Preparation and Characterization of Polyimide Aerogels with a

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Preparation and Characterization of Polyimide Aerogels with Uniform Nanoporous Framework Ya Zhong, Yong Kong, Junjun Zhang, Ying Chen, Boya Li, Xiaodong Wu, Sijia Liu, Xiaodong Shen, and Sheng Cui Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01756 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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Preparation and Characterization of Polyimide Aerogels with Uniform Nanoporous Framework Ya Zhong,a,b,c,d,* Yong Kong,a,c Junjun Zhang,a Ying Chen,a Boya Li,a Xiaodong Wu,a Sijia Liu,a Xiaodong Shen,a,b,d,* and Sheng Cuia,c,d a

b

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, PR China

c

Suqian Anjiao Technology Co., Ltd, Suqian 223800, PR China

d

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing 210009, PR China

ACS Paragon Plus Environment

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ABSTRACT Polyimide aerogels were successfully synthesized via straightforward sol-gel process under room temperature along with the supercritical CO2 drying using 4-amino-N-methylbenzamide (DABA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). 1,3,5-triaminophenoxybenzene (TAB) was used as the cross-linker. The chemical structure, pore structure, morphology, thermal performance, CO2 adsorption and mechanical performance of polyimide aerogels were investigated. The as-prepared polyimide aerogels had low bulk densities (0.091~0.167 g/cm3), low shrinkages (9.73~17.36%), low thermal conductivities (0.0307~0.0341 W/m·K), high specific surface areas (449.76~538.19 m2/g), small pore diameter (10.37~22.41 nm), high thermal stability (onset of decomposition above 560 °C) and excellent mechanical property. The CO2 adsorption capacities of polyimide aerogels were substantially higher than the values of the previous porous materials reported under the similar conditions and the CO2 uptake capacity was as high as 31.19 cm3/g at 25 °C and 1.0 bar. The resulting polyimide aerogels could be potentially used as thermal insulators and CO2 adsorbents.

Keywords Aerogel, Polyimide, Porous structure, CO2 adsorption


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■ INTRODUCTION Aerogels have three-dimensional (3D) network structures constructed via connection of nanoparticles. More than 90% of the space in aerogels is occupied by air, and only a very limited amount of solid phase is present. 1, 2 Because of the nanoporous structure, aerogels have various unique properties, such as low density, high porosity, low thermal conductivity, high surface areas and high acoustic attenuation.3, 4 In view of their unusual chemical and textural properties, aerogels have been investigated for a wide variety of applications, including thermal insulation, catalysis, catalyst support, adsorption, sensor, aerospace and aeronautics.5‒8 Over the past few decades, aerogels of various compositions include inorganic, organic and inorganic/organic hybrid aerogels, which were produced by sol-gel process followed by supercritical drying. More recently, aerogels made from the various engineering polymers have attracted much attention, such as polyurethane,9 polyurea,10 polybenzoxazine,11 syndiotactic polystyrene,12,13 polyimide (PI)14 and polyamide15. Compared to other engineering polymers, polyimide has been considered as a promising engineering polymer utilized under high temperature conditions because of its intrinsic advantages including excellent mechanical ability, good chemical resistance and superior stability at higher temperatures.16 Polyimide aerogels were obtained for the first time via the classic route of DuPont process, using both chemical dehydration and high temperature treatment to induce imidization.17 Another route for preparation of polyimide aerogels is the PMR method.18 Unfortunately, the polyimide aerogels generated from the above two routes have a disadvantage of the large shrinkage. Unlike the linear polyimide aerogels, the cross-linked polyimide aerogels were successfully fabricated and displayed excellent properties such as lightweight, strong and flexible.19 To date, types of cross-linkers have been used for the synthesis of polyimide aerogels. NASA’s Glenn Research


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Center20‒22 introduced an alternative route using an aromatic triamine, 1,3,5-triaminophenoxybenzene (TAB) as the cross-linker, resulting in the reaction between diamines and dianhydrides can occur even at room temperature to form polyimide gels. Especially, the as-prepared PI aerogels with the ultra-low dielectric constants (as low as 1.08) and low densities (0.078~0.094 g/cm3) still had compressive modulus of 4~8 MPa (40 times higher than silica aerogels at the same density), making them suitable as low dielectric substrates for lightweight antennas for aeronautic and space applications. More recently,

various

cross-linkers

1,3,5-benzenetricarbonyl

trichloride

including (BTC),24

silsesquioxane

(OAPS),23

2,4,6-tris(4-aminophenyl)pyridine

(TAPP)24,

octa(aminophenyl)

1,3,5-tris(aminophenyl)benzene (TAPB),25 polymaleic anhydride (PMA)26 and even 4,4-oxydianiline (ODA)-modified graphene oxides27 were investigated. These cross-linked polyimide aerogels can also be fabricated into continuous thin films with outstanding flexibility. Additionally, the recyclable adsorption28, acoustic29, dielectric24, moisture resistance23, pore structures30,

31

as well as thermal

properties32, 33 were researched. However, the reports about polyimide aerogels for CO2 adsorption are scarce. The gas capture performance of porous networks depends not only on the pore structures and specific surface area, but also on the chemical composition.34 Due to the Lewis-acidic character and quadrupole momentum of the polyimide, CO2 molecules are expected to strongly interact with the highly polar imide functionalities on the polymer surface. Some recent researches have suggested that the introduction of functional groups, such as amino, sulfonate35 and hydroxyl groups36, can effectively improve the adsorption property.

In this paper, polyimide aerogels were prepared via sol-gel technique, and supercritical fluid CO2 drying method. The wet-gels were synthesized by 4-amino-N-methylbenzamide (DABA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), and cross-linking with TAB, using pyridine and


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acetic anhydride as the catalyst and dehydrating agent, respectively. Finally, the structures and properties of the as-synthesized polyimide aerogels were investigated. Specially, the CO2 adsorption performance of polyimide aerogels were discussed at the 25 °C and in the pressure range of 0~1.0 bar.

■ EXPERIMENTAL SECTION Materials. DABA was obtained from Aldrich Industrial Corporation (China). Pyridine was purchased from Shanghai Shenbo Chemical Co., Ltd. (China). Acetic anhydride and acetone were obtained

from

Shanghai

Lingfeng

Chemical

Reagent

Co.,

Ltd.

(China).

BPDA

and

N-methyl-2-pyrrolidinone (NMP) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). TAB was purchased from ShenZhen Lijing Shenghua Co., Ltd. (China). All of the reagents were used directly as received without any treatment.


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Scheme 1. Synthetic route for polyimide aerogels cross-linked with TAB. Table 1. Formulations and textural properties of the as-prepared polyimide aerogels. Repeat unit

Polymer concen

Bulk density

n

(wt %)

(g/cm3)

(%)

1

15

5.0

0.091

2

20

5.0

3

25

4

Thermal conductivity

Surface aera

(W/m·K)

(m2/g)

(%)

9.73

0.0307

478.42

94.15

0.103

11.84

0.0312

494.80

5.0

0.127

13.57

0.0319

15

7.5

0.160

12.22

5

20

7.5

0.162

6

25

7.5

0.167

Sample

Average pore size

CO2 uptake

V(total)

V(micro)

(cm3/g)

(cm3/g)

22.41

1.43

0.124

27.21

93.13

18.62

1.26

0.112

25.82

449.76

91.58

13.59

0.99

0.109

25.17

0.0338

517.74

89.85

15.44

1.91

0.147

24.83

14.74

0.0332

538.19

90.73

13.65

2.29

0.166

31.19

17.36

0.0341

506.26

88.82

10.37

1.29

0.118

28.44

Shrinkage

Porosity


(nm)

(cm3/g)

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Preparation of Polyimide Aerogel Monoliths. Scheme 1 shows the schematic reaction process of preparing polyimide aerogels. Polyamic acid oligomer was initially synthesized using DABA and BPDA, which reacted rapidly in NMP. The repeat units in the oligomers, n, were 15, 20, and 25 by using a ratio of n diamine units to (n + 1) dianhydride units to fabricate the oligomers according to Table 1, and the total polymer concentrations (5.0 wt % and 7.5 wt %) were carried out in the gelation solution. As an example, the preparation of the 7.5% polyimide aerogels with n = 15. DABA (7.5 m mol, 1.704 g) was added to 50 ml of NMP and magnetically stirred until the DABA fully dissolved. Then, the solution was cooled to 0 °C, and (7.725 m mol, 2.2728 g) of BPDA was added. The mixture was stirred for 10 min at 0 °C and for 12 h at room temperature to yield a uniform solution. Subsequently, a solution of TAB (0.166 m mol, 0.066 g) in 24.7 ml of NMP was added with rapid stirring for approximately 10 min, followed by the addition of acetic anhydride (61.8 m mol, 5.8 ml, 8:1 M ratio to BPDA) and pyridine (61.8 m mol, 5.0 ml, 1:1 M ratio to acetic anhydride) to the solution. Immediately after mixing, the solution was poured into the molds and gelled within 30 min. Afterward, The gels were aged for 24 h in the mold at room temperature. Following aging, the wet gels were extracted into a solution of 75% NMP in acetone and soaked overnight. Subsequently, the solvent was exchanged in 24 h intervals with 25% NMP in acetone and finally with only 100% acetone. Finally, the monolithic polyimide aerogels were obtained using supercritical fluid CO2 drying (45 °C, 10 MPa, 12 h), followed by vacuum drying for 24 h at 55 °C to remove any residual acetone. Characterization. Samples of polyimide aerogels were prepared in cylinders (diameter 25 mm, height 25 mm) and the bulk density was determined by ρ=m/v where ρ, m and v are bulk density, mass and volume (obtained by v=πD2h/4 where D and h are diameter and height of the samples) respectively. Shrinkage given in Table 1 is the initial shrinkage as fabricated calculated based on the diameter


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shrinkage from the initial diameter of the cylindrical mold and diameter of polyimide aerogels post supercritical drying. Attenuated total reflectance (ATR) infrared spectroscopy was conducted using a Thermo Nicolet-IS5 Fourier transform infrared spectrometer. Thermogravimetric analyses (TGA) were performed using a simultaneous thermal analyzer (DCS1/1600LF), and samples were run at a temperature ramp rate of 10 °C per min from room temperature to 1000 °C under a nitrogen atmosphere. 13C NMR spectra of the polymers were obtained on a Bruker Avance III 400 spectrometer. The microstructure was surveyed by LEO-1530VP scanning electron microscopy (SEM) operating at the acceleration voltage of 10 kV. Pore structure properties were measured by Nitrogen adsorption/desorption porosimetry (Micromeritics ASAP2020 surface area). The specific surface areas were calculated using Brunaur-Emmett-Teller (BET) and the pore-size distribution were obtained from the desorption branch of isotherms by using the Barrett-Joyner-Halenda (BJH) model. The samples were outgassed at 75 °C for 10 h. The bulk densities (ρa) were calculated by measuring the weight and the volume of the samples (diameter 25 mm, height 25 mm). The skeletal densities (ρs) were measured using a G-DenPyc 2900 analyzer. The porosities were calculated using Eq. (1) Porosity = (1-ρα/ρs) × 100%

(1)

Carbon dioxide adsorption isotherms were collected on a Micromeritics ASAP2020 analyzer at 25 °C with the pressure range of 0~1.0 bar. The samples were outgassed at 150 °C under high vacuum for 3 h prior to the sorption measurements. The thermal conductivities were tested using a Hot Disk TPS 2500S thermal conductivity meter at 25 °C. The compressive strengths and modulus of the samples (15~20 mm in diameter and 30 mm in length) were measured by using an INSTRON 3382 testing machine. The test temperature was 25 °C and the test speed was 2.0 mm/min.


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■ RESULTS AND DISCUSSION

Figure 1. Polyimide aerogels: wet gel and aerogel. Figure 1 shows the photographs of the wet gel and polyimide aerogel, which present brown and maize-yellow in color, respectively. The final aerogel retains the monolithic morphology with little volume shrinkage compared with the original wet gels. According to previous reports37, the rigid DABA monomer may tend to form polymer repeat units with a high degree of planarity, which could lead to rigid chains and induce the chains to pack closely. The variables used to fabricate the samples are listed in Table 1 including the bulk density, shrinkage, thermal conductivity, porous structure and the CO2 uptake. As shown in Table 1, the bulk densities of the samples are 0.091~0.127 g/cm3 and 0.144~0.167 g/cm3 with polymer concentrations (5.0 wt % and 7.5 wt %), respectively. Additionally, all the samples with different bulk densities possess low thermal conductivities at 25 °C (0.0307~0.0341 W/m·K), which are mainly caused by the unique nanopores and framework structures of polyimide aerogels.


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Figure 2. FT-IR spectra for typical chemical groups in the solution at different reaction stages.

Figure 3. Solid NMR spectrum of polyimide aerogel (7.5 wt %, n = 15). Figure 2 shows the FTIR spectra of the samples at different reaction stages, which clearly illustrate the imidization process. First, there was only DABA in NMP and no obvious characteristic peaks appeared. Then, the reaction between the diamine and dianhydride occurred with the appearance of the secondary amide (CONH) at 1550 cm-1 after the introduction of BPDA. After adding the cross-linker of TAB, the peak at 1550 cm-1 was still present and no other peaks appeared that indicated


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the occurrence of cross-linking process. At last, the characteristic peak of CONH at 1550 cm-1 disappeared in the final aerogel sample after the chemical imidization and drying treatment. All of the about four reaction stages confirm the formation of the complete polyimide structure via chemical imidization. In addition, the two peaks at 1720 cm-1 and 1779 cm-1 are attributed to the symmetrical and asymmetrical stretch vibrations of C=O, two characteristic peaks in imide38. The broad weak transmittance at 3200~3400 cm-1 is mostly attributed to the stretch vibration of the –NH– group in DABA. With the ongoing imidization, the peak of –NH– group became more and more wide due to the formation of hydrogen bonds in the reaction process.39 The other characteristic peaks at 1662 cm-1, 1500 cm-1, 1368 cm-1 and 1320 cm-1 represent amic acid C=O, benzene ring, amide C-N, respectively. Figure 3 shows the

13

C NMR spectrum of the solid sample 4 of the polyimide aerogel. An imide

carbonyl peak at 166 ppm is associated with the carbonyl in amide group from DABA. Similarly, many previous reports indicated that the carboxyl groups in DABA and BPDA cannot be distinguished from the NMR spectra.40 The N atom connected aromatic carbon peak at 143.4 ppm, and other aromatic peaks between 120 and 145 ppm that are characteristic of polyimide.

Figure 4. TG-DSC curves under N2 of polyimide aerogel (7.5 wt %, n = 15).


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Figure 4 presents the TG and DSC curves of the as-prepared polyimide aerogel (7.5 wt %, n=15) under a nitrogen atmosphere from room temperature to 1000 °C, and the other samples curves exhibit the similar trends. The thermogram profile can be divided into four main stages: (1) 25~120 °C, (2) 120~560 °C, (3) 560~660 °C, (4) 660~1000 °C. At the first stage, the small weight loss of 2.8% is caused by the adsorption of H2O, CO2 and solvent. As is well known, because of the nano-sized porous structure and high porosity, the H2O, CO2 and solvent in the porous structure of the sample can not be removed completely during the supercritical CO2 drying process. At the second stage, the weight loss of 7.7% is attributed to decomposition of organic groups. At the third stage, there is an apparent weight loss of 33.5% due to the rupture of C-N bond and the continuous thermal decomposition of polyimide aerogel. Eventually, the polyimide aerogel is carbonized and decomposed completely with the weight loss of 11.0% at 1000 °C. Additionally, there is a slight mass loss in the range of 120~300 °C, indicating that the imidization process is not complete41. Furthermore, it is found that an obvious endothermic peak appears at 717 °C (shown as the DSC curve in Figure 4), which is primarily attributed to the thermal decomposition process of polyimide aerogel and other thermal behaviors.

Figure 5. SEM images of polyimide aerogels (a, b, c=5.0 wt %; d, e, f=7.5 wt %; n=15, 20, 25).


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Figure 5 shows the scanning electron microscopy (SEM) images of the polyimide aerogels with different polymer concentrations and n values. It is well known that the microstructures of the oxide aerogels42 such as silica, alumina, titanium, which present the three-dimensional network structures formed with the interconnection of nanoparticles( 2~10 nm). Intriguingly, the as-synthesized polyimide aerogels consist of a 3D network of polymer nanofibers tangled together with fiber diameters ranging from 15 nm to 50 nm. This result is similar to the morphology of some reported polyimide aerogels.26 Obviously, the increase of the densities of polyimide aerogels (as shown in Table 1) leads to the denser fibrillar structures and smaller pore sizes. In addition, polyimide aerogel made using 7.5 wt %, n=25 (Figure 5f) has densely packed strands. This may be due to the polyimide oligomers having greater chain rigidity, high planarity, and shorter chain length between cross-links, leading to the greater shrinkage. Therefore, this layout of above-mentioned polyimide nanofibers most likely forms caused by solvent interaction during the polymerization and subsequent gelation processes.

Figure 6. Typical N2 adsorption/desorption isotherms of polyimide aerogels. Inserts: corresponding pore size distribution curves. The pore structure characteristics and specific surface areas were further evaluated by nitrogen dsorption/desorption technique. The pore size distribution curves and adsorption/desorption isotherms


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of aerogels samples are shown in Figure 6. The relevant data are summarized in Table 1. All of the adsorption/desorption isotherms are Type IV according to the IUPAC classification, which indicates the presence of a significant fraction of mesopores. As shown in Figure 6 and Table 1, all the samples have high specific surface area (449.76~538.19 m2/g) and high porosity (88.87~94.15%), as shown by the change in volume of N2 adsorbed. More narrowly, the samples of 7.5 wt % have higher specific surface areas than the samples of 5.0 wt %, but the change in values undergo the similar trend of first increase and then decrease. This result might be attributed to the increase in the number of repeat units and polymer concentrations, which would cause the pore structures become more homogenous and close. However, with the densification of pore structures, there appears some agglomeration particles and sealing channels, which results in the decrease of specific surface areas, as indicated by the SEM images in Figure 5c, 5f. From the pore size distribution, it can be seen that the samples of 5.0 wt % show a broad pore size distributions ranging from 2 nm to more than 100 nm, while the pore size distribution curves of the samples of 7.5 wt % are narrow, presumably due to the smaller and more uniform pore structures without extra-large pores, as shown in Figure 5. In addition, with the increase of repeat units, the corresponding average pore size of the samples of 5.0 wt % decrease from 22.41 nm to13.59 nm, while from 15.44 nm to 10.37 nm for the samples of 7.5 wt %. Therefore, both repeat unit and polymer concentration are likely to affect the microscopic pore structures of polyimide aerogels, which is similar to the previous research results.43


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Figure 7. CO2 adsorption isotherms of polyimide aerogels. The CO2 adsorption properties of the polyimide aerogels were further investigated at 25 °C with the test pressure range of 0~1.0 bar. As seen in Figure 7, with an increase of the pressure, the uptake of CO2 also shows a prospective growing trend. Particularly, the uptake of CO2 exhibits a rapid increase in the low-pressure region, which indicates that CO2 molecules have advantageous interactions with the porous polymer skeleton. It can be seen that the adsorption capacity at low pressure reflects the close relationship between the adsorbates and the pore walls.44 Obviously, the sample (7.5 wt %, n = 20) shows the highest CO2 uptake capacity of 31.19 cm3/g and the detailed adsorption data are shown in Table 1. By contrast, the samples of 7.5 wt % have higher CO2 uptake than the samples of 5.0 wt % with the ranges of 24.83~31.19 cm3/g and 25.17~27.21 cm3/g, respectively. According to previous researches, the CO2 adsorption capacity had been studied at 1.0 bar and 25 °C, the triazine-based porous polyimide (9.57~27.82 cm3/g) discussed by Liebl45. In addition, the values of CO2 uptake for many other porous polymer materials were also mentioned, such as PI-2 (~22.26 cm3/g)46, PAF-1 (~24.27 cm3/g)47, micropore CEs (~20 cm3/g)48, and PSN-2 (~21 cm3/g)49. Thus, the CO2 uptake of 31.19 cm3/g in this study is substantially higher than the values of the previous porous materials reported under the similar conditions. The higher CO2 uptake may be attributed to the excellent microscopic porous structure of the as-prepared polyimide aerogel with high porosity, large pore


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volume and specific surface area, as shown in Table 1. It is explained that both the large pore volume and high specific surface area contribute to the interaction between the CO2 molecule and sample, which could increase the opportunity of adsorbent to contract with gas molecule and supply more adsorptive sites. Moreover, according to the theory of molecular dynamics, the kinetic diameter is equal to the intermolecular distance of two molecules colliding with zero initial kinetic energy, which indicates that the smaller pore size is beneficial to CO2 adsorption.45 Therefore, it could be concluded that the CO2 adsorption property dependent on both the pore structure and specific surface area, which represents the factual adsorptive site.

Figure 8. Adsorption enthalpy curves of polyimide aerogels. Furthermore, the isosteric enthalpies (Qst) of CO2 for polyimide aerogels were calculated from the adsorption isotherms measured at different temperatures using the Clausius-Clapeyron equation44:

lnP =

Qst +C RT

(2)

where, P, T, R and C are the pressure, the temperature at equilibrium, the gas constant, and the equation constant, respectively. The adsorption enthalpy curves of polyimide aerogels are shown in Figure 8. it is thought that the isosteric enthalpy is a heating effect due to the increased adsorption capacity, which is another measurement index for characterizing the adsorbing performance of a material. Generally, the superior CO2 adsorbents with larger adsorption capacity must have high adsorption enthalpy, which


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can reflect the intermolecular force between the adsorbent and the gas molecule. For some previous studies, a smaller pore size (more micropores) leads to higher Qst values. The Qst values of the samples in this study are slightly lower than the reported microporous polyimides (MPIs, 31~35 kJ/mol)50 because of their special mesoporous network structure. As shown in Figure 8, with the increase of the n value, the corresponding Qst values show the rising trend due to the greater number of the oligomers in the reaction system. Additionally, the Qst values apparently decrease with the increasing adsorption capacity, indicating that the interaction between CO2 and the pore wall is stronger than that between the CO2 molecules. On the other hand, the sample with higher n value has higher Qst value due to the higher content of oxygen and nitrogen atoms from the imide heterocycles in the network, which can produce strong dipole-quadrupole interactions between the pore surface and CO2 molecules. Therefore, the Qst value is another key influence factor for the CO2 adsorption performance of the porous polyimide aerogels. Combining with the above analysis of the adsorption isotherms (Figure 7), it was observed that the CO2 adsorption capacity in the porous polymers was mainly due to three aspects: 1) the larger pore volume and specific surface area; 2) the smaller and homogeneous pore structure; 3) the higher adsorption enthalpy.

Figure 9. The stress-strain curves of the selected polyimide aerogels (7.5 wt %).


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Figure 9 shows the uniaxial compression experiment and compressive stress-strain curves of the selected polyimide aerogels. The typical stress-strain curves are similar to the previous polyimide aerogels51 and other polymer reinforced aerogels52, 53, with a linear elastic region up to about 5~6% strain and exhibiting yield up to about 60~70% strain. Young’s modulus is taken as the initial slope of the stress strain curve. Generally, both compression strength and Young’s modulus of aerogels increase as the density increase, which is due to the increasing polymer concentration and n value. As presented in Figure 9, the stress at 10% strain and Young’s modulus of the selected samples (7.5 wt %, n = 15, 20, 25) are 0.46 MPa, 0.49 MPa, 0.56 MPa and 18.52 MPa, 19.47 MPa, 21.13 MPa, respectively. The test results are similar to the previous reports20, 24. Therefore, the as-synthesized polyimide aerogel with good mechanical property could possibly be used as thermal isolator and high pressure adsorbent for CO2 capture.

■ CONCLUSIONS Polyimide aerogels were successfully synthesized via sol-gel technique and supercritical CO2 drying by using DABA, BPDA and TAB as the cross-linker. The resulting aerogels vary in terms of bulk density, shrinkage, specific surface area, porosity, pore size, pore volume, thermal conductivity and CO2 absorption capacity depending on the number of repeat units, n, and the polymer concentration. In comparison to previously reported polyimide aerogels, the as-prepared polyimide aerogels showed lower bulk densities (0.091~0.167 g/cm3), lower shrinkages (9.73~17.36%) and lower thermal conductivities (0.0307~0.0341 W/m·K), which were mainly attributed to the unique nanopores and framework structures of polyimide aerogels. The samples had high thermal stability with the onset of decomposition above 560 °C and exhibited high char yield in nitrogen. Particularly, the CO2 adsorption capacities of polyimide aerogels were substantially higher than the values of the previous


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porous materials reported under the approximate conditions. The sample of 7.5 wt %, n = 20, showed the highest CO2 uptake of 31.19 cm3/g tested at 25 °C and 1.0 bar. The results showed that the CO2 adsorption capacity in the porous polymers was mainly due to three aspects: 1) the larger pore volume and specific surface area; 2) the smaller and homogeneous pore structure; 3) the higher adsorption enthalpy. Increasing the repeat unit resulted in increased compressive strength and Young’s modulus of the selected samples of 7.5 wt %. Therefore, the as-prepared polyimide aerogels with versatile properties can be the promising candidates for thermal insulators and CO2 adsorbents, reducing energy consumption and protecting the atmospheric environment.


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■AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. Zhong). *E-mail: [email protected] (X. Shen). ORCID Ya Zhong: 0000-0002-9995-9662 Author Contributions These two authors (Ya Zhong and Yong Kong) contributed equally to this work and should be considered co-first authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors acknowledge the supports from the National Natural Science Foundation of China (51702156 and 51602151), the Priority Academic Program Development of Jiangsu Higher Education Institution

(PAPD)—China, the

(BK20161002

and

Natural Science

BK20161003),

the

Scientific

Foundation and

of Jiangsu Province—China

Technologic

Start-ups

Incubation

Program—Entrepreneurship Competition Winning Project (BC2016036), and Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites.


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Table of Contents Graphic


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Preparation and Characterization of Polyimide Aerogels with a

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