ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия Б, 2011, том 53, № 11, с. 2028-2032
КОМПОЗИТЫ
УДК 541.64:539.3:536.4
MECHANICAL AND THERMAL PROPERTIES OF (Ag/C NANOCABLE)/EPOXY RESIN COMPOSITES1
© 2011 г. Honglu Liang",b and Demei Yu", b
a State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an 710049, China b Department of Applied Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, China
e-mail: dmyu@xjtu.edu.cn Received January 31, 2011 Revised Manuscript Received June 15, 2011
Abstract—A novel Ag/C nanocable and epoxy resin composite was obtained by compounding Ag/C nano-cables and epoxy resin. The nanocable is composed of a nanowire (core) wrapped with one or more outer layers (shell). Scanning electron microscopy images proved that the nanocables consisted of a silver nanowire core and a carbon outer shell. The Ag/C nanocables were modified by hyperbranched poly (amine ester) to improve their mechanical properties for further application. We separately compounded raw and modified Ag/C nanocables with epoxy resin, and then tested the thermal performance, tensile properties, and fracture morphology of each composite. We found that the tensile strengths of the two composite systems were enhanced by the epoxy resin, with the modified (Ag/C)/epoxy resin composite system improving more significantly. Differential scanning calorimeter (DSC) results showed that the glass transition temperature of the unmodified (Ag/C)/epoxy resin composite is increased when the Ag/C nanocable is filled, while that of the modified system slightly decreased. Fracture morphology results showed that both (Ag/C)/epoxy composite systems featured increased toughness. The modified Ag/C nanocables had better compatibility with the ep-oxy resin. The relationship between the properties and microstructure of the composites were discussed in detail to explain the mechanism behind the observed changes in material properties.
INTRODUCTION
Recently, nanometal/epoxy resin composites have attracted much attention because of their wide application in the electrical industry [1—4]. The properties of the nanocomposite are significantly dependent on nanoparticle size, structure, and geometry [5, 6]. Among all structures, one-dimensional (1-D) nano-structures, such as nanowires, nanotubes, and nano-belts, have been extensively studied because of their potential applications in nanoscale electronics, optoelectronics, and sensing devices [7—10]. Silver, an important metallic material having good electrical and thermal conductivity, offers great potential for such applications and can be enhanced further when fabricated in 1-D structures. When used in a nanometer scale, however, metallic materials become very sensitive to air and moisture, which subsequently degrade their performance [11, 12]. As a new kind of nano-structure, coaxial nanocables have been identified as good candidates for nanodevices, resulting in numerous studies in various fields [13—16]. The functions and properties of nanocables can be further enhanced by protecting metal nanowires from oxidation and corrosion on their outer shells and core-sheath het-erostructures.
Many efforts have been devoted to the synthesis of a special 1-D silver co-axial heterostructure (i.e.,
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"nanocable") by assembling single insulating nanotubes and/or silver nanowires in a radial direction. These special nanostructures are expected to have potential applications in nanoscale electronic devices because of their unique structure, particularly their highly conductive silver core and insulating outer shell. To the best of our knowledge, there is no study on Ag/C nanocable/epoxy resin composite exist, indicating their novel properties, especially with regard to the unique structure of the nanocables. In our paper, we prepare (Ag/C)/epoxy composites and explore the integration of nanocables in epoxy networks, including how they affect material properties. In a previous study, we found that the dielectric property of (Ag/C)/epoxy composites was improved [17]. However, the use of epoxy composites is often limited by their poor toughness, which affects the durability of components and places strong constraints on many design parameters. In order to better apply this new compound material in the field of electronics, we attempt to modify Ag/C nanocables. We graft flexible hyper-branched poly (amine ester) onto the surface of Ag/C nanocables to improve their surface defects, increase compatibility with the epoxy matrix, and improve the mechanical properties of the epoxy composites. We also compare the unmodified and modified systems, and discuss the mechanism of enhancement.
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MECHANICAL AND THERMAL PROPERTIES
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EXPERIMENTAL
Materials
The epoxy used was DER331 (Dow) with an equivalent weight of 185 g/equiv. The curing agent was composed of methyltetrahydrophthalic anhydride (MTHPA, Xi'an Resin Factory, Shaanxi, China). The accelerator used was 2,4,6-tri (dimethylami-nomethyl) phenol (DMP30, Shanghai Chemical Reagent Factory, Shanghai, China). Glucose, silver nitrate (AgNO3), and cetyltrimethylammonium bromide (CTAB) were of analytical grade. Diethanola-mine and methyl acrylate were reagent grade and were freshly distilled. Organic-free de-ionized water was used for all the experiments. Fig. 1. The morphology ofAg/C nanocable.
Sample Preparation
The Ag/C nanocables were synthesized under hydrothermal conditions. In a typical procedure, glucose and CTAB were dissolved in 25 ml de-ionized water. Next, 10 ml of aqueous solution containing AgNO3 was added slowly with vigorous stirring. The solution turned yellow because of the formation of AgBr. The solution was transferred into a 50 ml Teflon-lined stainless steel autoclave. Hydro thermal synthesis was performed at 160°C for 24 h [18]. After the reaction, the autoclave was allowed to cool naturally at room temperature. The suspension was centri-fuged at 4000 rpm for 10 min, and the resulting precipitates were washed thrice with ethanol and water and finally stored in ethanol. The product was dried at 40°C in vacuum.
Ag/C nanocables were typically modified as follows. Specific amounts of Ag/C nanocables and N,N-dihydroxyethyl-3-amino methyl propionate were transferred into three flasks. Using 1% p-toluene sul-fonic acid as a catalyst, the mixture was dissolved in acetone, and then ultrasonicated for 30 min until acetone was totally volatilized. The system was stirred vigorously for 10 h under N2 protection at 120°C. The product was washed and centrifuged with anhydrous alcohol several times until the filtrate no longer contained ungrafted hyperbranched poly (amine ester) and unreacted monomers. Finally, the grafted Ag/C nanocable products were dried under vacuum for 6 h at 50 oC.
A typical preparation of the (Ag/C)/epoxy composite is as follows: a measured amount of Ag/C nanocables were added to 10 g of epoxy resin. The mixture was stirred with a magnetic stirrer for 15 min at room temperature and then stoichiometric amounts of MTHPA and 0.2 ml promoters were added. After stirring for another 30 min and sonication, the mixture was poured into a mold and cured at 90°C for 2 h, then at 150°C for a further 4 h.
Characterization
The morphology of the Ag/C nanocable and fracture morphology of (Ag/C)/epoxy composite were studied using a JSM-7000F scanning electron microscope (SEM). The selected specimens of (Ag/C)/ep-oxy composite were coated with a thin layer of gold prior to microscopic analysis.
Differential scanning calorimeter (DSC) analysis of the epoxy and its blends was conducted using a Netzsh DSC 200PC. The calorimeter was previously calibrated with an indium standard. The temperature error in the DSC is ±0.2°C. Samples (~6 mg) were heated continuously from 50—250°C at a heating rate of 10 K/min. All measurements were done under nitrogen atmosphere.
Tensile strength tests were performed on a CMT electronic universal testing machine. In each case, at least 5 specimens were used and the average was taken.
RESULTS AND DISCUSSION
Morphology of the Ag/C Nanocables
The morphology and dimensions of the as-prepared Ag/C nanocables were examined by SEM. Figure 1 shows ultra-long Ag/C nanocables with outer diameters of 100—300 nm and lengths of up to tens of micrometers. Cores look brighter than the shells. Since materials with heavier atomic mass reflect more electrons, the cores of these cables are assumed to be metallic silver [17]. Cables are flexible, indicating better machinability for future nanodevice design [19].
FTIR Spectrum of the Ag/C Nanocable before and after Modification
From the FTIR spectra in Fig. 2, the grafted Ag/C nanocable shows new peaks compared with the spectrum of an ungrafted Ag/C nanocable. Peaks that appear at 1182 and 1161 cm-1 are ascribed to tertiary amines, while the peak at 1050 cm-1 is ascribed to pri-
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HONGLU LIANG, DEMEI YU
Transmittance, % 100 h
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Fig. 2. FTIR spectrum of (1) unmodified Ag/C nanocable and (2) modified Ag/C nanocable.
Fig. 3. DSC curve of the modified (Ag/C)/epoxy resin composites.
mary alcohols. The tertiary amine absorption peak at 1613 cm-1 is clearly enhanced, which shows the successful grafting of hyperbranched poly (amine ester) from Ag/C nanocables by one-step polycondensa-tion.
Thermal Properties of the (Ag/C)/Epoxy Composite
The DSC curves of the Ag/C nanocables and epoxy resin composite after modification are shown in Fig. 3. The glass transsition temperatures (Tg) of the two systems are listed in table shows different trends. Tg data for the unmodified system are obtained from our previous work [17]. In the unmodified system, the Tg first increases
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