Research : Erdogan Kiran
Research Interests
Examples of Current Research Projects
Supercritical fluids display properties that are between those of gases and liquids. However, unlike ordinary liquids or gases, their properties are readily adjustable through manipulations of pressure or temperature. Because of their tunable properties, they are very attractive solvents for a diversity of applications. Our research program is aimed at understanding both the equilibrium and time-dependent aspects of the physico-chemical processes involving these fluids. In particular, the solubility, reactivity, and stability of various substances ranging from relatively simple molecules to complex synthetic or natural polymeric systems in single and multi-component fluids are being studied. Transport properties under supercritical conditions are also being investigated. Special high-pressure thermal, optical, and spectroscopic techniques are in use and being further developed. Current applications are focused on polymers, pharmaceuticals and medical device applications. [return to top]A major focus of our current research is on polymer formation, modifications, processing, and recycling. Polymerization in supercritical fluids is explored as a method to control polymer molecular weight, molecular weight distributions, and chain microstructure. Interactions of high pressure gases and fluids with polymers and polymer solutions in supercritical fluids are evaluated for various polymer processing operations. Equilibrium and dynamic aspects of polymer solutions are investigated with respect to dissolution and phase separation. Pressure-jump techniques are used to study the kinetics of phase separation and for identification of "nucleation and growth" and "spinodal decomposition" regimes. Pressure-induced phase separation (PIPS) is used to produce micro or nanoporous or microstructured polymers and blends for applications related to foams, fibers, membranes, dielectric materials, and scaffolds. Polymer chain mobility and chain dimensions in different solvents, under different temperature and pressure conditions are studied. Crystallization from high-pressure conditions is also being explored. [return to top]Our program on natural materials is focused on understanding the physico-chemical properties and processing of natural polymers such as lignocellulosic materials-biomass and coal. Thermal, solvent, or chemical modification and separation procedures are being explored for novel processes such as supercritical delignification for sulfur and chlorine-free pulping and bleaching operations, environmentally acceptable solvents for processing of cellulose derivatives, supercritical impregnation processes for property modifications and wood preservation, supercritical recycling, and supercritical hydrolysis and depolymerization, supercritical fluid-based modifications and cleaning of coal. Biodegradable polymers and materials are being explored for medical and pharmaceutical applications ranging from controlled release to tissue engineering. [return to top]Selected Examples from Recent Research
Crystallization or re-crystallization of polymers in dense fluids at high pressures is creating new opportunities for formation of particles or novel morphologies from semi-crystalline polymers. Crystallization can be carried out at constant pressure, temperature, or density with different levels of undercooling. One of the recent efforts in our laboratory is focused on experimental determination of the solid-fluid phase boundary in polymer solutions, and the consequences of the path followed on the crystal morphology. Below are some examples of polyethylene crystals formed at different crystallization pressures.
Polymer foaming in or with supercritical fluids is of practical interest with wide range of applications. Miscibility of the fluid with the polymer is achieved under pressure which is then expanded with a pressure reduction. In the process the fluid-swollen polymer is transformed into a micro- or nanoporous material. The final morphology depends on the kinetics of phase separation, and the kinetics of the thermal transformation (crystallization and/or vitrification) of the polymer matrix, as well as the changing dynamics on viscosity and interfacial tension. Below are some examples of microcellular foams of a semi-crystalline biodegradable polymer, poly(epsilon caprolactone), and a glassy polymer poly(methyl methacrylate).
Polymerization in supercritical fluids is of great interest. Our current activity is in copolymerizations. These processes are intriguing in that with the progress of polymerization the fluid compositions and the viscosity of the medium changes. The change in the fluid composition along with the increase in the molecular weight of the polymer formed continually alters the phase boundaries. In copolymerizations, the initial monomer ratio employed and pressure and the temperature conditions determine the copolymer composition and the conditions of phase separation and the final morphologies. The end results may range from fine particles if the copolymer is semicrystalline and undergoes solid-fluid phase separation, to porous matrices if copolymer formed is glassy and is swollen in the reaction mixture. Below are some examples from precipitation polymerization of acrylonitrile and its copolymerization with 2-chlorostyrene and methyl methacrylate, forming nanoparticles.
Polymer miscibility and phase separation of polymers at high pressures in supercritical fluids is key to essentially all applications related to polymer modification and processing. One of our research focus areas is the investigation of the phase behavior and the miscibility of polymers in binary fluid mixtures. The fluid mixtures that we are specifically interested are those containing carbon dioxide as a component. Binary fluid mixtures introduce fluid composition as an additional tuning parameter to pressure or density in adjusting the miscibility and phase separation conditions for a given polymer. Such binary mixtures naturally arise for example in polymerization reactions in carbon dioxide, which are basically reactions that proceed in mixture of carbon dioxide and the monomer itself. The figure below shows the miscibility conditions for polysulfone in binary mixtures of carbon dioxide and tetrahydrofuran. The region above each curve represent the miscible region. As the solvent power is reduced with increasing the carbon dioxide content in the fluid mixture, pressures required to achieve miscibility increase, and the temperature sensitivity change sign.
Viscosity of polymer solutions is an extremely important transport property that plays a critical role in polymer formation, or in polymer processing ranging from formation of particles to fibers. A topic of interest is the reduction of viscosity in the presence of carbon dioxide or other supercritical fluids as diluents. Our group has been systematically investigating the viscosity of polymer solutions and their dependence on pressure, temperature, fluid composition, polymer type, concentration and molecular weight using a falling-cylinder type viscometer which also provides densities. These data are correlated with solution density. The figure below is such a plot. In the presence of carbon dioxide, at a given density, viscosity is lower in systems that contain more carbon dioxide. If same viscosity level is to be reached, the solutions contain in more carbon dioxide must be compressed to higher densities.
Selected Related Publications
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Department of Chemical Engineering Virginia Polytechnic Institute and State University 133 Randolph Hall Blacksburg, Virginia 24061 540 231 6631 540 231 5022 (fax) |