Anisotropy and Dynamic Ranges in Effective Properties of Sheared Nematic Polymer Nanocomposites

Forest, M. Gregory; Zheng, Xiaoyu; Zhou, Ruhai; Wang, Qi; Lipton, Robert

doi:10.1002/adfm.200500272

Cited by 15 publications

(7 citation statements)

References 23 publications

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“…These anisotropy/dimensional characterizations of percolation thresholds follow the authors' earlier volume-averaged characterizations of conductive [22,23] and mechanical anisotropy [24] in sheared nanorod dispersions. These homogenization results give coarse-grained properties that depend only on the second moment (for conductivity tensors), or second and fourth moments (for mechanical tensors), of the nanorod PDF.…”

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confidence: 68%

A Strategy for Dimensional Percolation in Sheared Nanorod Dispersions

et al. 2007

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As the aspect ratio of a unit (here, a stiff high-aspect-ratio rod) increases, the tendency increases for ensembles of these units to form hierarchical structures on scales quite removed from that of the single unit (particles). This is due in large part to geometrical packing considerations (based on excluded volume analysis) arising from the non-spherical shape. Nematic ordering of rod ensembles [1] is one illustration of this phenomenon, and percolation of rod or platelet inclusions is another. It happens that for thin rods, equilibrium percolation of isotropic distributions occurs well below the nematic transition. The introduction of a flow-induced distribution function leads one out of the highly studied domain of universal scaling laws for equilibrium, isotropic percolation thresholds. For materials processing applications, one is compelled to explore the impact of volume fraction of particles beyond the threshold of any desirable percolation-induced property transition and ascertain the role of anisotropy on macroscopic property tensors. This is required for practical applications in order to avoid statistical fluctuations across the preferred side of the threshold.There is a large quantity of literature on the onset of percolation among rods or "sticks" in two and three space dimensions, including seminal contributions of Balberg and collaborators [3,8,9]

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confidence: 68%

A Strategy for Dimensional Percolation in Sheared Nanorod Dispersions

et al. 2007

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“…The boundary conditions on the velocity v = (v x , 0, 0) are given by the Deborah number v x (y = ±1, t) = ±De. (12) Following previous studies [3,21,31,9], we assume homogeneous anchoring at the plates, given by the quiescent nematic equilibrium, f (m, y = ±1, t) = f e (m), (13) where f e (m) is an equilibrium solution of (3) with v = 0. At nematic concentrations, equilibria f e (m) are invariant under orthogonal rotations; orientational degeneracy is broken in the laboratory by mechanical rubbing or applied fields.…”

Section: Mesoscopic Closure Models Mesoscopic Models Replace the Smomentioning

confidence: 99%

Kinetic Structure Simulations of Nematic Polymers in Plane Couette Cells. II: In-Plane Structure Transitions

Forest¹,

Zhou

Wang³

2005

Multiscale Model. Simul.

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Nematic, or liquid crystalline, polymer (LCP) composites are composed of large aspect ratio rod-like or platelet macromolecules. This class of nanocomposites exhibits tremendous potential for high performance material applications, ranging across mechanical, electrical, piezoelectric, thermal, and barrier properties. Fibers made from nematic polymers have set synthetic materials performance standards for decades. The current target is to engineer multifunctional films and molded parts, for which processing flows are shear-dominated. Nematic polymer films inherit anisotropy from collective orientational distributions of the molecular constituents and develop heterogeneity on length scales that are, as yet, not well understood and thereby uncontrollable. Rigid LCPs in viscous solvents have a theoretical and computational foundation from which one can model parallel plate Couette cell experiments and explore anisotropic structure generation arising from nonequilibrium interactions between hydrodynamics, molecular short-and long-range elasticity, and confinement effects. Recent progress on the longwave limit of homogeneous nematic polymers in imposed simple shear and linear planar flows [

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“…[32,30,21,29]). Molecular orientation features in different flow regimes are of extreme importance for materials design, as they impart anisotropic and nonuniform material properties [34,18,19]. Another issue typical of non-Newtonian fluids is flow feedback, where elastic stresses alter apparent viscosity.…”

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On Weak Plane Couette and Poiseuille Flows of Rigid Rod and Platelet Ensembles

Cui¹,

Forest²,

Wang³

et al. 2006

SIAM J. Appl. Math.

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On weak plane Couette and Poiseuille flows of rigid rod and platelet ensembles (with Z. Cui, M.G. Forest and Q. Wang), SIAM J. Appl. Math. , 66 (4), 2006 Abstract. Films and molds of nematic polymer materials are notorious for heterogeneity in the orientational distribution of the rigid rod or platelet macromolecules. Predictive tools for structure length scales generated by shear-dominated processing are vitally important: both during processing because of flow feedback phenomena such as shear thinning or thickening, and postprocessing since gradients in the rod or platelet ensemble translate to nonuniform composite properties and to residual stresses in the material. These issues motivate our analysis of two prototypes for planar shear processing: drag-driven Couette and pressure-driven Poiseuille flows. Hydrodynamic theories for high aspect ratio rod and platelet macromolecules in viscous solvents are well developed, which we apply in this paper to model the coupling between short-range excluded volume interactions, anisotropic distortional elasticity (unequal elasticity constants), wall anchoring conditions, and hydrodynamics. The goal of this paper is to generalize scaling properties of steady flow molecular structures in slow Couette flows with equal elasticity constants [M. G. Forest et al., J. Rheol., 48 (2004), pp. 175-192] in several ways: to contrast isotropic and anisotropic elasticity; to compare Couette versus Poiseuille flow; and to consider dynamics and stability of these steady states within the asymptotic model equations.

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Anisotropy and Dynamic Ranges in Effective Properties of Sheared Nematic Polymer Nanocomposites

Cited by 15 publications

References 23 publications

A Strategy for Dimensional Percolation in Sheared Nanorod Dispersions

A Strategy for Dimensional Percolation in Sheared Nanorod Dispersions

Kinetic Structure Simulations of Nematic Polymers in Plane Couette Cells. II: In-Plane Structure Transitions

On Weak Plane Couette and Poiseuille Flows of Rigid Rod and Platelet Ensembles

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