Research Article www.acsami.org
Study on Thermal Conductivities of Aromatic Polyimide Aerogels Junzong Feng, Xin Wang, Yonggang Jiang, Dongxuan Du, and Jian Feng* Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, 109 De Ya Road, Changsha, Hunan 410073, China ABSTRACT: Polyimide aerogels for low density thermal insulation materials were produced by 4,4′-diaminodiphenyl ether and 3,3′,4,4′-biphenyltetracarboxylic dianhydride, cross-linked with 1,3,5-triaminophenoxybenzene. The densities of obtained polyimide aerogels are between 0.081 and 0.141 g cm−3, and the specific surface areas are between 288 and 322 m2 g−1. The thermal conductivities were measured by a Hot Disk thermal constant analyzer. The value of the measured thermal conductivity under carbon dioxide atmosphere is lower than that under nitrogen atmosphere. Under pressure of 5 Pa at −130 °C, the thermal conductivity is the lowest, which is 8.42 mW (m K)−1. The polyimide aerogels have lower conductivity [30.80 mW (m K)−1], compared to the value for other organic foams (polyurethane foam, phenolic foam, and polystyrene foam) with similar apparent densities under ambient pressure at 25 °C. The results indicate that polyimide aerogel is an ideal insulation material for aerospace and other applications. KEYWORDS: polyimide aerogels, cross-link, thermal conductivity, supercritical drying, low temperature
1. INTRODUCTION Polyimide aerogels as a functional organic aerogel have attracted much attention in recent years. They were produced by a sol−gel process, and then the solvent was removed in the wet gels using supercritical drying,1 similar to the traditional process and method for preparing aerogels.2 Inorganic aerogels such as the most widely studied silica aerogels have high use temperature but significant fragility. Hybrid silica aerogels neckreinforced by polymers have excellent mechanical strengths but poor thermal stability. Compared to silica aerogels, polyimide aerogels exhibited excellent mechanical and thermal stability, combined with excellent thermal insulation properties, low density, and flexibility, making them an ideal insulation material for aerospace and other applications, such as the measurement stations on the surface of Mars.3,4 For the synthesis of polyimide aerogels, the DuPont process is regarded as the classic route, and the polyimide aerogels have been synthesized through this typical two-step method using diamines and dianhydrides.1,3,5,6 First, the polyamic acid is synthesized using diamines and dianhydrides at ambient temperature, and subsequently is cyclized into polyimide by chemically method (acetic anhydride as dehydrating agent and pyridine as catalyst) or thermally at high temperature. Another route for preparation of polyimide aerogels is the PMR method. Polyimide aerogels have been synthesized using the PMR route from bis-NAD by Leventis, and the thermal conductivity is 31.00 mW (m K)−1.7 However, both of the two routes yield the polyimide aerogels showed large shrinkage. Recently, an alternative route was introduced by NASA Glenn Research Center. Using an aromatic triamine, 1,3,5-triaminophenoxybenzene (TAB), 1,3,5-benzenetricarbonyl trichloride (BTC), polyhedral oligomeric silsesquioxane, octa(aminophenyl) silsesquioxane (OAPS), or 2,4,6-tris(4-aminophenyl) pyridine © XXXX American Chemical Society
(TAPP) as cross-linker, the reaction between diamines and dianhydrides can occur even at room temperature to form polyimide gels.8 These cross-linked polyimide aerogels can be prepared into thin films with excellent flexibility.1,3 The moisture resistance had been studied by Guo,4 and dielectric properties were studied by Meador.9,10 Cellulose nanocrystalreinforced polyimide aerogels were researched by Nguyen.11 The thermal conductivity ranging from 20 to 200 °C had been studied by Guo.3 The thermal conductivity of the polyimide aerogel at 200 °C increases as expected, compared to that at 20 °C. However, thermal conductivities at ultralow temperature or different atmospheres have not been reported before. In this paper, we systematically examine the thermal conductivities of cross-linked polyimide aerogels, which are obtained via supercritical CO2 drying of wet-gels synthesized by 4,4′-diaminodiphenyl ether (ODA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), and cross-linking with TAB, using pyridine and acetic anhydride as catalyst and dehydrating agent, respectively. Different densities of polyimide aerogels were synthesized. The densities of polyimide aerogels vary by tuning the concentration of cross-linked polyimide in the solutions. The effects of temperatures (−130 to 50 °C), gas types (nitrogen and carbon dioxide), and gas pressures (5 Pa to 100 kPa) on the thermal conductivities of the aerogels were discussed, and the thermal conductivities were compared with those of other polymer foams (e.g., polyurethane foam, phenolic foam, and polystyrene foam). Received: February 22, 2016 Accepted: May 5, 2016
A
DOI: 10.1021/acsami.6b02183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of Polyimide Aerogels
2.3. Characterization. A Nicolet model 360 spectrometer was used for characterizing Fourier transform infrared spectrum. BET specific surface area was measured by nitrogen sorption method using a QuadraSorb SI (Quantachrome) analyzer. Barrett−Joyner−Halenda (BJH) pore size distribution was calculated by the desorption branch. The samples were outgassed at 80 °C for 6 h. The thermal conductivities of the polyimide aerogels were tested with specimen size of 50 mm × 50 mm × 15 mm using a Hot Disk thermal constants analyzer (TPS2500S, Sweden), and the 5501 detector was used. The pure polyimide aerogels are isotropic materials, and the thermal conductivities were measured by “bulk modules”. The uncertainty of measurement is ≤5%. The standard deviation of the measured thermal conductivity was calculated from 3 tests for each sample. The thermal conductivities were tested in the evacuable, controllable temperature equipment; the temperatures ranged from −130 to 50 °C, and were controlled by liquid nitrogen cooling or electronic heating. The gas pressures ranged from 5 Pa to 100 kPa, filling with CO2 or N2 gas. The thermal conductivities of the polyimide aerogels were tested under different temperature, pressure, and gas situations. A Hitachi S4800 SEM (Japan) was used to investigate the microstructure of the polyimide aerogels after coating the samples with gold. The apparent density (ρa) was calculated by measuring the weight and the volume of the samples. The skeletal density (ρs) was measured using a 3H2000TD helium pycnometer (Bei Shi de Instrument Technology Beijing Co., Ltd.). The porosity was obtained by calculation using eq 1.3
2. EXPERIMENTAL SECTION 2.1. Materials. BPDA was obtained from Changzhou Sunlight Pharmaceutical Co., Ltd. (China). Pyridine was bought from Xiya Chemical Co., Ltd. (China). Acetic anhydride was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (China). ODA and Nmethyl-2-pyrrolidinone (NMP) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. (China). TAB was bought from Aladdin Industrial Co. Acetone was obtained from Xilong Chemical Co., Ltd. (China). Before using the BPDA, it needed to be vacuumdried for 1 day. Other reagents were used directly. 2.2. Preparation. Scheme 1 shows the reaction process of preparing polyimide aerogels.1 Polyamic acid oligomer was synthesized using BPDA and ODA, which reacted rapidly in NMP at 20 °C. After the solution became transparent, TAB as a cross-linking agent was added to the reaction system. The solution became very sticky after stirring for a few minutes, and then by adding acetic anhydride and pyridine. Gelation occurred within 15 min after adding acetic anhydride and pyridine to the polyamic acid solution. After exchanging the NMP in the gels with acetone through a series of wash steps, the gels were dried by supercritical CO2 under 15 MPa at 70 °C. The obtained aerogels were dried at 80 °C under vacuum for 12 h.1 The chemical structure of TAB contains three amino groups. TAB was used as cross-linked polyamide acid, which form the threedimensional network structure. It is generally known that BPDA and ODA are common monomers used for the synthesis of the polyimides. BPDA provides the polyimides with rigid frame structure and improves the glass-transition temperatures and the thermal stability.12 The ether bond of ODA between phenyl rings enhances the flexibility of polyimides.1
porosity = (1 − ρa /ρs ) × 100% B
(1)
DOI: 10.1021/acsami.6b02183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
3. RESULTS AND DISCUSSION 3.1. Infrared Spectrum of Polyimide Aerogels. Figure 1 provides the Fourier tranform infrared spectrum of polyimide
Table 1. Textural Properties of the Polyimide Aerogels sample
conc of polyimide sol (%)
density (g cm−3)
porosity (%)
BET specific surface area (m2 g−1)
1 2 3
7.0 10.0 12.5
0.081 0.114 0.141
92.4 91.3 89.7
340 332 320
“bulk modules” at 20 °C under 100 kPa. The error bars were derived from 3 tests for the sample. As shown in Figure 4, the unapparent effect of the density is seen on thermal conductivity which ranged from 30.12 to 30.96 mW (m K)−1 for all aerogels studied. We chose the samples having the density 0.114 g cm−3 and measured the thermal conductivity under different temperature, pressure, and gas situation. The error bars were derived from 3 tests for each sample (Figures 5−7). Figure 5 shows the change of thermal conductivity with temperatures under 5 Pa and 100 kPa. The thermal conductivities of polyimide aerogels were measured between mean temperatures of −130 and 50 °C. As seen in the graph, the thermal conductivity is 23.55 mW (m K)−1 at 50 °C and drops largely to 8.42 mW (m K)−1 at −130 °C under 5 Pa. The thermal conductivity is 43.14 mW (m K)−1 at 50 °C and drops down to 11.93 mW (m K)−1 at −130 °C under 100 kPa. The thermal conductivity is composed of solid thermal conductivity, gas thermal conductivity, and radiation thermal conductivity, which decrease with the falling of temperature. The thermal conductivities under different gas pressures are shown in Figure 6, which were measured in an air atmosphere at 20 °C. It indicates that the thermal conductivity stays nearly constant between 5 and 2500 Pa. The thermal conductivities slowly increase from 20.44 to 23.48 mW (m K)−1 between 2500 Pa and 25 kPa. A dramatic increase from 23.48 to 30.85 mW (m K)−1 is observed between 25 and 100 kPa. This is because the mean free path of the gas lengthens with a decrease of the gas pressure. Figure 7 shows the relationship between thermal conductivities of the polyimide aerogels and temperatures, which were measured at −130 to 50 °C, under 1000 Pa N2 and CO2 atmospheres, respectively. As can be seen in Figure 7, the thermal conductivities are 30.53 and 33.67 mW (m K)−1 at 20 °C for CO2 and N2 atmosphere, respectively. The thermal conductivities under CO2 atmosphere are lower than those under N2 atmosphere. This may be due to a greater influence on the gaseous thermal conductivity. The gaseous thermal conduction is associated with the composition and structure of the gas. In general, the smaller the gas molecular weight and the simpler its composition and structure, the greater the
Figure 1. Fourier tranform infrared spectrum of polyimide aerogels.
aerogel with peaks for the CO bending vibration at 750 cm−1, cyclic CO stretching vibration at 1729 and 1780 cm−1, and CN stretching vibration at 1381 cm−1. The absence of a peak at 1550 cm−1 confirms the complete cyclization of the precursor polyamic acid to polyimide after the two-step imidization. 3.2. Textural Properties of the Polyimide Aerogels. Figure 2 shows the nitrogen sorption isotherms at 77 K and the BJH pore size distributions of the polyimide aerogels. In Figure 2a, the adsorption isotherms of polyimide aerogels exhibit IUPAC type IV curves, showing that they contain macropores (>50 nm). From Figure 2b, it can be seen that the pore size of polyimide aerogels is between 3 and 100 nm, ranging from mesopore to macropore, and the pore size distribution peaks are around 15−50 nm. Table 1 shows the textural properties of the polyimide aerogels. It is demonstrated that when the concentration of polyimide sol was increased in the preparation process, the densities of the resulting polyimide aerogels also increase. For different density samples, both the porosities and the BET specific surface areas of the polyimide aerogels are similar. The BET specific surface areas are 320−340 m2 g−1. SEM images of the polyimide aerogels are shown in Figure 3. They indicate that the polyimide aerogels are composed of a 3dimensional network of interconnected particles and continuously open pores. The sizes of the three-dimensional skeleton increase, and the porosity decreases with the increasing densities.13 3.3. Thermal Conductivity of the Polyimide Aerogels. The pure polyimide aerogels are isotropic materials. The thermal conductivities of polyimide aerogels were measured by
Figure 2. (a) Nitrogen sorption isotherms and (b) BJH pore size distribution of the polyimide aerogels. C
DOI: 10.1021/acsami.6b02183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. SEM images of polyimide aerogels with different bulk densities: (a) 0.081, (b) 0.114, (c) 0.141 g cm−3.
Figure 4. Thermal conductivity of aerogels vs density.
Figure 7. Thermal conductivity of polyimide aerogels at different temperatures in N2 (■) and CO2 (○) atmospheres.
coefficient of thermal conductivity. Since CO2 molecular weight is higher than N2, the thermal conductivity under CO2 atmosphere is lower. For comparison, the thermal conductivities of other polymer foams are shown in Table 2. It can been seen that, for the Table 2. Thermal Conductivities of Different Materials in Air at 25 °C
Figure 5. Thermal conductivities of the polyimide aerogels under 5 Pa and 100 kPa.
material
density/ (g cm−3)
thermal conductivity/ [mW (m K)−1]
polyurethane foam phenolic foam polystyrene foam polyimide aerogels
0.102 0.081 0.081 0.111
40.54 43.11 40.32 30.80
similar densities of polyimide aerogels and organic foams (polyurethane foam, phenolic foam, and polystyrene foam), polyimide aerogels have the lowest thermal conductivities. The reason may be that polyimide aerogels have porous nanostructure with high specific surface area. The thermal conductivities of polyimide aerogels are composed of solid thermal conduction, gaseous thermal conduction, and radiative thermal conduction. Gas phase heat transfer is an important method for heat transfer. The unique nanopores and network framework structure of aerogel limit the movement of gas molecules, causing the Knudsen rarefied gas effect, making the gaseous thermal conductivity of aerogel lower than the free space of gas thermal conductivity.14
Figure 6. Relationship between thermal conductivity of polyimide aerogels and air gas pressures.
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DOI: 10.1021/acsami.6b02183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Dielectric Polyimide Aerogels As Substrates for Lightweight Patch Antennas. ACS Appl. Mater. Interfaces 2012, 4, 6346−6353. (10) Meador, M. A. B.; McMillon, E.; Sandberg, A.; Barrios, E.; Wilmoth, N. G.; Mueller, C. H.; Miranda, F. A. Dielectric and Other Properties of Polyimide Aerogels Containing Fluorinated Blocks. ACS Appl. Mater. Interfaces 2014, 6, 6062−6068. (11) Nguyen, B. N.; Cudjoe, E.; Douglas, A.; Scheiman, D.; McCorkle, L.; Meador, M. A. B.; Rowan, S. J. Polyimide Cellulose Nanocrystal Composite Aerogels. Macromolecules 2016, 49, 1692− 1703. (12) Liu, Y. F.; Wang, Z.; Li, G.; Ding, M. X. Thermal and Mechanical Properties of Phenylethynyl-Containing Imide Oligomers Based on Isomeric Biphenyltetracarboxylic Dianhydrides. High Perform. High Perform. Polym. 2010, 22, 95−108. (13) Qian, J. J.; Chen, Y. R.; Feng, J.; Feng, J. Z.; Jiang, Y. G. Synthesis and Properties of Polyimide Aerogels. Gongneng Cailiao 2014, 45, 20122−20126. (14) Reichenauer, G.; Heinemann, U.; Ebert, H. P. Relationship between Pore Size and the Gas Pressure Dependence of the Gaseous Thermal Conductivity. Colloids Surf., A 2007, 300, 204−210.
4. CONCLUSIONS Polyimide aerogels by cross-linking BPDA and ODA with TAB were prepared, and the effects of temperatures, gas types, and gas pressures on the thermal conductivities of the polyimide aerogels were studied. When measured under pressure of 5 Pa at −130 °C, the thermal conductivity of polyimide aerogels with density of 0.114 g cm−3 is the lowest [8.42 mW (m K)−1]. The thermal conductivity is 30.53 mW (m K)−1 at 20 °C under CO 2 atmosphere, and 33.67 mW (m K) −1 under N 2 atmosphere. Environment gas atmosphere impacts the thermal conductivity, and the value of the measured thermal conductivity under CO2 atmosphere is lower than that under N2 atmosphere. The thermal conductivity of polyimide aerogel increases with increasing temperature and gas pressure. The total thermal conductivities of polyimide aerogels are 30.80 mW (m K)−1, which is lower than other organic foams (polyurethane foam, phenolic foam, and polystyrene foam) with similar density at 25 °C. Polyimide aerogels exhibit excellent thermal insulation properties, making them an ideal insulation material for aerospace and other applications, such as extravehicular activity suits, tents, or buildings.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51172279, 51302317) and Natural Science Foundation of Hunan Province China (14JJ3008).
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REFERENCES
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DOI: 10.1021/acsami.6b02183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
