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Research Interests

· Polymer processing (design and simulation) and rheology
· Composite materials and processing
· Polymeric materials and properties

Current Research Projects

Design of Macromolecular Architecture for Processing Performance Using Molecular Rheology

Sparse long chain branching (LCB) has been reported to have remarkable effects on the rheology and processing of polyolefins. By sparse LCB we are referring to branching levels typically less than one branch per 1000 backbone carbon atoms and arm molecular weights, Ma, significantly greater than the critical molecular weight (MW) for entanglements, Me. In most commercial polymeric systems the molecular architectural features of MW, molecular weight distribution (MWD), and branching topology contribute to their rheology in a convoluted manner, where, for example, extensional strain hardening can be observed in highly branched polyethylenes (PE's) as well as PE's with a very broad MWD. The development of metallocene catalysts provide the ability to synthesize resins with a relative narrow MWD (~2.0), which is desired for mechanical properties and to add long chain branches to commercial polyolefins in a controlled way to improve their processing performance. Hence, the potential now exists to design the catalyst system and polymerization conditions to produce the molecular architecture which will provide the desired melt rheology and processing performance. The recent development of a molecular theory for branched polymer systems and the possibility of connecting an ensemble of the model structure for branching topology to the statistical structure (i.e. seniority and priority) of metallocene catalyzed PE's containing long chain branching will be instrumental to designing molecules with a desired architecture. The goal of this research is to develop a quantitative theory for the rheological response and associated processing performance of a melt of known polymerization kinetics. This project will be part of an interdisciplinary program involving two U.S. universities and five UK universities. (NSF., PhD., pending/CP Chem., funded.)

 
Relaxation of branched polymer chains


Molecular Theory for Branched Polymers


Numerical Simulation of Injection Molding of Fiber Reinforced Thermoplastics

Particles of various aspect ratios are added to polymers to increase their mechanical properties. When glass fibers, high aspect ratio particles, are added to polymers, parts generated by means of injection molding exhibit significant increases in stiffness and strength which depend on the fiber orientation distribution. The ability to design molds which lead to parts with the optimum mechanical properties depends on the ability to simulate the flow and fiber orientation development during mold filling. The key to this work is developing an appropriate constitutive relation such as shown below, designing


fundamental rheological experiments to obtain material parameters, and developing the numerical method(finite element method) to solve the coupled equations. This project represents the combined efforts of chemical engineering and mathematics. (NSF/DOE funding).


Exfoliation of Nano-particles in Polymer Melts Using Super-crticial CO2

In order to capitalize on the potential offered by nano-particles in areas such as reinforcement, barrier, and electrical conductivity, higher levels of fully dispersed nano-particles must be obtained than presently possible with existing techniques. The objective of this work is to improve the degree of exfoliation and dispersion of clay nano-particles into polymeric matrices using supercritical carbon dioxide (SC-CO2). In this presentation we describe a technique by which nano-clays are mixed with SC-CO2, the pressure is partially released to expand the particles, and then the mixture is injected into the polymer melt as it is pumped through an extruder. Because SC-CO2 behaves as a polar organic solvent, it is believed that it readily enters the galleries of the nano-clay and swells the clay particles. When the pressure is partially released, CO2 expands the galleries and thereby further exfoliates the clay particles. Furthermore, CO2 is partially soluble in a number of polymers serving as a plasticizer and further facilitates the mixing process. Various techniques were used to quantify the degree of exfoliation of the clays in polypropylene. XRD was used to identify the d-spacing and TEM was used to observe the general features of the exfoliated structure. We also measured the melt rheology of nano-composites prepared by direct melt blending and those using SC-CO2 and found that the better the degree of exfoliation, the lower the values of the storage and loss modulus at low frequencies (i.e., no tail in the frequency sweep). More importantly materials were injecting molded and those prepared using CO2 had higher strength and modulus, which is a more critical assessment of the degree of exfoliation. The primary limitation of the technique at present is maintaining a uniform concentration of the nano-particles, but this will be eventually resolved (NSF).

       
Intercalated       Exfoliated



Improved Materials for Lower Cost Fuel Cells

About 50% of the cost of the fuel cell is in the bipolar plate and the polymer electrolyte membrane. We have been concerned with developing a process to produce cost-effective bipolar plates with high electrical conductivity, high corrosion resistance, excellent mechanical properties, and with the potential for rapid manufacturability. The composite consisting of graphite particles, thermoplastic fibers and carbon or glass fibers is generated by means of a wet-lay process to yield highly formable sheets. The porous sheets together with optional additives are then stacked and compression molded to form bipolar plates with gas flow channels. The key to the successful development of cost effective plates is the generation of materials which are sufficiently conductive and yet have the appropriate flow properties for shaping. Furthermore, the selection of an effective means of heating and cooling the composite materials during the manufacturing process is crucial to keeping the cost as low as possible. The other part of our effort is in developing a continuous process for generating the thin films which make up the polymer electrolyte membrane. This involves fluid flow, heat transfer, and mass transfer processes. Both experimental and modeling efforts are required (DOE and Nissan).

     
Film Casting Process       Fuel Cell Stack Showing the Bipolar Plates


High Performance Multi-functional Materials

Ion-containing polymers, or "ionomers", are part of a larger class of macromolecules known as polyelectrolytes. Although ion-containing polymers are already valuable in numerous applications including packaging, flocculants, cosmetics, pharmaceutical carriers, superabsorbents, etc., high performance systems of interest for other advanced technologies have not been as well explored. The mechanisms of ionic conduction that are critical for applications such as mechano-sensitive transducers, chemical sensors, and for membranes for fuel cells, polymer batteries and water purification are poorly understood. Moreover, ion transport is a fundamental process in a number of biological functions such as host-pathogen interactions, neural signal transduction, and heart function, in which relationships among ion transport and function(s) are enormously complicated. The ability to combine these materials with high performance structural materials such as thermoplastics reinforced with carbon and Kevlar fibers is unique and will be the emphasis of this research. Our goal will be to take advantage of the fact that these materials can be used to coat other materials such as reinforcing fibers used in composites to generate local sensing capabilities such as in the interior of a composite structure. In this way it may be possible to generate heat locally within the material to cause internal repair of the composite (thermoplastics). Furthermore, it may be possible to use these materials to change the shape of composite structures such as helicopter blade or wind structure. This could make it possible to eliminate mechanical parts in these structures. A group effort involving polymer synthesis, sensing, polymer and composites processing, and mechanical properties and performance will be employed. Furthermore, basic information about the process of healing and repair in humans will be explored in an effort to capitalize on these techniques. (DOD, pending)


High Performance PET Fibers Reinforced with Nano-particles

PET fibers are used both in textiles applications and as reinforcing materials for tires. There is a desire to increase their mechanical properties. One way to potentially do this is to use nano-particles as a reinforcing component. Here we explore the effect that nano-particles can have on the processing performance of PET and the level of improvement in mechanical properties. Part of this project will involve improving the dispersion of nano-particles in polymers using supercritical carbon dioxide. (Performance Fibers).


Benign Processing of Polymers

The effects of using near critical and supercritical carbon dioxide (CO2) to plasticize polymers that are difficult to melt process are studied, in particular acrylonitrile (AN) and methyl acrylate (MA) copolymers. Previous work with PAN/MA copolymers included differential scanning calorimetry (DSC), used to evaluate the resulting shift in the glass transition temperature (Tg) following plasticization, and pressurized capillary rheometry to evaluate the melt rheology and entry pressure effects prior to and after plasticization. A slit-die rheometer has been designed to allow the attachment of various nozzles to the exit to maintain high pressure and single-phase flow, suitable for measuring viscosity reduction with varying weight percentages of CO2 in a continuous process. In addition, a pressure chamber has been designed which attaches to the die exit to minimize the degree of foaming of the polymeric extrudate. In addition to viscosity reduction measurements, scanning electron micrographs of extruded materials were carried out on a AN/MA copolymer (modified with 10% rubber) to assess the degree of suppression of bubbles. This work serves as a basis for assessing the ability to use CO2 as a processing aid for unstable polymeric systems which would ordinarily have to be solution process.



Sintering of Viscoelastic Fluids

Background: Polymer sintering involves the process by which two polymer drops coalesce into a single drop. It is a process driven by surface tension and resisted by the melt rheology (in particular, viscosity). It is the basis for a number of polymer processing operations such as a rotational molding, powder coating, cold compression molding and selective laser sintering (a rapid prototyping technique). There is conflicting information pertaining to the role of viscoelasticity in polymer sintering. The experimental data indicate that the sintering rate increases with increasing fluid elasticity (i.e. longer relaxation time) while numerical simulation results indicate that the rate decreases with increasing relaxation time (or elasticity). Preliminary experiments on a complex viscoelastic fluid in our laboratory indicate that the time for coalescence of two drops into one drop is less than the time for the viscosity to reach steady state at the startup of shear flow. Hence, the average viscosity over the time of coalescence is lower than the steady state viscosity. This work suggests that modeling must involve not only an unsteady mechanical energy balance, but unsteady stresses. Furthermore, there are no reports of a combined program in which coalescence is observed experimentally and compared to numerical simulation on well characterized viscoelastic fluids.

Approach and Thrust: Our goal is to simulate the drop coalescence process for a series of polypropylenes of different molecular weights (MW's) and hence different degrees of viscoelasticity using multi-mode constitutive equations. There is strong evidence that the coalescence of two spherically shaped drops is dominated by biaxial extensional flow kinematics. Hence, these kinematics coupled with an unsteady mechanical energy balance and multimode constitutive equation will be solved numerically to predict the rate of sintering (i.e., neck growth of two spherical particles). Sintering of two drops will be observed under the microscope and the neck radius will be recorded as a function of time in order to assess the predictive ability of the model. By using spherical drops the complexities of using the finite element method may not be necessary. It is our hypothesis that the transient viscosity (i.e., time dependent viscosity measured at the startup of flow) is the key to accurately modeling sintering. This work will serve as a basis for modeling the sintering of more complex viscoelastic fluids referred to as thermotropic liquid crystalline polymers (TLCP's) which consist of rigid chain polymers. The numerical procedure developed for viscoelastic polymers will be extended to the rigid chain polymers but a more complex constitutive equation may have to be used. Finally, at least for the case of flexible chain polymers, if the approached described above is not adequate then the finite element method will be invoked to solve the coupled continuity, momentum, and constitutive equations.(NSF, pending).