of robust covalently cross-linked aerogels composed of polymers such as polyurea,13,14 polyurethane,15 polyimide,16 and polyamide.17,18 In recent work, processes for fabrication of multifunctional covalently cross-linked gels through formation of telechelic oligomers that gel after addition of a suitable cross-linker have been demonstrated and result in high mass-normalized strength and stiffness properties. This approach gives polymer aerogels with tailorable properties that depend on the oligomer backbone and the cross-linker. For example, polyimide aerogels have been fabricated as thin films with good moisture resistance,19 as mechanically strong materials,20 and with low dielectric constants,21 and have been demonstrated as effective low-k substrates for lightweight antennas.22 To fabricate cross-linked polyimide or polyamide aerogels, reactions are carried out in polar aprotic solvents at room temperature or lower and entail the condensation of bisnucleophiles with biselectrophiles to form step-growth oligomers. Control of the stoichiometric balance between the nucleophiles and electrophiles allows for control over the number of repeat units, n, of the oligomers formed in solution. Furthermore, it is possible to form oligomers end-capped with either two electrophilic or two nucleophilic sites based on the molar excess of bisnucleophile or biselectrophile. Cross-linkers reactive with electrophilic end groups such as anhydrides or isocyanates have included aromatic triamines, such as 1,3,5triaminophenoxybenzene (TAB), 1 5 , 2 0 , 2 1 1,3,5-tris(aminophenyl)benzene,23 2,4,6-tri(aminophenyl)pyridine,24 and octaaminophenylsilsesquioxane (OAPS).16,19 If the oligomers are capped with nucleophiles, such as amines, a reagent with three or more electrophilic moieties, such as 1,3,5benzenetricarbonyl trichloride (BTC)17,25 or poly(maleic anhydride),26 can be used as a cross-linker to react with the end groups of the oligomers to create a three-dimensional network that then forms the structure of the gel. Removal of the reaction solvent by supercritical fluid extraction provides lightweight, high-surface-area polymer aerogels with mechanical properties that are superior to noncovalently formed polymer (e.g., physical gels in which the polymer chains are bound by weak molecular interactions) and inorganic aerogels. Polyamide aerogels produced as described above using the inexpensive monomers isophthaloyl chloride (IPC) and pphenylenediamine (pPDA) and cross-linked with BTC had the highest compressive Young’s moduli reported to date for a polymer aerogel, compared on a same-density basis.17 Furthermore, these linear cross-linked materials could be made without the use of an inert atmosphere, unlike previously reported polyamide aerogels made using isocyanates.18 It should be expected that using para-substituted teraphthaloyl chloride, TPC, instead of IPC would result in a polymer

Scheme 1. Synthesis of Polyamide Aerogels Based on ParaSubstituted Monomers

also varied the rate of gelation by varying the formulated number of repeat units, n, in the oligomer as well as the calcium chloride concentration and the total concentration of polymer in solution. The effect of changing these parameters on the final properties of the aerogels is discussed.

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EXPERIMENTAL SECTION

Materials. Anhydrous N-methylpyrrolidinone (NMP), p-phenylenediamine (pPDA), anhydrous calcium chloride (CaCl2), and terephthaloyl chloride (TPC) were purchased from Sigma-Aldrich (St. Louis, MO). All reagents were used without further purification. General procedures for characterization are as described previously.17 Synthesis of the polyamide aerogels was carried out as shown in Scheme 1, by combining pPDA and TPC in NMP in the presence of CaCl2 and allowing gelation to occur, followed by solvent exchange into ethanol and supercritical CO2 extraction. A statistical experimental design study was carried out with CaCl2 concentration, the formulation number of repeat units in the polymer backbone, and the total polymer concentration in solution varied as shown in Table 1. A typical example, run 10 from Table 1, made using 30 wt % CaCl2, n = 40, 7.5 wt % total polymer is as follows: A solution of CaCl2 (2.25 g, 20.27 mmol) and p-PDA (3.45 g, 31.90 mmol) in NMP (89.98 mL) was prepared and cooled to 0 °C using an ice water bath. Upon cooling, an opaque suspension formed. Solid TPC (6.32 g, 31.12 mmol) was added at which point the suspension was dissolved again and a transparent yellow solution was formed. After several minutes of stirring, the solution became cloudy as the viscosity increased. After several more minutes of stirring, the thick suspension was poured into syringe molds prepared by removing the ends from 20 mL syringes. B


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ACS Applied Materials & Interfaces Table 1. Formulations of Polyamide Aerogel Studied with Measured Propertiesa run

repeat units, n

polymer conc. (wt %)

CaCl2 conc. (%)

density (g/cm3)

porosity (%)

shrinkage (%)

surface area (m2/g)

modulus (MPa)

stress @ 10% strain (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

60 40 20 20 20 40 40 60 40 40 60 20 40 60 40 40 60 20

5 5 7.5 5 10 7.5 7.5 7.5 10 7.5 5 5 7.5 10 7.5 7.5 10 10

40 30 30 20 40 20 30 30 30 30 20 40 30 20 30 30 40 20

0.143 0.106 0.152 0.117 0.265 0.164 0.164 0.155 0.226 0.158 0.124 0.097 0.158 0.206 0.160 0.156 0.286 0.190

90.7 93.0 89.6 92.0 81.8 88.7 88.8 89.5 84.5 89.4 92.0 93.9 89.4 85.6 89.7 89.7 80.5 87.3

27.5 21.3 20.3 24.9 25.3 21.5 21.9 19.7 22.7 20.6 25.2 17.6 20.7 19.5 20.9 20.9 28.1 18.3

295 249 285 260 290

57 17 39 2.0 160 7 29 64 63 54 0.5 26 26 25 70 43 83 12

1. 0.2 0.5 0.04 3 0.2 0. 6 0.7 1.5 0.6 0.04 0.3 0.6 0.6 0.7 0.6 3 0.5

259 240 286 247 215 258 248 281 278 274 274 250

a Note that polymer concentration is given as the total theoretical weight of polymer in solution, whereas the CaCl2 concentration is the based on the weight of the polymer.

Figure 1 shows solid 13C NMR spectra of three different formulations of polyamide aerogel from the study. Figure 1

Once covered with parafilm, the syringe molds and their contents were allowed to sit overnight at room temperature. The resulting yellow cylindrical monoliths were removed from the syringe molds and allowed to soak in ethanol for 5 days with the solvent being removed and replaced with fresh ethanol each day. Once the solvent exchange process was complete, the cylinders were subjected to supercritical drying using CO2. The monoliths were then dried in vacuum at 70 °C overnight, giving off-white aerogels with density of 0.15 g cm3. FT-IR (cm−1): 3321 (br), 1645, 1543, 1514, 1406, 1318, 1260. Solid 13C NMR (ppm): 166.6, 135.5, 128.8, 124.0. Larger rectangular samples for thermal conductivity were also prepared by pouring the sol prepared the same way into rectangular polyethylene molds and allowing the sol to gel. Large gels were sealed in a container with a small amount of NMP in the bottom to minimize water absorption and solvent evaporation. After standing overnight, the large rectangular monoliths were then subjected to the same solvent exchange and drying routine as the cylindrical samples.

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RESULTS AND DISCUSSION Polyamide aerogels were made according to Scheme 1 from pPDA and TPC using calcium chloride to keep the polyamide in solution until gel formation. Three variables were investigated in the study to assess their effect on the resulting aerogel properties. The amount of polymer (defined as the weight of the monomers in solution minus the byproducts of the condensation reaction) in solution was varied from 5 to 10 wt % in order to vary the density of the aerogels, whereas CaCl2 was varied from 20 to 40% of the polymer weight. In addition, a formulated ratio of (n + 1) pPDA to n TPC was used as a way of controlling the molecular weight of the polymers with excess amine. The study was carried out using statistical experimental design methodology. A face-centered central composite design in three variables was utilized with 15 different formulations made plus three repeats of the center point in the design to assess model reliability and reproducibility. Table 1 shows the formulations made along with the measured responses. The experiments were analyzed using multiple linear regression to create empirical models of the effect of the variables on all measured responses.

Figure 1. NMR spectra for two representative formulations in the study, including (top) run 17 and (bottom) run 14, along with structure of polymer showing peak assignments.

(top) shows the spectrum of run 17 from Table 1 which was made using 40% CaCl2 and a formulated n of 60. In the spectrum, the peak at 166.6 ppm can be assigned to the amide carbonyls (A). In addition, there are three distinct aromatic peaks at 135.5 ppm (carbons attached carbonyl or nitrogen, B), C


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Figure 2. Empirical models for (a) shrinkage (standard deviation = 0.90%, R2 = 0.92), (b) density (standard deviation = 0.007 g/cm3, R2 = 0.99), (c) CaCl2 content (standard deviation = 0.094 ppm, R2 = 0.75), and (d) porosity (standard deviation = 0.64%, R2 = 0.98) for polyamide aerogels in study.

completely independently from wt % polymer, the synergistic effect might not be significant. Density of the aerogels is expected to increase with increasing polymer concentration and does so at all CaCl2 concentrations as shown in Figure 2b, even though shrinkage decreases with increasing wt % polymer at 20% CaCl2. This is surprising since density is largely dependent on shrinkage. However, it also depends on all of the monomers being retained in the form of polymer after processing. As the NMR spectra show, polymerization is not complete when 20% CaCl2 is used. Thus, density may not go up as shrinkage increases at low CaCl2 concentration because some low molecular weight oligomers may be lost to solvent washes. It was initially thought that CaCl2 left in the aerogels as polymer concentration and CaCl2 are increased may increase density. However, as shown in Figure 2c, the CaCl2 is mostly washed out during solvent exchanges and supercritical fluid extraction and the measured amount of Ca remaining in the samples is less than 120 ppm for all aerogels in the study. Porosity as shown in Figure 2d, decreases with increasing polymer concentration and increasing CaCl2 concentration. This is the opposite of the trends seen for density, as expected. The number of repeat units, n, has a small though significant effect on shrinkage, density, and porosity, with density increasing at higher n and porosity decreasing at higher n. For Ca content remaining in the aerogels after processing, n was not a factor over and above standard error. Figure 3 shows scanning electron micrographs (SEM) at low and high magnification of selected samples from the study. As seen in Figure 3, there is a coarsening of the morphology as the polymer concentration and CaCl2 concentration are increased. At low magnification, the aerogel made with the lowest polymer

128.9 ppm (secondary carbons in amide phenyl ring, C) and 124.0 ppm (secondary carbons of amine phenyl ring, D). In contrast, Figure 1 (bottom) shows the spectrum of run 14, which is also formulated with an n of 60 but was made using only 20% CaCl2. In this spectrum, a fourth aromatic peak is appearing more downfield at 137.3 ppm. This peak can most likely be assigned to an aromatic carbon substituted with unreacted amine. In addition, two small peaks are seen at 62 ppm (F) and 18 ppm (G), which may belong to ethyl ester, formed from the reaction of an unreacted acid chloride and ethanol during the first solvent wash. Both of these observations taken together are strong evidence that lower molecular weight oligomers are obtained when too little CaCl2 is used in the synthesis. Since the formulations use excess amine, there should be no ester formed from acid chloride end groups. While unreacted amine should be present as the end group of the polymer, at the formulated n used, it should be too small to be detected. All spectra of aerogels in the study made using 20% CaCl2 display these extra peaks, whereas all other aerogels in the study show no evidence of end groups, indicating that high-molecular-weight polymers are obtained. Empirical models for shrinkage, density, and porosity are shown in Figure 2. As seen in Figure 2a, aerogels made using the highest wt % polymer and CaCl2 concentration shrink the most (as much as 28%) during processing. However, there is a synergistic, interactive effect of CaCl2 concentration and wt % polymer on shrinkage. Shrinkage greatly increases with increasing CaCl2 concentration at 10 wt % polymer, but slightly decreases with increasing CaCl2 at 5 wt % polymer. Because CaCl2 concentration is based on wt % polymer, this could be because higher amounts of CaCl2 were employed overall when 10 wt % polymer is used. If CaCl2 were varied D


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CaCl2 concentration (Figures 3b and 3d) have smaller diameter fibers, while the fibers appear to coalesce into larger bundles and sheets in the aerogels made with higher CaCl 2 concentration (Figure 3f, h). Nitrogen sorption porosimetry was performed on all of the aerogel formulations in the study and analyzed using the Brunauer−Emmett−Teller (BET) model applied to the adsorption arm of the isotherm for surface area and the Barrett−Joyner−Halena (BJH) model applied to the desorption arm of the isotherm for pore size distribution. Figure 4a shows the nitrogen adsorption/desorption isotherms for a selective sampling of aerogels from the study. The isotherms are IUPAC type-IV curves with an H1 hysteresis loop, which indicates that the materials are mesoporous. Figure 4b shows the pore size distributions of the same samples as shown in Figure 4a labeled the same way. The black traces show distributions for samples made with 5 wt % polymer concentration, clearly indicating that mostly very small pore sizes (under 4 nm) are obtained under this condition, with broad peaks growing in as CaCl2 concentration is increased (dashed line). In contrast, the red traces are from aerogels made using 10 wt % polymer and show a very broad distribution ranging from 10 up to 100 nm and very little of the 3 nm size pores. This is in keeping with the coarsening of the morphology as seen by SEM with increasing polymer concentration and CaCl2. It might be expected that surface areas might be smaller for aerogels made with higher CaCl2 and polymer concentration from the course appearance of the SEMs. In fact, the surface areas did not vary very much across the study, ranging from 200 to 300 m2/g. As shown in Figure 5, there is a slight though significant increase in surface area with increasing CaCl2 and polymer concentration. Compression tests of the aerogels were carried out to monitor changes in mechanical performance due to the variables studied. An empirical model for Young’s modulus taken as the initial slope of the stress strain curves of the aerogels is shown in Figure 6a. Figure 6b shows the empirical model for compressive strength taken as the stress at 10% strain. Both modulus and compressive strength increase with increasing polymer concentration and CaCl2 concentration. This is expected since density increases as polymer and CaCl2 are increased and mechanical properties typically track with density. However, as seen in log plots of modulus (Figure 6c) and compressive strength (Figure 6d) vs density, higher concentrations of CaCl2 lead to higher modulus and strength

Figure 3. SEM at low (left) and high (right) magnification of aerogels made using different concentrations of (a, b) 20% CaCl2 and 5 w% polymer; (c, d) 20% CaCl2 and 10 w% polymer; (e, f) 40% CaCl2 and 5 w% polymer; and (g, h) 40% CaCl2 and 10 w% polymer.

and CaCl2 concentration (Figure 3a) appears very uniform and fibrous, whereas that made with the highest polymer and CaCl2 concentration (Figure 3g) looks more sheetlike or papery. At higher magnification, it appears that the aerogels made with low

Figure 4. (a) Nitrogen adsorption/desorption curves of representative samples and (b) pore distributions of same samples. Note that the legend is the same for both graphs. E


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Thermal gravimetric analysis (TGA) of the polyamide aerogels from this study shows that the onset of decomposition is typically about 560 °C for those aerogels made using 40% CaCl2 as shown in Figure 7. This is also similar to TGA

Figure 5. Empirical model of BET surface area (standard deviation = 18.22) graphed vs polymer concentration and CaCl2 concentration.

at the same density. This would support the notion that higher concentrations of CaCl2 lead to more complete polymerization and higher molecular weight polymers. This is especially evident when polymer concentration is lowand the lowest density aerogels are producedwhere increasing CaCl2 concentration from 20 to 40% produces an order of magnitude higher modulus and compressive strength. The formulated number of repeat units did not have a significant effect on mechanical properties, further indicating that the molecular weight of the polymers is controlled by the CaCl 2 concentration and the ability of the salt to keep the polymer in solution.

Figure 7. TGA of polyamide aerogels made using different amounts of CaCl2.

reported for bulk poly(p-phenylene terephthalamide).31 In contrast, the aerogels made using 20% CaCl2 begin losing weight almost immediately, again demonstrating that polymerization is not complete for these formulations. The onset of decomposition of the polyamide aerogels is slightly lower than onsets of decomposition for polyimide

Figure 6. Empirical models for (a) compressive modulus (log standard deviation = 0.21, R2 = 0.91) and (b) stress at 10% strain (log standard deviation = 0.11, R2 = 0.99), and log−log plots of (c) modulus vs density and (d) stress at 10% strain vs density with different amounts of CaCl2 concentration. F


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Figure 8. Effect of aging on the polyamide aerogels, including the empirical model for (a) shrinkage and (b) density after exposure to 150 and 200 °C for 24 h.

Figure 9. Effect of aging on pore structure for aerogels made using (a) 20% CaCl2 and (b) 40% CaCl2; and (c) surface area of representative aerogels in the study made using increasing levels of CaCl2.

aerogels, which may be over 600 °C, depending on backbone. However, the polyimide aerogels tend to undergo shrinkage of 10 to 30% depending on backbone structure when heated to temperatures as low as 150 °C, which limits their use. This shrinkage tends to occur during the first 24 h of heating and then levels off; hence, the polyimide aerogels can be conditioned for use at a particular temperature, but the density is higher leading to higher thermal conductivity or higher dielectric constants. To evaluate the higher-temperature performance of the polyamides in regards to shrinkage, the aerogels in the study were heated in a flowing air oven at 150 and 200 °C for 24 h. Figure 8a shows the empirical model for shrinkage during aging. As shown in the graph, the polyamide aerogels are much more dimensionally stable than the polyimide aerogels, shrinking only 2 to 3% after heating at 150 °C and 2 to 7% after 200 °C. Shrinkage increases with increasing polymer

concentration, while CaCl2 concentration has a second order effect on shrinkage with minimum shrinkage occurring at about 30 to 35%. Since shrinkage is lower, the density increase due to shrinkage is much less than has been seen for polyimide aerogels as shown in Figure 8b. At 5% polymer concentration, the density increase due to shrinkage was negligible, whereas at 10 wt % polymer concentration, the density increased only by about 0.04 g/cm3 at all levels of CaCl2 concentration. Although shrinkage is reduced, porosity, and surface area are affected by heating to 150 or 200 °C as seen in Figure 9. For aerogels made using 20% CaCl2 (Figure 9a), the size of the distribution is greatly reduced after heating, indicating that much of the pores smaller than 100 nm are lost. In comparison, aerogels made using 40% CaCl2 do not lose as much of the fine porosity and in fact the pore distribution is somewhat narrower after heating. Figure 9c shows a comparison of surface area before and after heating to 200 °C. The surface area drops after G


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the use of cross-linker, and maintain their shape during processing, an improvement over the all-meta-substituted and meta−para-substituted cross-linked polyamide aerogels reported previously. Densities ranging from 0.1 to 0.3 g/cm3, depending on the concentration o calcium chloride, the formulated number of repeat units, and the concentration of polymer in the reaction mixture. Aerogels made using at least 30 wt % CaCl2 had the best strength when compared to aerogels of similar density. Furthermore, aerogels made using 30 wt % CaCl2 exhibited the lowest shrinkage when aged at elevated temperatures although all of the aerogels shrank less than 7% during heating, a great improvement over polyimide aerogels. The high thermal conductivity of these materials compared to other aerogel compositions of similar density allows them to be used in applications where the insulating properties of such aerogels is disadvantageous, for example, lightweight plastics replacements in consumer electronics, lightweight structures in automobile engine compartments, and galley furnishings in aviation interiors. In addition, thanks to their low cost and ease of fabrication with respect to other polymer aerogels and extremely high mass-normalized strength and stiffness properties, the polyamide aerogels in this work are uniquely promising for commercial applications where other polymer and inorganic aerogels have to date been unsuitable. Although the mechanical properties of the uncross-linked aerogels are comparable to previously reported meta−parasubstituted polyamide aerogels of the same densities, it might be anticipated that cross-linking would lead to even higher strength materials due to the more rigid backbone. Strategies to cross-link these all-para aerogels are currently being examined.

heating by about half for aerogels made using 20% CaCl2, whereas those made using 40% CaCl2 lose only 25% of the surface area. Thermal conductivity of polyimide aerogels made using 40% CaCl2 and n = 40 at varying polymer concentration was carried out using a calibrated hot plate device built by Aerogel Technologies. The measurement setup consisted of a flat sample (4−7 mm thick and at least 100 mm in both length and width) in direct contact with a plate of NIST reference material (SRM 1453) of similar planar dimensions with known thermal conductivity. The sample and reference material were sandwiched between an aluminum plate maintained at 37.5 °C and a flat-bottomed aluminum vessel filled with ice water, maintained at 0 °C. Temperatures were measured at the hot side, cold side, and at the interface between the two materials. The heat flux through the material can be calculated from the temperature drop across the NIST standard. This heat flux is then used with the thickness and temperature drop across the unknown sample to calculate the sample’s thermal conductivity. The thermal conductivity was found to vary linearly with polymer concentration as shown in Figure 10. This is expected

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Figure 10. Plots of thermal conductivity vs polymer concentration for aerogels prepared with 40% CaCl2 concentration and n = 40 (standard deviation =2.62 mW/(m K), R2 = 0.82).

Mary Ann B. Meador: 0000-0003-2513-7372 Notes

The authors declare no competing financial interest.

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as density increases with increasing polymer concentration and typically thermal conductivity in aerogels increases with increasing density. However, the values of thermal conductivity are much higher than would be expected with aerogels of this densityhigher than any aerogel reported in the literature for the same densities. The specific conductivity is 3−4 times higher than that of polyimide or silica aerogels, for example.16 The high effective thermal conductivity can be attributed to the high thermal conductivity of the polymer backbone due to efficient phonon transport. The backbone thermal conductivity as defined by Debye increases with the longitudinal wave speed in the solid material32 therefore it is expected that Kevlar, which has a high specific modulus thanks to its rigid-rod polymer structure, would also have high thermal conductivity. Comparison of bulk polymer properties supports this explanation, as Kevlar fiber has a thermal conductivity of about 3.8 W/(m K) at 20 °C33 compared to Kapton polyimide film, for example, which has a thermal conductivity of 0.12 W/ (m K).34,32

ACKNOWLEDGMENTS The authors thank NASA Glenn’s Director’s Discretionary Fund for support of this work.

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REFERENCES

(1) Schmidt, M.; Schwertfeger, F. Applications for Silica Aerogel Products. J. Non-Cryst. Solids 1998, 225, 364−368. (2) Hüsing, N.; Schubert, U. AerogelsAiry Materials: Chemistry, Structure and Properties. Angew. Chem., Int. Ed. 1998, 37, 22−45. (3) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243−4265. (4) Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741. (5) Kistler, S. S. Coherent Expanded Aerogels. J. Phys. Chem. 1932, 36, 52−64. (6) Pekala, R. W. Organic Aerogels from the Polycondensation of Resorcinol with Formaldehyde. J. Mater. Sci. 1989, 24, 3221−3227. (7) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. Aerogels Derived from Multifunctional Organic Monomers. J. Non-Cryst. Solids 1992, 145, 90−98. (8) Cardea, S.; Gugliuzza, A.; Sessa, M.; Aceto, M. C.; Drioli, E.; Reverchon, E. Supercritical Gel Drying: A Powerful Tool for Tailoring Symmetric Porous PVDF−HFP Membranes. ACS Appl. Mater. Interfaces 2009, 1, 171−180.

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CONCLUSIONS Polyamide aerogels composed of poly-p-phenylene-terephthalate, the same backbone chemistry as DuPont’s Kevlar, have been fabricated. The all-para-substituted polymers gel without H


Research Article

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Highly Porous, Rigid-Rod Polyamide Aerogels with Superior

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