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Macromolecular, Colloidal, and Complex Fluid Theory
Our overarching goal is the development and application of novel molecular-scale statistical mechanical theories of the equilibrium and dynamic properties of polymers, colloids, nanoparticles, liquid crystals, elastomers, nanocomposites and other complex fluids and soft materials. A common theme is to both understand existing systems at a fundamental level and develop predictive methods for guiding the experimental design of new materials. Five broad areas are of present interest.
The uniquely slow dynamics and elasticity of entangled polymer liquids is a fascinating scientific problem of high engineering and processing importance. Existing theoretical approaches are phenomenological. We are developing first principles statistical mechanical theories that explicitly capture the dynamical consequences of polymer connectivity and uncrossability for diverse macromolecular architectures (rigid rods, flexible chains, star-branched objects) in solution, melts, thin films and liquid crystalline states. Molecular level theories of the rheology of these systems under both constant constant stress and strain rate conditions are also being developed. The work is relevant not only to synthetic polymeric materials, but also dense collections of stiff biopolymers commonly found in cell biology and crucial for the mechanical function of the cytoskeleton.
"Particle-polymer" mixtures, in solution and melt states, are ubiquitous in diverse areas of science and technology. We have developed predictive theories of the equilibrium structure, properties and phase behavior of such polymer nanocomposites based on integral equation methods. Present work is focused on dense nanocomposites involving functional particles such as buckeyballs, or fillers that are soft and fluctuating such as crosslinked nanogels. We are building on our advances in predicting equilibrium structure to tackle the complex problem of the dynamics and mechanical response of these hybrid materials. Questions such as how fast do nanoparticles diffuse and hop in a polymer liquid or network, can nanoparticles gel or jam in the polymer matrix, how do particles modify polymer entanglements, elasticity (reinforcement) and viscosity, and the role of particle size, shape, interactions with the polymer, and nanocomposite statistical microstructure are being investigated.
A broad area of enduring interest is the slow dynamics of glass-forming fluids. We are developing molecular-level theories of relaxation, diffusion, viscoelasticity and vitrification of deeply supercooled liquids including molecular, polymer, metallic and colloidal and nanoparticle systems. New ideas about the origin of dynamic cooperativity and rare collective activated events have been conceived and are being quantitatively applied to experimental materials. This advance provides a foundation for treating polymer thin films which are of high engineering importance. A major aim is to understand the remarkably large speed up of dynamics observed when polymer films are thinner than 30-50 nanometers and have at least one surface exposed to air.
The development of functional colloidal assemblies in solution is of high interest. We are developing microscopic statistical mechanical theories of the self-assembly of patchy Janus colloids, the coupled translation-rotation dynamics, vitrification, and gelation of dense suspensions of nonspherical particles, electron conductivity in metallic nanoparticle gels, and how colloids organize and move in large mesh polymer networks. These topics connect with our efforts in the interdisciplinary area of basic energy sciences.
A new direction is to understand gas and molecule permeability in polymeric materials in the molten, crosslinked rubber, and glass states. This requires treating the effect of penetrant size, shape and interactions with the polymer on solubility, and the role of length scale dependent polymer dynamics and free volume on penetrant diffusion. A major goal is to establish molecular-level design rules for both optimizing permeability for membrane-based separation applications, and minimizing permeability to fabricate new barrier materials that protect polymers from degradation processes. In addition, the effect of nonequilibrium physical aging and external stress on these phenomena is of interest. This research also underpins the development of high performance “self-healing” materials where controlling the transport of small molecules and oligomers in polymeric microcapsules is essential.