Synthesis and Properties of Step-Growth Polyamide Aerogels Cross

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Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides Jarrod Williams, Mary Ann Babin Meador, Linda S McCorkle, Carl Mueller, and Nathan Wilmoth Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5012313 • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 6, 2014

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Chemistry of Materials

Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides Jarrod C. Williams*,†, Mary Ann B. Meador*,‡, Linda McCorkle§, Carl Mueller**, Nathan Wilmoth†† NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States KEYWORDS: Aerogel, polyamide, mesoporous, cross-linked. ABSTRACT: We report the first synthesis of step-growth aromatic polyamide (PA) aerogels made using amine end-capped polyamide oligomers cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC).

Isophthaloyl chloride (IPC) and/or terephthaloyl chloride (TPC) were

combined with m-phenylene diamine (mPDA) in N-methylpyrrolidinone (NMP) to give amine capped polyamide oligomers formulated with up to 40 repeat units. Addition of the cross-linker, BTC, typically induces gelation in under five minutes. Solvent exchange of the resulting gels into ethanol followed by supercritical CO2 drying gives colorless aerogels with densities ranging from 0.06 to 0.33 g/cm3, compressive moduli between 5 and 312 MPa, and surface areas as high as 366 m2/g. Dielectric properties were also measured in the X-band frequency range. It was

*

To whom correspondence [email protected]

should

be

addressed:

[email protected],

†

NASA Postdoc Program

‡

NASA

§

Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio, United States

**

Qinetiq NA

††

Vantage Partners, LLC

ACS Paragon Plus Environment

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found that relative dielectric constant decreased with density as seen with other aerogels with the lowest relative dielectric constant being 1.15 for aerogels with densities of 0.06 g/cm3. Because of their superior mechanical properties, these aerogels can be utilized in a number of aerospace related applications, such as insulation for rovers, habitats, deployable structures, and extravehicular activity suits, as well as low dielectric substrates for antennas and other electronics. Due to potentially lower cost relative to polyimide and other polymer aerogels, they also have potential for use in more terrestrial applications as well, such as insulation for refrigeration, building and construction, and protective clothing.

Introduction Aerogels are porous solids with high surface areas that are made by forming a gel network and removing the solvent without causing pore collapse.1 Due to characteristics such as high surface area, high porosity, low dielectric properties and low density, these lightweight aerogels are attractive for use as thermal insulators, low dielectric substrates,2, 3 catalyst supports,4 and as building and construction materials.5 Silica aerogels have poor mechanical properties, however.6 Thus, much effort recently has focused on improving mechanical properties of aerogels. One approach involves the reaction of styrene,7,

8

epoxy,9 or isocyanate,10,

11

with pendant

functionalities on the silica backbone such as amines, hydroxyl or vinyl groups to reinforce the silica backbone. More robust aerogels are obtained but at the expense of the maximum temperature at which they can be used. Furthermore, while the compressive moduli of, for example, epoxy reinforced silica aerogels are higher than silica aerogels of the same density (1240 MPa for aerogels ranging in density from 0.2 to 0.3 g/cm3),9 lower density polymer reinforced silica aerogels are still somewhat fragile.


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More recently, polymer aerogels have been fabricated through the formation of either chemically or physically cross-linked networks which avoid the use of silica altogether. Since factors such as polymer chain length, monomer type and cross-link density can be controlled it is possible to generate a broad spectrum of properties. For example, syndiotactic polystyrene aerogels have been produced by allowing hot solutions of the polymer to form physically interlinked semicrystalline domains that act as virtual cross-links upon cooling.12,13 These materials were found to be attractive substrates for removing impurities from air and fluids. However, due to the lack of covalent crosslinks, mechanical properties were poor. One strategy for improving the mechanical properties of polystyrene aerogels has been to investigate use of nanoscale reinforcements. A recent investigation has shown that the addition of carbon nanotubes to thermo-reversible polystyrene gels results in their homogenous dispersion and the formation of an interpenetrating 3D network of nanotubes and physically bonded polystyrene domains. 14 The net result was an enhancement of the compressive moduli of the materials with values as high as 6.4 MPa at densities of 0.06 g/cm3. Polystyrene aerogels of the same density, but without nanotube reinforcement, have compressive modulus of 4.2 MPa. While physical cross-links and use of nanotube reinforcements, as in the case of polystyrenes, give rise to modest increases in mechanical properties over silica aerogels, a superior approach relies on the use of covalently cross-linked polymers to form the aerogel. One of the best examples of this relies on the use of various di- and tri-isocyanates of varying geometries reacting with triethylamine and water in order to form oligomers, which are cross-linked to form polyurea (PU) aerogel networks with good mechanical properties.15 At a given density, these aerogels tend to have higher Young’s moduli than polystyrene aerogels and polymer reinforced silica aerogels. Young’s moduli ranging from 4 to 300 MPa were obtained in the density range of


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0.03-0.55 g/cm3. Even at densities of 0.03 g/cm3, half the density of the strongest polystyrene nanocomposite aerogel, covalently cross-linked PU aerogels have a higher Young’s modulus (7 MPa). When compared to epoxy reinforced silica9 aerogels of similar densities, PU aerogels tend to have Young’s moduli that are at least twice as large. For example, in the range of 0.19-0.20 g/cm3, epoxy reinforced silica has a modulus of 13 MPa while covalently cross-linked PU aerogels have moduli around 33 MPa. While these recent advancements lead to improved mechanical properties of organic polymeric aerogels and polymer reinforced silica aerogels16,17,18 over pure silica aerogels, low use temperatures due to poor thermal stability and/or low glass transition temperature limit their utility. A substantial improvement in both mechanical properties and use temperature is represented by the development of the polyimide (PI) aerogels.19,

20, 21

PI aerogels have been

reported with Young’s modulus as high as 102 MPa at a density of 0.18 g/cm3 18 and 20 MPa at a density of 0.08 g/cm3 20 making them, as a function of density, comparable or a little higher in modulus compared to the PU aerogels. The techniques used to fabricate them are simple and easily scalable, and even amenable to the fabrication of flexible thin films. However, the use of relatively

expensive

diamines

and

dianhydrides

and

cross-linkers,

triaminophenoxybenzene and octa(aminophenyl)silesquioxanes,18,

20

such

as

1,3,5-

which are not widely

available, are limiting factors in their widespread application. For this reason, we investigated a similar family of polymer aerogels, which can employ less expensive monomers and crosslinkers, namely aromatic polyamide (PA)22 which also have high onsets of decomposition and high glass transition temperatures (though not as high as PI). Furthermore, they can be made from monomers such as m-phenylenediamine (mPDA), isophthaloyl chloride (IPC), and


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terepthaloyl chloride (TPC) which are widely available at lower cost, making them an attractive alternative to polyimide aerogels. The only example of a PA aerogel previously reported relies on a non-conventional, high temperature reaction between a tri-isocyanate and a tricarboxylic acid in order to form the amide bond.23

It is an encouraging example of a PA aerogel but is limited by the fact that di-

isocyanates and dicarboxylic acids do not react in a fashion that is conducive to step-growth polymerization. It was reported that attempts at generating oligomer solutions from difunctional monomers with well-defined n values (number of repeat units) led to precipitation when diisocyanates and dicarboxylic acids were employed. In contrast, previous work has demonstrated that the classic step-growth reaction between diamines and diacid chlorides in Nmethylpyrrolidinone, NMP, to form PAs allows for the formation of oligomers without precipitation at controlled molecular weights.24 PA aerogels made in NMP also do not need base catalyst. Due to the Lewis basicity of NMP, the HCl generated during the polymerization is kept from protonating the aromatic diamines and hindering their reaction with difunctional acid chlorides.25, 26 In addition, the reaction proceeds at room temperature or below. Based on this previous work, we present here a route to PA aerogels by synthesizing amine end-capped oligomers from mPDA and diacid chloride in NMP and cross-linking with benzenetricarbonyl trichloride (BTC) as shown in Scheme 1. The resulting gels are dried using supercritical fluid extraction to form aerogels.

Several parameters are examined in the

fabrication of the aerogels, including formulated number of repeat units in the PA oligomers (n), concentration of polymer in solution (w/w%) and the diacid chloride used to make the oligomers. For the latter variable, either 100% IPC, or 100% TPC, or a 50/50 % combination was used. Properties of the aerogels are discussed and related to these parameters.


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Experimental Materials. Anhydrous N-methylpyrrolidinone (NMP), m-phenylene diamine (mPDA), 1,3,5benzenetricarbonyl trichloride (BTC), isophthaloyl chloride (IPC), and terephthaloyl chloride (TPC) were purchased from Sigma-Aldrich (St.Louis, MO). All reagents were used without further purification. General. Thermal gravimetric analysis (TGA) was run on a TA instruments Q500 with a ramp rate of 5°C per minute under nitrogen. Helium pycnometry was carried out on a Micrometrics Accupyc 1340 gas pycnometer. Samples were imaged by SEM on a Hitachi S4700 Field Emission Microscope after sputter coating the samples with platinum. Solid state 13C NMR was performed on a Bruker Avance-300 spectrometer with cross polarization and magic angle spinning (11 kHz). The resulting spectra were externally referenced to the carbonyl of glycine (176.1ppm 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 hours. IR spectra were obtained on a Nicolet Nexus 470 FT-IR spectrometer. Cylindrical molds to make the aerogels were made by cutting off the needle end of disposable, 20 ml polypropylene syringes, drawing the plunger to the opposite end and lining the inside with a rolled up Teflon sheet for easy removal of the monolith. Rectangular molds (nominally 1.5 x 1.0 x 0.5 in) made from silicone were also employed to make aerogels for dielectric testing.

Measurement of dielectric properties was carried out as previously

described using waveguide transmission.5 The effects of the n, type of acid chloride 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


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regression analysis was employed to develop empirical models that describe the effects of these variables on properties. Preparation of BTC Cross-Linked Polyamide Monoliths. Typically, gels were made from solutions of 5, 7.5, or 10 w/w % polymer in NMP, using n of 10, 20 or 30, and either 100% IPC, 100% TPC or a 50/50 combination of both as shown in Table 1. The formulated n is controlled by using a ratio of diacid chloride to diamine of n to (n + 1). The procedures for making the PA aerogels from different combinations of diacid chloride are nearly identical. Note that for 50% IPC/50% TPC aerogels, the calculated moles of acid chloride needed for a given formulated n are divided by two to give the amount of each acid chloride needed. The ratio of the combined moles of both acid chlorides with the diamine is still n to (n+1). A representative procedure for an IPC aerogel with an n of 30 and polymer concentration of 7.5% w/w (Sample 3 from Table 1) is described as follows. A solution of mPDA (6.832 g, 63.20 mmol) in NMP (179.96 ml) was cooled to 5 oC using an ice water bath. IPC (12.414 g, 61.14 mmol) was added in one portion as a solid and the cooled solution was allowed to stir for 60 minutes. BTC (0.360 g, 1.356 mmol) was added and the mixture was vigorously stirred for 5 minutes before being poured into four 20 ml cylindrical molds lined with Teflon and five small rectangular silicone molds. Gelation occurred within 5 minutes. After aging overnight at room temperature, the monoliths were removed from the molds and placed in 500 mL jars of ethanol. This was followed by seven solvent exchanges at 24 hour intervals to ensure that all of the NMP was removed from the gels. The gels, which were white and tapered at the ends, were then subjected to supercritical CO2 extraction followed by drying (75 °C) in a vacuum oven overnight. The aerogels from 50% IPC /50% TPC follow the same general procedure except that IPC was added to the reaction mixture first and allowed to stir for 30 minutes before TPC was added.


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After an additional 30 minutes of stirring, then BTC was added. The preparation of TPC aerogels followed the same procedure, except that for samples 19 (TPC, n=40, 10 wt%) and 17 (TPC, n=30, 7.5 wt%), TPC and mPDA were only allowed to react for 20 minutes before addition of BTC due to the fact that they gel quickly even without adding the cross-linker. IPC aerogels (representative sample 3): FTIR (neat, cm-1): 3315 (br), 1651, 1607, 1537, 1486, 1417, 1306, 1246, 1195, 1167, 1087. Solid 13C NMR (ppm): 116.9, 129.8, 137.2, 166.1. 50% IPC/ 50% TPC aerogel (representative sample 9): FTIR (neat, cm-1): 3301 (br), 1650, 1607, 1537, 1486, 1419, 1306, 1251, 1167, 1091. Solid 13C NMR (ppm): 117.0, 128.5, 137.4, 165.4. TPC aerogel (representative sample 17): FTIR (neat, cm-1):3315 (br), 1650, 1607, 1537, 1466, 1419, 1305, 1250, 1190, 1167. Solid 13C NMR (ppm):118.4, 127.9, 137.2, 165.9. Mechanical Characterization. The cylindrical monoliths were cut so that the length was 1.5 times the diameter and sanded in order to make sure that the ends were even and parallel. Samples made using TPC remained cylindrical throughout the gelation, solvent exchange, and drying processes. Samples containing IPC which tapered towards the ends of the cylinders during solvent exchange were reshaped into perfect cylinders by center-less grinding, and the ends were cut and sanded as previously described. The tests were carried out in accordance with the ASTM D695-10 method using a model 4505 Instron load frame. Young’s moduli, taken as the initial slope of the stress strain curve are reported in Table 1. Supercritical Drying Procedure. The prepared gels in ethanol were added to a one liter pressure vessel and sealed. The vessel was pressurized to 70 bar with CO2, at 25°C and the contents were allowed to soak for thirty minutes. The vessel was then flushed with sub-critical CO2 (approximately one liter at nine grams per minute) while maintaining the temperature and


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pressure. This process was repeated five times. Once the final sub-critical flush was completed, the temperature was increased to 35 °C and the pressure to 90 bar causing the CO2 to become supercritical. The vessel was then rinsed with approximately one liter of CO2 (9 grams per minute) in the supercritical phase. Following this final wash, the temperature was held constant at 35°C while the pressure in the vessel was reduced by venting 10 g of CO2 per minute.

Results and Discussion As shown in Scheme 1, three different amine terminated oligomers were made via low temperature condensation reaction between mPDA, and either 100% IPC, 100% TPC, or a combination of 50% IPC /50% TPC. The diamine and diacid chloride(s) react rapidly in NMP at 5 oC before addition of the triacid chloride, BTC. In fact, certain 100% TPC gels made with n = 30 or more and higher polymer concentration can only be stirred for 20 minutes prior to the addition of the cross-linker since gelation occurs rapidly even without adding cross-linker. In all other formulations, solutions were stirred for 60 minutes. Gelation occurs typically within five minutes of adding BTC to the oligomer solution. After exchanging the NMP in the gels with ethanol through a series of wash steps, the gels are supercritically dried using liquid CO2 extraction. The diacid chlorides are varied as shown in Scheme 1, in order to generate three different polymer backbones with differing degrees of para and meta substitution, to understand the effect of this variable on the properties of the aerogels. Meta substituted PA aerogels such as those made using IPC and mPDA are of interest because they are expected to be more flexible and less rigid as a result of their non-linear backbones. This is in contrast to the more para substituted PA aerogel species relying on TPC and mPDA which can be expected to exhibit higher rigidities arising from the para substituted backbone elements which facilitate more optimal packing and


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hydrogen bonding. Unfortunately, PA aerogels formulated using p-phenylenediamine (pPDA) with either TPC or IPC could not be examined in this study. Attempts to react pPDA under the conditions illustrated in Scheme 1 led to precipitation instead of gelation regardless of which acid chloride it was reacted with. Thus, as shown in Scheme 1, three different oligomers are produced. The 100% IPC oligomer consists of all meta substituted rings along the polymer backbone, while the 100% TPC oligomer consists of meta alternating with para substituted rings. In the oligomer made from 50% IPC/50% TPC, since the IPC was first combined with mPDA and stirred for a half hour before addition of TPC, a regular alternating pattern of three meta substituted rings and one para substituted ring (50% IPC/50 % TPC) should be produced. This is because the combination of the diamine and IPC in a near two to one ratio in the first step, should first produce an amine end-capped oligomer of n = 1 which subsequently reacts with TPC to form the oligomers in the second step. Other variables examined in this study include polymer concentration which was varied between 5, 7.5 and 10 w/w % and the number of formulated repeat units, n, in the oligomers which were varied between 20, 30 and 40. The range of n was chosen because oligomers formulated with n > 40 and low polymer concentration result in no gelation after addition of BTC while n < 20 oligomers gel too quickly after addition of BTC. A face centered central composite design was employed to minimize the number of experiments necessary to understand the relationship between the three variables and the resulting properties. A total of 15 different formulations were prepared with the middle point being repeated 4 times in order to assess model accuracy and reliability (See supporting information). The nineteen experiments are listed in Table 1 along with the corresponding measured data which was used to create empirical models. Note that the acid chloride used in


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each experiment is listed as the amount of TPC used as a percentage of total acid chloride in Table 1. It is understood that the amount of IPC used is 100% – TPC. A full quadratic model for each measured response was entertained, including all two way interactions between variables. Using backward stepwise regression to eliminate insignificant terms allows derivation of empirical models for all measured properties related to the variables studied. All PA aerogels produced in the study, regardless of acid chloride used, showed the same characteristic peaks, including peaks at 3315 cm-1 (s, amide bending, N-H), 1649 cm-1 (s, amide, C=O), and 1537 cm-1 (s, amide, N-H). The absence of any peaks or shoulders in the range of 1700-1750 cm-1 indicates that there is no remaining unreacted carboxylic acid and that no esters are formed from unreacted acid chlorides and ethanol during the solvent exchange step (See supporting information). Analysis of three spectra of representative PA aerogel samples by solid

13

C CP-MAS NMR

revealed that all three spectra contain a peak at 166 ppm corresponding to the amide carbonyl, as expected, and a peak at 137 ppm for the aromatic carbons substituted with nitrogen or carbonyl. The aromatic peaks corresponding to BTC cross-linker in all three spectra are anticipated to be small and overlap with the peaks arising from the aromatic rings of the oligomers. Samples made using 100% IPC in their backbone (sample 3, 7.5 w/w%, n=30) have two other aromatic peaks at 116.9 ppm (carbons from diamine moiety) and 129.8 ppm (other unsubstituted aromatic carbons from the dicarbonyl moiety). In samples made using 50% IPC / 50% TPC (sample 9, 7.5 w/w%, n=30), these same two peaks occur at 117.0 ppm and 128.5 ppm, slightly shifted from the 100% IPC. In samples that have 100% TPC in their backbones (sample 17, 7.5 w/w%, n=30), these two additional peaks occur at 118.4 ppm and 127.9 ppm, making differentiation of the three backbones possible (See supporting information).


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Figure 1 shows pictures of aerogel monoliths formulated using 100% TPC (sample 17, 7.5 w/w, n=30), 50% IPC/50% TPC (sample 9, 7.5 w/w, n=30), and 100% IPC (sample 3, 7.5 w/w, n=30). These are typical of those in the study made with the same backbone structures. Gels made using 100% TPC tend to become opaque during cross-linking and gelation. TPC based gels also do not shrink or swell during gelation. This is in contrast to formulations containing at least 50% IPC which remain clear and swell slightly in NMP, requiring that polypropylene molds used to form the monolith be lined with a Teflon sheet to ease removal from the mold. Aerogels based on 100% TPC, such as sample 17 shown in Figure 1, undergo shrinkage during solvent exchange and supercritical drying but do so uniformly during this process. The 50% IPC/50% TPC aerogels, such as sample 9 in Figure 1, do not shrink uniformly, but taper at the ends, and develop a skin during solvent exchange. Formulations using 100% IPC such as sample 3 in Figure 1, also form a skin and in most cases are similar in appearance to those using 50% IPC/50 % TPC. However, two formulations with 50% IPC/50% TPC, sample 2 (which is shown in Figure 1) and sample 4, also warped, cracked, and blistered during the solvent exchange process. The uniform shrinkage that occurs during processing with the TPC derived aerogels is not an unusual observation. Uniform volume change of some extent is observed in most aerogel processing and is often attributed to factors such as chain relaxation and macro-syneresis.27,

28

However, the non-uniform shrinkage, development of a skin, or warping and cracking observed in some IPC derived aerogels are not commonly reported. While information concerning the behavior of cross-linked aramid gel networks in swollen and non-swollen states interacting with different solvents is scarce, there are studies concerning the general behaviors of other well solvated networks and the types of syneresis that occurs when contacting non-solvents.29


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Furthermore, there are several studies carried out on the swelling and deswelling behaviors of polymeric hydrogels bearing pendant amide moieties when exposed to non-solvents.30,

31

These

behaviors are commonly linked to macrosyneresis if the sample undergoes shrinkage at the macro-scale, or micro-syneresis if the sample maintains its overall shape and size but undergoes pore size broadening. While micro- and macro-syneresis are typically discussed as phenomena that arise from the interactions between a solvent and a growing oligomer during polymerization, they are not often discussed as contributing factors leading to variations in pore structure after cross-linking and gelation. However, during numerous studies on the swelling, and deswelling behaviors of gelled polyacrylamide networks, Okay et al qualitatively identified the cause of some observed inhomogeneity in their cylindrical gel samples as arising from microsyneresis.24, 25 It was found that when a cylindrical gel sample that is swollen suddenly encounters a non-solvent, the parts of the sample in immediate contact with the non-solvent undergo a segregation into solvent-rich and polymer-rich regions forming an outer layer that undergoes deswelling while the inner core of the cylinder remains homogeneous. If the polymer rich region of the outer layer of the sample is in a glassy state, it can block or hinder diffusion to the inner part of the sample, causing a heterogeneous layer to form. It can be inferred then, that the inhomogeneities observed in the IPC containing aerogels arise due to similar segregation processes. As evidenced by the fact that the undried IPC containing gels will expand to a diameter larger than the molds during removal, they are in a swollen state before undergoing solvent exchange. The observed rounding of the sample ends arises from an outer layer that undergoes rapid deswelling and segregation upon contact with a non-solvent during the solvent exchange process while the inner core exchanges more slowly and uniformly


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undergoing a less drastic shrinkage. Examination of the cross sections of the supercritically dried aerogels confirms the presence of this different region as a narrow band extending a few millimeters in from the surface as shown in Figure 2. A comparison of scanning electron micrographs (SEM) of the different PA aerogels made in the study is shown in Figure 3. In Figure 3a, an SEM of a 100% TPC aerogel (sample 17, 7.5 w/w%, n = 30) is shown. The sample which shrinks uniformly has a uniform morphology consisting of a 3D network of separated polymer strands. A representative SEM of a 50% IPC/50% TPC aerogel (sample 9, 7.5 w/w%, n=30) shown in Figure 3b, looks similar to the 100 % TPC aerogel but has a more open pore structure, due to the fact that it undergoes less shrinkage during processing.

In contrast, a representative sample of a 100% IPC aerogel

(Sample 3, 7.5 w/w%, n=30), is shown in Figure 3c. In this sample, the polymer strands look as though they have partially coalesced, giving the appearance of thicker polymer walls with finer strands of polymer in between the walls. Measurement of surface area of all of the aerogels in the study was carried out using nitrogen sorption and analyzed using the Branuaer-Emmet-Teller (BET) method.32 Surface areas ranged from 48 to 366 m2/g. The empirical model for surface area (standard deviation=34.6, R2=0.88) is shown in Figure 4. As seen in the graph, in general, lower n and higher TPC concentration leads to higher surface area, with TPC concentration having the largest effect. Polymer concentration has a small, though significant effect on surface area as well. Aerogels made using 100% IPC have an order of magnitude lower surface area than those made with 100% TPC. This may be due to the observed deswelling induced micro-syneresis which, as discussed earlier for IPC containing aerogels, may play a predominant role in the morphology and overall appearance of the aerogels. As the segregation into polymer and solvent rich phases occurs during solvent


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exchange of IPC derived aerogels, the larger pores which form when the polymer strands collapse into walls lead to the decreased surface area. Macro-syneresis which leads to uniform shrinkage in the 100% TPC aerogels increases density of the aerogels, but since pores shrink uniformly, surface area is not reduced. Bulk density of the aerogels was calculated by dividing the measured weight by the measured volume, after sanding the monoliths to a uniform shape. It was not possible to measure density of samples 2 and 4 due to the previously discussed warping and cracking which occurred during solvent exchange. An empirical model for density (standard deviation=0.016, R2=0.98) derived from the measured data in Table 1 is shown in Figure 5a. As expected, polymer concentration has a large effect on the final density of the aerogel, with density increasing with increasing concentration for all combinations of TPC concentration and n. Aerogels derived from 100% TPC have higher densities since they experience more shrinkage than those made with IPC. There is also a small though significant synergistic effect of polymer concentration with n. When polymer concentration is high, n of 20 leads to lower density than n of 40, whereas at low concentration, the opposite is true. This may be due to the increased cross-link density at n = 20 having a larger reinforcement effect at lower polymer concentration. This would possibly lead to lower shrinkage of n = 20 aerogels compared to n = 40 aerogels.

At higher polymer

concentrations, cross-link density may be a less important factor. Porosity is calculated from the bulk density and the skeletal density measured by helium pycnometry. The empirical model for porosity (standard deviation=1.26, R2=0.98) shown in Figure 5b illustrates generally the opposite trends from density, that is, increasing TPC concentration and polymer concentration lead to decreased porosity. This is expected, since


15


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aerogels that are less dense will have a higher porosity.

Page 16 of 33

Figure 5b also shows the same

synergistic effect between n and polymer concentration. Dielectric properties (relative dielectric constant and loss tangent) of the PA aerogels from this study were characterized by permittivity measurements in the X-band range (11-12 GHz). The dielectric constant and loss tangent of the PA aerogels are dependent on the polymer concentration and TPC concentration, similar to the density models, as shown in the empirical model for relative dielectric constant (standard deviation = 0.042, R2 = 0.92) in Figure 6a and for loss tangent (standard deviation = 0.0014, R2 = 0.96) in Figure 6c. Relative dielectric constants of aerogels in general scale with density, and higher densities give rise to higher dielectric constants.2, 3, 33 PA aerogels are no exception to this observation. Graphing relative dielectric constant vs. density reveals a linear relationship (Figure 6b), confirming that the effect of the variables on dielectric properties is directly related to their effect on density, as observed for other aerogels. At lower densities, a dielectric constant as low as 1.15 is observed for a PA aerogel having a density of 0.06 g/cm3, making these materials similar to PI aerogels which have been recently demonstrated as substrates for lightweight antenna applications.2 However, as shown in Figure 6b, previously reported PI aerogels have a lower relative dielectric constant at a given density. Figure 6d shows a graph of loss tangent vs. density for PA aerogels compared with the PI aerogels previously reported. While loss tangents of PA aerogels also scale with density, loss tangents for the PI aerogels previously reported are nearly constant graphed vs. density, and lower than those of the PA aerogels.

Both the difference in dielectric constant and loss tangent

may be due to increased moisture content in the PA aerogels due to increased hydrogen bonding sights compared to the PI aerogels. Hrubesh et al showed a similar linear relationship between


16

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density and loss tangent or dielectric constant in as prepared silica aerogels.33 However, it was shown that after drying the silica aerogels at 700 K for 10 h, the slope of the line was greatly reduced. Hence, loss tangent is very sensitive to moisture. While dielectric constant of the aerogels was also shown by Hrubesh et al to be lower after drying, the effect is much more subtle, as is the difference in dielectric constant between PI and PA aerogels. Compression tests were carried out on the aerogels in the study to assess mechanical properties.

Young’s modulus is taken as the initial slope of the stress-strain curve, and

compressive strength is taken as the value of stress at 10 % strain, as is typically reported for low density foams.34 These values are listed in Table 1 and also used for empirical modeling. In Figure 7a, the empirical model for compressive strength (log standard deviation = 0.16, R2 = 0.90) is shown graphed vs. polymer concentration with separate lines for each of the three backbones. In the model, it is evident that compressive strength increases with increasing polymer concentration for all three backbones, while n is not a factor in the model over and above standard error. In addition, PA aerogels made with 100 % TPC were much higher in compressive strength than those made with at least 50% IPC. It can be expected that the 100% TPC containing formulations would be the strongest for two reasons. First, a high degree of para substitution allows for more intermolecular hydrogen bonding. Second, the more linear sections of an aromatic PA allow for additional strength through aromatic stacking interactions. PA aerogels using 50% IPC/50% TPC would have more randomized structures and less regioregularity which may disrupt stacking as well as hydrogen bonding. It is surprising that the 100% IPC aerogels are slightly higher strength than the 50% IPC/50% TPC aerogels, since they are typically lowest in density. As previously discussed, TPC aerogels and those made with higher polymer concentration are also higher in density. Indeed, the log-log plot of compressive


17


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Page 18 of 33

strength with density shown in Figure 7b shows the linear relationship typically seen with aerogels, though there is some scatter in the data. The increase in strength at a lower density may be due to the segregation occurring during solvent exchange which leads to consolidation of some of the polymer strands into thicker structures, though as previously mentioned, this does lead to a marked decrease in surface area. Figure 8a shows the empirical model for Young’s modulus (log standard deviation=0.25, R2=0.79) graphed vs. polymer concentration for the three different polymer backbones. Again, modulus increases with increasing polymer concentration, and the highest modulus aerogels are those made using 100 % TPC. Hydrogen bonding and the ability to stack probably also contribute to the increased modulus in these aerogels but again density also increases with increasing TPC concentration and increasing polymer concentration.

Modulus in aerogels

typically increases with increasing density. Thus, it is again surprising that the lowest density aerogels made using 100% IPC are also higher modulus than those made using 50% IPC/50% TPC. Again, the lower moduli observed for the 50% IPC/50%TPC systems is likely due to the fact that the the 100 % IPC and 100 % TPC systems have a higher degree of regioregularity allowing for more optimal packing of the oligomers. In Figure 8b, the Young’s moduli of the 100 % TPC and 100% IPC aerogels fabricated in this study are plotted as a function of density, and compared with other polymer aerogels recently reported to have high strength at lower density. As shown in the graph and as discussed in the introduction, PU aerogels23 and PI aerogels20 of a similar density are comparable in modulus. The PA aerogels shown, however, are nearly an order of magnitude higher in modulus than PI or PU aerogels and more than three orders of magnitude higher than silica aerogels at the lowest density shown.6


18

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Conclusion PA aerogels with three different backbones have been produced via the step-growth polymerization of inexpensive starting materials, mPDA and TPC or IPC, and crosslinking with triacid chloride, BTC. The chemistry employed is straightforward and simple, relying on no catalysts and should be applicable to a wide range of aromatic diamines, diacid chlorides and triacid chlorides unlike previously reported PA aerogel which is limited to a highly cross-linked structure produced from a tri-isocyanate and a tricarboxylic acid. A three variable study looking at the effect of formulated repeat units, n, between cross-links, polymer concentration and TPC concentration was carried out to understand the effects of the variables on properties of the PA aerogels. It was found that aerogels made using 100% TPC or 100% IPC had the highest reported moduli of any aerogel compared on a density basis. While TPC aerogels shrank more during processing, resulting in higher densities, they also exhibited uniform shrinkage and higher surface areas. This is in contrast to IPC aerogels which exhibited micro-syneresis during solvent exchange resulting in less homogeneous structures, and low surface area. Relative dielectric constants as low as 1.15 were also measured for the PA aerogels. The low density and low dielectric properties combined with higher compressive properties make the PA aerogels attractive materials for many aerospace applications.

Supporting Information Available: Central composite design diagram, FTIR for representative samples, 400 MHz 13C CP-MAS spectra for representative samples, and typical stress-strain curves. This material is available free of charge via the Internet at http://pubs.acs.


19


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Page 20 of 33

Scheme 1. Synthesis of cross-linked polyamide aerogels using 100% IPC, 100% TPC, or 50% IPC/50% TPC. Cl H2N

NH2

+ Cl

10 oC

O

H N

H N

H2N

NH2

O O

O

3( n + 1) eq. 3n eq. ( IPC, TPC or ( mPDA) combination)

n Cl

5

O

oC

O Cl

HN NH NH

O

O

HN O

n

O NH

O HN H N

HN

H N

n

O O

O

O

NH

N H

Networked Structure ( gel)

O

2 eq. BTC

O

O

H N

Cl

Supercritical H N

CO2 Extraction

O

Aerogel

NH O

O HN

O

NH NH

n

O


20

Page 21 of 33

a.

Loss tangent

Stress at 10% Strain (MPa)

Modulus, MPa

Dielectric constant, X-band

area, Surface m2/g

Porosity, %

Polymer conc., w/w % Density, g/cm3

TPC conc., %

n (repeat units)

Table 1. Properties of the different polyamide aerogels investigated in this study

1

20

0

5

0.06

96

111

1.15

58.0

0.34

0.006

2a

20

0

10

-----

-----

119

-----

-----

-----

-----

3

30

0

7.5

0.10

94

137

1.19

125.2

0.61

0.006

4a

40

0

5

-----

-----

47.8

-----

-----

-----

-----

5

40

0

10

0.16

89

95.5

-----

47.8

1.34

0.015

6

20

50

7.5

0.13

91

247

1.22

24. 6

0.66

0.007

7

30

50

5

0.08

95

166

1.17

5.6

0.19

0.007

8

30

50

7.5

0.12

92

228

1.23

26.3

0.58

0.007

9

30

50

7.5

0.12

93

198

1.18

24.0

0.52

0.006

10

30

50

7.5

0.13

91

190

1.24

19.8

0.58

0.008

11

30

50

7.5

0.10

93

209

1.17

12.8

0.30

0.005

12

30

50

7.5

0.11

93

252

1.21

16.1

0.39

0.006

13

30

50

10

0.15

90

225

1.26

40.9

0.77

0.009

14

40

50

7.5

0.13

91

162

1.21

64.8

1.43

0.007

15

20

100

5

0.22

85

366

1.34

169.2

1.59

0.014

16

20

100

10

0.33

77

385

1.53

101.3

3.77

0.019

17

30

100

7.5

0.30

79

275

1.45

312.1

3.00

0.014

18

40

100

5

0.18

88

347

1.28

57.6

0.67

0.011

19

40

100

10

0.36

75

271

1.58

213.9

5.03

0.024

Sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Monoliths of this sample were cracked and distorted during the drying process. Some properties could not be measured.


21


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17

9

3

Page 22 of 33

2

Figure 1. Picture of representative aerogels produced in the study, including sample 17, 100 % TPC, 7.5w/w%, n=30), sample 9, 50 % IPC/ 50 % TPC, 7.5 w/w%, n=30), sample 3, 100 % IPC, 7.5w/w%, n=30 and sample 2, 100 % IPC, 10 w/w%, n=20).


22

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Cross section of a 50 % IPC/50% TPC aerogel (sample 6, 7.5 w/w%, n=20) with segregated region apparent as a narrow band along outer the edge.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a. 0.30 g/cm3, 275 m2/g, 79 % porous

b. 0.12 g/cm3, 198 m2/g, 92 % porous

Page 24 of 33

c. 0.10 g/cm3, 137 m2/g, 94 % porous

Figure 3. Comparison of SEMs of a) 100 % TPC aerogel (sample 17, 7.5 w/w%, n=30), b) 50 % IPC/50% TPC aerogel (sample 9, 7.5 w/w%, n=30) and c) 100 % IPC aerogel (sample 3, 7.5 w/w%, n=30).


24

Page 25 of 33

n=20 n=40

500 400 300 200

100 75

100

25 5

0

TP C

9 8 7 Polym 6 er c o nc., %

co

50

0

nc .,

%

2

/g Surface area, m

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The empirical model for surface area graphed vs. polymer concentration and TPC concentration at two levels of n


25


n = 20 n = 40

0.3 0.2 %

100 80 60 40 20

0.0

a.

con c.,

0.1

10

9 8 7 6 Polyme r conc., w/w %

0

TP C

3 Density, g/cm

0.4

5

105 100 Porosity, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

b. Figure 5.

95 90 85 80 75 70 6 Po lym 7 8 er 9 co nc ., w 10 t%

n = 20 n = 40

20 40 60 ., % onc 80 c 100 TPC

0

The empirical models for a) density and b) porosity graphed vs. polymer

concentration and TPC at two levels of n.


26

Page 27 of 33

1.6 Relative dielectric constant

c constant Relative dielectri

1.6

1.4

1.2 10

9 8 Po 7 lym er 6 co nc ., %

a.

40 5

20 0

60

Cc TP

80

c. , on

100 %

1.5

1.3 1.2 1.1

0.025 0.020 Loss tangent

0.015 0.010 100 80 60 40 20 %

co nc .,

8 Polym e

c.

7 r c on

c., %

6

5

0

0.20

0.25

0.30

0.35

PA aerogels PI aerogels, ref. 3

0.015 0.010 0.005 0.000 0.05

TP C

9

0.15

b.

0.020

0.000

0.10

Density, g/cm3

0.025

0.005

PA aerogels PI aerogels, ref. 3

1.4

1.0 0.05

n = 20 n = 40

Loss tangent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10

d.

0.15

0.20

0.25

0.30

0.35

Density, g/cm3

Figure 6. Graphs of dielectric properties, including a) empirical model for relative dielectric constant; b) the linear relationship between relative dielectric constant and density for PI and PA aerogels; c) the empirical model for loss tangent; and d) the linear relationship between loss tangent and density for PI and PA aerogels. Error bars are one standard deviation of the pooled data from the model.


27


Stress at 10% strain, MPa

10

1

100 % IPC 50 % IPC/50% TPC 100% TPC

0.1 4

a.

5

6 7 8 9 Polymer conc., w/w %

10

11

0.3

0.4

10 Stress at 10% strain, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

1

0.1 0.05 0.06 0.070.080.090.1

b.

Density, g/cm

0.2

3

Figure 7. Graphs of the a) empirical model for compressive strength at 10% strain and b) log stress at 10% strain vs. log density. Error bars are one standard deviation of the pooled data from the model.


28

Page 29 of 33

1000

Modulus, MPa

100

10 100 % IPC 50 % IPC/50% TPC 100% TPC

1 4

a.

5

6 7 8 9 Polymer conc., w/w %

10

11

1000 100 Modulus, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 1 PA aerogels PI aerogels, ref. 20 PU aerogels, ref. 23 silica aerogels, ref. 6

0.1 0.01 0.1

b.

1

Density, g/cm3

Figure 8. Graphs of a) the empirical model for compressive Young’s modulus with data used to derive the model and b) log modulus plotted vs. low density. Error bars for PA and PI aerogels are one standard deviation of the pooled data from the models.


29


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Page 30 of 33

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ASTM D695


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Table of Contents Graphic


33

Synthesis and Properties of Step-Growth Polyamide Aerogels Cross

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