Electrically conductive hybrid elastomeric nanocomposites

A novel synthesis method uniformly disperses polyaniline and carbon nanotubes in an elastomeric matrix, resulting in a nanocomposite with tunable conductivity and enhanced mechanical properties.Modern technologies increasingly call for advanced composite materials whose balance of properties can be controlled, tuned,…

A novel synthesis method uniformly disperses polyaniline and carbon nanotubes in an elastomeric matrix, resulting in a nanocomposite with tunable conductivity and enhanced mechanical properties.Modern technologies increasingly call for advanced composite materials whose balance of properties can be controlled, tuned, and matched to the requirements of specific applications. Elastomeric nanocomposites have attracted interest in recent decades because the high surface area of nanoparticles allows significant mechanical properties to be achieved (e.g., reinforcing the elastomer) at relatively low filler loadings. Using carbon nanotubes (CNTs) as the filler opens up further application opportunities by creating a material that combines high electrical conductivity with the mechanical properties of a polymer. To accomplish this, it is crucial for the nanofiller to be efficiently dispersed in the matrix.Known methods for dispersing CNTs in an elastomeric matrix include melt mixing and roll milling followed by hot-press molding or drop casting. However, in all of these the CNTs tend to agglomerate, and so fail to impart their intrinsic properties (e.g., high strength, electron mobility, and thermal conductivity) to the nanocomposite. One potential solution is to incorporate polyaniline (PANI), an intrinsically conducting polymer, along with CNTs to reduce the van der Waals interactions among CNTs and prevent them from agglomerating.1,2 Conductive polymers such as PANI can be introduced into an elastomeric matrix using techniques such as thermomechanical mixing and solution mixing. Typically, composites are prepared by the solution method and formed via drop casting on a silicon wafer or glass plate. The main disadvantages of the above routes are nonuniform distribution of conductive particles in the polymer matrix, complex preparation procedure, and slow polymerization reactions.To get around these difficulties and tackle the challenge of achieving a good dispersion of conductive particles in an elastomeric matrix, we recently developed a new approach for preparing a conductive hybrid nanocomposite.3,4We obtained a novel electrically conductive elastomeric nanocomposite composed of CNTs and PANI dispersed in a styrene-isoprene-styrene (SIS) block copolymer matrix. The synthesis procedure consisted of an in situ inverse emulsion polymerization of aniline doped with dodecyl benzene sulfonic acid (DBSA) in the presence of CNTs and dissolved SIS block copolymer. This was followed by a precipitation-filtration step, consisting of precipitation of the dispersions in methanol and vacuum microfiltration through a 20–25μm filter paper. The nanocomposites formed in this way were then gently separated from the membrane, dried overnight, and characterized.We found that the nanofiller dispersions prepared by this method were stable, with no notable precipitation over time, which we attribute to the presence of the PANI. High-resolution scanning electron microscopy (HRSEM) images of the individual CNTs (see Figure 1) show that the PANI chains had wrapped themselves in a dense layer around the CNTs to form a core-shell nanostructure of CNTs uniformly coated in PANI. We believe this PANI layer reduced the van der Waals forces between the CNTs and so largely prevented them from agglomerating.Figure 1.Left: High-resolution scanning electron microscope (HRSEM) image of a film cross section at 100K magnification shows an individual carbon nanotube (CNT) covered with polyaniline (PANI). Right: Schematic representation of the styrene-isoprene-styrene (SIS) elastomeric nanocomposite system composed of a CNT enveloped by PANI and dodecylbenzene sulfonic acid (DBSA).We also observed that the CNTs were not randomly distributed in the polymer matrix but instead possessed an organized structure composed of evenly distributed elastomeric ‘islands’ surrounded by CNTs, to form a 3D CNT network (see Figure 2). This unique nanoarchitecture appears to be a result of the preparation method. The completion of polymerization of aniline and the breakdown of dispersions occurred during the precipitation step in methanol. The SIS block copolymer self-assembled in the methanol and led to the formation of the islands during the ensuing vacuum filtration step. The conductive CNT/PANI particles were displaced by the SIS particles and formed a continuous conductive network around them.Figure 2.HRSEM images of a cross section of SIS/CNT/PANI film prepared by precipitation-filtration.The dependence of conductivity on filler concentration can be predicted by statistical percolation theory, following a scaling law. This allows us to calculate, via best fit with the equation, a ‘percolation threshold’ for electrical conductivity, which is the minimum loading at which the conductive filler particles form a continuous network inside the polymer matrix. Using this method, the calculated percolation threshold was estimated at ~0.4% by weight (wt%): see Figure 3. This relatively low percolation threshold is attributed to the unique island-network structure of the conductive filler.Figure 3.Volume electrical conductivity against CNT concentration by weight (wt%) and (inset) best fit of experimental conductivity values using the power law equation of percolation theory. σ: Conductivity of the reinforced polymer. φc: Percolation threshold. R2: Coefficient of determination.We also investigated the mechanical properties of ternary SIS/CNT/PANI systems with different CNT loadings (see Figure 4). As the CNT concentration increases, the value of Young’s modulus increases with a corresponding decrease in the strain at break.5 However, for CNT loadings between 3 and 7wt% we notice a sharp increase in Young’s modulus together with a sharp decrease in strain at break. This phenomenon can be better understood by examining the HRSEM images of elastomeric nanocomposites with different CNT loadings (see Figure 4). We can see that the conductive network of the nanocomposite with 3wt% CNT in Figure 4(a) has a more open architecture, with larger-sized elastomeric islands, whereas the nanocomposite containing 10wt% CNT in Figure 4(b) is more densely packed, with smaller-sized elastomeric islands. These findings suggest that the structures and nanoarchitecture of the material, and hence its properties, can be controlled by varying the amount of filler.Figure 4.Graph showing the mechanical properties of SIS/CNT/PANI nanocomposite systems with different CNT loadings, and HRSEM images of cross sections of SIS/CNT/PANI nanocomposite films containing (a) 3wt% and (b) 10wt% CNT.In summary, we investigated a hitherto unstudied method for the preparation of conductive elastomeric nanocomposites. We obtained a hybrid SIS/CNT/PANI nanocomposite in which a continuous 3D CNT/PANI network formed within the SIS matrix, conferring the desired combination of electrical and mechanical properties to the material. This novel synthesis approach also offers the opportunity for tuning the structures within the material to achieve remarkably distinctive architectures and properties. We plan to follow up this work by assessing the possibility of using the developed nanocomposites as efficient and low-cost strain sensor materials.AuthorsIrena BrookDepartment of Chemical Engineering Technion – Israel Institute of Technology (IIT)Irena Brook is a PhD candidate in the Interdepartmental Program in Polymer Engineering. Her research interests include nanocomposites, conductive nanoparticles, elastomers, and electrochemical and electromechanical sensors.Guy MechrezDepartment of Chemical Engineering Technion – Israel Institute of Technology (IIT)Rosa TchoudakovDepartment of Chemical Engineering Technion – Israel Institute of Technology (IIT)Shiran LupoDepartment of Chemical Engineering Technion – Israel Institute of Technology (IIT)Moshe NarkisDepartment of Chemical Engineering Technion – Israel Institute of Technology (IIT)Ran Y. SuckeverieneDepartment of Water Industries Kinneret College in the Jordan ValleyReferencesR. Y. Suckeveriene, E. Zelikman, G. Mechrez, A. Tzur, I. Frisman, Y. Cohen and M. Narkis, Synthesis of hybrid polyaniline/carbon nanotube nanocomposites by dynamic interfacial inverse emulsion polymerization under sonication, J. Appl. Polym. Sci. 120, pp. 676-682, 2011. E. Zelikman, R. Y. Suckeveriene, G. Mechrez and M. Narkis, Fabrication of composite polyaniline/CNT nanofibers using an ultrasonically assisted dynamic inverse emulsion polymerization technique, Polym. Adv. Technol. 21, pp. 150-152, 2010. I. Brook, G. Mechrez, R. Y. Suckeveriene, R. Tchoudakov and M. Narkis, A novel approach for preparation of conductive hybrid elastomeric nano-composites, Polym. Adv. Technol 24, pp. 758-763, 2013. I. Brook, G. Mechrez, R. Y. Suckeveriene, R. Tchoudakov, S. Lupo and M. Narkis, The structure and electro-mechanical properties of novel hybrid CNT/PANI nanocomposites, Polym. Compos., 2013. Published online in advance of printG. Mechrez, R. Y. Suckeveriene, R. Tchoudakov, A. Kigly, E. Segal and M. Narkis, Structure and properties of multi-walled carbon nanotube porous sheets with enhanced elongation, J. Mater. Sci. 47, pp. 6131-6140, 2012. DOI:  10.2417/spepro.005248

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