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Fibers with diameters less than a micron can be formed using an electrospinning process where a droplet of a polymer solution is elongated by a strong electrical field. The resulting nanofibers are collected as nonwoven mats with extremely large surface to volume ratios which are being used, or finding uses, in filtration, protective clothing, biomedical applications and reinforced composites. The electrospinning process also has advantages for investigation of fiber formation processes with new materials and solvents including the ability to work with small sample sizes, rapid time scale of the spinning/solvent removal process and high elongation of as spun fibers. We have been working on nanofibers formation of various advanced materials: i) inorganic nanofibers using electrospinning/sol-gel method, ii) polymer-inorganic nanocomposites, iii) newly developed biodegradable polymers. We would like to explore the possibility of tailoring the properties of these fibers on a nanoscale (~ 100 nm) via the electrospinning process. The group is also involved in continuum modeling of the fluid jet of the electrospinning process and the instability that the jet experiences while travelling to the collector. Learn More

We are developing structural and molecular models which are of great importance in interrelating the microstructure with processing conditions, and numerical methods to couple efficiently simulation of the macroscopic flow with evolution of the underlying molecular configurations over many different time and length scales. By first understanding the dynamics of a polymer chain by using Brownian dynamics simulations, we systematically incorporate the physical insight into a new, coarser-scale model. The new model captures the finer scale physics of the polymer chain model in a wide range of flows, but it is sufficiently simple to be used in the analysis of flows in complex geometries. The efficient, parallel, time integration scheme is effectively utilized when even more complicated constitutive descriptions such as molecular models for liquid crystalline polymers, multi-mode models, and multi-phase models are required in complex viscoelastic flow simulations. Additionally, the crystallization kinetics including stress-induced crystallization are incorporated into the simulation of various polymer processing operations such as fiber spinning, film casting, film blowing, and injection molding to obtain favorable comparisons between experimental data and numerical simulations. Learn More

Instabilities in viscoelastic flows are emerging as a rich class of fluid mechanical phenomena that are of much importance in polymer processing and coating operations. These instabilities are often found in nearly inertialess flows, and are driven solely by the non-Newtonian behavior of complex fluids such as polymer melts and solutions. However, most of the progress in the analysis of the stability of viscoelastic flows has been restricted to simple viscometric flows. The current analysis is based on finite element calculation of the steady base flows in complex geometries and the long-time solution of the time-dependent equations formed by linearization about the base flow. The equation set for the perturbations is solved either by direct time integration or by the Arnoldi method in which the most dangerous eigenvalues for the eigenvalue problem that corresponds to the long-time solution of the disturbance equations are determined. We also apply an energy analysis to examine poorly understood mechanisms of elastic instabilities in complex flows such as axisymmetric and planar contraction/expansion flows. Learn More

Finally, we are interested in understanding solid state processing (at temperatures lower than the melting point of a polymer) of polymeric materials for economic production with enhanced material properties. Although many attempts to apply the concept of solid state processing to many polymeric systems have been made for several decades, the practical impact of the process has been rather insignificant, because the lack of productivity seems to be inherently imbedded in the process. The interplay between material preparation and process development is the key here, and one good example of this interplay in solid state processing is the synthesis of ultra high molecular weight polyethylene (UHMWPE) for which the yield is high even at relatively low polymerization temperatures. Such UHMWPE nascent reactor powders can directly be processed to continuous multi-stage orientation drawing to produce high strength, high modulus materials or microporous membranes. Finite element analysis of solid-state drawing, which incorporates stress induced crystallization and morphology evolution, is applied to predict the structure and properties of the product. Learn More