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Experimental Physics:
NMR Group
Dr. von Meerwall
Self-diffusion of polymer melts and blends:
Prof. von Meerwall and his students currently are studying the self-diffusion of viscous polymer liquids (alkanes and polyethylene; polybutadiene), pure and in binary blends. This work is done in collaboration with Prof. Mattice and recently also with Prof. Wang (both Polymer Science) and some of their students. The point is to understand exactly how the temperature, molecular weight, and concentration affect the diffusion coefficients, which are measured with the nuclear magnetic resonance (NMR) method. The results are compared in detail with theoretical models and with numerical simulations of self-diffusion in these systems. Recent extensions of this work concentrate on cyclic polymers and on binary blends of highly entangled linear polymers.
Recycling of various industrial rubbers:
Prof. von Meerwall and his group are also interested in the recycling of various industrial rubbers (styrene-butadiene; natural rubber; silicone rubber) to help give them a second life. He collaborates with Prof. Isayev (Polymer Engineering) and some of his students to investigate at the molecular level how powerful ultrasound causes rubbery materials to disintegrate. NMR relaxation and diffusion measurements reveal the molecular motions of various components of the polymer network after it is broken up. The idea is to prepare for industrial implementation of the ultrasound method and to maximize its effectiveness. The project is now evolving to include industrially important rubbers containing solid filler particles.
Development and characterization of biocompatible polymer materials:
Prof. von Meerwall and members of his group are collaborating with Prof. Kennedy (Polymer Science) and with Profs. Cheung and Lopina (Chemical Engineering) and their students to develop and characterize biocompatible polymer membranes and bicontinuous composites . These are suitable for implants into the body, and will either enclose bioactive substances subject to attack by the immune system, e. g., in the treatment of diabetes, or else directly deliver drugs at carefully controlled rates. NMR diffusion measurements indicate the permeability of various membranes or composites to molecules of widely different sizes and shapes.
In addition, Prof. von Meerwall's group collaborates widely with polymer scientists whose studies call for measurements of molecular motions and diffusion in rubbery polymers, networks, and colloids. The laboratory frequently hosts graduate students and colleagues from other departments at Akron, and from other universities and laboratories.
Surface and Low Temperature Physics
Condensed Matter
Dr. Mallik
Certain polymers, and other smaller molecules, chemisorb on metal oxides via acid-base reactions to form covalent bridges or resonant bonds. It is important to understand the surface physics and chemistry of such systems from a fundamental standpoint, but also because they have important technological applications in aircraft construction, adhesion, corrosion, lubrication, and catalysis. We have used Multiple Reflection Absorption Infrared Spectroscopy (MRAIRS) and Inelastic Electron Tunneling Spectroscopy (IETS) to record spectra of quasi-monolayer films of molecules as large as polymers adsorbed on metal oxides to reveal information on the molecules? adsorbed geometrical configurations. Interpretation of the adsorption mechanisms for these systems depends on the ability to detect carbonyl, phosphoryl, phosphonyl, sulphonyl and other vibrational modes of surface-adsorbed species. Our results for carbonyl containing molecules show that in addition to stoichiometric and steric differences between the various adsorbates, the sample fabrication processes involved also affect the intensity of the associated spectral lines. Comparison of MRAIRS and IETS spectra in the past has largely ignored these constraints limiting the effectiveness of these techniques. Our work suggests that these two techniques, when used in tandem, can provide valuable information for a wider range of systems of adsorbates.
Surface coatings on glassy materials:
Common silica glass, SiO2, is one of the most widely used materials in the world. Various types of glass, either pure silica or those with metal or other inorganic ions incorporated in their amorphous structure, have had many uses over the centuries. Currently, silica glasses are used primarily for the production of various optical elements such as lenses, mirrors, plates, and diffraction gratings and printed circuit boards. Those doped with metal and/or inorganic ions are used in a diverse range of applications. These include, amongst many other things, the manufacture of optical filters, storage vessels, and fiber optics, the production of steel, and in the nuclear power industry for radiation screening. In many instances surface coatings are applied to the glasses for protection against erosion, or for optical filtering. We have fabricated ultra-thin (~ 1-2 nm) sputtered amorphous films of SiO2 and GeO2, characterized their topography and that of the underlying substrates using Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM), and recorded their vibrational spectra with and without various adsorbates of commercial significance such as silane coupling agents. Our results reveal the nature of the bonding mechanism at the glass/coating interface, which cannot be obtained easily by other means. Currently we are studying different types of glasses.
Surface states of amorphous materials:
Over approximately the last two decades much work has been done to investigate the optical end electronic properties of various semiconducting materials as candidates for pholtovoltaic applications (in particular solar cells); they represent a renewable energy source, which has few detrimental effects on the environment. Currently, about 95% of commercially available photovoltaic cells are made from crystalline silicon wafers, similar to those in the computer chip industry. The remainder is primarily amorphous thin-film semiconductor materials such as rare-earth doped silicate and fluoride glasses, the so-called III-V materials (e.g., GaAs) and most recently the promising II-VI class of materials (e.g., CdTe, and CdSe). Semiconducting materials such as these are much cheaper to produce but, at present, their efficiency (at best approximately 7%) is vastly inferior to crystalline materials (as high as 18%). The efficiency is lower mainly because charge carriers or other impurities are absorbed at defect sites such as dangling bonds and unwanted impurities thus reducing the magnitude of the photocurrent.
Commercially viable manufacture of thin-film amorphous photovoltaic devices relies primarily on Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE), and sputtering; the goal is to increase device efficiency by minimizing the number of defects created during the fabrication process. In order to do this, the various types of defects and their sources must be identified. Several workers have used Photoluminescence Spectroscopy (PLS), Raman Spectroscopy, Infrared Spectroscopy (IR) and other related techniques to study thin films of photovoltaic materials. These spectroscopies can record the energies and intensities of phonons, excitons and vibrational modes of the films. Many studies have been done to characterize the electronic and optical properties of amorphous hydrogenated silicon (a-Si:H), polycrystalline silicon, and their oxides. The chemical structure and composition has also been studied by a variety of spectroscopic techniques including Fourier Transform Infrared Spectroscopy (FTIR) infrared ellipsometry, neutron scattering, X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES). Our work focuses on our experience in the fabrication of ultra-thin (~ 1-2 nm) sputtered amorphous films of Si, Ge, and their oxides with and without various adsorbates. We have published their vibrational spectra using IETS and are now working on III-V and II-VI materials where we plan to identify adsorption mechanisms for surface coatings.
Dr. Henriksen
... biological applications of AFM...
A major area of research in surface science is thin film growth and characterization. Of
particular interest is the onset of epitaxial growth and critical island size for metal films. Four vacuum chambers are available for film growth. Presently growth characterization is limited to scanning probe microscopy (one ambient STM/AFM and one UHV STM), reverse-view electron diffraction, and measurements of electronic properties such as Hall voltages and magnetoresistance. Related to the thin film work is the study of mechanisms by which molecular compounds adsorb on the surface of highly reflective metal films. Chemical compounds of particular interest are those related to adhesion or lubrication. Techniques used for these studies are inelastic electron tunneling spectroscopy (IETS), infrared (IR) spectroscopy, and programmed thermal desorption (PTD) spectroscopy.
Dr. Griffin: Physics education and computer assisted instruction.
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