Alejandro Rey , McGill University
Carbon-carbon (c/c) composites are high performance multifunctional structural materials that are widely used in the transportations, microelectronics, and aerospace sectors. Composites based on carbonaceous mesophase (CM) matrices and carbon fibers are obtained by flowing the nematic CM into an array of carbon fibers. Since nematic liquid crystal with embedded second phases invariably result in the nucleation of topological defects in the mesophase matrix, the c/c structure-material properties relations need to be based on knowledge of how defects nucleate and organize within arrays of fibers. Texture generation in CMs is well described by liquid crystal surface science, defect physics and rheology. The path to the ultimate understanding of texturing processes in c/c composites builds on theory and simulation of: (i) LC coating fiber flow, (ii) LC fiber wetting models, (iii) wetting properties of fiber bundles, (iv) mesophase mixtures, (v) geometric optimization of fiber cross-sectional shape, and (vi) nanofiber structures. Objectives (i-iv) were completed in the first part of the grant. In particular new wetting laws for nematic coatings on cylindrical fibers were formulated and experimentally validated in collaboration with Prof. M. Srinivasarao at Geogia Institute of Technology. The fifth objective seeks to elucidate the role fibre cross-sectional geometry on texturing. It is well known that CM pitches undergo structuring when they flow through square screens and meshes, leaving behind long lived disclination lattices, since the net topological charge in the mesophase is zero. Faceted particles, inclusions, and fibres will also have the ability to inject disclinations in a controlled fashion into the mesophase matrix. In contrast to circular fibres. for faceted particles, edges also contribute to defect emission and absorption. Texturing due to single faceted particle has been characterized. Defects in an aligned nematic mesophase with homeotropic anchoring surrounding a facetted particle include bulk +1/2 disclinations, surface corner disclinations, and disclination strings that join vertices. The phase diagram of a typical single faceted particle in terms of temperature as a function of particle size consist of : (I) string defect mode (II) mixed surface defect mode (II) surface-bulk defect mode. The single particle investigation was extended to multiple particle, as it is relevant to c/c materials. Two dimensional texture simulations, based on the Landau-de Gennes equations of nematodynamics in the absence of flow, were carried out for nematics with embedded faceted binary and multiple particles using temperature, size, and density as topological control variables. The stable modes obtained from kinetic simulations are summarized in texture phase diagrams in terms of temperature, particle separation and particle size. The key novelty in 2D faceted particles is the presence of corners that are sources/sinks of disclination lines or active corner defects. Under close corner-to-corner proximity, disclination lines form inter- or intra- particle links, while under larger distances, surface defects decorate the corners. This generic geometric behaviour is modulated by temperature since the line tension of disclinations decreases with increasing temperature. The characteristic texture diagrams show that for nanoparticles, increasing particle separation results in a disclination mode sequence of linked particles, cross-linked particles, and intra-particle disclinations. For colloidal particles, increasing size results in a mode sequence of linked particles and isolated particles with corner defects. For intermediate particle size, a continuous evolution of the phase diagrams from the former to the latter is demonstrated. At low temperatures, increasing interparticle distances leads to a disclination defect breakage at a critical value independent of particle size. Multimer particle assemblies form stable filaments via a corner-to-corner interparticle disclination linkage mechanism that only exist due to facetting. The stability of the multimers to bending is characterized using curvature as a topological control variable. As stretching-driven instabilities in binary particles give rise to surface defects modes, bending-driven instabilities give rise also to multi-particle rings with corner decorated surface defects. The integrated texture phase diagrams, topological transitions, tension and bending-driven instabilities demonstrate the potential functionality of faceted particle liquid crystal composites and nanocomposites. Finally, reducing the facetted particles size to the nanoscale results in a liquid crystal nanocomposite which may display orientatioonal order in the matrix phase as well as nanoparticle positional order. The two order parameter composite was investigated with the aim of extracting information on the exact relation between polygonal nanopartciel arrangements and topological defects. A systematic analysis of defect textures in facetted nanoparticles with polygonal configurations embedded in a nematic matrix is performed using the Landau-de Gennes model, homeotropic strong anchoring in a square domain with uniform alignment in the outer boundaries. Defect and textures are analyzed as functions of temperature T, polygon size R, and polygon number N. For nematic nanocomposites, the texture satisfies a defect charge balance equation between bulk and surface (particle corner) charges. Upon decreasing the temperature, the central bulk defects split and together with other -1/2 bulk defects, are absorbed by the nanoparticle's corners. Increasing the lattice size decreases confinement and eliminates bulk defects. Increasing the polygon number increases the central defect charge at high temperature and the number of surface defects at lower temperatures. The excess energy per particle is lower in even than in odd polygons, and it is minimized for a square particle arrangement. These discrete modeling results show for first time that even under strong anchoring, defects are attached to particles as corner defects, leaving behind a low energy homogeneous orientation field that favors nanoparticle ordering in nematic matrices. These new insights are consistent with recent thermodynamic approaches to nematic nanocomposites that predict the existence of novel nematic/crystal phases and can be used to design nanocomposites with orientational and positional order.