A bio-surfactant for defect control: Multifunctional gelatin coated MWCNTs for conductive epoxy nanocomposites
Introduction
Carbon fiber reinforced epoxy composites have been extensively applied in the aerospace and aviation industries due to their excellent specific strength and stiffness, fatigue resistance, environmental adaptability and tailoring property [[1], [2], [3]]. However, the relatively inferior strength and the insulation nature of the epoxy matrix hinder the realization of high mechanical and multifunctional performances of the composites, for example, as potential lightning strike suppression and electromagnetic shielding structural material [[4], [5], [6], [7], [8]]. Therefore, an advanced epoxy with superior mechanical and electrical properties is desirable in order to exploit its various application areas.
Carbon nanotube is widely accepted as a promising filler candidate for achieving epoxy matrix materials with phenomenal mechanical and electrical improvements due to the excellent mechanical, electrical and thermal properties [9]. However, if pristine CNTs are directly mixed with the polymer matrix, strong van de Waals interaction will drive CNTs to agglomerate into large bundles that can lead to the following negative consequences [10]: (1) preventing the fully display of their high performances; (2) increasing the viscosity of the resin matrix; (3) introducing defects at CNTs/epoxy interfaces and in the bulk resin matrix. Therefore, significant efforts have been contributed to seek solutions to untangling of CNT agglomerates, or improving its dispersion in various polymer systems. The reported solution can be summarized into two types of major methods: covalent modification (chemical functionalization) and noncovalent modification (physical treatment or surfactant treatment) of the CNTs [11].
The covalent method is the oxidizing using acids followed by introducing functional groups onto the surface of the CNTs to relax strong inter-tube interactions and improve dispersion of the CNTs in a resin material. After acid treatment, functional moieties such as carboxyl groups [12,13], amino groups [14,15], silane groups [16], epoxide groups [17] etc. have been successfully grafted to the CNTs, which result in a remarkable increase of the mechanical property for the bulk nanocomposites. However, the covalent modification can make damages to the structural integrity of the CNTs, not only decreasing its original conductivity and mechanical toughness but also introducing defects in the composites [18,19]. In addition, the covalent method usually involves complex chemical reactions at the expenditure of various hazardous acids, such as sulfuric acid, nitric acid, hydrochloric acid, hydrogen peroxide and organic solvents, then takes long times and great effort to filter and wash nanofillers with deionized water, ethanol or acetone, all of which resulting in large amount of energy waste, manufacturing cost and more importantly, bringing about potential environmental and human health concerns.
It was reported that some noncovalent methods, on the other hand, could effectively exfoliate the entangled CNT structure while largely maintain the original material properties of CNTs via wrapping special surfactants or polymer chains on the surface of the CNTs. Sodium dodecyl sulfate (SDS) [[20], [21], [22]], sodium dodecylbenzene sulfonate (SDBS) [23,24], polystyrene sulfate (PSS) [25], polyphenyleneethynylene (PPE) [26], polyaniline (PANI) [27], Triton X [28], Tergitol [29] etc. are reported to be effective media to improve dispersion of the CNTs by noncovalent interactions. However, it is noted that some of the surfactants cannot interact firmly with the CNTs and can be easily removed during filtering or washing processes, resulting in re-agglomeration of the CNTs [19]. Recently, bio-treatment using natural materials as the bio-surfactants has attracted great attention for making functionalized nanomaterials due to their ubiquity, low cost, environmental-friendly and easy processability. The rich functional groups, such as amino, carboxyl, hydroxyl, methyl, on their molecular structures provide multiple reactive sites for interactions with their surrounding agents or functionalize with the CNTs [30]. Proteins, such as soy protein [31], bovine serum albumin [32], chitosan [33], glucose oxidase [34], ferritin [35] etc., have been reported to show impressive improvement on the dispersion of single-walled CNTs, multi-walled CNTs, graphene platelets and carbon blacks [31,36,37], resulting in good electrical, mechanical and bio-compatibility properties in the energy, medical and biological applications. From the reported studies the authors discovered that protein could be an effective surfactant for improving the dispersion of nanomaterials when it was used for their treatment in the polymer matrices. However, to the author's knowledge, no studies on investigation of protein-assisted dispersion of CNTs in the epoxy system that can be used in the engineering structural composite materials have been reported.
In this study, abundant gelatin powder has been successfully coated on the surface of MWCNTs and mixed with epoxy matrix to fabricate a gelatin-CNT/epoxy nanocomposite. The dispersion state, wettability, electrical and mechanical performances, as well as microscopic morphology of the fracture surface of the gelatin treated nanocomposite have been characterized and compared with pristine and chemical functionalized nanotubes (NH2-CNTs) epoxy composites. It was shown that the gelatin protein treated CNTs, gelatin-CNTs, effectively dispersed in the polymer, resulting in significant improvement in wettability, and mechanical and electrical performances. The mechanism of the property enhancement using gelatin to treat CNTs has been discussed. This work provides the first step for the application of multifunctional protein modified epoxy matrix to be used in the high performance CFRP composite structures.
Section snippets
Materials
Multiwalled carbon nanotubes (MWCNTs) were supplied from Nanocyl SA (Sambreville, Belguim) with average diameter of 9.5 nm, average length of 1.5 μm and carbon purity of 90%. Amino functionalized MWCNTs were purchased from Chengdu Organic Chemicals Co. Ltd with average diameter of 10 nm, average length of 50 μm and purity of 95%. Gelatin powder produced from porcine skin (type A) was purchased from Sigma-Aldrich (MO, USA). Acetic acid (99.9% purity) was purchased from J. T. Baker company (PA,
Fabrication and characterizations of gelatin-CNT/epoxy nanocomposite
The fabrication processes of gelatin-CNT/epoxy nanocomposite were shown in Fig. 1 and the detailed procedures of the sample preparation is described in the above experimental section. It is known that natural gelatin powder is consisted of large amount of entangled gelatin chains that cover their abundant functional groups inside. As such, to most effectively release the potential of gelatin as bio-surfactant, the first step is to disentangle gelatin chains from microscale level to nanoscale
Conclusion
In the paper, we demonstrated that the gelatin can be used as an effective bio-surfactant for CNT treatment that is necessary for reinforcing polymers in order to achieve multifunctional nanocomposites, and even more effective than some chemical treatment methods using toxic acids. This can be reflected from the multiple results that indicate the epoxy nanocomposites with gelatin-treated CNTs possess simultaneously enhanced mechanical properties and electrical conductivity than those with the
Acknowledgements
This work is sponsored by Project funded by China Postdoctoral Science Foundation (Grant No. 2016M602304). We are grateful to the School of Biological Sciences Franceschi Microscopy & Imaging Center (FMIC) at Washington State University for using the field-emission electron microscope.
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