Increased Flexibility in Polyimide Aerogels Using Aliphatic Spacers in

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Increased Flexibility in Polyimide Aerogels Using Aliphatic Spacers in the Polymer Backbone Marcos Pantoja,† Nicholas Boynton,‡ Kevin A. Cavicchi,† Bushara Dosa,‡ Jessica L. Cashman,‡ and Mary Ann B. Meador*,‡ †

Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States

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ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 02/22/19. For personal use only.

S Supporting Information *

ABSTRACT: Polyimide aerogels are mechanically strong porous solids with high surface area, low density, and dielectric constants close to 1, making them ideal materials for use as substrates for lightweight antennas. Increasing the flexibility of the polyimide aerogels extends the usefulness for conformal antennas for use on small aircraft such as unmanned air vehicles or personal air mobility vehicles. To this end, polyimide aerogels made with aromatic amines with 4−10 methylene units as flexible spacers between aromatic rings in the backbone have been fabricated. Substituting 25−75 mol % of fully aromatic 2,2′-dimethylbenzidine with these flexible diamines increases the flexibility of polyimide aerogels, making them bendable at thicknesses up to 2−3 mm. The density, dielectric constants, thermal and moisture stability, and mechanical properties of these aerogels were assessed to understand the effect of the amount and length of the methylene spacers on these properties. KEYWORDS: polyimide aerogels, mesoporous, dielectric constant, porosity, surface area

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INTRODUCTION Aerogels are low-density solids with high surface area, high porosity, small pore sizes, and dielectric constants approaching one as density is decreased.1 Polymer or polymer−silica hybrid aerogels have many of the same properties as inorganic silica aerogels. 2 However, many such aerogels also possess mechanical properties exceeding those of silica aerogels, making them much more applicable as low dielectric substrates, durable insulation, or lightweight multifunctional structures. Among polymer aerogels, polyimide aerogels stand out as combining higher temperature stability with good mechanical properties, making them particularly useful for aerospace and other applications.3 Polyimide aerogels are typically rigid substrates unless made into nominally 0.5 mm thin films. The stiffness or modulus of polyimide aerogels typically depends on both backbone chemistry and density with more flexible diamines such as 4,4′-oxydianiline giving lower modulus than the more rigid 2,2′-dimethylbenzidine (DMBZ) when compared on a same density basis.4 Just like silica aerogels, dielectric constants of polyimide aerogels vary linearly with the density regardless of backbone chemistry, establishing a good understanding of how to tune the dielectric properties.5 Many electronic devices such as antennas could benefit from materials possessing low dielectric constants.6 Recently, we demonstrated the use of the more rigid DMBZ-based aerogels as a low dielectric substrate for antennas.7,8 Patch antennas © XXXX American Chemical Society

with 8−32 elements were made from aerogels with a relative dielectric constant of 1.16 and compared to antennas made from commercial fiberglass and fluorocarbon composite substrates with a dielectric constant of 2.1. The aerogel antennas were found to be higher in gain across a wider frequency range and more than 75% lighter in weight. In general, decreasing the weight and complexity of antenna systems allows aircraft designs to include additional payload electronics to enhance flying, navigation, and surveillance capabilities or add more fuel to extend flight range.9,10 In particular, lightweight antennas are desirable for small aircraft such as unmanned air vehicles (UAV) or urban air mobility vehicles. Such aircraft cannot travel beyond the horizon from the operator since satellite links are required to maintain communication. Typical communication antennas used on larger UAV such as the Global Hawk have large dish antennas on gimbals for pointing. These antennas weigh hundreds of pounds, take up a great deal of space and sit under a radome that may project more than 30 cm above the surface of the aircraft, creating added drag. Phased array antennas, which use beam forming to point the antenna, can be made to be conformal to the wing or fuselage surfaces. Conformal phased array antennas that can close a link with a satellite are being Received: November 20, 2018 Accepted: February 8, 2019

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DOI: 10.1021/acsami.8b20420 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

concentration in the solution during sample preparation was varied between 7, 8.5, and 10 wt %. A statistical experimental design approach was used to carry out the study. The densities, dielectric constants, thermal and moisture stability, and mechanical properties of these aerogels were assessed to understand the effect of the amount and length of the methylene spacers on these properties.

designed using aerogel substrates that may occupy less than a meter of the vehicle surface. In addition, because the antennas are relatively thin (1−2 cm), little or no drag is added to the aircraft. A limitation of the polyimide aerogel antenna is the stiffness of the aerogel substrate, as previously mentioned. To design conformal phased array antennas for the best performance, it is desirable to have flexible aerogel substrates that are two or more millimeters thick. The polyimide aerogels are typically rigid at thicknesses larger than a millimeter. Flexible antennas would be universally conformal to any location of any aircraft, facilitating the manufacturing and installation processes. The rigidity of the existing polyimide aerogels results from the aromatic diamine and dianhydride constituents, which increase bulk aerogel stiffness due to their inherent molecular rigidity and high intermolecular association forces.11,12 Many studies have incorporated flexible linkages within the backbone chemistry of polyimides to reduce molecular order and provide torsional mobility.13 Of particular interest is the use of aliphatic spacers as links between aminophenoxy groups to increase the solubility and processability of polyimides.14−16 These studies used aromatic diamines with different length aliphatic spacers consisting of between 3 and 12 methylenes to produce tough, soluble, and flexible polyimide films. The fabrication of the diamines is a straightforward Williamson ether synthesis as previously described14 using a dichloroalkane and a protected amine, p-phenoxyacetaminophen, in a one to two ratio, followed by the deprotection of the amine as shown in Scheme 1 for 1,4-bis(4-aminophenoxy)butane (BAP4) with four methylenes in the spacer.

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EXPERIMENTAL PROCEDURES

Materials. 4-Acetamidophenol (≥99%), potassium iodide (≥99.5%), potassium hydroxide (≥85%), 1,4-dichlorobutane (99%), 1,6-dichlorohexane (98%), 1,10-dichlorodecane (99%), acetic anhydride (≥98%), triethylamine (≥99%), hydrochloric acid (37%), potassium carbonate (≥99%), and anhydrous ethyl acetate (99.8%) were purchased from Sigma-Aldrich, 200 proof ethanol was purchased from Decon Labs, Inc., acetone (ASTM D329) (≥99.5%) was purchased from Fisher Chemical, and N-methylpyrrolidinone (99.5%) (NMP) was purchased from Tedia. 1,3,5-Triaminophenoxybenzene (TAB) was obtained from Triton Systems (200 Turnpike Road #2, Chelmsford, MA 01824-4053). 2,2′-Dimethylbenzidine (DMBZ) and biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (BPDA) were obtained from Chriskev, Inc. (13920 W 108th Street, Lenexa, KS 66215). BPDA was dried under vacuum at 125 °C for 24 h before being used. Deionized water was prepared using a reverse osmosis desalination system. All other reagents were used as received. General. Attenuated total reflectance infrared spectroscopy was obtained using a Thermo Scientific Nicolet iS10 Fourier transform infrared (FTIR) spectrometer equipped with a germanium crystal. Solution 1H and 13C NMR spectroscopy was performed using a Bruker Avance-300 spectrometer with TopSpin 2.1 software. Samples were dissolved in deuterated dimethyl sulfoxide. 13C solid-state NMR spectroscopy was performed using a Bruker Avance-300 spectrometer equipped with a probe for solid samples. The experiments were conducted using cross-polarization and a magic angle spinning frequency of 11 kHz. The 13C spectra were externally referenced to the carbonyl of glycine (176.1 ppm, relative to tetramethylsilane). Scanning electron micrographs (SEM) were obtained using a Hitachi S-4700 field-emission SEM system after sputter-coating the samples with platinum. The weight and volume of cylindrical samples were measured using a digital balance and a digital caliper, respectively, and were used to calculate bulk density. The skeletal density of cylindrical samples was measured using a Micromeritics AccuPyc 1340 helium pycnometer and was used to calculate the porosity. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 TGA. Samples were heated from room temperature to 800 °C at a rate of 10 °C/min under nitrogen. The TA Instruments Universal Analysis 2000 software was used to analyze the TGA data. The onset of thermal decomposition was defined using the step tangent method which is the intersection between the lines extrapolated for the near horizontal baseline where there is no weight loss and the tangent line to the inflection point on the weight retention versus temperature curve where major weight loss occurs.17,18 Dielectric constant measurements were performed using a Keysight 85072A 10 GHz Split Cylinder Resonator. The split cylinder resonator measures relative permittivity and loss tangent at RF (∼10 GHz/∼30 mm). The cylindrical resonant cavity separates into two halves to load the sample and is adjustable, allowing the gap to accommodate varying sample thicknesses. To measure the electric modes in the resonator, a small coupling loop is introduced through a small hole in the side of each cylinder half. To achieve the highest sensitivity of the split cylinder resonator, the sample is placed where the electric field is parallel, at maximum and perpendicular to the z axis of the cylinder halves. Other equipment associated with the measurement vector network analyzer calculates permittivity and loss tangent using algorithms developed at National Institute of Standards and Technology’s Electromagnetics Division in Boulder, Colorado. The surface area and pore volume of the monoliths were measured by nitrogen physisorption using a micromeritics accelerated surface area

Scheme 1. Synthesis of Diamines with Aliphatic Spacers

In this study, to improve the bend radius of thicker polyimide aerogel parts and understand the relationship between both spacer length and amount of spacer, we have incorporated chains of 4, 6, and 10 methylenes as linking groups in aromatic diamines. As shown in Scheme 2, we substitute up to 75 mol % of DMBZ in the polyimide backbone with these flexible diamines to assess the effect on properties. Using biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (BPDA) as the dianhydride component, the diamine component was a mixture of DMBZ with BAP4, 1,6-bis(4aminophenoxy)hexane (BAP6), or 1,10-bis(4-aminophenoxy)decane (BAP10), more generally referred to as BAPx. This series of aliphatic diamines allowed for aliphatic chain length variation between 4, 6, and 10 carbons. The concentration of BAPx as a percentage of the total moles of diamine (with mol DMBZ equal to 100 mol % minus mol BAPx) was varied between 25, 50, and 75 mol %. Furthermore, the polymer B


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ACS Applied Materials & Interfaces Scheme 2. Synthesis of Polyimide Aerogels with 50 mol % BAPx and 50 mol % DMBZ

and (3) the length of the methylene spacer between the aromatic rings in the flexible diamines was varied between 4, 6, and 10 carbons. Table 1 also shows the data obtained from each of the formulations studied listed in the order of which they were run. Using DesignExpert 10 software from StatEase, an I-Optimal statistical experimental design strategy, where the experiments are chosen by computer algorithm to minimize the average prediction variance over the design space. This strategy was employed because, strictly speaking, the number of methylene units in the spacer cannot be considered a continuous variable. Rather it is a discrete numeric variable (fractions of methylene are not allowed), making more classic designs for response surface modeling, such as a central composite design, not relevant. Unique formulations (14) were fabricated to derive empirical models, and some formulations were repeated two or three times (13 total repeats) to assess model error and reliability. Empirical models were fitted to the data using backward stepwise

and porosity 2020 system and the Brunauer−Emmett−Teller (BET) analysis method.19 Synthesis of Polyimide Aerogels. Polyimide aerogels were prepared, as shown in Scheme 2. Polyimides were prepared using DMBZ in combination with diamines with methylene spacers (BAP4, BAP6, or BAP10) and BPDA as the dianhydride component. The BAPx diamines were synthesized following the procedure established by Garcı ́a et al.14 1H and 13C NMR spectra of BAP4, BAP6, and BAP10 can be found in Figures S1−S3 in the Supporting Information. Using a constant formulated number of repeat units (n) of 30, polyimide formulations were prepared according to Table 1 with three main experimental variables: (1) concentration of polyimide in NMP was varied between 7, 8.5, and 10 wt %; (2) concentration of BAPx used as mol % of the total diamine (DMBZ concentration is taken as 100 minus BAPx mol %) was varied between 25, 50, and 75 mol %; C


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Table 1. Variables Used to Make Aerogels from the Statistical Experimental Design Study along with the Measured Data; Rows of the Same Color Represent Repeat Formulations

regression to define the effect of the variables on the measured properties. Standard error for each model is included in the discussion. As an example, we describe the preparation of sample 15 from Table 1, which is made using 7 wt % polymer concentration and 25 mol % BAP4. BPDA (4.23 g, 14.4 mmol) was added to a solution of BAP4 (0.95 g, 3.48 mmol) in 80 mL of NMP with stirring to form short anhydride-capped oligomers. Once all of the BPDA dissolved, DMBZ (2.22 g, 10.4 mmol) was added to the solution and stirring was continued until all of the DMBZ dissolved to chain extend the

short oligomers and form mostly alternating co-polymers with anhydride endcaps, as shown in Scheme 2. A solution of TAB (0.12 g, 0.31 mmol) in 10 mL of NMP was then added. After 10 min of stirring (long enough to obtain a clear solution), acetic anhydride (10.9 mL, 120 mmol, 8:1 molar ratio relative to BPDA) was added to the solution followed by triethylamine (4.0 mL, 30 mmol, 3:1 molar ratio relative to BPDA). Once the solution turned clear, it was poured into prepared molds which gelled within 20 min. These molds included a 20 mL syringe prepared by cutting off the needle end of the syringe and extending the plunger all of the way out, a dog bone D


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the beam. Flexural Young’s modulus (Ef) was taken as the slope of the initial linear portion of the stress−strain curve, which gives the flexural modulus as

silicone mold with dimensions in compliance with ASTM D882 standards, a rectangular closed mold (2.4 mm x 25 cm x 18 cm) which opens to facilitate the demolding of the sheet sample and a smaller rectangular open mold (3 cm x 2 cm x 1 cm). The gels were aged for 24 h in the molds and were covered by Teflon sheets to prevent the evaporation of the solvent. Following aging, the gels were extracted into a solution of 75% NMP/25% acetone and soaked overnight. This was followed by a 24 h soak in a solution of 25% NMP/75% acetone. Finally, the gels were soaked in 100% acetone for three subsequent 24 h cycles. The solvent was removed by supercritical CO2 extraction, followed by vacuum drying overnight at 80 °C. The resulting aerogels had a bulk density of 0.121 g/cm3. FTIR (cm−1): 2931.5, 2854.1, 1776.4, 1721.9, 1610.1, 1512.3, 1490.6, 1416.9, 1364.9, 1251.7, 1229.0, 1094.3, 827.4, 738.6. 13C solid-state NMR (ppm): 18.7, 26.0, 67.2, 123.3, 130.9, 157.5, 165.4. Compression Testing. The faces of the cylindrical samples were grinded with sand paper to ensure that they were flat and parallel to each other. The samples were compressed at room temperature at a rate of 0.05 in./min to meet the ASTM Standard D695-10, with the sample sizes nominally 13−16 mm in diameter and 19−24 mm in length (maintaining an approximate 1:1.5 diameter to length ratio). The samples were compressed between a pair of compression plates on an Instron 5567 load frame using the Bluehill data acquisition software. The aerogels were compressed to the full capacity of the load cell (samples 1 and 20 from Table 1 broke before reaching the full capacity of the load cell). The Young’s modulus was taken as the slope of the initial linear portion of the stress−strain curve. The initial diameter of the samples was used to calculate engineering stress by dividing the measured force by the initial cross-sectional area, whereas their initial length was used to convert travel distance to engineering strain. Tensile Testing. Dog bone specimens nominally 4.2−5.4 mm wide, 4.7−6.6 mm thick, 31−40 mm long (this variation resulted due to differences in shrinkage) were tested at room temperature using an MTS Tytron 250 microforce testing system operating with an extension speed of 2 mm/min (ASTM D882) and a 500 N load cell. The Young’s modulus was taken as the slope of the initial linear portion of the stress−strain curve. The initial width and thickness of samples were used to calculate engineering stress by dividing the measured force by the initial cross-sectional area, and their initial length was used to convert travel distance to engineering strain. Three-Point Bending. Rectangular pieces 25 mm long x 14 mm wide x nominally 2 mm thick were cut from the sheet obtained from the square silicone sandwich mold previously described. These pieces were tested at room temperature using a TA Instruments Q800 dynamic mechanical analysis (DMA) instrument with a three-point bending fixture operating using the controlled force mode. The total length of the sample is 25 mm, which is enough to clear the 20 mm total span between the two outer posts on the three-point bending clamp. After placing the samples in the center of the three-point bending fixture, the driving shaft of the DMA was lowered to apply a force normal to the length of the samples at a rate of 0.2 N/min. Although the three-point bending fixture does feature low-friction roller bearings, friction was further reduced by lightly coating the rectangular samples with silica powder. Flexural stress was calculated using the flexural stress equation specified by ASTM D790

σf =

3F(t )L 2WT 2

Ef =

F(t )L3 4WD(t )T 3

(3)

Water Uptake. Rectangular samples nominally 2.7 cm x 1.8 cm x 0.9 cm were dried in a vacuum oven at 80 °C for 24 h. The samples were immediately weighed, and the dimensions were measured after being removed from the oven. The samples were then placed in glass jars filled with deionized water. The samples were fully submerged in the jars, and lids were placed on the jars. The samples remained in the deionized water for 24 h. The samples were then extracted from the jars, and the excess water was wiped from the sample surface before measuring the final mass (ASTM D570-98). The water uptake was calculated using the following equation water uptake =

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final mass − initial mass initial external surface area

(4)

RESULTS AND DISCUSSION Cross-linked polyimide aerogels with varying amounts of aliphatic content were synthesized, as shown in Scheme 2. Table 1 summarizes the process variables and properties of the polyimide aerogel formulations prepared in this study. All of the formulations gelled within 20 min. In general, aerogels made with BAP4 were dark brown in color, whereas those made with BAP6 and BAP10 adopted a lighter yellow color. Selected 13C solid-state NMR spectra of three sample formulations which compare the effect of aliphatic spacer length and mol % aliphatic diamine are shown in Figure 1. The NMR spectra of all of the samples analyzed contain an imide carbonyl peak (1) at 165.4 ppm and broad aromatic peaks between 105.4 and 149.6 ppm characteristic of these polyimides. The aliphatic peak (2) at 18.7 ppm corresponds to the pendant methyl groups from DMBZ. The

(1)

Here, σf is the three-point flexural stress, F(t) is the force applied at a given time t to the center of the beam, L is the distance between the posts (20 mm), and W and T are the width and thickness of the sample, respectively, measured prior to testing. Furthermore, this test standard also specifies a flexural strain as εf =

6D(t )T L2

(2)

Figure 1. 13C solid-state NMR spectra of polyimide aerogels reacted in 7 wt % solutions with (a) 25 mol % BAP10; (b) 25 mol % BAP4; and (c) 75 mol % BAP4.

where εf is the three-point flexural strain of the sample’s outer surface and D(t) is the maximum deflection at a given time t of the center of E


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ACS Applied Materials & Interfaces intensity of this peak decreases as DMBZ is decreased from 75 mol % in the samples from spectra (a) and (b) to 25 mol % in the sample from spectrum (c) as BAPx content is increased. Similarly, the peak (3) at 67.2 ppm corresponds to the aliphatic ether carbon from the methylene segment of the BAPx and the peak (4) at 157.5 ppm corresponds to the aromatic ether carbon.14 The intensity of these peaks increases in spectrum (c) for the sample containing 75 mol % BAP4 as expected. The peaks (5) at 26.0−29.8 ppm correspond to the other methylene carbons in the aliphatic spacer.14 Also as expected, the intensity of this peak is higher for the samples containing BAP10 (more methylenes per BAPx unit) and 75 mol % BAP4 (more of BAPx content). The FTIR spectra of the same sample formulations seen in Figure 2 have characteristic bands for polyimides, including

Figure 3. Empirical models for (a) shrinkage of aerogels and (b) density of aerogels made with BAP4 and BAP10 graphed vs BAPx concentration and total polymer concentration.

Figure 2. FTIR spectra of polyimide aerogels reacted in 7 wt % solutions with (a) 25 mol % BAP10, (b) 25 mol % BAP4, (c) and 75 mol % BAP4.

concentration causes shrinkage to increase from 15 to over 30% for aerogels made using 7 wt % polymer, independent of the length of the methylene spacer in each of the different diamines. This is attributed to the flexibility of the aliphatic units, which are less able to resist the Laplace forces driving the collapse of the pores. The increase in shrinkage is less when the polymer concentration is higher. As expected, density increases with increasing polymer concentration and also increases due to the increase in shrinkage with increasing BAPx concentration. The length of methylene spacers in each of the diamines has a very small though significant effect on the density as seen in Figure 3b, which shows surfaces for BAP4 and BAP10 (the surface for BAP6 falls in between these two surfaces). Interestingly, the density decreases slightly with increasing length of methylene spacer in the BAPx. This might be due to a decrease in shrinkage among the different flexible diamines even though the effect is not evident in the shrinkage model. The standard deviation is larger for shrinkage and perhaps masks the effect. Nitrogen adsorption−desorption experiments were performed on all of the aerogel formulations in the study. Typical nitrogen adsorption−desorption isotherms of the select aerogels are shown in Figure 4a. According to the IUPAC conventions developed to classify gas sorption isotherms and their relationship to the porosity of materials, these curves best classify as type IV adsorption isotherms and as type H1 hysteresis loops which identify the porosity as being in the mesoporous range defined as between 2 and 50 nm.22,23 The hysteresis behavior characteristic of this type of isotherm

those at 1364.9 cm−1 (υ imide C−N), 1721.9 cm−1 (symmetric υ imide CO) and 1776.4 cm−1 (asymmetric υ imide CO). Evidence of complete room-temperature imidization is seen through the lack of bands at ∼1660 cm−1 (υ amic acid CO), ∼1535 cm−1 (υ amide C−N), and at ∼1860 cm−1, which would indicate the existence of unreacted anhydride. Furthermore, bands at ∼1807 and ∼980 cm−1, which are expected for the isoimide structure, are not observed in the FTIR spectra. The aliphatic content in these polyimides is evidenced by the peaks at 2931.5 and 2854.1 cm−1 (aliphatic C−H stretching).20 The intensity of these peaks increases with mol % BAPx but is highest for the sample with BAP10 which, similar to the NMR spectra, further shows the increase in the methylene content. Similarly, the peak at 1512.3 cm−1, which is usually assigned to the ring breathing of para-substituted phenyl rings,21 also increases with increasing BAPx content as seen in spectrum (c). Analysis of the data in Table 1 was done to understand the effect of the variables on the properties of the aerogels. Figure 3 shows empirical models of (a) shrinkage (standard deviation = 1.7%, R2 = 0.91) and (b) density (standard deviation = 0.0137 g/cm 3, R 2 = 0.94) of the aerogels vs BAPx concentration and the total polymer concentration used to make the aerogels. Only BAPx concentration and total polymer concentration have an effect on shrinkage (Figure 3a). The number of methylenes in the spacers have no significant effect on shrinkage over and above standard error. Increasing BAPx F


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Figure 4. (a) Typical N2 adsorption−desorption isotherms (at 77 K) and (b) the graph of relative pore volume vs pore diameter of polyimide aerogels derived from 10 wt % solutions; (c) the empirical model for BET surface area.

density is somewhat higher for the latter. The aerogels appear to be quite uniform with the fibrous structure typically seen for polyimide aerogels. The SEM of the aerogel made using 25 mol % of BAP10 (Figure 5e,f) also appears to be similar to those made with the shorter four-methylene spacer. However, the aerogel made using 75 mol % of BAP10 has a much coarser microstructure, as seen in Figure 5g,h. Having the largest amount of the longer 10 methylene spacers clearly causes the fiber strands present in the other SEMs to cluster together and give rise to a hierarchical structure with larger pores between the clusters. The thermal stability of the polyimide aerogels formulated with BAPx is summarized in Figure 6. Figure 6a shows thermal gravimetric analysis (TGA) plots of samples tested under nitrogen environments. There is no significant weight loss around 200 °C, indicating that imidization was complete and the solvent was efficiently removed during the drying process. Overall, the onset of thermal degradation, Td, occurs at lower temperatures for samples containing larger mol % BAPx and longer methylene spacers, as expected, since the aliphatic content is less thermally stable than fully aromatic polyimide. Onset of decomposition for polyimide aerogels made using 100 mol % DMBZ with BPDA and TAB cross-links is about 511 °C,3 whereas previously reported polyimides made using 100% BAP6 with BPDA have onsets of decomposition of about 450 °C.16 The onsets of decomposition reported here all fall between those values as might be expected since these polyimide aerogels are hybrid structures containing both BAPx and DMBZ. The initial onset of decomposition is followed by a second dip in the TGA most likely due to the

results from capillary condensation of nitrogen in the mesopores.24,25 In addition, Figure 4a shows that shorter methylene spacers and lower concentration of BAPx increase adsorption intensity and slightly decrease the peak P/Po value. This indicates that aerogels with more aliphatic content form less well-defined porous structures which reduces the capillary condensation. This is further seen in Figure 4b which shows the relation between relative pore volume vs pore diameter for the length of methylene spacer and concentration of BAPx. As seen, the pore size distributions of the different samples peak right around 50 nm for all of the aerogels. However, the relative pore volume decreases with spacer length, and increasing BAPx concentration indicating a decrease in the amount of mesopores. Surface area was calculated from the nitrogen sorption data using the Brunauer−Emmett−Teller (BET) method. The empirical model for BET surface area (standard deviation = 29.9 cm2/g, R2 = 0.84) is seen in Figure 4c and shows that surface area decreases with increasing BAPx concentration and with increasing methylene spacer length. This might be expected since the amount of mesopores decreases under these same conditions, as seen in Figure 4a,b. Increasing polymer concentration also leads to a small though significant decrease in surface area, due to the overall increase in density and subsequent decrease in overall porosity. Scanning electron micrographs (SEM) of the same four aerogel formulations shown in the graphs in Figure 4a,b are shown in Figure 5 at high and low magnification. The SEM of the aerogels made with 25 mol % BAP4 (Figure 5a,b) and 75 mol % BAP4 (Figure 5c,d) look similar, even though the G


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Figure 5. Low- (left) and high (right)-magnification SEM of aerogels made using 10 wt % polymer concentration, and (a) and (b) 25 mol % BAP4; (c) and (d) 75 mol % BAP4; (e) and (f) 25 mol % BAP10; and (g) and (h) 75 mol % BAP10.

Figure 6. (a) Graph of representative TGA curves and (b) empirical models of decomposition temperature. H


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Figure 7. (a) Compressive stress−strain curves of aerogels made with 10 wt % solutions containing BAP4 and BAP10 at concentrations of 25 and 75 mol %, (b) plot of log modulus vs log density of aerogels made with BAP4 and BAP10 at concentrations of 25 and 75 mol %, and graphs of empirical models for (c) modulus from compression and (d) for compressive stress at 10% strain of aerogels graphed vs BAPx concentration and polymer concentration used to make the gels.

investigation. Figure 7a shows compressive stress−strain curves of selected formulations. These curves follow the typical compressive behavior of cellular solids with three regions.26 The initial rise in the first region is due to the elastic bending of the cell walls, followed by a plateau in the second region corresponding to wall buckling and yielding, and finally a sharp rise in the third region due to densification once the cell walls meet. This behavior is best seen in the stress−strain curves for 25 mol % BAPx-derived aerogels, where varying the methylene length of the aliphatic diamine does not appear to affect the compressive properties. Increasing the amount of aliphatic diamine to 75 mol % BAPx decreases the second region plateau portion of the curve due to softer structures that do not buckle and instead easily collapse and begin the densification process at smaller strains. However, the initial rise in stress does increase with shorter methylene lengths. Typical of all aerogels, modulus does scale with density when the same backbone chemistry is compared as seen in the log−log plots of modulus vs density shown in Figure 7b. As seen in Figure 7b, aerogels with either 25 mol % BAP4 or BAP10 (closed symbols) are similar in modulus, with samples made with BAP4 having slightly higher overall modulus. In comparison, previously reported aerogels with similar density made using only BPDA and DMBZ in the backbone (no BAPx) have modulus values three times those made using 25 mol % BAPx, representing a significant reduction in stiffness.4 The 75 mol % BAP10 aerogels are much lower in modulus

degradation of the aromatic imide structures. Typical with TGA of polyimides in nitrogen, there is a significant amount of char yield. The difference in the char yield is related to the weight of the aliphatic content, including the methylene spacers and methyl groups from DMBZ in each of the aerogels. Aerogels made using 25 mol % BAP4 have about 7.3 wt % aliphatic content, whereas those using 25 mol % BAP10 are about 11 wt % aliphatic. Those made using 75 mol % BAP4 and BAP10 have 9.3 and 19 wt % aliphatic content, respectively. Figure 6b shows the empirical model for Td (standard deviation = 1.69, R2 = 0.93), which is consistent with the previous discussion. The highest Td is measured for the aerogels with the smallest methylene spacer. There is also a second-order effect of BAPx concentration on Td. The Td is highest for aerogels with 25 mol % BAPx and decreases with increasing BAPx concentration up until about 50−60 mol % BAPx, where it reaches a minimum. The slight increase in Td as BAPx goes from 50 to 75 mol % may be due to a concomitant increase in surface area and/or density over the same span. Increasing density or surface area could increase thermal conductivity in aerogels. It is expected that in the aerogels with lower thermal conductivity, there could be a temperature lag in the interior of the sample which would mean that the whole specimen is not seeing the temperature that is read by the thermocouple. Figures 7−9 summarize the mechanical properties of the different polyimide aerogel formulations analyzed in this I


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Figure 8. (a) Tensile stress−strain curves of aerogels made with 10 wt % solutions containing BAP4 and BAP10 at concentrations of 25 and 75 mol %, (b) plot of log modulus vs log density of aerogels made with BAP4 and BAP10 at concentrations of 25 and 75 mol %, and (c) graph of empirical model for modulus from tensile tests.

more methylene spacer groups. Only the formulation made using 75 mol % BAP10 (10 wt % polymer) was able to be tested. The formulation made using 75 mol % BAP10 and 7 wt % polymer was too fragile, and samples broke before they could be tested. Nevertheless, the model correctly predicts this to be the lowest modulus formulation at 9.4 MPa, similar to the measured value by compression. Formulations containing 25 mol % BAP4 again have a slightly higher modulus compared to those made using 25 mol % BAP10 when compared on a same density basis. The empirical model for tensile modulus (log standard deviation = 0.053, R2 = 0.95) in Figure 8c closely resembles the model for compressive modulus in Figure 7c. Modulus decreases with increasing number of methylenes in the spacer and increases with increasing polymer concentration. There is also a synergistic effect of BAPx concentration with the number of methylenes in the spacer. When BAP4 is used, the modulus slightly increases with increasing BAPx concentration due to an increase in density, but for BAP6- and BAP10derived aerogels, modulus decreases with increasing BAPx concentration, even though the density is increased. The results presented so far agree with our initial hypothesis which predicts a decrease in modulus and an increase in flexibility with more and longer methylene spacers. Three-point bending tests were conducted to directly address how introducing aliphatic spacers into the backbone chemistry affects the bend flexibility of the polyimide aerogels. Three-point bending test data is presented in Figure 9. The three-point bending stress−strain curves illustrated in Figure

than the 75 mol % BAP4 aerogels when compared at the same density. Empirical models of the compression Young’s modulus (log standard deviation = 0.11, R2 = 0.93) and the stress at 10% strain (log standard deviation = 0.09, R2 = 0.91) of all of the formulations are shown in Figure 7c,d, respectively. These graphs show an increase in modulus and stress at 10% strain with increasing polymer concentration due to an increase in density as discussed in the previous paragraph. Furthermore, the modulus and stress at 10% strain increase with decreasing methylene length in the aliphatic diamine. These surfaces illustrate the differences in flexibility between the different BAPx used. In agreement with the stress−strain curves in Figure 7a, the difference in modulus is smaller between BAPx at 25 mol % BAPx (practically overlapping at 10 wt % polymer solution) and becomes more pronounced at 75 mol % BAPx. Results of tensile testing are shown in Figure 8. Figure 8a shows stress−strain curves from tensile tests of selected formulations. These curves contain a linear elastic region followed by yielding and the eventual tensile failure of the material. Similar to the compression curves in Figure 7a, the tensile behavior of aerogels made using 25 mol % BAPx is very similar. In addition, 25 and 75 mol % BAP4 formulations also behave similarly. However, the 75 mol % BAP10 formulations have a much lower modulus than those made using 25 mol % BAP10. The tensile modulus graphed vs density in Figure 8b is lower for the 75 mol % BAPx samples compared to 25 mol % BAPx samples, which is expected because these formulations have J


Research Article


Figure 9. (a) Three-point bend stress−strain curves of aerogels made with 10 wt % solutions containing BAP4 and BAP10 at concentrations of 25 and 75 mol %; (b) plot of log modulus vs log density of aerogels made with BAP4 and BAP10 at concentrations of 25 and 75 mol %; (c) graph of empirical model for modulus from three-point bend tests; and (d) picture showing the bend radius of a 2 mm thick aerogel made using 25% BAP10 at a polymer concentration of 10 wt %.

samples used for flexural tests are thinner (nominally 2 mm thick films) than those used for compression (cylinders) or tensile (dog bones). Thinner samples of the same formulation of polyimide aerogel are often higher in density than thicker samples since the outer surface, which is typically higher in density, makes up a larger portion of the cross-section of the sample.27 The aerogels were also subjected to water uptake tests to understand how the amount of BAPx and size of the spacer groups affect moisture resistance. Rectangular samples of the aerogels in the study, nominally 2.7 cm x 1.8 cm x 0.9 cm in size, were soaked in water for 24 h and weighed before and after testing. The empirical model for water uptake (log standard deviation = 0.064, R2 = 0.998) is shown in Figure 10. As shown in Figure 10, only the BAP10 formulations were moisture resistant with very little water absorbed (1−4 wt %). The BAP10-formulated samples remained floating atop the water throughout the test, whereas the BAP4 samples (which absorbed 200−600 wt %) sank to the bottom. The BAP6 samples remained floating but sank a little into the water. The concentration of BAPx and total polymer had little effect on the water uptake. Figure 11 shows the relative dielectric constant vs the aerogel density for BAPx containing materials. As seen with previously studied polymer aerogels, dielectric constant follows a linear increase with density.5,28 Aerogels made using 25 mol % BAPx have the lowest density and, therefore, the lowest dielectric constant. Given their good combination of flexibility, low density, thermal and moisture resistance, and dielectric

9a show an initial linear increase corresponding to elastic deformation, followed by a leveling portion. Similar to the tension and compression curves, this change in slope corresponds to the onset of yielding. Further straining the samples results in a pinching on the underside of the sample, whereas the top of the sample is under tension. Plotting modulus as a function of aerogel density in Figure 9b shows that BAP4 containing formulations have the highest modulus at both 25 and 75 mol % BAPx. In the empirical model for modulus from three-point bend tests (log standard deviation = 0.10, R2 = 0.92) shown in Figure 9c, modulus decreases with increasing number of methylenes in the spacer and increases with increasing polymer concentration as density increases, consistent with the other mechanical tests. There is only a small second-order effect of BAPx concentration on the modulus. Figure 9d shows an aerogel made using 25 mol % BAP10 and 10 wt % polymer being bent to a radius of about an inch. Similar formulations using BAP4 and BAP6 exhibit the same behavior. Those made using 75 mol % BAP10 tend to be more fragile and break more easily. Thus, again, only the 10 wt % polymer formulation could be tested. The 7 wt % polymer sample broke before testing. Nevertheless, the model correctly predicts the modulus for that formulation to be the lowest at about 10 MPa, similar to that of compression and tensile tests. Although their relative scaling is the same, there are some differences between the compression, tensile, and flexural modulus of the polyimide aerogels. Overall, the compression and tensile moduli are in good agreement, whereas the flexural modulus values are slightly higher. This may be because the K



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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b20420. 1 H and 13C NMR spectra of BAP4, BAP6, and BAP10 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marcos Pantoja: 0000-0003-1267-5374 Nicholas Boynton: 0000-0003-4593-7457 Kevin A. Cavicchi: 0000-0002-6267-7899 Mary Ann B. Meador: 0000-0003-2513-7372

Figure 10. Empirical model for water uptake graphed vs BAPx concentration and polymer concentration.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the following employees of the Ohio Aerospace Institute: Daniel Scheiman for thermal analysis testing and FTIR, Baochau Nguyen for NMR, Heidi Guo for nitrogen sorption measurements, and Linda McCorkle for SEM. We are grateful to the Transformative Aeronautics Concepts Program for funding this work under the Convergent Aeronautic Solutions sub-project: Conformal Lightweight Antenna Systems for Aeronautical Communication Technologies (CLASACT). M.P. acknowledges support under NASA grant NNX17AB54G.

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Figure 11. Graph of dielectric constants vs density for aerogels in the study using all three BAPx diamines.

(1) Hrubesh, L. W.; Keene, L. E.; Latorre, V. R. Dielectric Properties of Aerogels. J. Mater. Res. 1993, 8, 1736−1741. (2) Randall, J. P.; Meador, M. A. B.; Jana, S. C. Tailoring Mechanical Properties of Aerogels for Aerospace Applications. ACS Appl. Mater. Interfaces 2011, 3, 613−626. (3) 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. (4) Meador, M. A. B.; Aleman, C. R.; Hanson, K.; Ramirez, N.; Vivod, S. L.; Wilmoth, N.; McCorkle, L. Polyimide Aerogels with Polyamide Cross-links: A Low Cost Alternative for Mechanically Strong Polyimide Aerogels. ACS Appl. Mater. Interfaces 2015, 7, 1240−1249. (5) Meador, M. A. B.; McMillon, E.; Sandberg, A.; Barrios, E.; Wilmoth, N. G.; Mueller, C. H.; Miranda, F. A. Dielectric and Other Properties of Polyimide Aerogels Containing Fluorinated Blocks. ACS Appl. Mater. Interfaces 2014, 6, 6062−6068. (6) Volksen, W.; Miller, R. D.; Dubois, G. Low Dielectric Constant Materials. Chem. Rev. 2010, 110, 56−110. (7) Meador, M. A. B.; Wright, S.; Sandberg, A.; Nguyen, B. N.; Van Keuls, F. W.; Mueller, C. H.; Rodríguez-Solís, R.; Miranda, F. A. Low Dielectric Polyimide Aerogels as Substrates for Lightweight Patch Antennas. ACS Appl. Mater. Interfaces 2012, 4, 6346−6353. (8) Meador, M. A. B.; Miranda, F. A. Design and Development of Aerogel-Based Antennas for Aerospace Applications: A Final Report to the NARI Seedling; Technical Report to NARI Seedling Fund. NASA TM2014-218346, August 2014. (9) Callus, P. J. Conformal Load-Bearing Antenna Structure for Australian Defence Force Aircraft; Defense Science and Technology Report; DSTO-TR-1963; file number 2006/1151925, 2007. (10) Leflour, G.; Calnibalosky, C.; Jaquet, H. In Reduction of Time and Costs for Antennas Integration through Computational Electro-

properties, aerogels made using 25 mol % BAP10 are the most suitable for future exploration of antenna applications.

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REFERENCES

CONCLUSIONS

Polyimide aerogels are mechanically strong porous solids with high surface area, low density, and relative dielectric constants close to 1, making them ideal materials for use as substrates for lightweight antennas. Increasing flexibility of the polyimide aerogels extends the usefulness for conformal antennas for use on small aircraft such as unmanned air vehicles or personal air mobility vehicles. In this study, we fabricated polyimide aerogels with aromatic amines with 4−10 methylene units as flexible spacers between aromatic rings in the backbone. Substituting 25−75 mol % of fully aromatic DMBZ with these flexible diamines increases the flexibility of polyimide aerogels, making them bendable at thicknesses up to 2−3 mm. The density, dielectric constants, thermal and moisture resistance, and mechanical properties of these aerogels were assessed to understand the effect of the amount and length of the methylene spacers on these properties. Formulations made using 25 mol % BAP10 with 10 methylene units in the spacer have the best combination of thermal and moisture resistance, low density, good mechanical properties, and low dielectric constant for use as a substrate for lightweight conformal antennas. L


Research Article

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Increased Flexibility in Polyimide Aerogels Using Aliphatic Spacers in

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