Mechanically Strong Polyimide / Carbon Nanotube Composite Aerogels with Controllable Porous Structure

Composites Science and Technology 156 (2018) 186e191

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Composites Science and Technology

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Mechanically strong polyimide / carbon nanotube composite aerogels with controllable porous structure

** * Wei Fan a, Lizeng Zuo b, Youfang Zhang b, Ye Chen a, , Tianxi Liu a, b, a State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, PR China b State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, 220 Handan Road, Shanghai 200433, PR China article info abstract

Article history: Developing aerogels with controllable pores, outstanding mechanical properties and excellent thermal Received 6 September 2017 stability still remains a key challenge in evolution of aerogels. In the present work, polyimide (PI) Received in revised form composite aerogels with controllable porous structures and tunable multi-functions have been fabricated 6 December 2017 with the addition of carbon nanotubes (CNTs) with different functional groups and aspect ratio via an Accepted 30 December 2017 eco-friendly freeze-drying method followed by a thermal imidization process. The interactions between Available online 5 January 2018 PI chains and CNTs can form crosslinking points through physical or chemical bonding, which could overcome the expansive force in the ice crystal growth process and the capillary force during ice sub- Keywords: Carbon nanotubes limation, thus forming a three-dimensional porous structure. Through increasing the content of func- Polymer-matrix composites (PMCs) tional groups on CNTs, the PI/CNT composite aerogels show enhanced structural stability with a less Nano composites shrinkage (<20%) during processing, further resulting in improved mechanical properties. As a result, the Mechanical properties PI composite aerogels exhibit low density (~0.1 g cm 3), improved compression modulus (33.5 MPa), and Aerogel high thermal stability (above 580 C), showing great potential for application as lightweight and high- performance materials. Additionally, this work develops a new strategy to realize the controllable preparation and tunable properties of polymer aerogels by utilizing nanoﬁllers as an effective crosslinking agent. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction of silica aerogel monoliths have been severely restricted on account of their inherent fragility, poor mechanical properties, and high- The term “aerogel” comes from the fact that they are synthe- cost preparing process. The compression modulus of pure silica sized from gel materials, in which the liquid component has been aerogels is mostly in the kpa range while typical polymer reinforced replaced with a gas to leave intact solid nanostructures [1,2]. This silica aerogels exhibit compression modulus ranging from hun- unique microstructure endows aerogels with impressive proper- dreds of kpa to several MPa. Therefore, to broaden the applications ties, such as extremely high porosity, quite low apparent density, of aerogels while fully realizing their outstanding performance, and considerably high surface area. Outstanding properties of mechanically strong and environmentally stable aerogels are aerogels enable them to be attractive materials for applications in needed. In contrast to silica, polymer matrices showing better thermal insulating, chemical adsorbents, catalytic carriers, and mechanical properties, good environmental stability, and cost- electrical applications [3e5]. Although silica aerogels are the most efficient manufacturing process, have raised considerable atten- widely studied systems at the present time, potential applications tion for constructing polymer aerogels with versatile properties [6,7]. Unfortunately, typical polymer aerogels, such as polyurethane aerogels, poly (vinyl alcohol) aerogels and poly (vinyl chloride) * Corresponding author. State Key Laboratory for Modification of Chemical Fibers aerogels, exhibit poor thermal stability, which tend to be stable up and Polymer Materials, College of Materials Science and Engineering, Donghua to at most 100e200 C. For many aerospace applications, such as University, 2999 North Renmin Road, Shanghai 201620, PR China. insulation for launch vehicles or for planetary entry, descent and ** Corresponding author. landing systems, much higher use temperatures are needed. E-mail addresses: [email protected] (Y. Chen), [email protected], txliu@dhu. edu.cn (T. Liu). Therefore, aerogel materials with both good mechanical properties https://doi.org/10.1016/j.compscitech.2017.12.034 0266-3538/© 2018 Elsevier Ltd. All rights reserved. W. Fan et al. / Composites Science and Technology 156 (2018) 186e191 187 and thermal stability are urgently desired. and high thermal stability (above 580 C), implying their great Polyimide (PI), because of their excellent thermal stability, good potential as lightweight and high-performance materials for aero- mechanical properties and high glass transition temperatures, are space applications. the polymer of choice for high-performance applications [8,9]. Previously, PI aerogels have been synthesized by gelation of 2. Experimental section chemical imidization of poly (amic acid) (PAA) gels in dilute solution, followed by supercritical drying. In this approach, the aerogels 2.1. Functionalization of CNTs have a strong tendency to shrink during processing up to as much as 40%. In order to maintain the highly porous structure and inhibit Pristine multi-walled carbon nanotubes with different outer the shrinkage during the drying process, crosslinking agents, diameters: 50e60 nm, 10e20 nm, 6e8 nm were noted as p-CNT50, typically aromatic triamine, such as 1,3,5-triaminophenoxybenzene p-CNT20, p-CNT8, respectively. Oxidized CNT50 (o-CNT50) was pre- (TAB) and octa (aminophenyl) silsesquioxane (OAPS), are usually pared by refluxing the p-CNT50 in the mixture of concentrated added during the gelation process [10,11]. These TAB or OAPS H2SO4 and HNO3 (see details in supporting information). Oxidized- crosslinked PI aerogels can be produced with very little shrinkage amino-CNTs, which contain both oxygen-containing and amino during the processing and exhibit up to 90% porosity. However, groups, noted as o-a-CNT50 was synthesized by thermal treatment these triamines are usually expensive and not easy to synthesize. of o-CNT with hydrazine hydrate and ammonia water at 95 C for Compared with supercritical drying, freeze-drying method is 1 h. A dispersion of amino-functionalized CNTs (a-CNT50) was ob- environmentally friendly and cost effective, which is promising for tained by refluxing the above solution at 95 C for another 5 h. The fabrication of polymer aerogels. More recently, freeze-drying overall preparation process of functionalized CNTs is shown in method has been reported for fabricate PI aerogels by using Fig. 1. water-soluble or dispersible triethylamine modified PAA [12e14]. However, PI aerogels prepared by freeze-drying technique usually 2.2. Fabrication of PI-based aerogels display large shrinkage and uncontrollable porous structures with macropores ranging from several to hundred micrometers, which PI/CNT composite aerogels were prepared via a sol-gel process, results in the loss of dimensional stability and mechanical strength. followed by freeze-drying and thermal imidization process. Water- Therefore, seeking for alternative crosslinking agents to develop PI soluble precursor of PI, triethylamine capped poly (amic acid) (TEA- aerogels with good dimensional stability and controllable pores via PAA), was mixed with o-CNT50 suspension to obtain a homogenous freeze-drying method is still a challenge. PAA/o-CNT50 sol. Subsequently, the sol was cast into cylindrical Polymer hydrogels can be crosslinked chemically, which are molds and frozen in liquid nitrogen followed by drying in a freeze- composed of polymer networks with covalent bonding, or physi- dryer for ice sublimation. After 48 h in the lyophilizer, the samples cally, which are composed of physical networks with noncovalent were heated for complete imidization. Finally, the PI/o-CNT50 interactions [15]. Previously, it has been reported that inorganic composite aerogel was obtained. For revealing the effects of func- materials, such as clay platelets, can act as effective multifunctional tional groups and aspect ratios of CNTs on structures and properties crosslinking agents for polymer hydrogels. The organic/inorganic of composite aerogels, PI composite aerogels incorporated with o- hybrid gel exhibited improved structural stability as well as me- a-CNT50, a-CNT50, o-CNT20, and o-CNT8 were also fabricated ac- chanical properties as compared to a conventional chemically cording to the same procedure, and noted as PI/o-a-CNT50, PI/a- crosslinked gel [16]. In this case, adding inorganic nanofillers into CNT50, PI/o-CNT20, and PI/o-CNT8, respectively. the PI aerogel would reinforce the aerogel acting as either chemical or physical crosslinkers. Specifically, carbon nanotubes (CNTs) are 2.3. Characterization typical nanofillers that have been widely applied for reinforcing polymer matrix [17e20]. Non-functionalized CNTs, however, Fourier transformation infrared (FT-IR) spectra were recorded cannot be stably and finely dispersed in polymer matrices due to with a 4 cm 1 spectral resolution on a Nicolet Nexcus 6700 spec- their strong physical interaction resulted in heavy aggregations. A trometer equipped with a DTGS detector. Thermogravimetric common technique to incorporate CNTs into such polymers is analysis (TGA) was carried out on a Mettler Toledo TGA1 instrument through chemical functionalization of the CNTs, which is used to under a constant nitrogen flow from 100 to 1000 C with a heating enhance the nanotube-polymer interfacial interactions [21]. rate of 10 C min 1. X-ray photoelectron spectroscopy (XPS) spectra Increasing the interfacial binding between CNTs and the PI aerogel were performed by an X-ray photoelectron spectroscopy (XPS) with will improve the interfacial strength and enhance crosslinking a VG ESCALAB 220I-XL device, and XPS spectra were all corrected density, with the goal of improving the macroscopic structure using C 1s peak at 284.5 eV. Morphologies of the samples were integrity and mechanical properties of the polymer aerogels. Be- investigated using a field emission scanning electron microscopy sides, the CNTs could also serve as nanoscale soft building blocks for (FESEM) (Ultra 55, Zeiss) at an acceleration voltage of 5 kV. Pore size aerogel materials, further inhibiting the shrinkage and reinforcing distributions of aerogels were investigated by counting all the pore the dimensional stability of the composite aerogels. sizes in a 500 SEM image using a Smart TiffV2 software. The Hence, in the present study, functionalized CNTs are employed apparent densities of aerogels were calculated by measuring the as both crosslinkers and reinforcing phases for fabricating PI aer- mass and dimension using an electronic balance and a slide caliper, ogels using an environmentally friendly freeze-drying method and five samples were tested for each composition. Compression followed by a thermal imidization process. The interactions be- moduli of aerogels were calculated from the slope of the linear tween PI chains and functionalized CNTs can form crosslinking portion of the stress-strain curve investigated on a SANS CMT4104 points through physical or chemical bonding, thus forming a three- test machine according to the ASTM D3575 method. dimensional (3D) porous structure for the PI/CNT composite aerogels. Through the precisely control over the content of functional 3. Results and discussions groups and aspect ratio of CNTs, the PI/CNT composite aerogels show good structure stability with a little shrinkage (<20%) during Functionalized CNTs were fabricated via acid-treated oxi- processing. As a result, the PI composite aerogels exhibit low dization and amino-functionalization process (Fig. 1a). The con- density (~0.1 g cm 3), improved compression modulus (33.5 MPa), tents and types of functional groups on the surface of CNTs were 188 W. Fan et al. / Composites Science and Technology 156 (2018) 186e191

Fig. 1. (a) Schematic illustration for preparation of functionalized CNTs. (b) FTIR spectra, (c) XPS survey spectra, and (d) TGA curves of p-CNT50, o-CNT50, o-a-CNT50 and a-CNT50. investigated before use. As shown by FTIR spectra in Fig. 1b, almost composite aerogels show much smaller pore sizes and more uni- no functional groups can be observed for p-CNT50. In the FTIR form pore distributions. PI/o-CNT50 composite aerogel exhibits a 1 spectra of o-CNT50, the peak at ~1720 cm is assigned to the C¼O smaller pore size of ~20e30 mm(Fig. 2h), which may be due to the stretch of carboxylic (COOH) groups. For o-a-CNT50 and a-CNT50, strong interaction between polymer matrix and oxygen-containing 1 both display two characteristic peaks including C-N (~1400 cm ) groups on CNTs. For the PI/o-a-CNT50 composite aerogel (Fig. 2i), and N-H (~3120 cm 1), giving a strong indication of the amino smallest pore sizes and relatively narrow distributions (~5e10 mm) groups (-NH2) attached to CNTs. Meanwhile, the intensities of C-N are observed that may be attributed to the increased content of and N-H peaks are observed to be stronger in a-CNT50 than that of functional groups on CNTs (see TGA data). Besides, BET analysis also o-a-CNT50, revealing the increase of amino groups in a-CNT50. The indicates that there are mesopores mainly within the range of chemical composition of functionalized CNTs was further investi- 2e4 nm for the PI/o-a-CNT50 composite aerogel, which may be gated by XPS. As depicted in Fig. 1c, C 1s peak at 284.5 eV and O 1s resulted from the loss of functional groups during the thermal peak at 532.0 eV are observed for o-CNT50, indicating the oxygen- imidization process (Fig. S1). However, the pore sizes of PI/a-CNT50 containing functional groups on o-CNT50. Besides these two main show slightly increase as compared with PI/o-a-CNT50 (Fig. 2j), peaks, an additional N 1s peak at 400 eV is observed for both o-a- probably due to the decreased contents of functional groups on CNT50 and a-CNT50, indicating the presence of nitrogen-containing CNTs. All the above results indicate that the increased contents of groups on the surface of CNTs. The atomic ratio of carbon, nitrogen functional groups on CNTs contribute to smaller pore size of the and oxygen (C:N:O) for a-CNT50 is 85.5:4.5:10.0, while for o-a- corresponding composite aerogels. CNT50 is 83.1:2.7:14.2. In comparison to that of o-a-CNT50, oxygen To facilitate a better understanding of the pore formation content of a-CNT50 slight decreases, indicating the reduction of mechanism, the microstructures of PAA-based precursor aerogels oxygen functional groups under the treatment of hydrazine hy- were also investigated by SEM. As indicated in Fig. 3aec, the PAA/ drate, which is similar with the reduction of graphene oxide by CNT aerogels have 3D structures in which the pore sizes are in hydrazine hydrate. The weight percentage of functional groups on accordance with those of their corresponding PI/CNT aerogels. CNT are estimated from the TGA curves between 150 and 300 C Thus, the porous structures of the aerogels have been shaped before (Fig. 1d). The contents of functional groups are 8 wt%, 21 wt%, 10 wt the thermal imidization process, which is highly dependent on the % for o-CNT50, o-a-CNT50, and a-CNT50, respectively, indicating that crosslinking points formed during the gelation process. In the PAA/ o-a-CNT50 has the highest content of functional groups. CNT hydrogel, except the interchain interaction of the entangled The functionalized CNTs were blended with PAA precursor to PAA chains, the interactions between PAA and CNTs can also form construct PI-based aerogels via a sol-gel process followed by freeze- crosslinking points through physical or chemical bonding, which drying and thermal imidization process, which is schematically could overcome the expansive force in the ice crystal growth pro- illustrated in Fig. 2a. Unless speciﬁcally stated, the content of CNT cess and the capillary force during ice sublimation, thus forming a nanoﬁllers is 2 wt% in composite aerogels. The digital photos of the porous structure. As shown by the TEM image in Fig. S2, the CNTs as-prepared PI aerogel and PI/CNT composite aerogels are shown in can be clearly observed and homogeneously dispersed in the PAA Fig. 2b. It can be seen that PI/CNT composite aerogels with any hydrogel. Based on this analysis, three structural models of the desired sizes and geometries, such as cylinder, disk and rod can be crosslinking points depended on the types of functional groups on fabricated. The morphology of PI-based composite aerogels is CNTs are depicted in Fig. 3def. In PAA/o-CNT50 hydrogel, the shown in Fig. 2cef. It can be seen that well-organized and 3D oxygen-containing groups on o-CNT50 can form hydrogen bond porous network structures have been formed for all the aerogels. with carboxyl and amino groups on PAA, which form the cross- Moreover, the pore size and distribution of the aerogels prepared linking points (Fig. 3d). For the PAA/o-a-CNT50 hydrogel, a short from different CNTs exhibit much difference, which has been time treatment with hydrazine hydrate directly results in increase summarized in Fig. 2gej and Table 1. Neat PI aerogel shows a broad of amino groups on o-a-CNT50 surface, leading to the formation of pore distribution, with a pore size mainly centered in 30e40 mm amide bond between o-a-CNT50 and PAA chains in addition to the (Fig. 2g). With the incorporation of functionalized CNTs, all the hydrogen bond (Fig. 3e). The abundant amount of functional groups W. Fan et al. / Composites Science and Technology 156 (2018) 186e191 189

Fig. 2. (a) Schematic illustration of preparation of PI/CNT composite aerogels. (b) Photographs of PI-based aerogels (1: neat PI aerogel, 2e5: PI/CNT composite aerogels with different sizes and shapes). SEM images of (c) neat PI aerogel, (d) PI/o-CNT50 aerogel, (e) PI/o-a-CNT50 aerogel, (f) PI/a-CNT50 aerogel, and (gej) corresponding pore size distribution of the aerogels.

Table 1 Mechanical properties and densities of PI-based aerogels.

Sample Contents of functional groups on CNTs (wt CNT diameter Pore sizea Apparent Densityb (g Porosityc Shrinkaged Compression modulus %) (nm) (mm) cm 3) (%) (MPa)

PI ee30e40 0.115 ± 0.018 92.0% 30.6 ± 0.6 19.2 ± 1.4 PI/o-a- 21 50 5e10 0.084 ± 0.005 94.2% 15.1 ± 1.0 33.5 ± 1.8

CNT50 PI/a-CNT50 10 50 10e20 0.103 ± 0.013 92.8% 17.7 ± 0.9 29.2 ± 1.6 PI/o-CNT50 85020e30 0.094 ± 0.007 93.5% 20.1 ± 1.1 26.5 ± 2.1 PI/o-CNT20 82020e25 0.117 ± 0.013 91.9% 18.9 ± 0.8 29.6 ± 1.5 PI/o-CNT8 10 8 10e15 0.100 ± 0.008 93.1% 17.9 ± 0.8 32.2 ± 2.5 a Estimated from SEM image analysis of cross-sections of the aerogel. b Apparent densities (rapp) were calculated from the weight and volumes of the aerogels. c The porosity is deﬁned as (1errel) 100%, where rrel is the relative density. The relative density is calculated as rapp/rsc where rsc is the skeletal density, which is estimated from the density of polyimide (1.44 g cm 3). d Estimated from the volume change between the aerogel and hydrogel before freeze-drying.

on o-a-CNT50 result in the increased crosslinking points formed in before freeze-drying, which is an important parameter for evalu- the hydrogel, thus leading to a decreased pore size in the final ating dimensional stability of the aerogels. As shown in Fig. 4c, the aerogel. Increasing treatment time of hydrazine hydrate leads to shrinkage of PI composite aerogels is significantly decreased decrease of functional groups on a-CNT50 (see TGA analysis), which compared with that of neat PI aerogel, which can be attributed to results in the decrease of crosslinking points, and further causes the the increased interfacial interaction derived from the crosslinking increase of pore sizes (Fig. 3f). Therefore, it can be concluded that points between PAA and functionalized CNTs. Among the com- increasing amount of functional groups on CNTs can form more posite aerogels, PI/o-a-CNT50 exhibits a much smaller shrinkage as crosslinking points with PAA chains, contributing to the effective compared with other aerogels, and the shrinkage of aerogels de- decrease of pore sizes of the aerogels, while the type of functional creases with the increased contents of functional groups, which groups hardly have any influence on the porous structure. may be due to higher contents of crosslinking points would sustain The shrinkage, apparent densities, and compression modulus of the capillary force during drying, maintaining the dimensional the obtained aerogels were investigated to evaluate the dimen- stability of the aerogels. The compression modulus of neat PI aer- sional stability and mechanical strength of the composites. As ogel is about 19.2 ± 1.4 MPa, which is much higher than that of shown in Table 1, all the aerogels possess very low density in the typical polymer reinforced silica aerogels with similar density range of 0.08e0.12 g cm 3, and high porosity that higher than 90%. exhibiting compressive modulus ranging from 0.1 to 2 MPa. As a result, the aerogel can stand on two pieces of petal of a flower Compared with bare PI aerogel, the compression modulus of (Fig. 4a), implying the high porosity and lightweight of aerogels. composite aerogels shows significant improvement. PI/o-a-CNT50 Notably, this lightweight aerogel can withstand 10000 times of its composite aerogel exhibits highest modulus of 33.5 ± 1.8 MPa, own weight without any deformation, exhibiting high anti- showing 74% improvement as compared with that of neat PI aer- compressibility (Fig. 4b). The shrinkage of the aerogels is esti- ogel. As depicted in Fig. 4c, it is interesting to find that the modulus mated from the volume change between the aerogel and hydrogel of composite aerogels has an opposite trend in contrast to the 190 W. Fan et al. / Composites Science and Technology 156 (2018) 186e191

Fig. 3. SEM images of (a) PAA/o-CNT50, (b) PAA/o-a-CNT50, (c) PAA/a-CNT50 aerogels, and (def) their corresponding structural models.

Fig. 4. Digital photographs showing the PI/o-a-CNT50 aerogel (a) standing on the petal of a flower, and (b) compressed by a copper block. Shrinkage and compression modulus of PI/ CNT aerogels with (a) different functional groups on CNTs, and (b) different aspect ratios of CNTs. shrinkage for all the samples. PI/o-a-CNT50 with the lowest sizes decrease with the decrease of diameters of CNTs. The PI/o- shrinkage exhibits the highest modulus, and the modulus of aero- CNT8 has pore size located at 10e15 mm(Fig. S6d) while the pore gels increases with the increased contents of functional groups on size of PI/o-CNT20 is located in the range of 20e25 mm(Fig. S6e). CNTs, indicating that the modulus has a positive correlation with Therefore, the increase of aspect ratio of CNTs can decrease the pore amount of crosslinking points between functionalized CNTs and size of composite aerogels, favoring the dimensional stability and PAA. mechanical properties, which can be explained by that CNTs with The mechanical properties of PI/CNT aerogels prepared with larger aspect ratio can easily tangle with PAA chains, forming CNTs with different aspect ratios are also compared. Three kinds of stronger interactions during the gelation process. CNTs with different outer diameters (~8 nm, 20 nm, 50 nm) and The thermal stabilities of PI-based aerogels were evaluated by lengths (as shown in Table S1 and Fig. S3) were employed to TGA (Fig. 5). It can be observed that the first weight loss stages, fabricate composite aerogels. It is worth to mention that the con- which appeared at 100e200 C, are attributed to the removal of the tents of functional groups (see TGA data in Fig. S4) and C/O atomic abundant surface functional groups in the aerogel system. The ratio (see XPS data in Fig. S5) of o-CNT8, o-CNT20 and o-CNT50 are major weight loss occurs at 550e650 C mainly due to the almost the same to eliminate the influence of functional groups. As decomposition of PI matrix. All the PI-based aerogels have high shown in Fig. 4d, with the decrease of diameter, i.e., increase of decomposition temperature (Td, 10%) over 580 C, which is much aspect ratio of CNTs, the shrinkage of the composite aerogel slightly higher than that of the reported general polymer-based aerogels decreases and the compression modulus increases. The changes of (typically below 250 C). Therefore, the PI/CNT composite aerogel the mechanical properties can be inferred from the morphology of possesses excellent thermal stability, which can be applied in the composite aerogels. As shown in Table 1 and Fig. S6, the pore extended areas that require heat resistance. W. Fan et al. / Composites Science and Technology 156 (2018) 186e191 191

https://doi.org/10.1016/j.compscitech.2017.12.034.

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