Eswaraiah et al. Nanoscale Research Letters 2011, 6:137 http://www.nanoscalereslett.com/content/6/1/137 reinforcing materials possessing excellent electrical and mechanical properties and their unique one-dimensional structure [16,17] make them ideal for creating overlap- ping conductive network for high-performance EMI shielding at low loadings [18-21]. CNT-polymer compo- sites either based on solvent casting or melt-based tech- niques have been studied with various polymer matrices, including PMMA [22], liquid crystal polymers, and mel- amine formaldehydes [23], PVA [24], and fused silica [25] for various applications such as radiation protec- tion, EMI shielding, and electrostatic discharge materi- als. There are many reports on EMI shielding of carbon nanotubes reinforced polymer composites [26-30] in the X-band region because of its use in military communi- cation satellites, weather monitoring, air traffic control, defense trackingand high-resolution imaging radars. But the disadvantage is the high loading of carbon nano- tubes which is at present economically not feasible. So, there is a critical need for the development of low-cost EMI shielding materials at this particular frequency. Yonglai et al. [31] reported low-cost EMI shielding materials with the combination of carbon nanofiber and carbon nanotube composites in polystyrene (PS) matrix. They could achieve electromagnetic interference shield- ing effectiveness (EMI SE) of 20 dB for the combination of 10 wt.% carbon nanofiber and 1 wt.% carbon nano- tubes in PS matrix in the range 12-18 GHz. In the pre- sent study, we have developed a low-cost hybrid EMI shielding material comprising of manganese dioxide nanotubes and low loading of multiwalled carbon nano- tubes (MWCNTs) in PVDF matrix. EMI shielding effi- ciency and electrical conductivity of the composites with different weight fractions of functionalized multiwalled carbon nanotubes (f-MWCNTs) and MnO2 nanotubes (MNTs) were investigated to optimize polymer compo- sites with less content of carbon nanotubes that exhibit enhanced electrical properties and serve as a better EMI shielding material. The focus of the present work is to fill the space between the MNTs using a low weight percent of f-MWCNTs within the polymer matrix and thereby making utmost use of the advantages of f-MWCNTs and eventually achieve low-cost and improved EMI shielding materials. Experimental section Materials PVDF was used as polymer matrix with a molecular weight of 100,000 g.mol and it was purchased from Alfa Aesar. MWCNTs were synthesized by chemical vapor deposition technique. MNTs were prepared by hydro- thermal route and N,N-dimethyl formamide was used as the solvent for carbon nanotubes and MnO2 nanotubes. Laboratory grade acids, bases, and organic solvents were used. Page 2 of 11 Synthesis of functionalized multiwalled carbon nanotubes MWCNTs were synthesized by chemical vapor deposi- tion technique using misch metal (approximately 50% cerium and 25% lanthanum, with small amounts of neo- dymium and praseodymium)-based AB3 alloy hydride catalysts [32]. The as-grown MWCNTs not only contain pure MWCNTs but also amorphous carbon, fullerenes, and other metal catalysts. In order to remove these cata- lytic impurities and amorphous carbon, air oxidation was performed at 350°C for 4 h followed by acid treat- ment in concentrated HNO3. After purification, MWCNTs were functionalized with 3:1 ratio of H2SO4 and HNO3 at 60°C for 6 h in order to impart hydroxyl and carboxyl functional groups over the side walls. Synthesis of MnO2 nanotubes MNTs were prepared by hydrothermal route [33]. Briefly, 0.608 g of KMnO4 and 1.27 ml of HCl (37 wt.%) were added to 70 ml of de-ionized water with continuous stir- ring to form the precursor solution. After stirring, the solution was transferred to a teflon lined stainless steel autoclave with a capacity of 100 ml. The autoclave was kept in an oven at 140°C for 12 h and then cooled down to room temperature. The resulting brown precipitate was collected, rinsed, and filtered to a pH 7. The as-pre- pared powders were then dried at 80°C in air. Synthesis of f-MWCNTs-MNTs-PVDF composites MNTs and f-MWCNTs reinforced polymer matrix com- posites were prepared by mixing the respective compo- site solutions at high-speed rotations per minute followed by solvent casting. Here, we describe the method of preparation of the composites. Initially, 10 mg of MNTs and 990 mg of polymer were dispersed separately in dimethylformamide (DMF) with the help of an ultrosonicator for 1 h at room temperature for the preparation of 1 wt.% MNTs in polymer matrix. These two solutions were mixed by sonicating together for 1 h and the composite solution was transferred to a melt mixer and stirred at room temperature at 4,000 rpm for 2 h and at 80°C for 30 min. The resulting solution was transferred into the beaker and kept in an oven to remove the solvent. Finally, dried thin films were put in a mold and pressed to form 1-mm thick structures. A similar procedure was followed for the preparation of functionalized multiwalled carbon nanotubes (f- MWCNTs)/PVDF composite films. For the preparation of f-MWCNTs/MNTs/PVDF composite, fixed amount of MNTs, f-MWCNTs, and PVDF were added to DMF separately for a desired composition, and the above- mentioned procedure was followed to prepare the com- posite films. A series of composites were prepared in a similar way by varying the amount of polymer, MNTs, and MWCNTs.