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Alexandru D. Asandei (Ph.D. 1998, Macromolecular Science, Case Western Reserve University)
Early Transition Metal Mediated Living Radical Polymerization
Peter Burkhard (PhD Structural Biology (1995), University of Basel )
Design of peptide nanoparticles for medical applications
Andrey V. Dobrynin (Ph. D., Polymer Physics, 1991, Moscow Institute of Physics & Technology, Moscow, Russia)
Protein-Polyelectrolyte Complexes
Brian Huey (Ph.D., Materials Science, 1999, University of Pennsylvania)
Nanoscale Lithography of Polymer Resists
Fotios Papadimitrakopoulos (Ph.D., Chemistry, 1992, Univ. of Massachusetts, Amherst)
Nano Bio Chemistry Using Single Walled Carbon Nanotubes
Thomas A. P. Seery (Ph.D., Chemistry, 1991, Univ. of Southern California)
Surface Initiated Polymerization
Montgomery T. Shaw (Ph.D., Chemistry, 1970, Princeton)
1. Shear Actuator Based on Tilted Electorheological Elastomers
2. Robust Solvent Sensors Based on Partially Swollen ER Elastomers
Gregory A. Sotzing (Ph.D., Organic Chemistry, 1997, University of Florida)
Development of Detectors for an Electrochemical Artificial Nose
Robert A. Weiss (Ph.D., Chemical Engineering, University of Massachusetts, Amherst, MA)
Shape Memory Polymers Based On Elastomeric Ionomers
Lei Zhu (Ph.D., Polymer Science, 2000, University of Akron)
Tuning Surface Properties with Block Copolymer Brushes
Alexandru D. Asandei (Ph.D. 1998, Macromolecular Science, Case Western Reserve University)
Early Transition Metal Mediated Living Radical Polymerization
Preparation of well-defined functional polymers is of fundamental importance for developing sensors and actuators. Conventional free radical polymerization affords polymers with poor control over the molecular weight and polydispersity. In contrast, living radical polymerization enables the synthesis of polymers with predetermined molecular weights and narrow polydispersity. This polymerization is based on the suppression of side reactions using the persistent radical effect.
The objective of the project is to investigate the ability of early transition metal (Ti and Zr) complexes to perform the radical ring opening of epoxides and to act as persistent radicals in living radical polymerization. The success of this project will be leveraged by the recent efforts in our laboratories developing new Ti and Zr complexes with labile bonds that undergo facile homolytic cleavage. Students in this project will be introduced to the concepts of polymerization kinetics and their relationship to molecular weight distributions as well as the principles involved in the design consideration for organometallic species that are involved in mediating radical polymerizations. The REU student will acquire skills in a large variety of laboratory and analytical techniques such as purification of monomer and reagents, handling of air-sensitive compounds, degassing techniques, reaction sampling, and use of analytical instruments such as the gel permeation chromatograph (GPC) and nuclear magnetic resonance (NMR) spectrometer.
Peter Burkhard ( PhD Structural Biology (1995), University of Basel )
Design of peptide nanoparticles for medical applications
Peptide Nanoparticles
In our lab we are designing nanoparticles based on peptides as building blocks. This is achieved by rational protein de novo design. These nanoparticles are composed of protein oligomerization domains with different oligomerization states linked by a short linker peptide. The nanoparticles are characterized by regular icosahedral symmetry like small virus particles. Currently we are developing a prototype of self-assembling functionalized peptidic nanoparticles, which can be used as a drug targeting and delivery system for the visualization and treatment of cancer. These nanoparticles will be modified to carry a drug entity (radionuclide) as well as a pathfinder molecule on each of the 60 peptide chains of the icosahedral nanoparticle.
Furthermore, such nanoparticles with regular polyhedral symmetry represent an ideal repetitive antigen display system. Surface proteins of pathogens or fragments of such proteins can easily be engineered into the peptide sequence of the nanoparticle. Notably, many surface proteins of pathogens contain coiled-coil sequences. For example, by simply extending the trimeric coiled-coil of the nanoparticle by the coiled-coil sequence of the HIV surface protein gp41, a candidate HIV vaccine can be designed. Whereas in the past, different kinds of adjuvants were tested to improve the immunogenicity of an antigen or a specific epitope, such a repetitive antigen display can strongly augment the immunogenicity of a certain epitope, thus avoiding the need for sometimes highly toxic adjuvants.
De novo design of a-helical proteins
The parallel two-stranded a-helical coiled coil is the most frequently encountered subunit oligomerization motif in proteins. We have de novo designed several highly charged two-heptad repeat long peptides which are stabilized by a complex network of different possible inter- and intra-helical salt bridge arrangements in addition to the well-known hydrophobic interactions occurring along the dimer interface. We are accessing the biophysical properties of these peptides by CD-spectroscopy, analytical ultracentrifugation and X-ray structure determination. The X-ray structure of these peptides confirmed our predicted intra- and inter-helical salt bridge network. Such de novo designed peptides can be used as very short a-helical coiled coils for applications such as two-stage drug targeting and delivery systems, coiled coils as templates for combinatorial helical libraries for drug discovery, and as synthetic carrier molecules.
Andrey V. Dobrynin (Ph. D., Polymer Physics, 1991, Moscow Institute of Physics & Technology, Moscow, Russia)
Protein-Polyelectrolyte Complexes
Electrostatic interactions play an important role in physics, chemistry and biology. For example, in complex fluids they stabilize colloidal suspensions against phase separation. In biological systems electrostatic interactions regulate the organization of the cytoskeleton, the compaction of genetic material, and the complexation between proteins and DNA (polyelectrolytes). Moreover, protein-polyelectrolyte complexes control the rheology and lubrication properties of synovial fluid in mammalian joints Ð a natural example of actuation. An industrial application of protein-polyelectrolyte complexes is to use polyelectrolytes to boost the viscosity of protein solutions for coating photographic film and paper. In materials science electrostatic self-assembly is a new and promising technique for fabricating layered polymeric nanocomposites by alternatively exposing a substrate to molecules of opposite charges. This technique is now routinely employed for making ultrathin films from synthetic polyelectrolytes, DNA, proteins, charged nanoparticles (e.g., metallic, semiconducting, magnetic, ferroelectric materials), and other supramolecular species.
The undergraduate students will be involved in the development of the molecular level models of the solutions containing polyampholyte-polyelectrolyte complexes using a combination of analytical and numerical techniques. The plan is to develop molecular models describing the formation of polyampholyte-polyelectrolyte complexes in a wide range of polymer and salt concentrations, solution pH and such properties of polymers as their molecular weight and charge distribution. In dilute solutions the resulting model will provide details of the internal structure of polyampholyte-polyelectrolyte complexes and of the effects of counterion release and condensation on the complex formation. The formation of intercomplex associations and reversible gelation will be studied in semidilute solutions. This will allow prediction of polymer conformations, and solution properties such as viscosity, diffusion coefficient, and relaxation time that can be compared with experimentally measured values.
Another research initiative will include the molecular simulation of multilayer formation by alternating deposition of oppositely charged polyelectrolytes and proteins. It will systematically study the effects of surface charge density, fraction of charged monomers, chain stiffness, salt concentration and solution pH, strength of the short-range interactions and molecular weight of polymers on the multilayer buildup process. Such molecular simulations will allow us to elucidate the molecular mechanisms for multilayer formation process.
Brian Huey (Ph.D., Materials Science, 1999, University of Pennsylvania)
Nanoscale Lithography of Polymer Resists
Continued progress in semiconductor device performance depends strongly on lithographic capabilities for preparing structures of diminishing size. This is generally achieved by optically exposing a polymer thin film (resist) through a patterned mask, though this method is rapidly approaching the limits of optical resolution. To generate still smaller features, patterned resists are often prepared by fine positioning of an electronic current. This is possible in an atomic force microscope (AFM), which contacts a fine probe with the resist surface and applies varying currents and forces with nanometer scale precision.
The aim of this project is to determine the necessary fabrication steps to achieve sub-50 nm lithography using the AFM. Several polymer resists, film thickness and preparation parameters, development currents, and fabrication speeds will be assessed. Structures including dots, lines, and whimsical patterns will be generated (for example the 4 um wide UConn crest as shown). After fabricating these novel features, the AFM will also be used for their assessment. Ultimately, nanoscale piezoelectric sensors and actuators will be fabricated using the procedures developed during the REU project. The student will therefore gain skills in semiconductor resist technology, surface characterization, and nanofabrication.
Fotios Papadimitrakopoulos (Ph.D., Chemistry, 1992, Univ. of Massachusetts, Amherst)
Nano Bio Chemistry Using Single Walled Carbon Nanotubes
The research effort of Papadimitrakopoulos' group includes a wide range of materials and devices in the area of nano-bio-systems. Much of our expertise is concentrated at the supramolecular assembly of man-made artificial nanostructures and their unique interactions with biological entities (such as proteins, DNA and biocompatible polymers). These endeavors span into single wall carbon nanotubes (SWNTs), semiconductor nanocrystals, DNA-assisted assembly of photonics crystals and implantable biosensors. In particular there is considerable effort within Papadimitrakopoulos' group in the separation of SWNTs into diameter and metallicity (metallic (met-) vs. semiconducting (sem-)). Our group was the first to report that using wet chemistry one can separate and/or enrich fractions of SWNTs according to type (or otherwise termed "metallicity") and diameter. Aside from the immense technological importance of enhancing the structural purity and homogeneity of SWNTs, obtaining well-fractionated samples could also enable us to better characterize and model the effects of diameter and chirality. Our group is working to advance this separation methodology and obtain a better description of the physicochemical properties of solution-dispersed SWNTs. Separated SWNTs are poised to enhance considerably the properties of nanostructured devices. Building on our initial finding of self-assembled SWNT forest arrays, a number of enzymatic electrochemical biosensors have been developed in collaboration with Prof. Rusling's group. More recently, we have been able to demonstrate SWNT forest-based electrochemical immunoassays with sensitivity exceeding that of traditional ELISA. Patterning of these SWNTs forest arrays at the nanometer level is currently used to produce nanosized needles that could electrochemically interface with living bacteria and cells, without disturbing their normal physiology (in collaboration with Professors Marcus, Rusling, Noll and Huey).Thomas A. P. Seery (Ph.D., Chemistry, 1991, Univ. of Southern California)
Surface Initiated Polymerization
The ability to control molecular structure on a scale intermediate between organic molecules and bulk matter has become increasingly important in technologies relying on organic films of 1 Ð 100 nanometers thickness. Miniaturization of sensors and actuators is driving the need for control of physical and chemical properties on exactly these distance scales. Our research has advanced the concept of surface initiated polymerization (SIP) as a means to prepare conformal polymer monolayers on hard substrates. Gold and silicon oxide surfaces have been used as model systems to demonstrate this concept.
Idealization of SIP:
(A) Surface coverage of initiator is formed.
(B) Active catalyst is generated.
(C) Chain growth polymerization provides polymer with catalyst on the living ends.
(D) Endcapping gives control over ultimate surface properties.
The REU student working on this project would contribute to our efforts to extend this concept to gallium arsenide surfaces. This substrate is of interest primarily due to its applications in the field of microelectromechanical (MEMs) devices. These are a family of actuators that can be lithographed out of GaAs to make working devices with micron dimensions. This small distance scale brings some unique problems of device failure that we are attempting to solve through control of the surface properties, e.g. elasticity and surface energy. The first task would involve functionalizing the surface with thiol coupling agents that have terminal norbornyl groups. The next step is detection of these species at the surface using ellipsometry, contact angle, surface IR techniques, and x-ray photoelectron spectroscopy measurements. The norbornyl species will be reacted with ruthenium alkylidene complexes to form catalytic species on the surface. Polymerization then occurs after addition of strained cyclic olefins. This project is modular, and the interdisciplinary nature will allow a student the freedom to focus on either synthetic or physical chemistry. This choice of focus will provide the students with either synthetic skills and the manipulation of organometallic reagents and air sensitive chemistry or skills in physical characterization of surface bound polymers.
Montgomery T. Shaw (Ph.D., Chemistry, 1970, Princeton)
1. Shear Actuator Based on Tilted Electorheological Elastomers
In anticipation of a growing need for actuators with custom sizes and shapes, we have been investigating electrorheological (ER) elastomers. ER elastomers are soft materials that exhibit large and rapid changes in properties on application of an electric field. While considered mainly for applications where a change in stiffness is required (e.g, active damping) it is clear that structures with actuating properties can be made, in the following manner: In the normal ER elastomer, a soft matrix is filled with polarizable 5-mm particles that are aligned in the direction normal to the sample by application of an aligning field. If these particles are deformed away from this direction in the presence of a field, there is a restoring force because the particles are forced to separate. However, if the particle chains are tilted with respect to the field in the off position, as shown in the photomicrograph, it is clear that application of the field will result in a tendency for the sample to deform. With enough area, the forces generated should be quite high.
Scientific Objectives: To investigate the role of composite structure on actuator response (force, deformation , efficiency). Structural variables include tilt angle, particle volume fraction and particle pretreatment. Simple electrostatic theories will be tested using the results.
Student activities: The undergraduate researcher would make the elastomers using silicone RTV technology, and characterize their voltage-stress responses.
Skills: The student will gain experience with polymerization reaction kinetics, electrostatics, dielectric response, and engineering calculations (efficiency, power consumption, etc.).
2. Robust Solvent Sensors Based on Partially Swollen ER Elastomers
Sensors for the partial pressure of solvent can exploit the changes in conductivity of a filled polymer with swelling. The key to sensitivity is to have the maximum change in resistance with minimum changes in solvent fugacity. This means that the conducting particles in the elastomer should be on the verge of percolation and that the polymer is able to swell freely. It is well known that it is possible to created conductive paths in an elastomer by magnetic alignment of small nickel or iron particles. We have found that the simple way of controlling the conductivity is to swell slightly the elastomer to separate the particle by a very small amount; this is shown in the photomicrograph at the left. By selecting the polymer and swelling solvent carefully, one can tailor the response of the material to various vapors that need to be detected and measured.
Scientific Objectives: The objective is to investigate the role of particle volume fraction and geometry on response to swelling, and to compare these results with percolation models.
Student Activities: The undergraduate researcher would make sensors of various structures and measure their sensitivity, i.e., the change in resistance with change in partial pressure of a vapor.
Skills Taught: The researcher will gain experience in thermodynamics, mass transfer, simple electronics, and instrumentation technology.
Gregory A. Sotzing (Ph.D., Organic Chemistry, 1997, University of Florida)
Development of Detectors for an Electrochemical Artificial Nose
ÒNoseÓ technology is currently being sought after for applications such as landmine detection, monitoring the quality of foods, beverages and air, disease detection, drug detection, etc. Several local and international companies either sell or are involved in the commercialization of this device. However, this technology is not at the detection limits required to meet the needs for many of these applications. Therefore, significant advances have to be made at increasing the sensitivity of the detectors which are housed in these instruments.
ÒNosesÓ consist of an array of detectors that typically utilize either conductivity, fluorescence, capacitance or frequency as the mode of transduction. The REU student working on this project will be actively involved in the preparation of sensors for chemical vapor detection. During the course of the summer, the student will be involved in the synthesis of electrically conducting and/or electroactive polymers that have different sensing capabilities depending on their band properties. The magnitude of the current response as measured by chronoamperometry will depend on charge transfer interactions that occur between analyte and polymer. An array of detectors consisting of different conducting polyheterocycles will be tested in a vapor delivery system in order to assess the ability of the detector array (ÒnoseÓ) to distinguish chemically different analytes.
We are currently developing new materials for use in chemosensors based on Astaxanthin, 1, which is one of the most abundant carotenoids found in nature. Astaxanthin can serve as a bifunctional monomer consisting of a conjugated pi system having structural similarity to polyacetylene, the iodine doped form of which is the first organic intrinsically conductive polymer. We have carried out several condensation polymerizations of astaxanthin, one of which is depicted in the figure on the left below. We were able to alter the light absorbing properties of astaxanthin via incorporation into the main chain of a polymer as shown in the spectra below right. Our future studies for this REU project will focus on the use of polyastaxanthin as a radical scavenger. Specifically, we propose to investigate the interaction of our poly(astaxanthin)s with NO with possible application as a sensor and a means by which to remediate NO.
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Polymerization of Astaxanthin |
UV-Vis-NIR spectra: dashed = polyastaxanthin, solid = astaxanthin; A = methanol, B = tetrahydrofuran |
Robert A. Weiss (Ph.D., Chemical Engineering, University of Massachusetts, Amherst, MA)
Shape Memory Polymers Based On Elastomeric Ionomers
Shape memory materials represent a class of materials that change shape as a response to temperature. These materials can be formed into temporary shapes using a prescribed thermal and stress history. Because of their rubber-like elasticity, shape memory polymers (SMP) can realize much greater deformations than conventional shape memory metal alloys, which creates many new applications for these materials, e.g., biomedical, robotics, and actuators. Most SMPs are crosslinked elastomers that are ÒfrozenÓ into a temporary shape by first subjecting the polymer in its melt state to a controlled deformation and then cooling below a glass transition temperature (Tg) or below a melting point (Tm) and allowing the material to crystallize before the stress is removed. Upon heating again above either Tg or Tm, the polymer chains relax to their unperturbed state, which results in a change in shape of the material.
The objective of this project is to assess the feasibility of developing a SMP from a plasticized ionomer. Ionomers possess physical crosslinks as a result of interchain association of bonded salt groups. We have previously shown that the ionic crosslinks can be replaced by a crystalline phase by adding a crystallizable plasticizer that associates with the salt groups of the ionomer, e.g., metal stearates. However, when the crystalline plasticizer melts, the ionic aggregates reassume the role as physical crosslinks.
During the course of this project, the student will prepare sulfonated EPDM (S-EPDM) ionomers by sulfonating a commercial EPDM rubber, produce mixtures of various metal stearates and S-EPDM by melt processing, characterize the thermal and dynamic mechanical properties of the blends using differential scanning calorimetry and dynamic mechanical analysis and characterize the microstructure of the blends using small angle x-ray scattering. The shape memory characteristics will be preparing oriented structures (e.g., fibers or films) of the materials and measuring the shape changes and the kinetics of the shape change that occur upon heating.
Lei Zhu (Ph.D., Polymer Science, 2000, University of Akron)
Tuning Surface Properties with Block Copolymer Brushes
Environment-responsive polymer coatings have great potential for cell growth control, micro-reactor, drug delivery, and chemical and/or biological sensors. Surface-tethered amphiphilic block copolymer brushes provide great opportunity to fine-tune surface properties due to their ability to respond to different hydrophobic and hydrophilic environments. Our research focuses on the fundamentals of block copolymer brush formation on flat substrates, including chemsorption and physisorption of ABC-type block copolymers, and surface-initiated block copolymerization. These block copolymer brushes may form distinct surface nanopatterns, depending on the composition, immiscibility between different blocks, and interaction with environments. Onion, garlic, flower, dumbbell, and checkboard structures have been suggested based on computer modeling.
This research provides REU students a broad spectrum of knowledge in polymer science with both synthesis and characterization training. Teamed with experienced graduate students, undergraduate students will be actively involved in polymer synthesis, surface, and morphology characterization. In the research, the physisorption of a poly(2-vinyl pyridine)-b-poly(butylene oxide)-b-poly(ethylene oxide) (P2VP-b-PBO-b-PEO) triblock copolymer on flat mica surface will be systematically studied (see figure 
above). First, the effect of P2VP molecular weights on the PBO-b-PEO brush length will be studied by ellipsometry measurement. It is expected that lower P2VP molecular weight results in closer inter-chain distance, and thus, more stretching of the block copolylmer chains. Second, the block copolymer brushes will be exposed to ethanol (hydrophilic) and hexane (hydrophobic) solvents to study the environment-responsive effect. Selective solvents may induce ordered surface nanopatterns, and these morphologies will be visualized using atomic force microscopy. The fine-tuning of the surface free energy will be studied by contact angle. The surface chemical compositions will be studied by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements. At last, the crystallization of the tethered PEO blocks on the two-dimensional confined surface will be studied by X-ray diffraction techniques.