Rheology and Electrorheology of Nanorod-Loaded Liquid Crystalline Polymers
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1129-V05-03
Rheology and Electrorheology of Nanorod-Loaded Liquid Crystalline Polymers Ana R. Cameron-Soto, Sonia L. Aviles-Barreto, and Aldo Acevedo-Rullán Department of Chemical Engineering, University of Puerto Rico, Mayagüez P.O Box 9046, Mayagüez, PR, 00681
ABSTRACT The effect of carbon nanotube concentration and dispersion on the rheology of liquid crystalline solutions of hydroxypropylcellulose (HPC) has been experimentally studied. The rheology of nanocomposites of HPC and multiwalled carbon nanotubes (MWCNT) in m-cresol was characterized in steady-state and transient dynamic tests. The rheology as particle loading increases shows a very distinct response in the magnitude and scaling of the steady-state viscosity, and the storage and loss modulus. The liquid crystalline phase was characterized by direct observations by reflected polarized light microscopy. Additionally, an electric-field effect was observed on the rheology of the HPC/MWCNT in m-cresol soft composites. The HPC in mcresol matrix is non-responsive, thus the electrorheological effect is due to the presence of the carbon nanotubes. The mechanism for this effect is still uncertain, since it does not follow the scaling predicted by simple models for heterogeneous or homogeneous ER fluids.
INTRODUCTION Inclusion of nanoparticles in a polymer matrix may provide value-added properties, such as increased electrical conductivity, increased thermal conductivity, low flammability, or reduced permeabilities, to mention a few, in addition to modifying the mechanical properties [1, 2]. Addition of anisotropic nanorods results in very different properties primarily due to the ability to tune the physical properties by controlling the size, aspect ratio, and assembly of the rods. A special case of field-assisted alignment includes the use of a liquid crystalline (LC) matrix and its ability to orient along an applied field, which allows for control of the orientation [3-5]. Lower applied fields and thus, lower energy requirements would be necessary to obtain orientation in a LC phase-mediated alignment. Liquid crystalline phase behavior have been observed in rodlike nanoparticle dispersions, such as MWNT [6], SWNT [7], CdSe nanorods [8], goethite [9], and natural clays [10], upon increase of the concentration (lyotropic LC). Similar to the phase behavior of rodlike polymers, as rod length increases the onset of liquid crystallinity decreases and the biphasic gap increases with increasing length distribution. Duran and coworkers (2005) reported an enhancement of the isotropic-nematic phase transition temperature of low molecular weight liquid crystal E7 with inclusions of MWCNT [11]. No changes were observed when spherical nanoparticles were added to E7 under similar conditions. These experiments suggest a significant contribution of inclusions’ anisotropy to the entropy of the nematic system. Therefore, inclusion of rodlike nanoparticles into an LC polymer matrix may not have a detrimental impact on the orientational order and phase behavior of the mixture.
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