Poly(maleic anhydride) cross-linked polyimide aerogels: synthesis and properties

Abstract

A series of aerogels was fabricated by cross-linking amine end-capped polyimide oligomers with poly(maleic anhydride)s. Poly(maleic anhydride)s are commercially available with various aliphatic side groups and are less costly than other cross-linkers used for polyimide aerogels. Thus they are used here as possible substitutes to form cross-linked polyimide aerogels at a lower cost. The effects of the different side groups of the cross-linkers and oligomer backbone structures on the density, porosity, shrinkage, surface area, morphology, and mechanical properties of the aerogels are discussed. Aerogels with low density (0.12–0.17 g cm⁻³), high porosity (>88%), high surface area (360–550 m² g⁻¹), and Young's modulus (2–60 MPa) were produced in the study. The thermal stability and water uptake of the samples were also studied. The aerogels may be potential candidates in a variety of aeronautic and space applications, such as space suit insulation for planetary surface missions, insulation for inflatable structures for habitats, and cryotank insulation for advanced space propulsion systems.

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Introduction

Various aeronautic and space applications require novel thermal insulation materials with lower weight, better mechanical properties and environmental stability. These may include thermal insulation for extravehicular activity (EVA) suits for planetary surface missions,¹ inflatable habitat structures, inflatable aerodynamic decelerators for entry, descent and landing operations,^2–4 and cryotank insulation for advance space propulsion systems.⁵ Aerogels, made by removing the solvent in a gel by some means that preserves the solid structure of the gel, are superior thermal insulators because of their low density, high porosity, and high surface area.⁶ However, many types of aerogels are fragile, and thus are limited for those applications.⁷

Previously reported octa(aminophenyl)silsesquioxane (OAPS) or 1,3,5-triaminophenoxybenzene (TAB) cross-linked polyimide aerogels^8,9 have good mechanical properties and can be fabricated as flexible thin aerogel films. At room temperature, the OAPS-cross-linked polyimide aerogels made with bisaniline-p-xylidene (BAX) (14.4 mW m⁻¹ K⁻¹, 760 torr^8,10) and 4,4′-oxydianiline (ODA) (20 mW m⁻¹ K⁻¹, 760 torr¹⁰) have thermal conductivity similar to silica aerogels¹¹ of the same density but are robust compared to silica aerogels. Recently reported cellulose aerogels have higher thermal conductivity (56–81 mW m⁻¹ K⁻¹) than the cross-linked polyimide aerogels.¹² OAPS-cross-linked polyimide aerogel films were tested to simulate heat loads for planetary re-entry in a layered insulation system.¹³ The tests demonstrate that the polyimide aerogel can be potentially used as part of the insulation for inflatable decelerators for entry, descent and landing operations to replace silica aerogel composite blankets.

Previous studies of cross-linked aerogels have mainly focused on cross-linkers with amine groups, including the aforementioned TAB and OAPS, as well as 2,4,6-tris(4-aminophenyl)pyridine (TAPP)¹⁴ or 1,3,5-tris(aminophenyl)benzene (TAPB)¹⁵ to cross-link anhydride end capped oligomers. However, all of those cross-linkers are either quite expensive or not commercially available. Recently, we reported aerogels made using 1,3,5-benzenetricarbonyl trichloride (BTC) as cross-linker, which is commercially available, and can be used to cross-link amine terminated polyimide oligomers.¹⁶ Mechanical properties of the resulting polyimide aerogels were similar to TAB and OAPS cross-linked aerogels with the same backbone.

In addition to thermal conductivity of the aerogel, moisture resistance is also an important consideration in many applications.¹⁷ Hydrophobic silica aerogels^18–20 are generally fabricated by modifying the silica surface using hydrophobic groups. Hydrophobicity of cellulose aerogels,²¹ resorcinol-formaldehyde aerogel,²² and graphene aerogels²³ have also been reported. We have also reported tailoring the properties of the OAPS cross-linked polyimide aerogels by using ODA in combination with rigid diamines, p-phenylene diamine (PPDA) or 2,2′-dimethylbenzidine (DMBZ).²⁴ Moisture resistant formulations are obtained if at least 50% of the ODA is replaced by DMBZ, although water contact angles are only measured in the range of 85–90°. The recently reported BTC cross-linked polyimide aerogels, however, absorb water even when the amine in the backbone is 100% DMBZ.¹⁹

In this study, cross-linkers with multiple maleic anhydride groups along a polymer backbone are used to cross-link amine end capped polyimide oligomers. Poly(maleic anhydride) has been used since the 1940's as emulsion stabilizers, detergent compositions and viscosity modifiers.²⁵ Since they are commercially available and less expensive than other potential cross-linkers, it is of interest to examine these in the synthesis of polyimide aerogels as a way of making large scale manufacturing feasible. Thus, in this study, we use poly(maleic anhydride)s as cross-linkers to prepare aerogels as shown in Scheme 1. There are many poly(maleic anhydride)s with different side groups and molecular weights available. We chose these three commercially available poly(maleic anhydride)s because they represent a wide range of molecular weights and different length side groups. The differences in the structures may have an effect on the 3D network of the polyimide aerogels, thus affecting the final properties. The cross-linkers studied include poly(ethylene-alt-maleic anhydride) (PMA, M_w 100 000–500 000), poly(maleic anhydride-alt-1-octadecene) (PMA-O, M_n 30 000–M_n 50 000), and poly(isobutylene-alt-maleic anhydride) (PMA-D, M_w 6000). All three cross-linkers are comprised of alternating ethylene and maleic anhydride groups along the polymer backbone. PMA has no aliphatic side groups, while PMA-O has a long aliphatic chain on each ethylene unit and PMA-D has two methyl groups on each ethylene unit, as shown in Scheme 1.

Scheme 1 The formation of 3D network of aerogels made with ODA and/or DMBZ and cross-linked by various poly(maleic anhydride).

As in previous studies, we also compare aerogels with backbones containing DMBZ, ODA or a combination of both and biphenyl-3,3′,4,4′-tetracarboxylic anhydride (BPDA) to fabricate the amine capped poly(amic acid) oligomers as shown in Scheme 1. In this way, a direct comparison can be made to polyimide aerogels made using other cross-linkers with the same polyimide backbone.

Experimental

Materials

BPDA was purchased from UBE, Inc. ODA and DMBZ were purchased from Omni Specialty Chemicals, Inc. PMA, PMA-O, PMA-D, HPLC grade N-methyl-2-pyrrolidinone (NMP), anhydrous acetic anhydride, and TEA were purchased from Sigma-Aldrich. BPDA was dried at 125 °C in vacuum for 24 h before use. All other reagents were used without further purification.

General

Attenuated total reflectance (ATR) infrared spectroscopy was obtained using a Nicolet Nexus 470 FT-IR spectrometer. Solid ¹³C NMR spectroscopy was carried out with a Bruker Avance-300 spectrometer, using cross-polarization and magic angle spinning at 11 kHz. The solid ¹³C spectra were externally referenced to the carbonyl of glycine (176.1 relative to tetramethylsilane, TMS). Scanning electron micrographs were obtained using a Hitachi S-4700 field emission scanning microscope after sputter coating the samples with gold. The samples were out-gassed at 80 °C for 8 hours under vacuum before running nitrogen-adsorption porosimetry with an ASAP 2000 surface Area/Pore Distribution analyzer (Micromeritics Instrument Corp.). The skeletal density was measured using a Micromeritics Accupyc 1340 helium pycnometer. Using bulk density (ρ_b) and skeletal density (ρ_s) measured by helium pycnometry, the percent porosity was calculated using eqn (1):

Porosity = (1 − ρ_b/ρ_s) × 100% (1)

Thermal gravity analyses (TGA) was performed using a TA model 2950 HiRes instrument. Samples were run at a temperature ramp rate of 10 °C per minute from room temperature to 750 °C under nitrogen or air.

Mechanical characterization

The specimens were cut and polished to make sure that the top and bottom surfaces were smooth and parallel. Samples were conditioned at room temperature for 48 hours prior to testing. The diameter and length of the specimens were measured before testing. The specimens were tested with sample sizes close to 1 : 1.25 ratio of diameter to length. The samples were tested between a pair of compression plates with an AT4, 500 lbf load cell. All testing was carried out at nominal room conditions, and at a crosshead speed of 0.05 in min⁻¹ as dictated by the ASTM guidelines. The aerogels were crushed to 80% strain or the full capacity of the load cell (whichever occurred first). The Young's modulus was taken as the initial linear portion of the slope of the stress strain curve.

Preparation of aerogel monoliths using a combination of different diamines and different PMA cross-linkers

Poly(amic acid) oligomer was formulated in NMP using a ratio of total diamine to BPDA of (n + 1) to n, where n, the number of repeat units in the polyimide backbone between cross-links, was formulated from 20 to 30. This provides oligomers with an average of n repeat units terminated with amine, as shown in Scheme 1 and Table 1. Three different poly(maleic anhydride)s were used to react with the terminal amine groups on the poly(amic acid) oligomers. The mole percent of rigid diamine, DMBZ, in place of ODA ranges from 0 to 100% in this study. All aerogels were produced using a concentration of 10 w/w% total polymer in solution. A sample procedure for PMA-D cross-linked for an oligomer (n = 25) made with 50% ODA and 50% DMBZ is as follows: to a stirred solution of DMBZ (0.9218 g, 4.34 mmol) and ODA (0.8694 g, 4.34 mmol) in 33 mL NMP was added BPDA (2.4567 g, 8.35 mmol). The mixture was stirred until all BPDA was dissolved, and reacted to form poly(amic acid) oligomers. In order to fully cross-link the diamine groups on the poly(amic acid) oligomers, a stoichiometric solution of PMA-D (0.1035 g, 0.017 mmol) in 2.405 mL NMP was added where the amount of anhydride groups on the PMA-D was equal to the number of amine end groups on the poly(amic acid) oligomers. The resulting solution was stirred for 15 minutes, after which acetic anhydride (6.555 mL, 69.4 mmol) was added followed by addition of TEA (2.420 mL, 17.4 mmol). These amounts represent an acetic anhydride to BPDA ratio of eight to one and a TEA to BPDA ratio of two to one. The solution was continually stirred for 10 minutes and then poured into 20 mL syringe molds (2 cm in diameter), prepared by cutting off the needle end of the syringe and extending the plunger all the way out. The gels which formed within 30 minutes were aged in the mold for one day before extracting into 75% NMP in acetone solution and soaked for 24 hours to remove acetic acid and TEA. The solvent within gels was gradually exchanged to acetone in 24 hour intervals starting with 75% NMP in acetone, followed by 25% NMP in acetone and finally three more time with 100% acetone. The gels were then placed in a supercritical fluid extraction chamber in acetone, and washed with liquid CO₂; then CO₂ was converted into a supercritical state by increasing the temperature to 35 °C and gaseous CO₂ was slowly vented out. The resulting aerogel was further vacuum dried at 75 °C overnight. The dry aerogels produced in this way have a density of 0.138 g cm⁻³ and porosity of 89.8%. FTIR (cm⁻¹): 1774, 1718, 1618, 1500, 1440, 1417, 1367, 1308, 1288, 1242, 1172, 1115, 1088, 1010, 879, 829, 781, 763, 737, 706.

Table 1 The experimental design and properties of polyimide aerogels cross-linked by various poly(maleic anhydride), with different n and 0–100% ODA and (100-ODA)% DMBZ

Run#	n	% DMBZ	Cross-linker	Density (g cm^−3)	Shrinkage (%)	Porosity (%)	Surface area (m^2 g^−1)	Modulus (MPa)	10% stress (MPa)	Water uptake (%)
1	25	50	PMA	0.148	18.6	89.4	431	23.9	0.88	166.1
2	25	50	PMA-O	0.144	18.7	89.3	470	8.6	0.27	83.2
3	25	50	PMA	0.149	18.6	89.3	426	23.5	0.92	123.9
4	25	50	PMA-D	0.138	15.9	89.8	442	2.74	0.17	74.0
5	30	50	PMA-O	0.156	20.4	89.0	447	16.3	0.74	59.9
6	25	0	PMA	0.164	21.1	89.3	370	25.4	0.92	461.8
7	25	50	PMA-O	0.154	20.9	88.9	475	8.4	0.42	73.0
8	25	50	PMA-D	0.149	18.3	89.4	448	3.4	0.18	62.4
9	30	100	PMA-D	0.137	16.0	90.0	526	47.3	1.71	615.2
10	25	0	PMA-O	0.153	20.4	89.9	377	25.8	0.82	511.1
11	20	0	PMA-O	0.142	18.8	90.6	395	20.6	0.68	549.6
12	30	0	PMA-D	0.163	21.0	89.9	373	20.2	0.80	490.4
13	25	100	PMA	0.132	14.8	90.3	507	53.3	1.52	631.8
14	25	100	PMA-D	0.127	15.0	90.7	529	44.4	1.42	630.4
15	20	50	PMA-D	0.134	15.7	90.3	474	3.1	0.14	95.5
16	20	0	PMA	0.155	19.5	89.9	362	33.2	0.80	449.8
17	25	50	PMA-O	0.155	17.6	88.8	450	2.7	0.21	70.2
18	20	0	PMA-D	0.145	18.1	90.3	405	23.2	0.73	532.2
19	20	100	PMA-D	0.119	12.6	91.3	540	47.1	1.26	701.9
20	25	50	PMA	0.160	20.3	88.4	404	20.4	1.01	131.2
21	25	100	PMA-O	0.121	13.4	91.2	522	55.9	1.26	689.7
22	30	100	PMA-O	0.126	14.8	90.4	499	57.2	1.40	651.6
23	30	0	PMA	0.164	20.6	89.5	378	23.3	0.94	463.6
24	20	100	PMA	0.125	13.2	91.2	554	55.8	1.48	680.7
25	25	0	PMA-D	0.156	19.9	89.8	397	27.7	0.85	511.0
26	20	50	PMA-O	0.129	15.9	90.6	472	11.1	0.43	111.4
27	25	50	PMA	0.143	17.1	89.6	430	23.1	0.91	138.1
28	20	50	PMA	0.128	14.5	90.5	480	16.5	0.53
29	30	50	PMA	0.164	21.4	88.1	379	42.6	1.38	67.9
30	25	50	PMA-D	0.136	15.7	90.2	488	6.4	0.26	109.6
31	25	50	PMA-D	0.144	17.9	89.8	445	12.6	0.50	89.4
32	30	50	PMA-D	0.156	20.0	88.3	421	32.8	1.07	74.6
33	25	50	PMA-O	0.153	20.0	89.0	450	23.2	0.95	82.9
34	30	0	PMA-O	0.164	21.9	90.2	363	25.6	0.84	482.6
35	20	100	PMA-O	0.119	13.6	91.4	515	39.8	1.22	703.7
36	30	100	PMA	0.146	17.5	89.5	511	60.5	1.90	564.4

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Water uptake experiment

The vacuum dried samples were weighed and put in water in a closed jar for 48 h. Then the samples were removed from the water, the surface of the aerogels was wiped off, and the samples were reweighed. Water uptake of the aerogel was calculated using following equation:where w is the weight of the aerogel samples after absorbing water, w₀ is the original weight of the aerogel samples before soaking.

Statistical analysis

For the samples prepared in Table 1, experimental design and analysis was conducted using Design Expert Version 9.1, available from Stat-Ease, Inc., Minneapolis, MN. Three variables, PMA type, n value (20–30) and DMBZ% (0–100%), are studied in the experimental design. Multiple linear regression analysis was used to derive empirical models to describe the effect of each of the process variables studied on measured properties. A full quadratic model including all main effects, second-order effects and all two way interactions was entertained, and continuous variables were orthogonalized (transformed to −1 to +1 scale) before analysis. Terms deemed to not be significant in the model (<90% confidence) were eliminated one at a time using a backward stepwise regression technique.

Results and discussion

A statistical design study in three variables was carried out to understand the effect of the type of poly(maleic anhydride) cross-linker, number of repeat units (n) and backbone on polyimide aerogel properties. The cross-linkers used are all based on poly(maleic anhydride), but different side groups and different molecular weights may lead to differences in the final aerogels. Gels were made in NMP, with equivalent ratios of total diamine to BPDA of (n + 1) to n in solution. Amber colored poly(amic acid) oligomers with terminal amine groups were formed initially. A stoichiometric amount of poly(maleic anhydride) was added to cross-link the amine end capped poly(amic acid) oligomers and then the poly(amic acid)s were chemically imidized using acetic anhydride and triethylamine (TEA). Aerogels were produced by drying the wet gels using supercritical drying.

Table 1 shows the formulations and properties of polyimide aerogels, all made using 10 w/w% total polymer concentration in the gelation solution. As shown in the table, three different poly(maleic anhydride)s, including PMA, PMA-O and PMA-D, were used in the study along with three different formulated repeat units (n = 20, 25 or 30). Varying n, in effect, is equivalent to varying the amount of cross-linker in the aerogel. In addition, the diamine was varied as shown in Table 1 from 0 to 100 mol% DMBZ (amount of ODA is 100 – DMBZ mol%). To show that imide bond forms between maleic anhydride and diamine moieties, model compounds were produced by reaction of poly(maleic anhydride) and aniline. Fig. 1 shows solid ¹³C NMR spectra of the PMA-O cross-linker before reaction, the product of PMA-O reacted with aniline, and the same product after chemical imidization. The carbons in the aliphatic side chain from PMA-O appear between 13 and 60 ppm in all three spectra. The chemical shift of the carbonyl carbon of poly(maleic anhydride) before reaction (Fig. 1a) appears at 172.0 ppm, and is shifted slightly to 175.7 ppm after reaction of aniline to form amic acid (Fig. 1b). Chemical imidization shifts the carbonyl carbon to 176.7 ppm (Fig. 1c). More evidence of reaction is provided by the aromatic carbons. Carbon 3 (Fig. 1b), which is attached to nitrogen in the amic acid appears at 137.3 ppm before imidization, while the carbons next it (4 and 8) appear at 120 ppm. After imidization (Fig. 1c), the imide ring is less electron donating and therefore the attached carbon 3 shifts upfield to 132.6 ppm, while the rest of the carbons on the aromatic ring shift to 128.1 ppm. These assignments are in reasonable agreement with those previously reported for the reaction of maleic anhydride and aniline followed by solution NMR.²⁶ Fig. 2 shows the ¹³C NMR spectra of the aerogels made with ODA, and n = 20 using PMA-O, PMA-D, and PMA as cross-linkers. All the cross-linked aerogel samples contain an imide carbonyl peak at approximately 165 ppm belonging to the aromatic rings in the repeat units, and aromatic peaks between 115 ppm and 143 ppm. The peak at 153 ppm corresponds to the aromatic ether carbon of ODA. The small peaks at 28 ppm and 23 ppm are attributed to the aliphatic carbons on the cross-linkers. The maleimide peaks from the cross-linker which should appear at about 176 ppm are barely visible due to their relatively small size compared to the rest of the polymer.

Fig. 1 ¹³C NMR of (a) PMA-O, (b) PMA-O-aniline, and (c) PMA-O-aniline after chemical imidization.

Fig. 2 ¹³C NMR of (a) PMA-O, (b) PMA-D, and (c) PMA cross-linked polyimide aerogels made with ODA.

Fig. 3 shows the FTIR spectra of (a) PMA-O cross-linker and (b) n = 20 polyimide oligomer cross-linked with PMA-O. FTIR of PMA-O (Fig. 3a) contains a peak for the anhydride at 1860 cm⁻¹ and peaks for aliphatic groups at 2924 cm⁻¹ and 2852 cm⁻¹. The spectrum of the cross-linked polyimide aerogel in Fig. 3b, contains characteristic bands for polyimides, including peaks at 1373 cm⁻¹ (ν imide C–N), 1716 cm⁻¹ (symmetric ν imide C=O), and 1774 cm⁻¹ (asymmetric ν imide C=O); the aliphatic peaks from the cross-linker are not observed because only a small amount of the cross-linker is used compared to polyimide oligomer. Also not present are bands at ∼1660 cm⁻¹ (ν amic acid C=O) and ∼1526 cm⁻¹ (ν amide C–N) or unreacted anhydride at 1860 cm⁻¹. These observations coupled with little weight loss in the thermogravimetric analysis (TGA) at 200 °C (Fig. 4) indicates that imidization is complete.

Fig. 3 FTIR of PMA-O and PMA-O cross-linked polyimide aerogel with BPDA/ODA at n = 20.

Fig. 4 TGA curves in N₂ of PMA-O cross-linked aerogels made with n = 25, ODA, 50% DMBZ + 50% ODA, and DMBZ.

The onset of decomposition is also observed in Fig. 4 by TGA and is found to change with the polyimide backbone structures. Similar to other aerogel cross-linked with TAB, OAPS or BTC, aerogels made with ODA have the highest onset of decomposition temperature. All the cross-linked samples have high char yield above 60%. Again, because the cross-linker is present in only a small amount, weight loss due to cross-linker is not readily observed. The onset of thermal decomposition is lower for DMBZ containing aerogels, due to the presence of pendent methyl groups. Aerogels made with 50% DMBZ + 50% ODA and aerogels made with 100% DMBZ have similar onsets of decomposition, but because the methyl groups make up only a small percentage of the total weight of the polymer, the difference in total weight loss is very small.

Graphs of empirical models of density (R² = 0.85, standard deviation = 0.0058 g cm⁻³), shrinkage (R² = 0.81, standard deviation = 1.21), and porosity (R² = 0.78, standard deviation = 0.43) of the aerogels are shown in Fig. 5. Density ranges from 0.12 g cm⁻³ to 0.17 g cm⁻³, as shown in Fig. 5a. Density significantly increases as n and DMBZ concentration decrease in agreement with previous studies using different cross-linkers, and is slightly higher for aerogels cross-linked with PMA compared to PMA-O or PMA-D. Since all aerogels were prepared from solutions with a total polymer concentration of 10 w/w%, the difference in density must be due to differences in shrinkage. However, while shrinkage increases with decreasing n and DMBZ concentration the same as density (Fig. 5b), the cross-linker did not have a significant effect on shrinkage over and above random error. This is probably because the standard deviation is very small for density but larger for shrinkage compared to the range of the data; thus, the density model is more discerning and able to distinguish more subtle effects. The porosities of the aerogels ranged from 88 to 92%, as shown in Fig. 5c. As expected, porosity increases as density decreases. Thus, PMA cross-linked aerogels have the lowest porosity, while PMA-O and PMA-D cross-linked aerogels have slightly higher porosity.

Fig. 5 The empirical model of (a) density, (b) shrinkage, and (c) porosity of the aerogels made with ODA and/or DMBZ cross-linked with PMA-O, PMA-D and PMA.

All the cross-linked aerogels made with ODA have slightly lower densities compared to previously reported TAB cross-linked aerogels^9,16 made with the same backbone (0.19 to 0.20 g cm⁻³ for 10 w/w% total polymer concentration) but are within the same range (0.14 to 0.18 g cm⁻³) of OAPS⁸ and BTC¹⁶ cross-linked aerogels made with ODA. The cross-linked aerogels made with DMBZ have slightly higher densities (0.12–0.14 g cm⁻³) than OAPS⁸ and BTC cross-linked aerogels made with DMBZ (0.10–0.12 g cm⁻³),¹⁶ but are similar to TAB cross-linked aerogels (0.13–0.15 g cm⁻³).⁹

Fig. 6a shows typical nitrogen sorption isotherms for cross-linked polyimide aerogels. The hysteresis loop characterizing this type of isotherm is associated with capillary condensation of nitrogen in the mesopores. The cross-linked polyimide aerogels made with ODA or DMBZ have IUPAC type IV curves with an H1 hysteresis loop, indicating that the monoliths consist predominately of three dimensional continuous meso–macropores.²⁷ For polyimide aerogels made with 50% ODA + 50% DMBZ, an abnormal hysteresis loop at P/P₀ in 0.6–0.8 range is observed, which might relate to the narrow pore size distribution centered around 6 nm, as shown in Fig. 6b. According to IUPAC definition, pores are classified by the pore diameter, with micropores having diameters less than 2 nm, mesopores having diameters between 2 and 50 nm, and macropores having pore diameters larger than 50 nm. The plot of pore volume vs. pore diameter of the aerogels demonstrates that the cross-linked aerogels have mesoporous structures. As seen before using other cross-linkers, the greatest effect on pore size distribution in the aerogels is from the oligomer backbone structure. Aerogels made with ODA have smaller pores on average (∼23 nm) than aerogels made with DMBZ (28–32 nm). Slight differences in the size of the distributions are observed for the different cross-linkers with PMA typically giving a larger distribution. It may be that the side groups of PMA-O and PMA-D reduce infiltration of N₂ during the test.

Fig. 6 (a) Typical N₂ adsorption and desorption isotherm curves and (b) plots of pore volume vs. pore size of the cross-linked aerogels.

Formulations made with 50% DMBZ and 50% ODA and 10 w/w% total polymer concentration, have a sharp pore size distribution peak centered at 6 nm. This arises from macro-phase separation during gelation when the synthesis is carried out using a mixture of the two diamines. This is indicative of formation of hierarchical pore structures with finer pores structure inside of a coarser framework as evidenced by scanning electron microscope (SEM) images at high and low magnification (Fig. 7). At lower magnification Fig. 7a–c, the aerogels made using 50% DMBZ + 50% ODA look very different from the aerogels made with ODA (Fig. 7a) or DMBZ (Fig. 7c), displaying a coarser structure (Fig. 7b) with micron sized features. Coarser morphologies were also seen with BTC cross-linked polyimide aerogels made using 50% DMBZ + 50% ODA in random synthesis, which leads to macro-phase separation and hierarchical structures. As demonstrated with BTC cross-linked aerogels, if synthesis is carried out either to force an alternating structure or at lower polymer concentration, macro-phase separation is prevented and the pore size distribution and SEM resemble the ones for aerogels made from a single diamine. As in polyimide aerogels previously reported using TAB, OAPS, or BTC as cross-linkers, the aerogels have a fibrous network structure as seen in SEM at higher magnification. The ODA containing formulations (Fig. 7d) had strands with smaller diameters than those derived from 50% DMBZ + 50% ODA (Fig. 7e) and DMBZ alone (Fig. 7f). The type of poly(maleic anhydride) used as cross-linker had no effect on the morphology as seen by SEM.

Fig. 7 Lower magnification SEM images of PMA-O cross-linked polyimide aerogels at n = 25 made with (a) ODA, (b) 50% DMBZ + 50% ODA, and (c) DMBZ; and higher magnification SEM images of PMA-O cross-linked polyimide aerogels at n = 25 made with (d) ODA, (e) 50% DMBZ + 50% ODA, and (f) DMBZ.

As seen from Fig. 8, the empirical model of the surface area (R² = 0.91, standard deviation = 18.00 m² g⁻¹) using Brannuer–Emmet–Teller analysis method (BET) of the cross-linked aerogels ranges from 350 m² g⁻¹ to 550 m² g⁻¹. The largest effect on surface area comes from the diamine, increasing with increasing amount of DMBZ. The number of repeat units, n, also has a small but significant effect on surface area with surface area decreasing slightly as n increases. There is also a small but significant effect of cross-linker type on the surface area with PMA cross-linked aerogels having slightly lower surface area than those made with PMA-D and PMA-O cross-linked aerogels.

Fig. 8 The empirical model of the surface area of the cross-linked polyimide aerogels.

All cross-linked aerogels made with ODA have higher surface area than those previously reported for OAPS cross-linked polyimide aerogel,⁸ while they are similar to the TAB cross-linked polyimide aerogels,⁹ and slightly lower than those made with BTC.¹⁶ The aerogels made with DMBZ or 50% DMBZ + 50% ODA have similar surface areas to BTC cross-linked aerogels, but are higher in surface area than OAPS and TAB cross-linked aerogels.

Compression tests were performed on all aerogel formulations in the study. Young's modulus of the aerogels was measured as the initial slope of the stress strain curves, some examples of which are shown in Fig. 9a. Generally in aerogels, the modulus increases as density increases.²⁸ As seen from the empirical model of the modules (log standard deviation = 0.24, R² = 0.65) in Fig. 9b, as n increases, modulus slightly increases. This is due to higher shrinkage and thus higher density as n increases as seen in Fig. 5. PMA cross-linked aerogels also have slightly higher density, so PMA cross-linked aerogels are higher in modulus than the PMA-O and PMA-D cross-linked polyimide aerogels. Similar to OAPS, TAB and BTC cross-linked aerogels, aerogels made with DMBZ have higher modulus, even though density is lower with increasing DMBZ concentration. This is due to the higher stiffness of the DMBZ backbone. However, the modulus of each of the poly(maleic anhydride) cross-linked aerogels made with 50% DMBZ + 50% ODA, are lower than the aerogels made with DMBZ or ODA only, which is most likely due to the hierarchical morphology as demonstrated with BTC cross-linked aerogels. When BTC cross-linked aerogels made using 50% DMBZ + 50% ODA were fabricated in such a way as to force an alternating structure, the modulus fell between 100% ODA and 100% DMBZ formulations.

Fig. 9 (a) The stress–strain curves of the cross-linked polyimide aerogels cross-linked with PMA-D, (b) the empirical model of Young's modulus of the cross-linked polyimide aerogels, (c) the relationship plot of modulus to density, and (d) the empirical model of 10% stress of the cross-linked polyimide aerogels.

Fig. 9c shows a graph of modulus vs. density, comparing polyimide aerogels using different cross-linkers with the same polyimide backbone, along with silica aerogels.^16,24,29 The poly(maleic anhydride) cross-linked aerogels from this study shown in green have for the most part similar modulus at comparable densities as the POSS and BTC cross-linked aerogels. The exception is the aerogels made with 50% DMBZ + 50% ODA in the polyimide backbone (open green squares on the plot) which have lower modulus due to the hierarchical structure as already discussed. Fig. 9d shows the empirical model of compressive strength (log standard deviation = 0.19, R² = 0.66), taken as the stress at 10% strain graphed vs. n and DMBZ concentration. Compressive strength has the same trends as modulus with the highest strength aerogels being those made from DMBZ at n = 30 with PMA as cross-linker.

While thermal stability of the polyimide backbone is high as evidenced by TGA, the polyimide aerogels have been known to undergo shrinkage at temperatures below the onset of decomposition. To assess this behaviour in the poly(maleic anhydride) cross-linked aerogels, they were heat treated at 200 °C for 24 h, 48 h and 120 h in flowing air. The masses and sizes the samples were measured before and after heat treatment. The weight loss at 200 °C over the 120 h of aging was less than 2% for all of the samples studied. Fig. 10a shows the empirical models for shrinkage occurring during aging of the cross-linked polyimide aerogels at 200 °C for 24 h (standard deviation = 2.06, R² = 0.96) and 120 h (standard deviation = 2.24, R² = 0.95). Shrinkage at 200 °C is not affected by the cross-linker type, but decreases slightly with increasing n. Also as seen in Fig. 10a, shrinkage occurs in the first 24 hours and heating does not result in more shrinkage. Formulations made with 50% DMBZ + 50% ODA shrink slightly less than the formulations made with ODA and much less than those made with DMBZ, similar to trends seen with BTC cross-linked aerogels. Due to shrinkage, the aerogels change in density during the first 24 hours of aging and then level off as illustrated in the example made with 50% DMBZ + 50% ODA, n = 25, shown in Fig. 10b. Fig. 10c shows the empirical model of the density of the cross-linked polyimide aerogels after heating at 200 °C for 120 h (standard deviation = 0.024, R² = 0.98). As seen with BTC cross-linked aerogels, the largest effect on density after heating is due to the polyimide backbone. After being heat treated for 120 h, n has no effect on the density of the aerogel samples. The lowest density after aging is seen for 50% DMBZ formulations which are slightly lower that for 100% ODA formulations. As seen with BTC aerogels, density of DMBZ formulations increases to 0.7 g cm⁻³, leaving very little porosity. There is slight difference in density due to the poly(maleic anhydride) structure. PMA-D cross-linked polyimide aerogels have the lowest density after aged.

Fig. 10 (a) The empirical models of shrinkage of polyimide aerogels heated at 200 °C for 24 h and 120 h; (b) the cross-linked polyimide aerogels made with 50% DMBZ + 50% ODA and n = 25 heated at 200 °C for 24 h, 48 h, and 120 h; (c) the empirical models of density of the samples heated at 200 °C for 120 h.

After heat treating at 200 °C for 120 h, the surface area of the polyimide aerogels significantly decreases. There is no effect of the cross-linkers on the surface area after aging. For example, as shown in Fig. 11a, the surface area of PMA-D cross-linked aerogel at n = 20 made with ODA dropped about 58%, from 405 to 169 m² g⁻¹. Although there is less shrinkage of the 50% DMBZ + 50% ODA formulations during aging, the surface area after heat treatment significantly decreases 87% from 474 to 87 m² g⁻¹.

Fig. 11 The surface area of the polyimide aerogels cross-linked with PMA-D at n = 20 before and after being heated at 200 °C for 120 h.

Water contact angle measurements could not be done on all of the aerogels since in many cases, the droplet was absorbed fairly quickly by the aerogel. To be able to compare the moisture resistance of the aerogels, water uptake experiments were done on all of the samples. The samples were weighed, then placed in water for 48 hours, and weighed once again. All of the samples made with 50% DMBZ + 50% ODA floated on water during the whole 48 hours, while the other samples eventually sank as water was absorbed. Fig. 12 shows the empirical model (standard deviation = 30.80, R² = 0.99) of the water uptake of the samples. The water uptake of the samples ranged from 60 wt% to 700 wt%. Formulations made using 50% DMBZ + 50% ODA have the lowest water uptake, about 60 wt%, which accounts for about 15% of the pore volume of the aerogel samples, while the highest uptake was for DMBZ derived aerogels, which absorbed about 700 wt% water (or about 93% of the pore volume). DMBZ derived aerogels have the largest pore sizes compare to other kinds of aerogels, which might allow water in more easily. As n was increased, water uptake slightly decreased. The aliphatic side groups on the different cross-linkers had no significant effect on the water uptake of the samples, over and above random error.

Fig. 12 The empirical model of the water uptake of the cross-linked polyimide aerogels.

Conclusions

A series of polyimide aerogels were synthesized using three different poly(maleic anhydride) cross-linkers and amine capped polyimide oligomers made from BPDA and two different diamines. The effect of the cross-linkers on the final properties of the aerogels was discussed. The cross-linkers which differed in aliphatic substitution and molecular weight gave very similar results for all properties measured. PMA-D and PMA-O cross-linked aerogels have slightly lower density, slightly higher porosity, and slightly higher surface area than PMA cross-linked aerogels. PMA cross-linked aerogels have similar modulus values compared to TAB, OAPS or BTC cross-linked aerogels of similar density and similar polyimide backbone structure, while modulus of PMA-D and PMA-O aerogels were lower, especially when the backbone was derived from 50% ODA and 50% DMBZ. The poly(maleic anhydride) cross-linked polyimide made with 50% ODA + 50% DMBZ demonstrated the lowest shrinkage on aging at 200 °C, similar to BTC cross-linked aerogels previously reported. These aerogels also had the lowest water uptake. Since thermal conductivity of aerogels typically is related to density and surface area, it is expected the thermal conductivity of poly(maleic anhydride) cross-linked polyimide aerogels will be similar to that of the OAPS cross-linked polyimide aerogels although we have not yet completed this characterization. Over all, any of the poly(maleic anhydride)s studied can be used as a low cost substitute to cross-link polyimide aerogels, giving similar properties to other reported cross-linkers.

Acknowledgements

We gratefully acknowledge support from the Center Institutional Capabilities and ETD GCD Entry system modelling Programs. We thank Baochau N. Nguyen, Ohio Aerospace Institute for NMR spectra. We also thank the NASA Lewis' Educational and Research Collaborative Internship Project (LERSIP) intern student program.

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