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

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Mechanically Strong Polyimide / Carbon Nanotube Composite Aerogels with Controllable Porous Structure Composites Science and Technology 156 (2018) 186e191 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech 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 À 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 cross- linking 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 solu- tion, 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 À 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
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