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    1700357 (1 of 9) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


    Electrically Insulated Epoxy Nanocomposites Reinforced with Synergistic Core–Shell SiO2@MWCNTs and Montmorillonite Bifillers

    Zijian Wu, Sheng Gao, Lei Chen, Dawei Jiang, Qian Shao, Bing Zhang, Zhaohui Zhai, Chen Wang, Min Zhao, Yingyi Ma, Xiaohong Zhang, Ling Weng, Mingyan Zhang,* and Zhanhu Guo*

    DOI: 10.1002/macp.201700357

    1. Introduction

    Carbon nanotubes (CNTs) have remark- able electrical, thermal, and mechanical properties such as extremely high con- ductivity and Young’s modulus, which can efficiently improve the physical properties of polymers even at low CNT loadings.[1–4] Especially for electrical properties, the addition of a small amount of conductive CNTs to an insulating polymer matrix results in nanocomposites with highly enhanced electrical conductivity.[5] The critical content of CNTs that character- izing a drastic increase in conductivity is commonly termed as the electrical per- colation threshold. When the CNTs con- tent exceeds this electrical percolation threshold, the CNTs conductive pathway inside the poly mer matrix is formed. The electrical percolation threshold of most CNTs/polymer nanocomposites is much below 1 vol%.[6–9] The polymer nanocomposites with high electrical con- ductivity are of high demand for electro- magnetic interference shielding, antistatic device, energy storage/conversion/saving,

    Electrical Insulation

    With unique physicochemical properties, multiwalled carbon nanotubes (MWCNTs) have enabled major achievement in polymer composites as rein- forcing fillers. Nevertheless, high conductivity of raw MWCNTs (R-MWCNTs) limits their wider applications in certain fields, which require outstanding thermal conductivity, mechanical, and insulation properties simultane- ously. In this article, silica (SiO2) coated MWCNTs core–shell hybrids (SiO2@MWCNTs) and organically modified montmorillonite (O-MMT) are employed to modify epoxy (EP) simultaneously. The epoxy-clay system is cured by using anhydride curing agent. The impact strength and flexural strength of final nanocomposites are greatly improved. Meanwhile, the final composites remain in high electrical insulation. Compared to mixed acid treated MWCNTs (C-MWCNTs) (0.5 wt%)/EP nanocomposites, the volume resistivity of the O-MMT(4 wt%)/SiO2@MWCNTs(0.5 wt%)/EP nanocom- posites increases more than six orders of magnitude. Synergistic toughening effect occurs when using core–shell SiO2@MWCNTs and MMT bifillers. The electrical insulation is attributed to the suppressed electron transport effect by SiO2 layer on the CNTs surface, and the blocked conductive CNTs network by the buried 2D structural O-MMT. The SiO2@MWCNTs core–shell hybrids also benefit to decrease the dielectric constant and dielectric loss of CNTs/ EP composites. This work provides guidance to using CNTs as reinforcement fillers to toughen the polymers for electric insulating applications.

    Dr. Z. Wu, Prof. X. Zhang, Dr. L. Weng, Prof. M. Zhang Key Laboratory of Engineering Dielectrics and Its Application Ministry of Education Harbin University of Science and Technology Harbin 150040, China E-mail: taotaozh@hrbust.edu.cn Dr. Z. Wu, S. Gao, Z. Zhai, C. Wang, Dr. Y. Ma, Dr. L. Weng, Prof. M. Zhang College of Material Science and Engineering Harbin University of Science and Technology Harbin 150040, China Dr. L. Chen, Dr. M. Zhao School of Chemical Engineering and Technology Harbin Institute of Technology Harbin 150001, China

    Dr. D. Jiang Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials Northeast Forestry University Harbin 150040, China Dr. Q. Shao, Dr. B. Zhang College of Chemical and Environmental Engineering Shandong University of Science and Technology Qingdao 266590, China Dr. M. Zhao, Dr. Z. Guo Integrated Composites Laboratory (ICL) Department of Chemical & Biomolecular Engineering University of Tennessee Knoxville, TN 77966, USA E-mail: zguo10@utk.eduThe ORCID identification number(s) for the author(s) of this article

    can be found under https://doi.org/10.1002/macp.201700357.

    Macromol. Chem. Phys. 2017, 218, 1700357

  • © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700357 (2 of 9)

    www.advancedsciencenews.com www.mcp-journal.de

    actuators, anticorrosive coating, and sensors.[10–35] Nevertheless, high conductivity of CNTs was an obstacle to wider applications of the CNTs/polymer nanocomposites in some special fields, which require not only outstanding thermal and mechanical properties but also insulation simultaneously (e.g., electronic packaging). However, few researchers pay attention to the remarkable thermal and mechanical properties of CNTs in the insulating fields.[36,37]

    A great synergistic reinforcing effect of nanofillers with two different dimensions such as 2D nanomaterials (e.g., gra- phene and clay platelets) and 1D nanomaterials (e.g., CNTs and nanofibers) in polymer matrix has been observed.[38,39] The mechanical properties of the polymer nanocomposites are sig- nificantly improved when compared with the neat polymer, which is due to the formation of much jammed fillers net- work with 1D nanomaterials and 2D nanomaterials combined together.[40,41] Layered silicate as a good nanoscale reinforce- ment for polymer matrix has a similar 2D planar nanostructure to graphene.[42] Polymer/layered silicate nanocomposites have attracted great industrial and academic interests, because at low filler content (i.e., lower than 5 wt%) they show a remarkable improvement of mechanical, thermal, and physical properties compared with the pristine polymer, conventional micron and macrocomposites. Montmorillonite (MMT) clay, consisting of nanometric silicate layers, has been used as inorganic fillers to modify epoxy (EP)-based materials. Compared to conductive graphene, the insulating properties of layered silicate make it better suited to strengthen insulating polymer. Some function- alized CNTs have been fabricated and used as fillers to improve the properties of polymers, especially aimed to enhance the thermal conductivity while remaining high electrical insula- tion.[43] However, no attempts have been made to obtain the EP composites with both improved mechanical properties and high electrical insulation using silica (SiO2) coated multiwalled carbon nanotubes (SiO2@MWCNTs) and MMT together.

    In this work, an insulating silica layer coated on the surface of MWCNTs was used to create SiO2@MWCNTs core–shell hybrids, which were used as reinforcement fillers to modify the epoxy composites with anhydride as curing agent. Bisphenol- A epoxy/anhydride system was synergistically reinforced using core–shell SiO2@MWCNTs and montmorillonite as fillers, while remaining electrical insulation of epoxy composites. A comprehensive analysis on synergistic toughening mechanism and dielectric properties of EP nanocomposites is presented.

    2. Experimental Section

    2.1. Materials

    E-44 epoxy resins with epoxy value between 0.41 and 0.47 were purchased from Wuxi Resin Phoenix Co. Ltd., China. Methyl- hexahydrophthalic anhydride and pyridine supplied by Zhe- jiang Alpharm Chemical Technology Co., Ltd , China were used as curing agent and accelerator, respectively. Raw sodium montmorillonite (R-MMT) with a layer spacing of 1.26 nm was purchased from Xi′an Comprehensive Test Center Rock Min- eral Chemical Institute, China. Octadecyl quaternary ammo- nium salt was purchased from Sinopharm Chemical Reagent

    Co. Ltd, China. R-MWCNTs (length: 5–10 µm, diameter: 40–60 nm) prepared by chemical vapor deposition method were purchased from Shenzhen Nanotech Port Company. Tetraethy- lorthosilicate (TEOS) and other reagents were purchased from Aladdin Chemical Reagent Co. Ltd. All the chemicals were used as received without any further treatment.

    2.2. Preparation of O-MMT

    In a typical preparation, a certain amount of the original mont- morillonite was placed into a three-necked flask, and stirred at 80 °C in deionized water for 3 h. Next, a certain amount of octa- decyl quaternary ammonium salts were dissolved in deionized water to prepare organic modifier and stirred at 80 °C for 3 h, then slowly added to the MMT/water mixture and stirred vig- orously for 10 h. The mixture was washed several times with deionized water until no white precipitate was detected by drop- ping AgNO3 solutions in the filtrate. At last, organically modi- fied MMT (O-MMT) were collected after washing and drying. Finally, the MMT clay was modified with octadecyl quaternary ammonium salts and was intercalated into the interlayer region of MMT-Clay.

    2.3. Preparation of SiO2@MWCNTs

    2 g R-MWCNTs were first placed into a flask, then concentrated H2SO4 and HNO3 in a volume ratio of 3:1 were poured into the flask, the mixture was treated at 65 °C for 10 h under ultrasoni- cation. The resulting mixture was washed with acetone several times by high-speed centrifuge until the pH of the filtrate was neutral and then collected by freeze-drying treatment, marked as C-MWCNTs. Next, 1 g C-MWCNTs, 160 mL ethanol, and 40 mL deionized water were mixed together in a three-necked flask, and ultrasonically dispersed for 1 h to form a stable dispersion. The solution containing 2 g TEOS and 20 mL ethanol was added drop- wise to the C-MWCNTs solution. The solution was then stirred at the speed of 400 rpm for 12 h at 50 °C. Finally, the dispersion was vacuum filtered through a 0.45 µm teflon membrane, and thoroughly rinsed with dimethyl formamide (DMF) and deion- ized water at least 5 times. The as produced SiO2@MWCNTs nanoparticles wer