Graphene Oxide: A Versatile Agent for Polyimide Foams with

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Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility Linli Xu, Shidong Jiang, Baopeng Li, Wenpeng Hou, Guoxing Li, Mushtaque A. Memon, Yong Huang, and Jianxin Geng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00981 • Publication Date (Web): 01 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Chemistry of Materials

Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility Linli Xu, Shidong Jiang, Baopeng Li, Wenpeng Hou, Guoxing Li, Mushtaque A. Memon, Yong Huang, Jianxin Geng* Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China; Email: [email protected] Abstract: Close-celled aromatic polyimide (PI)/graphene foams with low density and improved flexibility were fabricated by thermal foaming of poly(amic ester)/graphene oxide (PAE/GO) precursor powders. The PAE/GO precursor powders were prepared by grafting GO nanosheets with PAE chains, which led to efficient dispersion of the GO nanosheets in PAE matrix. Incorporation of GO resulted in an enhanced foaming capability of the precursor, i.e. enlarged cell size and decreased foam density. Notably, a decrease of 50% in the foam density was obtained by addition of only 2 wt% GO in the precursor. In the foaming process, the GO nanosheets functioned as a versatile agent that not only provided heterogeneous nucleation sites but also produced gaseous molecules. By analyzing the foaming mechanism, the excellent features of GO in heat transfer, gas barrier, and strength reinforcement also facilitated to obtain large and uniform cells in the foams. In addition, the PI/graphene foams exhibited a prominent flexibility and enhanced flexural strength, as an elastic-to-nonelastic conversion of the initial stage of the compressive stress-strain curves was observed by increasing the content of graphene in the PI matrix and an increase of 22.5% in flexural strength was obtained by addition of 0.5 wt% GO in the precursor.

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Introduction Development of functional polymer-based foams with light weight and high specific properties by incorporation of carbon nanomaterials has recently attracted an enormous attention among scientists.1 Various conventional carbon-based nanofillers, including graphite, carbon nanofibers (CNFs), and carbon nanotubes (CNTs), have been incorporated into polymer matrixes for preparation of polymer foams with light weight, high toughness, and improved electrical conductivity.2-5 Although the cell size, the uniformity of the cells, and foam density can be regulated by properly controlling the foaming conditions, the incorporation of the conventional carbon nanofillers, which function as heterogeneous nucleation agents, commonly leads to decreased cell size.2-5 Indeed, the decrease of cell size results in increased foam density. However, light weight is quite valuable for the polymer-based foams used in weight-sensitive aerospace, transportation, and automobile industries, as light weight means high efficiency and saving in material and energy. As a rising star of carbon nanomaterials, graphene has been extensively employed in polymer-based composites owing to its nucleation, high strength, high Young's modulus, as well as excellent electrical and thermal properties.6-10 As the precursor of chemically converted graphene, graphene oxide (GO) is generally prepared by oxidation of graphite and possesses an abundance of oxygen-containing groups, which enable modification of GO nanosheets with desirable molecules or polymers covalently or noncovalently.11-15 As a result, the modified GO nanosheets show good compatibility with the interested polymers and can markedly improve the properties of the resultant composites even with a very low addition,16-20 due to its low percolation threshold in polymer matrixes (ca. 0.1 vol% for randomly oriented oblate ellipsoids with an aspect ratio of 1000).21 For example, an electrical conductivity of ca. 0.1 S m-1, which is more than 14 orders of magnitude higher than that of pristine polystyrene, was obtained with ca. 1 vol% of graphene loaded in polystyrene.18 Mechanical properties and thermal 2 ACS Paragon Plus Environment

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stability of polymers were also markedly improved through addition of graphene materials.19,22,23 The crumpled morphology of graphene nanosheets, along with their facile chemical tunability, allowed the fabrication of polymer/graphene nanocomposite films with enhanced gas barrier performance at extremely low loading.23-25 Graphene has also been employed in polymer-based foams, which have been demonstrated potential applications in areas such as sensors, supercapacitors, high-performance flexible electrode materials, oil-water separation, and electromagnetic interference shielding.1,22,26-35 Template methods, including either using a three-dimensional graphene foam as template to impregnate polymers26-28 or using a polymer foam as skeleton to assemble graphene sheets,29,30 turned out to be a straightforward approach for making polymer/graphene foams. But, the interfacial affinity between polymers and graphene, as well as the scalability of production, may limit the practical applications of such foams. Other approaches for fabricating polymer/graphene foams include subcritical CO2 assisted foaming of polymer/graphene nanocomposites,31 freeze drying of polymer/GO nanocomposite hydrogels,22,32 a water vapor induced phase separation process in polymer/graphene nanocomposite solution films,33,34 and in situ polymerization of vinyl monomers in GO stabilized Pickering high internal phase emulsions.35 Among these methods, graphene merely functions as skeleton or nanofiller in the polymerbased foams; therefore, the graphene only materialize their functionalities in the resultant polymer/graphene foams. As an ultrathin two-dimensional material, GO nanosheets contain plenty of oxygen-containing groups, which provide various routes for tuning the interface interactions with polymers. However, little attention has been paid on the function of GO nanosheets and the interface interactions between GO and polymers in the process of foaming, which are actually worthy of consideration because interfacial issues would influence the foaming behaviors.



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Herein, we report the fabrication of aromatic polyimide (PI)/graphene foams with low density and prominent flexibility by thermal foaming of poly(amic ester) (PAE)/GO nanocomposite precursor, which was synthesized from GO and classic monomers 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4'-oxydianiline (ODA). In contrast to the previously reported polymer/graphene foams that were based on general plastics, aromatic PI make the foams attractive for aeronautics and space structural materials due to their high thermal stability (> 300 ºC), high glass transition temperature (Tg > 250 ºC), and excellent mechanical properties. The foaming temperature of PAE is in line with the removal temperature of the oxygen-containing groups from the surfaces of GO nanosheets, making the GO nanosheets function as both heterogeneous nucleation sites and foaming agent in the foaming process. As a result, the incorporation of GO into the precursor not only improved the foaming capability of the precursor but also enhanced the flexibility of the resultant foams. To the best of our knowledge, this is the first report to disclose the effect of GO nanosheets on the foaming process of polymers and fabricate large-sized polymer/graphene foams using solid precursor powders. Experimental Section Materials and characterization methods: GO was synthesized by following a modified Hummers’ method.12,36 Experimental details for obtaining a GO slurry in a solvent compatible with the precursor system was provided in Supporting Information. Monomers BTDA and ODA (extra purity grade) were purchased from Aladdin Industrial Corp. BTDA was dried under vacuum at 120 °C for 5 h before used. All the other chemicals were purchased from Sinopharm Chemical Co. Ltd and used as obtained. The characterization methods were included in Supporting Information. Synthesis of PAE precursor: In a 1000 mL round-bottom flask, BTDA (136.7036 g, 0.4200 mol) was dissolved in a mixture solvent of tetrahydrofuran (THF) (95 mL) and methanol (MeOH) (65 mL) at room temperature under the protection of N2 atmosphere. The solution was kept at 70 °C for 6 h with 4 ACS Paragon Plus Environment

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magnetic stirring until the color changed from milky white to clear yellow, indicating conversion of BTDA into benzophenone-4,4'-dimethoxycarbonyl-3,3’-dicarboxylic acid (BDMDA). Then, ODA (86.2419 g, 0.4221 mol) were added to the BDMDA solution and stirred for 2 h to obtain a homogeneous PAE solution. The mole ratio of BTDA to ODA was 1:1.005, leading to amino-capped PAE chains. The PAE solution had a viscosity of 19000 mPa·s at room temperature and the solid content of the PAE solution was determined to be ca. 70 wt%. A portion of the PAE solution was dried at 70 °C for 14 h to evaporate the most solvent of THF and MeOH. The solid material was ground and sieved using an 80 mesh sieve to obtain fine PAE powders, which were labelled as P0. Synthesis of PAE/GO nanocomposite precursors: To PAE solutions, GO with different loadings (0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 wt% of PAE) was added with the aid of high-speed stirring (6000 rpm for 15 min at room temperature) and subsequent magnetic stirring (2 h at 70 °C). In addition, during the magnetic stirring, reactions took place between GO and PAE, leading to formation of uniform PAE/GO solutions in THF. The PAE/GO solutions containing different contents of GO were also dried at 70 °C for 14 h. The solid materials were ground and sieved using an 80 mesh sieve to obtain fine PAE/GO precursor powders, which were labelled as P0.1, P0.2, P0.5, P1, P2, and P5, respectively. To investigate the chemical reaction between GO nanosheets and PAE, the PAE/GO solutions were subjected to vacuum filtration using polytetrafluoroethene membranes having 0.2 µm pores and washed with copious THF to remove free PAE. The filtration cakes containing PAE-grafted GO, designated as GO-g-PAE, were dried at 70 °C for 4 h in a vacuum oven. Fabrication of PI foams and PI/graphene foams: PI and PI/graphene foams were prepared through a thermal foaming process in a mold (50 × 50 × 10 mm3) using the P0, P0.1, P0.2, P0.5, P1, P2, and P5 powders as precursor. The mold was preheated at 50 °C for 4 h in an oven. After loaded with the precursor powders, the mold was placed on a hot press for 20 min, with the temperature of the top and



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bottom heating plates set at 170 °C and a pressure between the two plates set at 15 MPa. And then, the mold was rapidly transferred into an oven set at 250 °C and kept in the oven for 60 min. After that, the temperature was heated up to 300 °C and held for 30 min. Finally, the mold was cooled to room temperature, and the foam was demoulded. The foams, obtained with P0, P0.1, P0.2, P0.5, P1, P2, and P5 powders, were labelled as F0, F0.1, F0.2, F0.5, F1, F2, and F5, respectively. The minimum quantities of the PAE and PAE/GO precursor powders needed to fill up the mold after foaming were determined by using different amounts of the precursor powders. To obtain intact individual cells of PI and PI/graphene, insufficient precursor powders were used in the thermal foaming and the external surfaces of the foams were used for SEM observation. To investigate the chemical process taking place on the surfaces of GO nanosheets during the thermal foaming, the GO-g-PAE nanosheets were thermally treated under the same condition, and the obtained material was designated as graphene-g-PI. The density of the foams was determined by the ratio between the mass and the volume of the foams. Results and Discussion In this work, we fabricated PI/graphene foams containing various contents of graphene by a two-step method that consists of synthesis of PAE/GO nanocomposites (the precursor of PI/graphene foams) and thermal foaming (Scheme 1). PI system was selected because of the following factors: (1) PI has high thermal stability and excellent mechanical properties, which make itself a promising structural material in aeronautic and space, transportation, and automobile industries; (2) the precursor of PI contains plenty of functional groups such as carboxylic acid, ester, and amine, which result in intimate interactions between the precursor and GO nanosheets through chemical bonding, and homogeneous dispersion of the modified GO nanosheets in the precursor matrix. PAE was adopted because the ester groups provide higher stability to the precursor than the carboxylic acid groups and release small molecules MeOH as foaming agent during thermal foaming. Notably, the foaming temperature for the 6 ACS Paragon Plus Environment

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PAE adopted in our research is coincident with the removal temperature of the oxygen-containing groups from GO nanosheets. Therefore, the presence of GO not only results in enhanced mechanical properties of the resultant PI/graphene foams, but also leads to improved foaming capability of the PAE/GO nanocomposites (see discussion later). As we know, this is the first report that discloses the effect of GO nanosheets on the foaming process of polymers. Scheme 1. (a) The synthesis of PAE precursor, (b) a representative molecular structure of GO, (c) the schematic illustration of grafting PAE chains onto the surfaces of GO nanosheets, and (d) the fabrication of PI/graphene foams through thermal foaming.



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In Scheme 1a is displayed the synthesis of PAE, which underwent an esterization of BTDA with MeOH to form BDMDA and subsequent polyconsendation between BDMDA and ODA to form PAE. In the polyconsendation, 0.5 mol% excess of ODA was loaded so that the ends of PAE chains were capped with amine groups. Considering the epoxide, hydroxyl, and carboxylic acid groups contained on GO nanosheets, we speculate that the PAE chains were grafted onto GO most possibility through the reaction between the epoxide groups and the amine groups, as schematically illustrated in Scheme 1c. As a result, the GO nanosheets were homogeneously dispersed in PAE solution.

Figure 1. FT-IR spectra of pristine PAE and the GO-g-PAE that was isolated from the PAE/GO solution prepared with 1 wt% GO. In order to clearly investigate the interactions between PAE and GO, the GO-g-PAE nanosheets were separated by filtration. Figure 1 shows the FT-IR spectra of pristine PAE and the GO-g-PAE that was isolated from the PAE/GO solution prepared with 1 wt% GO. The FT-IR spectrum of GO is presented in Supporting Information (Figure S1). The PAE powders yielded a spectrum containing a strong broad peak from 3200 to 3700 cm-1 corresponding to the stretching vibration of N-H bonds and a weak broad peak at ca. 2600 cm-1 due to the vibration of ammonium salts that situated at the ends of PAE chains. In addition, the PAE also exhibited peaks at 1720 cm-1 corresponding to the stretching vibration of C=O 8 ACS Paragon Plus Environment

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bonds, as well as peaks at 1500 and 1250 cm-1 attributed to the stretching vibrations of benzene rings and C-O-C bonds, respectively. Such spectroscopic features supported the chemical structure of PAE. In contrast, the peak at ca. 2600 cm-1 was disappeared in the spectrum of GO-g-PAE nanosheets, indicating disappearance of the ammonium salts due to the reaction between the terminal amine groups and the epoxide groups on the surfaces of GO nanosheets (a ring-opening reaction of the epoxide groups).11 Therefore, the FT-IR data supported that the surfaces of GO nanosheets were covalently grafted with the PAE chains.

Figure 2. TEM images of (a) GO nanosheets and (b) the GO-g-PAE nanosheets that were isolated from the PAE/GO solution prepared with 1 wt% GO. 9 ACS Paragon Plus Environment


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The morphologies of GO nanosheets and the GO-g-PAE nanosheets were studied using transmission electron microscopy (TEM). One can see that the GO nanosheets are very flat and have very low contrast (Figure 2a), indicating thin thickness of the sheets. In contrast, needle-like crystals can be clearly seen on the surfaces of the GO-g-PAE nanosheets (Figure 2b), directly proving the modification of GO nanosheets by PAE. Since the GO-g-PAE nanosheets was subjected to washing with a copious of good solvent of PAE during isolation, one can figure out that the PAE chains must be linked onto the GO nanosheets covalently. Moreover, the uniform attachment of the needle-like crystals indicated that the functionality, through which the GO was grafted, uniformly distributes on the surfaces of the GO nanosheets.

Figure 3. TGA curves of GO, pristine PAE, and the GO-g-PAE nanosheets that were isolated from the PAE/GO solution prepared with 1 wt% GO. 10 ACS Paragon Plus Environment

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The thermal stability and composition of GO, pristine PAE, and the GO-g-PAE nanosheets were investigated using thermogravimetric analysis (TGA). Figure 3 shows the TGA and differential TGA (DTGA) curves. GO exhibited a marked weight loss in the temperature range from ca. 90 to 220 °C (31.5 wt%) due to the decomposition of oxygen-caontaining groups such as hydroxyl, epoxide, and carboxylic acid groups.37 The pristine PAE powders displayed two marked weight losses in the temperature ranges from ca. 80 to 160 °C (7.50 wt%) and from ca. 160 to 280 °C (10.82 wt%) and a weak weight loss in the temperature range from ca. 300 to 350 °C (0.65 wt%), derived from the DTGA curve (the black line in Figure 3b), before the thermal decomposition of PI that happened higher than ca. 430 °C. Based on the chemical structure of PAE, we speculate that the weight losses happened at the aforementioned three temperature ranges might be contributed to the release of THF that were associated in PAE through intermolecular interactions, the removal of MeOH which resulted in conversion of PAE to PI, and the thermal decomposition of un-imidized PAE fragments, respectively. It is noteworthy that the temperature for the removal of oxygen-containing groups from GO nanosheets is coincident with that for the release of THF from the PAE precursor. Therefore, from the viewpoint of release of gaseous small molecules, the presence of GO may faciliate the foaming of PAE (see discussion later). As for GO-g-PAE nanosheets, the major weight losses occurred in temperature ranges from ca. 100 to 240 °C (3.08 wt%) and from ca. 280 to 350 °C (5.61 wt%), which were higher than the corresponding temperatures for prisine PAE probably due to the confined molecular movement of the PAE chains linked on the surfaces of GO nanosheets. In addition, the thermal decomposition temperature of the un-imidized PAE fragments (2.91 wt%) was higher (from ca. 375 to 450 °C) than that in the case of pristine PAE.13,22 In addition, the thermal decomposition temperature of PI linked on the surfaces of graphene nanosheets was ca. 50 °C higher than that of the pristine PI (ca. 430 °C for the former vs. 480 °C for the latter). 11 ACS Paragon Plus Environment


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Figure 4. (a) Normalized Raman spectra of GO, pristine PAE, and P1 (the PAE/GO precursor prepared with 1 wt% GO), and (b) Raman map of P1 film plotted by the intensity of G band (scanning area: 50 × 50 µm2). Raman characterization was carried out to further investigate the interactions of GO nanosheets with PAE and the dispersibility of the grafted GO nanosheets in PAE matrix. Figure 4a shows the normalized Raman spectra of GO, pristine PAE, and P1 obtained with an excitation wavelength of 532 nm. GO exhibited characteristic D and G bands at 1347 and 1598 cm-1, whereas no peak was detected for pristine PAE in the wavelength range scanned. As expected, P1 yielded a Raman spectrum that contained the D and G bands of graphene. But, the D band moved to a high-frequency position 1351 cm1

compared with that of GO due to the chemical modification of GO surfaces or charge transfer between

PAE and GO.15 In addition, the intensity of the D band for P1 was weaker than that of GO because of 12 ACS Paragon Plus Environment

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partial conversion from sp3 to sp2 carbon probably due to the removal of the hydroxyl and epoxide groups on the surfaces of GO nanosheets through dehydration reaction under an acidic condition (the pH value of the PAE solution was ca. 5).11,38 In experiments, the color of the P1 solution was found to change from brown yellow to black yellow as the reaction proceeded. In Figure 4b is displayed the Raman map of a P1 film plotted by the intensity of G band. It can be seen that the signals uniformly distribute in the image, indicating that the grafted GO nanosheets are uniformly distributed in PAE matrix. Based on the FT-IR, TEM, TGA, and Raman data, we can conclude that the GO nanosheets were covalently grafted with PAE chains, and that the resultant GO-g-PAE nanosheets homogeneously dispersed in PAE matrix.

Figure 5. The schematic illustration of the thermal foaming process and digital images of PI and PI/graphene foams. A ruler was used to scale the size of the foams. 13 ACS Paragon Plus Environment


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Table 1. The minimum weights of the precursors for fabricating one piece of PI or PI/graphene foam, and the densities of the foams. Samples The minimum weight of precursor for 1 piece of foam (g) Foam density -3 (g cm )

F0

F0.1

F0.2

F0.5

F1

F2

4.0

3.7

3.6

3.5

3.0

2.0

0.150

0.137

0.133

0.130

0.110

0.075

With the precursors in hand, next efforts were directed toward fabricating PI/graphene foams. Figure 5 shows the procedure of fabricating PI and PI/graphene foams through thermal foaming and the photos of PI and PI/graphene foams. Such stepwise procedure was optimized according to the TGA curves of PAE and PAE/GO precursor powders (Figure S2). Various amounts of precursor powders were tried for thermal foaming so that the minimum quantities of PAE and PAE/GO precursors were determined. FTIR data supported the conversion of PAE to PI upon such a thermal treatment procedure (Figure S3). With increasing the content of GO in the precursor, the color of the foams changed from light yellow to light green, then to dark green. The uniform color indicated the uniform quality of the foams. The content of foaming agent, i.e. the quantity of the gas released in the foaming process, was a very important factor to determine the foaming degree. The foaming agent in our PAE/GO system included the released THF associated to PAE by intermolecular interactions (from 80 to 160 °C in PAE’s TGA curve), the released MeOH at the initial stage of thermal removal of the easter groups (from 160 to 280 °C in PAE’s TGA curve), and the released small molecules such as CO2 due to the thermal removal of oxygen-containing groups from GO (from 90 to 220 °C in GO’s TGA curve). The content of the foaming agent released from PAE and the PAE/GO precursors was derived from the TGA curves by calculating the weight loss in the temperature range from 50 to 170 °C (Figure S2 and Table S1). The measured content of foaming agent gradually increased as the loading of GO increased in the precursor. 14 ACS Paragon Plus Environment

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The increased release of foaming agent may be attributed to the gaseous small molecules released from the GO nanosheets. The minimum quantities of the precursors required for filling up the mold (50 × 50 × 10 mm3) and the densities of the obtained foams were summarized in Table 1. It is seen that the minimum quantity decreased gradually as the loading of GO increased. Notably, the minimum quantity was decreased from 4 to 2 g with only 2 wt% GO loaded in PAE precursor, meanwhile the density of the foam decreased from 0.15 to 0.075 g cm-3. Therefore, a conclusion can be drawn that the presence of GO is beneficial to enhance the foaming capacity of PAE. Such a unique effect is exclusive for GO and it has never been reported for the conventional carbon-based nanofillers such as graphite, CNFs, and CNTs.2-4

Figure 6. (a) TGA and (b) DSC curves of PI and PI/graphene foams. 15 ACS Paragon Plus Environment


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The thermal properties of PI and the PI/graphene foams were evaluated using TGA and differential scanning calorimetry (DSC). PI and the PI/graphene foams all exhibited similar TGA curves (Figure 6a), from which a thermal decomposition temperature was detected at ca. 545 °C. The undetectable change in the thermal decomposition temperature upon incorporation of graphene is different from the case of general polymers,22 and this phenomenon might be ascribed to the rigid mainchain structure of PI. From the DSC curves of PI and the PI/graphene foams (Figure 6b), one can see that the Tg gradually decreased as the content of graphene increased in the foams. It was reported that the presence of huge rigid graphitic structures restricted the movement of polymer chains, leading to increased Tg.16,23 But, in this research, we observed an inverse changing trend of Tg, which might be due to the decreased molecular weight of the precursor because of the ester exchange reaction between PAE and GO nanosheets. Gao et al reported the decreased degree of polymerization upon increasing the weight percentage of GO, being in line with the condensation polymerization theory proposed by Flory.39 Similar results were also obtained through in situ polymerization in the presence of GO for preparation of poly(butylene succinate) and polyamide-6.40,41


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Figure 7. SEM images of intact individual cells (the left column) and fractured surfaces (the right column) of PI and the PI/graphene foams: (a, b) F0, (c, d) F0.1, (e, f) F0.2, (g, h) F0.5, (i, j) F1, and (k, l) F2. 17 ACS Paragon Plus Environment


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To further investigate the impact of GO on the thermal foaming of PAE, PI and PI/graphene foams were observed by scanning electron microscopic (SEM). Intact individual cells of PI and PI/graphene were obtained by thermal foaming of insufficient precursor powders (the left column in Figure 7). One can see that all the cells display round and intact morphology. According to the foaming mechanism, each cell is derived from one piece of precursor powder. In contrast, in the compact PI and PI/graphene foams, the cells squeeze each other to make the foams completely fill up the mold. For SEM observation, samples were torn out off the foams. PI was so tough, even at a low temperature of liquid nitrogen, that most of the cells in the fractured surfaces were torn up (the right column in Figure 7). Further analysis of the impact of GO on the thermal foaming of PAE was performed by combining the SEM images of both the intact and the fragmented cells. One can see that F0 was mainly composed of cells with a diameter ranging from 100 to 300 µm (Figure 7a and 7b). With increasing the loading of GO in PAE (Figure 7c-7l), we found the following changes in the morphology of the corresponding foams. First, the cell size increased and uniformized as the content of GO increased. For example, the size of the cells in F2 ranged from 400 to 500 µm (Figure 7k and 7l), which was ca. 2 times larger than the cells in F0. Therefore, the density of F2 was just a half of that of F0 (Table 1). Second, the quantity of the unfoamed particles decreased as the content of GO increased. These two features microscopically emphasized the enhanced foaming capability of the PAE due to the incorporation of GO. Third, the surfaces of the cells became rougher as the content of GO increased. The rough surfaces should be ascribed to the incorporated graphene nanosheets in the walls of the cells. The aligned graphene or GO nanosheets in the walls of the cells resulted in a reinforcing effect on the strength of the cells, leading to enlarged cells before the cells were blown out during foaming. In addition, the incorporation of graphene in polymers was also reported to improve the gas barrier of the polymer composites.42 This effect might be also beneficial to obtain large-sized cells since the PAE/GO walls, or the PI/graphene 18 ACS Paragon Plus Environment

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walls, could hold the released gas tightly in the cells during the foaming process. Finally, the content of GO was increased to 5 wt% in the PAE/GO composite. It was found that the foaming capability of precursor P5 was decreased, as more P5 powders were required to fill up the mold during foaming. SEM observations indicated that the cells of foam F5 were slightly smaller than the cells of foam F0, and that some cells were even blown out (Figure S4). These findings could be ascribed to deteriorated tensile property of the PAE/GO or PI/graphene composite due to overloading of GO.43



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Figure 8. (a) Compressive stress-strain curve of PI foam, with the initial section of the curve shown in the inset, (b) the initial section of the compressive stress-strain curves of PI and the PI/graphene foams, and (c) flexural strength of PI and the PI/graphene foams as function of the loading of GO.


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Table 2. Summary of the maximum endurable compressive strain and stress of PI and PI/graphene foams before the sliding of the cells. Samples

F0

F0.1

F0.2

F0.5

F1

F2

The compressive strain (%)

0.6

1.8

2.1

4.1

4.5

8.3

The compressive stress (MPa)

0.007

0.011

0.013

0.016

0.016

0.016

Next, efforts were directed towards measuring the mechanical properties of PI and the PI/graphene foams. Figure 8a shows a compressive stress-strain curve of PI foam, which includes an apparent plateau region, an elastic range, and a strain-hardening range after the yield point. Close inspection found that the apparent plateau region consisted of a short elastic range (within ca. 0.6% compressive strain) and a real plateau, where no change of compressive stress was detected within ca. 4% compressive strain (the inset in Figure 8a). We speculate that the short elastic range is ascribed to deformation of the cells and the following plateau is caused by the sliding between the cells in the foam. Such sliding leads to further packing of the cells. After that, PI foam underwent the elastic range, corresponding to the intrinsic deformation of the foam material, followed by the yield point, and strainhardening range. Upon incorporation of graphene (Figure 8b), the compressive stress-strain curves of F0.1 and F0.2 also began with a short elastic range and a plateau, but the elastic range ended at larger compressive strains (1.8 and 2.1% for F0.1 and F0.2) and greater compressive stresses (0.011 and 0.013 MPa for F0.1 and F0.2) (Table 2). With further increasing the content of graphene, the compressive stress-strain curves of F0.5, F1, and F2 began with a non-plastic range, which ended at further increased compressive strains (4.1, 4.5, and 8.3% for F0.5, F1, and F2) and further elevated compressive stresses (0.016 MPa for all F0.5, F1, and F2). Such different behaviors of the foams might be correlated to the percolation threshold: when the content of graphene was less than the percolation threshold, the cells of F0.1 and F0.2 behaved similar to that of F0, showing an initial elastic response to compressive strain; 21 ACS Paragon Plus Environment


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when the content of graphene was higher than the percolation threshold, the cells of F0.5, F1, and F2 showed an initial non-elastic response to the compressive strain. In addition, the endurable compressive stresses for F0.5, F1, and F2 were almost identical before their cells began sliding because the cells had the similar surface roughness, as evidenced from SEM data (Figure 7h, 7j, and 7l). The gradually increased endurable compressive stain, before the cells began sliding, with increasing the content of graphene, combining the conversion from the initial elastic deformation to the initial non-elastic deformation of the cells when the content of GO was higher than the percolation threshold, implied the enhanced compliance of the PI foams by incorporation of graphene. Therefore, PI/graphene foams could be used as cushion material. In Figure S5a are displayed the compressive stress-strain curves of PI and PI/graphene foams with the compressive stain up to 50%. The F0.1 and F0.2 showed the similar behaviors as F0, as a yield point could be recognized between the intrinsic elastic range and the strainhardening range of the foam materials. In contrast, once the content of graphene was higher than the percolation threshold, i.e. F0.5, F1, and F2, the yield point was not readily recognized in the compressive stress-strain curves, indicating that the formed graphene network in PI matrix played an important role in determining the mechanical properties of PI/graphene foams. For close-celled polymer foam materials, the interface adhesion between the cells determines the strength of the materials. Basically, the adhesion strength between the cells should be smaller than the intrinsic strength of the bulk material. Therefore, the fracture preferentially occurs at the interfaces between the cells if PI and the PI/graphene foams are stretched or bent. In order to ascertain the effect of graphene on the interface adhesion between the cells, flexural strength of PI foam and the PI/graphene foams was measured. Figure S5b shows the loading-displacement curves of PI and PI/graphene foams in flexural tests, with the flexural strength summarized in Figure 8c. One can see that F0.1, F0.2, F0.5, and F1 showed smaller flexural modulus and greater flexural strength than F0, indicating the improved


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interface adhesion between the cells and the enhanced flexibility of the foams. However, an exception was found to happen to F2, which exhibited an extremely small flexural strength. In this sample, the content of graphene is much higher than the percolation threshold so that some graphene nanosheets may be situated in between the cells. Therefore, the weak interlayer interaction of graphene nanosheets would lead to deterioration in the flexural strength of F2. PI/graphene foams possess larger cell size, narrower size distribution, lower density, and higher flexibility, and higher flexural strength than pristine PI foam, demonstrating that GO nanosheets function as a versatile agent in thermal foaming of polymers. Compared with other carbon-based nanofillers such as graphite, CNFs, and CNTs, graphene and GO are two-dimensional ultrathin nanosheets and GO additionally contains removable oxygen-containing groups. The functions of GO nanosheets in thermal foaming can be summarized as follows: (1) GO nanosheets function as nucleation agent in the foaming process. The gaseous molecules removed from the surfaces of GO nanosheets generate nucleation sites, which drastically lower the critical free energy of nucleation and thus facilitate rapid cell nucleation. Furthermore, the uniformly distributed nucleation sites further lead to uniform growth of the cells in the foaming process, leading to narrower size distribution. (2) GO nanosheets act as an effective gas barrier in the foaming process. It was reported that GO and graphene nanosheets improved the gas barrier of polymers.24,25,44-46 Therefore, the cell size is increased under the elevated internal pressure because the released gas is tightly held inside the nucleated bubbles. (3) GO nanosheets play an important role in enhancing the heat transfer in the foaming process. The foaming degree of the precursor, the cell size distribution, and the size of the foam materials are all affected by the heat transfer rate of the precursor. It was reported that the thermal conductivity of polymer materials was dramatically improved by incorporation of graphene.10,40 Therefore, the foaming degree of the precursor was enhanced by addition of GO in this research, as SEM images revealed that the quantity of



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unfoamed precursor particles decreased and the cells were enlarged and uniformized in the presence of GO. (4) GO nanosheets improve the mechanical properties of the PAE or PI nanocomposites, as illustrated in other polymer systems.10,44 The improved mechanical properties of PAE or PI nanocomposites satisfied the required stretching force during the growth of cells in the foaming process, leading to enlarged cells. In addition, the resultant graphene nanosheets also enhance the mechanical properties of both the individual cells and the foam materials. Conclusions Aromatic PI/graphene foams were prepared through thermal foaming of PAE/GO precursor powders, which were prepared by synthesis of PAE precursor and in situ grafting of GO nanosheets with PAE chains. FT-IR and Raman data proved that GO nanosheets were covalently grafted with PAE, resulting in homogeneous dispersion of the GO-g-PAE nanosheets in PAE matrix. TGA data indicated that the PAE/GO composites exhibited more weight loss calculated from 50 to 170 °C as the loading of GO increased in the precursor. Therefore, the incorporation of GO not only served as heterogeneous nucleation sites but also produced more foaming agent due to the removal of oxygen-containing groups from the surfaces of GO nanosheets. By analyzing the foaming mechanism, the well-known features of GO including heat transfer, gas barrier, and strength reinforcement also facilitated to obtain uniform and large-sized cells in the foams. Notably, the density of the PI/graphene foam was reduced to the half of that of pristine PI foam by addition of only 2 wt% GO in precursor PAE. In addition, the incorporation of graphene nanosheets improved the flexibility and the flexural strength of the foam materials. Considering the wide variety of polyimide derivatives, as well as other types of polymer foam systems, we expect the significant findings in this research will bring a new avenue for preparation of highperformance polymer foam materials.


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Acknowledgement This work was supported by the “Hundred Talents Program” of Chinese Academy of Sciences, the National Natural Science Foundation of China (21274158 and 91333114). Supporting Information Available: Synthesis of GO, characterization methods, FT-IR spectrum of GO, TGA of precursor powders, FT-IR spectra of precursor powders and the corresponding foams, SEM images of individual cells and fractured surface of foam F5, and compressive and flexural tests of PI and PI/graphene foams. This material is available free of charge via the Internet at hip://pubs.acs.org. References (1) Antunes, M.; Ignacio Velasco, J. Multifunctional Polymer Foams with Carbon Nanoparticles. Prog. Polym. Sci. 2014, 39, 486-509. (2) Lee, L. J.; Zeng, C. C.; Cao, X.; Han, X. M.; Shen, J.; Xu, G. J. Polymer Nanocomposite Foams. Compos. Sci. Technol. 2005, 65, 2344-2363. (3) Antunes, M.; Mudarra, M.; Ignacio Velasco, J. Broad-Band Electrical Conductivity of Carbon Nanofibre-Reinforced Polypropylene Foams. Carbon 2011, 49, 708-717. (4) Zhi, X.; Zhang, H.-B.; Liao, Y.-F.; Hu, Q.-H.; Gui, C.-X.; Yu, Z.-Z. Electrically Conductive Polycarbonate/Carbon Nanotube Composites Toughened with Micron-Scale Voids. Carbon 2015, 82, 195-204. (5) Yeh, S.-K.; Huang, C.-H.; Su, C.-C.; Cheng, K.-C.; Chuang, T.-H.; Guo, W.-J.; Wang, S.-F. Effect of Dispersion Method and Process Variables on the Properties of Supercritical Co2 Foamed Polystyrene/Graphite Nanocomposite Foam. Polym. Eng. Sci. 2013, 53, 2061-2072.



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Graphene Oxide: A Versatile Agent for Polyimide Foams with

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