- Email: [email protected]
Solution-processable polyimide aerogels with high hydrophobicity
Materials Letters 176 (2016) 118–121
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solution-processable polyimide aerogels with high hydrophobicity Shuai Wu, Ai Du n, Shangming Huang, Wei Sun, Youlai Xiang, Bin Zhou n Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 23 March 2016 Accepted 12 April 2016 Available online 13 April 2016
Hydrophobic polyimide aerogels with high solubility were prepared by combining sol-gel technology and supercritical CO2 extraction. Polyimide gels were derived from hybrid diamines 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP) with chemical imidization. Introduction of BAPP enhanced the hydrophobicity and water resistance of the obtained polyimide aerogels, as well as the solution-process ability. Overall, the obtained polyimide aerogels have low thermal conductivity in the range of 0.032–0.059 W/mK. Comparing to the conventional polyimide aerogels, solution-processable polyimide aerogels with high hydrophobicity are more applicable as thermal insulators in harsh environments. & 2016 Published by Elsevier B.V.
Keywords: Porous materials Polyimide aerogels Thermal insulation Sol-gel preparation
1. Introduction Polyimide aerogels have high thermal stability, high mechanical strength, low dielectric constant, and low thermal conductivity. These properties makes them excellent materials for lightweight substrates for antennas, flexible insulation for space suits, and inflatable structures for entry, descent, and landing applications [1]. Previously, most of the researches about the polyimide aerogels have been focused on their shrinkage, strength, thermal stability, or the cost of production [1–3]. Indeed, the significant achievements have promoted the development of the polyimide aerogel. In 2015, FLEXcon helped NASA develop ideas into reality with polyimide aerogel technology for many aerospace applications such as insulation for cryotanks and spacesuits, as well as more down-to-Earth uses in construction, refrigeration and pipe insulation [4]. Despite their outstanding properties and great potential for application, most of the conventional polyimide aerogels are insoluble and infusible due to their rigid backbones and strong interchain interactions, leading to processing difficulties [5]. Generally, solubility of polyimide can be improved by the structural modifications of monomers [5–7]. Among them, incorporation of the ether bond in the repeats units is an effective way [7]. However, oxygen links in the backbones make the aerogels susceptible to moisture. For the polyimide aerogel, water in its porous networks has negative effects on the structures and properties, especially the excellent thermal insulation. As previously reported, moisture n
Corresponding authors. E-mail addresses: [email protected] (A. Du), [email protected] (B. Zhou). http://dx.doi.org/10.1016/j.matlet.2016.04.099 0167-577X/& 2016 Published by Elsevier B.V.
resistance can be enhanced by the hydrophobic segments [7,8]. In this paper, polyimide aerogels with high solubility and hydrophobicity were formed by the hybrid diamines ODA and BAPP. The gels were derived from the PAA cross-linked by the crosslinker of BTMSPA and dried by supercritical CO2 extraction. Solubility, hydrophobicity, and moisture resistance of the obtained samples were investigated.
2. Experimental Preparation of polyimide aerogel: The molar ratio of dianhydride to total diamines is 26:25. BPDA (8.7 mmol, InnoChem, AR) was slowly added to a stirred solution of hybrid diamines (InnoChem, AR) in 40 ml NMP (Sinopharm, AR). The mixture was stirred for nearly 0.5 h until the formation of polyamide acid (PAA), then BTMSPA (0.68 mmol, InnoChem, AR) was slowly added to end capped with PAA oligomers. The reaction mixture was vigorously stirred for 30 min, after which acetic anhydride (6.57 ml) and pyridine (5.62 ml) were added to finish the chemical imidization process. The mixture solution was continually stirred for 10 min, then was poured into a polytetrafluoroethylene mold. The gel was removed from the mold after gelation, and soaked in fresh acetone to remove the acetic anhydride and pyridine. Characterization: The morphology of the aerogels was observed by SEM (Philips-XL30FEG). The specific surface area and pore size distribution of the aerogels were obtained from nitrogen adsorption–desorption isotherms at 77 K, analyzed by Quantachrome Autosorb-1 analyzer. Thermal conductivity was measured on thermal constants analyzer (Hot Disk, TPS 2500S) at room temperature. Thermo gravimetric (TG) analysis was conducted from room temperature to 900 °C on a TGA-7 instrument (Perkin Elmer,
S. Wu et al. / Materials Letters 176 (2016) 118–121
119
Table 1 The detailed properties of the obtained polyimide aerogels. Samples
BAPP (wt%)
Density (g/cm3)
Shrinkage (%)
Surface area (m2/g)
Pore volume (cm3/g)
Thermal conductivity (W/mK)
Onset of decomposition (°C)
PI/BAPP-0 PI/BAPP-25 PI/BAPP-50 PI/BAPP-75 PI/BAPP-100
0 25 50 75 100
0.122 0.137 0.124 0.172 0.246
10.6 12.2 8.2 15.3 15.7
662.1 446.2 357.5 76.1 28.9
2.61 1.96 1.52 0.39 0.07
0.0322 0.0334 0.0336 0.0495 0.0594
553 551 553 564 530
Fig. 1. (a-1) Nitrogen absorption-desorption isotherms and (a-2) pore-size distribution; (b) SEM images and possible mechanism of the polyimide aerogels.
a)
b) BAPP content
Water absorption(g/g)
Contact angle(deg.)
160 140 120 100 80 60 40 20 0 0
25
50 (%)
75
100
PI-0
5
PI-25
PI-50 PI-75 PI-100
4 3 BAPP content
2 1 0 0
25
50 (%)
75
Fig. 2. (a) Water contact angle and (b) water adsorption of the polyimide aerogels.
100
120
S. Wu et al. / Materials Letters 176 (2016) 118–121
Fig. 3. Solubility test of the PI-75 and PI-100 in NMP.
America). Contact angles (CA) were measured using a JC2000A optical contact-angle software.
3. Results and discussion The hybrid diamines consist of 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP). As list in Table 1, mass fraction (wt%) of BAPP in the total weight of the hybrid diamines was varied from 0% to 100% to fabricate polyimide aerogels PI/BAPP-0, PI/BAPP-25, PI/BAPP-50, PI/BAPP-75, and PI/ BAPP-100. The precursor polyamide acid (PAA) was cross-linked by the Si-O-Si, which is derived from the hydrolysis and condensation of the BTMSPA via the absorbed water (detailed reaction mechanism was illustrated in Fig. S1). The gels was dried by supercritical CO2 extraction to obtain the polyimide aerogels with density in the range of 0.12–0.25 g/cm3. Fig. S3 shows the FT-IR spectra of all the obtained polyimide aerogels, the characteristic bands containing 1776 cm 1 (asymmetric imide C ¼O), 1716 cm 1 (symmetric imide C ¼O) and 1375 cm 1 (imide C-N) indicated that the imidization is complete [1–3]. Moreover, the high onset decomposition temperature of about 550 °C in N2 for all the samples (as list in Table 1) provides a further confirmation for the complete imidization process [1–3]. Adsorption-desorption isotherms of nitrogen of the samples are shown in Fig. 1a-1. According to IUPAC, all the isotherms are type IV with H1 hysteresis loop. H1 hysteresis loop was often reported for materials with compacts of approximately spherical particles or cylindrical pore geometry [9]. From Fig. 1a-2, we can see that the pore size of the obtained samples is mainly in the range of meso porous and macro porous and the distribution peak are almost at 20 nm. However, the microstructures of the obtained samples show in SEM images are quite different. From Fig. 1b, we can see that the PI/BAPP-0 is nano fibrous. And the polymer chains become stubs or particles in PI/BAPP-25, PI/BAPP-50, PI/BAPP-75, and PI/BAPP100, following the increasing amount of BAPP. There are even many big clusters in the PI/BAPP-75 and PI/BAPP-100. The difference on nanostructure may be attributed to the rigidity of their monomers. Introduction of BAPP tend to increase the flexibility of the repeat units. These flexible building blocks have more opportunities to meet together and form the structure like a ball of wool. The possible mechanism is illustrated in Fig. 1b-6. As list in Table 1, transformation on the microstructure from shaggy fibrous networks to particle packing backbones cause a decrease in pore volume from 2.61 to 0.07 cm3/g, as well as the
decrease in specific surface area from 622 to 29 g/cm3. The main reason is that the big clusters cause a decrease in the micro and meso pores (as show in Fig. 1a-2), which result in a reduction on specific surface area [10]. In addition, decrease in micro and meso pores also increase the gas thermal conductivity of the obtained polyimide aerogels [11,12]. Meanwhile, big cluster in the frameworks increases the solid thermal conductivity [11,12]. Therefore, total thermal conductivity of the samples increase from 0.032 to 0.059 W/mK (as list in Table 1). As we can see from the Fig. 2a, the PI/BAPP-0 is hydrophilic with water contact angle of about 80°, because the imide rings in the polyimide backbones are hydrophilic. After replacing ODA with an increasing amount of BAPP, the water contact angle increase from 105° to 131°. The main reason for the enhanced hydrophobicity is the introduction of the BAPP with hydrophobic methyl groups. Water-absorption test was carried out by immersing the samples into deionized water. Fig. 2b show the water resistance of the obtained polyimide aerogels and the inset presents the samples with different amount of BAPP in the test. The PI/BAPP-0 and PI/BAPP-25 absorbed water into their pores and sank into the bottom after a short time. However, the PI/BAPP-50, PI/BAPP-75, and PI/BAPP-100 are water-resistant, which remained floating on the surface of the water indefinitely. The histogram in Fig. 2b show that the moisture content in the first two samples are around 500%, which are much higher than that in the latter three ones (less than 10%). In nature, the process of water absorption on the polyimide aerogels take place by wetting and capillary action. On one hand, the contact angle provides an inverse measure of wettability. A contact angle less than 90° usually indicates that wetting of the surface is very favorable, and contact angles greater than 90° generally means that wetting of the surface is unfavorable. On the other hand, capillary force is often depending on the porous structure of the samples. Small pores such as micro pores and meso pores can cause a higher surface area which can greatly aid in the process [13]. As mentioned above, introduction of BAPP tend to enhance the hydrophobicity of the polyimide aerogels. The enhanced hydrophobicity is a key factor for the decrease in the water absorption capacity, which significantly restraints the wetting process. In addition, decrease in specific surface area for the increasing amount of BAPP, to a large extent, weakens the capillary action and improves the water resistance. Solubility of the obtained samples is tested by dissolving the samples in NMP. As we can see from Fig. 3, PI/BAPP-100 and PI/ BAPP-75 have high solubility and are dissolved completely in NMP at around 15 °C. However, PI/BAPP-0, PI/BAPP-25, and PI/BAPP-50
S. Wu et al. / Materials Letters 176 (2016) 118–121
have limited solubility and were partially dissolved in the NMP until the temperature increases to 40 °C. In nature, the ether linkage tends to weaken the regularity and crystallinity of polymer molecules, leading to high solubility [7]. The excellent solubility of PI/BAPP-50 and PI/BAPP-100 can be ascribed to the introduction of ether bridge segments in BAPP, which have one more ether than ODA. Meanwhile, methyl groups in the side chains, to some extent, also weaken the molecular polarity and improve the solubility of the polyimide aerogels [7].
4. Conclusion Hydrophobic polyimide aerogels with high solubility are derived from the combination of 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP). Over all, the obtained polyimide aerogels exhibit excellent thermal insulation with thermal conductivity in the range of 0.032–0.059 W/mK.
Acknowledgements This work was supported by National High Technology R&D Program of China (2013AA031801), National Natural Science Foundation of China (51172163).
121
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.04. 099.
References [1] M.A.B. Meador, C.R. Alemán, K. Hanson, et al., ACS Appl. Mater. Interfaces 7 (2015) 1240–1249. [2] M.A.B. Meador, L. McCorkle, D.J. Quade, et al., ACS Appl. Mater. Interfaces 3 (2011) 546–552. [3] M.A.B. Meador, E.J. Malow, R. Silva, et al., ACS Appl. Mater. Interfaces 4 (2012) 536–544. [4] BusinessWire. 〈http://search.proquest.com/docview/1709152929/fulltext?ac countid ¼28839〉, (accessed 3.09.15). [5] Y.C. Kunga, S.H. Hsiao, Mater. Chem. 21 (2011) 1746–1754. [6] L.L. Liu, X.G. Zheng, Z. Sun, et al., Liq. Cryst. 10 (2015) 1382–1390. [7] L. Xiong, Xl Wang, H.X. Qi, et al., J. Appl. Polym. Sci. 10 (2013) 1493–1501. [8] L.W. Hrubesh, P.R. Coronado, Jr Satcher, J. Non-Cryst. Solids 285 (2001) 328–332. [9] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169–3183. [10] S.J. He, Y. Zhang, X.P. Shi, et al., Mater. Lett. 144 (2015) 82–84. [11] G.Q. Zu, J. Shen, W.Q. Wang, et al., Chem. Mater. 26 (2014) 5761–5772. [12] G.H. Tang, C. Bi, Y. Zhao, et al., Energy 90 (2015) 701–721. [13] H.J. Wang, A. Kleinhammes, T.P. McNicholas, et al., J. Phys. Chem. C 118 (2014) 8474–8480.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Solution-processable polyimide aerogels with high hydrophobicity Shuai Wu, Ai Du n, Shangming Huang, Wei Sun, Youlai Xiang, Bin Zhou n Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Tongji University, Shanghai 200092, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 February 2016 Received in revised form 23 March 2016 Accepted 12 April 2016 Available online 13 April 2016
Hydrophobic polyimide aerogels with high solubility were prepared by combining sol-gel technology and supercritical CO2 extraction. Polyimide gels were derived from hybrid diamines 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP) with chemical imidization. Introduction of BAPP enhanced the hydrophobicity and water resistance of the obtained polyimide aerogels, as well as the solution-process ability. Overall, the obtained polyimide aerogels have low thermal conductivity in the range of 0.032–0.059 W/mK. Comparing to the conventional polyimide aerogels, solution-processable polyimide aerogels with high hydrophobicity are more applicable as thermal insulators in harsh environments. & 2016 Published by Elsevier B.V.
Keywords: Porous materials Polyimide aerogels Thermal insulation Sol-gel preparation
1. Introduction Polyimide aerogels have high thermal stability, high mechanical strength, low dielectric constant, and low thermal conductivity. These properties makes them excellent materials for lightweight substrates for antennas, flexible insulation for space suits, and inflatable structures for entry, descent, and landing applications [1]. Previously, most of the researches about the polyimide aerogels have been focused on their shrinkage, strength, thermal stability, or the cost of production [1–3]. Indeed, the significant achievements have promoted the development of the polyimide aerogel. In 2015, FLEXcon helped NASA develop ideas into reality with polyimide aerogel technology for many aerospace applications such as insulation for cryotanks and spacesuits, as well as more down-to-Earth uses in construction, refrigeration and pipe insulation [4]. Despite their outstanding properties and great potential for application, most of the conventional polyimide aerogels are insoluble and infusible due to their rigid backbones and strong interchain interactions, leading to processing difficulties [5]. Generally, solubility of polyimide can be improved by the structural modifications of monomers [5–7]. Among them, incorporation of the ether bond in the repeats units is an effective way [7]. However, oxygen links in the backbones make the aerogels susceptible to moisture. For the polyimide aerogel, water in its porous networks has negative effects on the structures and properties, especially the excellent thermal insulation. As previously reported, moisture n
Corresponding authors. E-mail addresses: [email protected] (A. Du), [email protected] (B. Zhou). http://dx.doi.org/10.1016/j.matlet.2016.04.099 0167-577X/& 2016 Published by Elsevier B.V.
resistance can be enhanced by the hydrophobic segments [7,8]. In this paper, polyimide aerogels with high solubility and hydrophobicity were formed by the hybrid diamines ODA and BAPP. The gels were derived from the PAA cross-linked by the crosslinker of BTMSPA and dried by supercritical CO2 extraction. Solubility, hydrophobicity, and moisture resistance of the obtained samples were investigated.
2. Experimental Preparation of polyimide aerogel: The molar ratio of dianhydride to total diamines is 26:25. BPDA (8.7 mmol, InnoChem, AR) was slowly added to a stirred solution of hybrid diamines (InnoChem, AR) in 40 ml NMP (Sinopharm, AR). The mixture was stirred for nearly 0.5 h until the formation of polyamide acid (PAA), then BTMSPA (0.68 mmol, InnoChem, AR) was slowly added to end capped with PAA oligomers. The reaction mixture was vigorously stirred for 30 min, after which acetic anhydride (6.57 ml) and pyridine (5.62 ml) were added to finish the chemical imidization process. The mixture solution was continually stirred for 10 min, then was poured into a polytetrafluoroethylene mold. The gel was removed from the mold after gelation, and soaked in fresh acetone to remove the acetic anhydride and pyridine. Characterization: The morphology of the aerogels was observed by SEM (Philips-XL30FEG). The specific surface area and pore size distribution of the aerogels were obtained from nitrogen adsorption–desorption isotherms at 77 K, analyzed by Quantachrome Autosorb-1 analyzer. Thermal conductivity was measured on thermal constants analyzer (Hot Disk, TPS 2500S) at room temperature. Thermo gravimetric (TG) analysis was conducted from room temperature to 900 °C on a TGA-7 instrument (Perkin Elmer,
S. Wu et al. / Materials Letters 176 (2016) 118–121
119
Table 1 The detailed properties of the obtained polyimide aerogels. Samples
BAPP (wt%)
Density (g/cm3)
Shrinkage (%)
Surface area (m2/g)
Pore volume (cm3/g)
Thermal conductivity (W/mK)
Onset of decomposition (°C)
PI/BAPP-0 PI/BAPP-25 PI/BAPP-50 PI/BAPP-75 PI/BAPP-100
0 25 50 75 100
0.122 0.137 0.124 0.172 0.246
10.6 12.2 8.2 15.3 15.7
662.1 446.2 357.5 76.1 28.9
2.61 1.96 1.52 0.39 0.07
0.0322 0.0334 0.0336 0.0495 0.0594
553 551 553 564 530
Fig. 1. (a-1) Nitrogen absorption-desorption isotherms and (a-2) pore-size distribution; (b) SEM images and possible mechanism of the polyimide aerogels.
a)
b) BAPP content
Water absorption(g/g)
Contact angle(deg.)
160 140 120 100 80 60 40 20 0 0
25
50 (%)
75
100
PI-0
5
PI-25
PI-50 PI-75 PI-100
4 3 BAPP content
2 1 0 0
25
50 (%)
75
Fig. 2. (a) Water contact angle and (b) water adsorption of the polyimide aerogels.
100
120
S. Wu et al. / Materials Letters 176 (2016) 118–121
Fig. 3. Solubility test of the PI-75 and PI-100 in NMP.
America). Contact angles (CA) were measured using a JC2000A optical contact-angle software.
3. Results and discussion The hybrid diamines consist of 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP). As list in Table 1, mass fraction (wt%) of BAPP in the total weight of the hybrid diamines was varied from 0% to 100% to fabricate polyimide aerogels PI/BAPP-0, PI/BAPP-25, PI/BAPP-50, PI/BAPP-75, and PI/ BAPP-100. The precursor polyamide acid (PAA) was cross-linked by the Si-O-Si, which is derived from the hydrolysis and condensation of the BTMSPA via the absorbed water (detailed reaction mechanism was illustrated in Fig. S1). The gels was dried by supercritical CO2 extraction to obtain the polyimide aerogels with density in the range of 0.12–0.25 g/cm3. Fig. S3 shows the FT-IR spectra of all the obtained polyimide aerogels, the characteristic bands containing 1776 cm 1 (asymmetric imide C ¼O), 1716 cm 1 (symmetric imide C ¼O) and 1375 cm 1 (imide C-N) indicated that the imidization is complete [1–3]. Moreover, the high onset decomposition temperature of about 550 °C in N2 for all the samples (as list in Table 1) provides a further confirmation for the complete imidization process [1–3]. Adsorption-desorption isotherms of nitrogen of the samples are shown in Fig. 1a-1. According to IUPAC, all the isotherms are type IV with H1 hysteresis loop. H1 hysteresis loop was often reported for materials with compacts of approximately spherical particles or cylindrical pore geometry [9]. From Fig. 1a-2, we can see that the pore size of the obtained samples is mainly in the range of meso porous and macro porous and the distribution peak are almost at 20 nm. However, the microstructures of the obtained samples show in SEM images are quite different. From Fig. 1b, we can see that the PI/BAPP-0 is nano fibrous. And the polymer chains become stubs or particles in PI/BAPP-25, PI/BAPP-50, PI/BAPP-75, and PI/BAPP100, following the increasing amount of BAPP. There are even many big clusters in the PI/BAPP-75 and PI/BAPP-100. The difference on nanostructure may be attributed to the rigidity of their monomers. Introduction of BAPP tend to increase the flexibility of the repeat units. These flexible building blocks have more opportunities to meet together and form the structure like a ball of wool. The possible mechanism is illustrated in Fig. 1b-6. As list in Table 1, transformation on the microstructure from shaggy fibrous networks to particle packing backbones cause a decrease in pore volume from 2.61 to 0.07 cm3/g, as well as the
decrease in specific surface area from 622 to 29 g/cm3. The main reason is that the big clusters cause a decrease in the micro and meso pores (as show in Fig. 1a-2), which result in a reduction on specific surface area [10]. In addition, decrease in micro and meso pores also increase the gas thermal conductivity of the obtained polyimide aerogels [11,12]. Meanwhile, big cluster in the frameworks increases the solid thermal conductivity [11,12]. Therefore, total thermal conductivity of the samples increase from 0.032 to 0.059 W/mK (as list in Table 1). As we can see from the Fig. 2a, the PI/BAPP-0 is hydrophilic with water contact angle of about 80°, because the imide rings in the polyimide backbones are hydrophilic. After replacing ODA with an increasing amount of BAPP, the water contact angle increase from 105° to 131°. The main reason for the enhanced hydrophobicity is the introduction of the BAPP with hydrophobic methyl groups. Water-absorption test was carried out by immersing the samples into deionized water. Fig. 2b show the water resistance of the obtained polyimide aerogels and the inset presents the samples with different amount of BAPP in the test. The PI/BAPP-0 and PI/BAPP-25 absorbed water into their pores and sank into the bottom after a short time. However, the PI/BAPP-50, PI/BAPP-75, and PI/BAPP-100 are water-resistant, which remained floating on the surface of the water indefinitely. The histogram in Fig. 2b show that the moisture content in the first two samples are around 500%, which are much higher than that in the latter three ones (less than 10%). In nature, the process of water absorption on the polyimide aerogels take place by wetting and capillary action. On one hand, the contact angle provides an inverse measure of wettability. A contact angle less than 90° usually indicates that wetting of the surface is very favorable, and contact angles greater than 90° generally means that wetting of the surface is unfavorable. On the other hand, capillary force is often depending on the porous structure of the samples. Small pores such as micro pores and meso pores can cause a higher surface area which can greatly aid in the process [13]. As mentioned above, introduction of BAPP tend to enhance the hydrophobicity of the polyimide aerogels. The enhanced hydrophobicity is a key factor for the decrease in the water absorption capacity, which significantly restraints the wetting process. In addition, decrease in specific surface area for the increasing amount of BAPP, to a large extent, weakens the capillary action and improves the water resistance. Solubility of the obtained samples is tested by dissolving the samples in NMP. As we can see from Fig. 3, PI/BAPP-100 and PI/ BAPP-75 have high solubility and are dissolved completely in NMP at around 15 °C. However, PI/BAPP-0, PI/BAPP-25, and PI/BAPP-50
S. Wu et al. / Materials Letters 176 (2016) 118–121
have limited solubility and were partially dissolved in the NMP until the temperature increases to 40 °C. In nature, the ether linkage tends to weaken the regularity and crystallinity of polymer molecules, leading to high solubility [7]. The excellent solubility of PI/BAPP-50 and PI/BAPP-100 can be ascribed to the introduction of ether bridge segments in BAPP, which have one more ether than ODA. Meanwhile, methyl groups in the side chains, to some extent, also weaken the molecular polarity and improve the solubility of the polyimide aerogels [7].
4. Conclusion Hydrophobic polyimide aerogels with high solubility are derived from the combination of 4, 4′-oxydianiline (ODA) and 2, 2-Bis [4-(4-aMinophenoxy) phenyl] propane (BAPP). Over all, the obtained polyimide aerogels exhibit excellent thermal insulation with thermal conductivity in the range of 0.032–0.059 W/mK.
Acknowledgements This work was supported by National High Technology R&D Program of China (2013AA031801), National Natural Science Foundation of China (51172163).
121
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.04. 099.
References [1] M.A.B. Meador, C.R. Alemán, K. Hanson, et al., ACS Appl. Mater. Interfaces 7 (2015) 1240–1249. [2] M.A.B. Meador, L. McCorkle, D.J. Quade, et al., ACS Appl. Mater. Interfaces 3 (2011) 546–552. [3] M.A.B. Meador, E.J. Malow, R. Silva, et al., ACS Appl. Mater. Interfaces 4 (2012) 536–544. [4] BusinessWire. 〈http://search.proquest.com/docview/1709152929/fulltext?ac countid ¼28839〉, (accessed 3.09.15). [5] Y.C. Kunga, S.H. Hsiao, Mater. Chem. 21 (2011) 1746–1754. [6] L.L. Liu, X.G. Zheng, Z. Sun, et al., Liq. Cryst. 10 (2015) 1382–1390. [7] L. Xiong, Xl Wang, H.X. Qi, et al., J. Appl. Polym. Sci. 10 (2013) 1493–1501. [8] L.W. Hrubesh, P.R. Coronado, Jr Satcher, J. Non-Cryst. Solids 285 (2001) 328–332. [9] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169–3183. [10] S.J. He, Y. Zhang, X.P. Shi, et al., Mater. Lett. 144 (2015) 82–84. [11] G.Q. Zu, J. Shen, W.Q. Wang, et al., Chem. Mater. 26 (2014) 5761–5772. [12] G.H. Tang, C. Bi, Y. Zhao, et al., Energy 90 (2015) 701–721. [13] H.J. Wang, A. Kleinhammes, T.P. McNicholas, et al., J. Phys. Chem. C 118 (2014) 8474–8480.