Electric Field Effect on the Rheology of MWCNT Dispersions in Liquid Crystalline Polymers Ana R. Cameron-Soto and Aldo Acevedo-Rullán Department of Chemical Engineering, University of Puerto Rico, Mayagüez, PO Box 9046, Mayagüez, PR 00680, email: aldacevedo@uprm.edu ABSTRACT To achieve the full potential of nanocomposites anisotropic particles have to be well dispersed and their orientation controlled. In this research, we combine the self-organizing properties of liquid crystal polymers (LCP) and the fact that the carbon nanotubes are electronically polarizable to develop novel oriented LCP nanocomposite materials. The electrorheological (ER) effect of multi- walled carbon nanotubes dispersions on LCP matrices was observed and quantified experimentally. A non-electric field responsive, hydroxypropylcellulose (HPC), was used as the polymeric matrix. A negative ER effect on the steady-state viscosity was observed. Simple mechanisms for homogeneous or heterogeneous ER fluids do not capture the observed behavior. Additionally, the electric field effect on quiescent samples has been analyzed by polarized optical microscopy to elucidate the mechanisms for the observed rheological response. Neither bundle aggregation nor migration to the electrodes was observed at the micro-scale. . Keywords: MWCNT, liquid crystalline polymer, nanocomposite, electrorheology 1 INTRODUCTION Carbon nanotubes (CNTs) composites have attracted attention due to their exceptional mechanical, such as high tensile strength, and electronic properties. Proposed applications for light and high strength nanotube-reinforced composites span from biomedical to aeronautic industries. However, the control of the dispersion and orientation of CNTs in polymeric matrixes is an important issue to obtain better and efficient nanocomposites applications and to obtain multifunctionality. Many techniques have been proposed to achieve their alignment such as the introduction of argon gas in a laser ablation reactor[1], application of an external electric or magnetic field [2-6], deposition of individual nanotubes using chemically functionalized nanolithography templates [7], electrophoresis [8], flow-induced alignment [9], and using the self-organizing properties of the liquid crystals (LC) [10-12]. Most of these methods are not suitable for the preparation of nanocomposites, since they cannot be transferred to continuous processing or are limited to laboratory scale processes. For example, orientation with LC surfactants is not feasible because there is no efficient method to solidify the composite without adding stress to it, without losing the LC phase and the CNT disorientation [11, 13]. In spite of all the efforts to achieve a better orientation, no successful technique which allows a controlled and reproducible production of CNT nanostructures has been developed. In this work, we take advantage of the self-organizing properties of a liquid crystalline polymer (LCP) matrix and its inherent processing flexibility to orient MWCNTs, and explore the use of the LCP’s ability to orient with external flow and electric fields to orient the nanotubes. We present the electrorheological characterization of MWCNT dispersion on LCP matrices, and the electric field effect on quiescent samples using polarized optical microscopy. The chosen LCP matrix does not have a response with an electric field. These experiments provide additional understanding of the mechanisms of particle orientation on the LCP matrix. 2 EXPERIMENTAL METHODS 2.1 Materials Hydroxypropylcellulose (HPC) (M w = 100 kDa) and multiwalled carbon nanotubes (>95%, O.D. = 20-30 nm, L = 0.5-2 μm) were purchased from Sigma Aldrich, while m- cresol (97% purity) was purchased from Fisher Scientific. A completely liquid crystalline phase was observed for polymer concentrations above 35 wt% in a Micromaster II polarized optical microscope. The HPC/MWCNT/m-cresol solutions were prepared by mixing of HPC in MWCNT/m- cresol stock solutions up to a 45 wt% of polymer. Solutions were left to equilibrate for at least one week. Stock solutions and final mixtures were dispersed by ultrasonification in a Branson 450W sonicator. Previous work has shown that MWCNT particle loadings below one volume percent do not have an effect on the scaling of the linear viscoelastic moduli, which suggested no effect on the liquid crystalline structure of the solution [14, 15]. In the case of solutions for the optical study, sonication time was minimized to produce samples where MWCNT bundles could be identified on the micron scale. NSTI-Nanotech 2009, www.nsti.org, ISBN 978-1-4398-1783-4 Vol. 2, 2009 485