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Continuum Mechanical and Computational Aspects of Material Behavior

Description: The authors develop a theory for uniaxial nematic elastomers with variable asphericity. As an application of the theory, they consider the time-independent, isochoric radial expansion of a right circular cylinder. Numerical solutions to the resulting differential equation are obtained for a range of radial expansions. For all expansions considered, there exists an isotropic core of material surrounding the cylinder axis where the asphericity vanishes and in which the polymeric chains are shaped as spherical coils. This region, corresponding to a disclination of strength + 1 along the axis, is bounded by a narrow transition layer across which the asphericity increases rapidly and attains a non-trivial positive value. The material thereby becomes anisotropic away from the disclination so that the polymeric chains are shaped as ellipsoidal coils of revolution prolate about cylinder radius. In accordance with the area of steeply changing asphericity between isotropic and anisotropic regimes, a marked drop in the free-energy density is observed. The boundary of the disclination core is associated with the location of this energy drop. For realistic choices of material parameters, this criterion yields a core on the order of 10{sup -2} {micro}m, which coincides with observations in conventional liquid-crystal melts. Also occurring at the core boundary, and further confirming its location, are sharp transitions in the behavior of the constitutively determined contributions to the deformational stress and a change in the pressure. Furthermore, the constitutively determined contribution to the orientational stress is completely concentrated at the core boundary. The total energy shows a definitive preference for disclinated states.
Date: March 14, 2004
Creator: Fried, Eliot & Gurtin, Morton E.
Partner: UNT Libraries Government Documents Department

Electrostriction in Field-Structured Composites: Basis for a Fast Artificial Muscle?

Description: The electrostriction of composites consisting of dielectric particles embedded in a gel or elastomer is discussed. It is shown that when these particles are organized by a uniaxial field before gelation, the resulting field-structured composites are expected to exhibit enhanced electrostriction in a uniform field applied along the same axis as the structuring field. The associated stresses might be large enough to form the basis of a polymer-based fast artificial muscle.
Date: January 27, 1999
Creator: Anderson, R.A. & Martin, J.E.
Partner: UNT Libraries Government Documents Department

Final Technical Report

Description: The goal of the project was to research and develop a biorefinery technology platform for adhesives, elastomers and foams. The program developed new bio-based products which can replace petrochemical-based polyurethane technology in film laminating and other adhesive, sealant and elastomer applications. The technology provides faster cure, lower energy consumption and safety enhancements versus incumbent urethane technology.
Date: March 30, 2007
Creator: Kauffman, Thomas F.
Partner: UNT Libraries Government Documents Department

Effects of Tritium in Elastomers

Description: A number of elastomers, used as flange gaskets in the piping system of the Savannah River Plant tritium facilities, are being examined to identify those compounds more radiation-resistant that the currently specified Buna-N rubber and to study the mechanism of tritium radiation damage. This paper discusses this study.
Date: March 4, 2003
Creator: Zapp, P.E.
Partner: UNT Libraries Government Documents Department

Functionalized Materials From Elastomers to High Performance Thermoplastics

Description: Synthesis and incorporation of functionalized materials continues to generate significant research interest in academia and in industry. If chosen correctly, a functional group when incorporated into a polymer can deliver enhanced properties, such as adhesion, water solubility, thermal stability, etc. The utility of these new materials has been demonstrated in drug-delivery systems, coatings, membranes and compatibilizers. Two approaches exist to functionalize a material. The desired moiety can be added to the monomer either before or after polymerization. The polymers used range from low glass transition temperature elastomers to high glass transition temperature, high performance materials. One industrial example of the first approach is the synthesis of Teflon(reg. sign). Poly(tetrafluoroethylene) (PTFE or Teflon(reg. sign)) is synthesized from tetrafluoroethylene, a functionalized monomer. The resulting material has significant property differences from the parent, poly(ethylene). Due to the fluorine in the polymer, PTFE has excellent solvent and heat resistance, a low surface energy and a low coefficient of friction. This allows the material to be used in high temperature applications where the surface needs to be nonabrasive and nonstick. This material has a wide spread use in the cooking industry because it allows for ease of cooking and cleaning as a nonstick coating on cookware. One of the best examples of the second approach, functionalization after polymerization, is the vulcanization process used to make tires. Natural rubber (from the Hevea brasiliensis) has a very low glass transition temperature, is very tacky and would not be useful to make tires without synthetic alteration. Goodyear's invention was the vulcanization of polyisoprene by crosslinking the material with sulfur to create a rubber that was tough enough to withstand the elements of weather and road conditions. Due to the development of polymerization techniques to make cis-polyisoprene, natural rubber is no longer needed for the manufacturing of tires, but ...
Date: May 31, 2003
Creator: Salazar, Laura Ann
Partner: UNT Libraries Government Documents Department

The structure of carbon black and its composites with elastomers: A study using neutron scattering

Description: The authors have been exploring the use of small-angle neutron scattering and the method of contrast variation to give a new look at a very old problem--reinforcement of elastomers by carbon black in durable rubber products. The method has yielded some interesting information on the structure of an experimental carbon black, HSA, and on the associations of HSA in polyisoprene composites. Carbon black has a hierarchy of structures consisting of particles covalently bound into aggregates, which in turn associate by weak interactions into agglomerates. The authors found that in HSA the aggregates are rodlike, containing an average of 4--6 particles. The aggregates have an outer graphitic shell and an inner core of lower density carbon. The core is continuous throughout the carbon black aggregate. Contrast variation of swollen HSA-polyisoprene gels show that the HSA is completely embedded in polyisoprene and that the agglomerates are formed predominantly by end on associations of the rodlike aggregates. The surface structure of the carbon black appears smooth over length scales above about 10 {angstrom}. Further studies on production carbon blacks suggest that the shell-like aggregate structure is present in commercial carbon blacks.
Date: July 1, 1996
Creator: Hjelm, R.P.; Wampler, W. & Gerspacher, M.
Partner: UNT Libraries Government Documents Department

The in-situ generation of silica reinforcement in modified polydimethylsiloxane elastomers

Description: Structure and properties of a series of modified polydimethylsiloxane (PDMS) elastomers reinforced by {ital in situ} generated silic particles were investigated. The PDMS elastomer was modified by systematically varying the molecular weight between reactive groups incorporated into the backbone. Tetraethoxysilane (TEOS) and partial hydrolyzate of TEOS were used to generate silic particles. Chemistry and phase structure of the materials were investigated by {sup 29}Si magic angle spinning nuclear magnetic resonance spectroscopy and swelling experiments.
Date: June 1, 1996
Creator: Prabakar, S; Bates, S.E.; Black, E.P. & Ulibarri, T.A.
Partner: UNT Libraries Government Documents Department

Technical basis for storage of elastomer-sealed produce cans in the DOE-STD-3013-94 package

Description: Department of Energy standard DOE-STD-3013-94 establishes criteria for the long-term packaging of plutonium metal and oxide. The inclusion of organic materials in sealed packages of plutonium may produce gases that contribute to container pressurization. To expedite processing, it would be desirable to permit, within the DOE-outlined criteria, limited amounts of organic materials to be used as a sealing gasket in some packaging containers. This paper presents a technical basis for allowing elastomer-sealed cans to be packaged inside the sealed inner container of a double weld-sealed DOE-STD-3013-94 container system.
Date: November 1, 1996
Creator: Horrell, D.R.; Stakebake, J.L. & Szempruch, R.W.
Partner: UNT Libraries Government Documents Department

The structure of carbon black-elastomer composites by small-angle neutron scattering and the method of contrast variation

Description: We have been exploring the use of small-angle neutron scattering and the method of contrast variation to give a new look at a very old problem: reinforcement of elastomers by carbon black in durable rubber products. Carbon black has a hierarchy of structures consisting of particles covalently bound into aggregates, which in turn associate by weak interactions into agglomerates. We found that in one carbon black, HSA, the aggregates are rodlike, containing an average of 4-6 particles. The aggregates have an outer graphitic shell and an inner core of lower density carbon. The core is continuous throughout the carbon black aggregate. Contrast variation of swollen HSA-polyisoprene gels shows that the HSA is completely embedded in polyisoprene and that the agglomerates are formed predominantly by end on associations of the rodlike aggregates. The surface structure of the carbon black appears smooth over length scales above about 10 {angstrom}. Further studies using production carbon blacks suggest that these structural characteristics are generally present in commercial rubber composites.
Date: July 1, 1996
Creator: Hjelm, R.P.; Wampler, W. & Gerspacher, M.
Partner: UNT Libraries Government Documents Department

The Effect of Surface Contamination on Adhesive Forces as Measured by Contact Mechanics

Description: The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to mate the two surfaces and subsequently applying a tensile load to peel the surfaces apart. The contact region is assumed the superposition of elastic Hertzian pressure and of the attractive surface forces that act only over the contact area. What are the effects of the degree of surface contamination on adhesive forces? Clean aluminum surfaces were coated with hexadecane as a controlled contaminant. The force required to pull an elastomeric hemisphere from a surface was determined by contact mechanics, via the JKR model, using a model siloxane network for the elastomeric contact sphere. Due to the dispersive nature of the elastomer surface, larger forces were required to pull the sphere from a contaminated surface than a clean aluminum oxide surface.
Date: December 18, 2000
Creator: EMERSON,JOHN A.; GIUNTA,RACHEL K.; MILLER,GREGORY V.; SORENSEN,CHRISTOPHER R. & PEARSON,RAYMOND A.
Partner: UNT Libraries Government Documents Department

Mechanical Properties of Cellular Materials

Description: The authors calculated the stress-strain relation for elastomeric foam from an ab initio theory, which shows that the plateau and densification regions should be described by a hyperbola. The theory seems to agree reasonably well with experiment.
Date: July 9, 1999
Creator: Solem, J.C. & Dienes, J.K.
Partner: UNT Libraries Government Documents Department

Time-domain dynamic opto-rheology study of polymer films using step-scan FTIR time-resolved spectroscopy (S{sup 2}FTIR TRS)

Description: Step-scan Fourier transform infrared spectroscopy in conjunction with impulse stress on polymer films has been used to monitor dynamic rheological responses in real time. A novel piezo-electrically-driven polymer microrheometer was employed to apply repetitive impulses to the polymer sample while time-domain spectra were recorded. Recent results include the study of both semi-crystalline polymers such as isotactic polypropylene (iPP) and elastomers such as Estane polyester/polyurethane copolymer and Kraton tri-block copolymer. The spectral changes of iPP are consistent with frequency-domain results. For iPP at room temperature, large differences in the response times of different absorption bands are not seen. However, the orientation response of the CH{sub 3} rocking mode is slightly slower than the responses of the backbone modes. To the authors` knowledge, this is the first reported successful step-scan FTIR time-domain dynamic polymer opto-rheology experiment. The advantages of the time-domain experiment over the frequency-domain experiment are also discussed briefly. This technique appears to be applicable to a variety of polymer samples, and examples from additional results are illustrated.
Date: July 1, 1998
Creator: Wang, H.; Palmer, R.A.; Manning, C.J. & Schoonover, J.R.
Partner: UNT Libraries Government Documents Department

Performance testing of elastomeric seal materials under low and high temperature conditions: Final report

Description: The US Department of Energy Offices of Defense Programs and Civilian Radioactive Waste Management jointly sponsored a program to evaluate elastomeric O-ring seal materials for radioactive material shipping containers. The report presents the results of low- and high-temperature tests conducted on 27 common elastomeric compounds.
Date: June 1, 2000
Creator: BRONOWSKI,DAVID R.
Partner: UNT Libraries Government Documents Department

Dynamic modulus estimation and structural vibration analysis.

Description: Often the dynamic elastic modulus of a material with frequency dependent properties is difficult to estimate. These uncertainties are compounded in any structural vibration analysis using the material properties. Here, different experimental techniques are used to estimate the properties of a particular elastomeric material over a broad frequency range. Once the properties are determined, various structures incorporating the elastomer are analyzed by an interactive finite element method to determine natural frequencies and mode shapes. Then, the finite element results are correlated with results obtained by experimental modal analysis.
Date: November 18, 1998
Creator: Gupta, A.
Partner: UNT Libraries Government Documents Department

Modeling the behavior of an earthquake base-isolated building.

Description: Protecting a structure against earthquake excitation by supporting it on laminated elastomeric bearings has become a widely accepted practice. The ability to perform accurate simulation of the system, including FEA of the bearings, would be desirable--especially for key installations. In this paper attempts to model the behavior of elastomeric earthquake bearings are outlined. Attention is focused on modeling highly-filled, low-modulus, high-damping elastomeric isolator systems; comparisons are made between standard triboelastic solid model predictions and test results.
Date: November 26, 1997
Creator: Coveney, V. A.; Jamil, S.; Johnson, D. E.; Kulak, R. F. & Uras, R. A.
Partner: UNT Libraries Government Documents Department

An Ultrasonic Measurement System for the Precise Determination of the Elastic Moduli of Single Crystals

Description: The measurement of the ultrasonic wave velocities in a sufficient number of known directions in a single crystal makes possible the direct computation of the elastic moduli of the crystal. For specimens having linear dimensions on the order of 2 to 3 mm, phase comparison techniques were found useful. The equipment described is based on the technique developed by McSkimin. The measurement system is actually a type of acoustic interferometer adapted to small solid specimens, and it requires specialized electronic equipment which cannot be purchased off the shelf. (auth)
Date: May 1, 1961
Creator: Renken, C. J.
Partner: UNT Libraries Government Documents Department

Effects of a Surface Engineered Metallic Coating on Elastomeric Valve Stem Seal Leakage

Description: Valve stem seal leakage is a major source of fugitive emissions, and controlling these emissions can result in added expense in leak detection and repair programs. Elastomeric O-rings can be used as valve stem seals, and O-ring manufacturers recommend lubrication of elastomeric seals to prevent damage and to assure proper sealing. In this research, a metallic coating was applied as a lubricant using a vacuum vapor deposition process to the surface of elastomeric valve stem seals. Valve stem leak measurements were taken to determine if the coated O-rings, alone or with the recommended lubrication, reduced valve stem seal leakage. This research determined that the metallic coating did not reduce valve stem leakage.
Date: December 2000
Creator: Taylor, John Abner
Partner: UNT Libraries