Publications

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Both conventional and combinatorial approaches were used to study the pore formation process in epoxy based polymer systems. Sandia National Laboratories conducted the initial work and collaborated with North Dakota State University (NDSU) using a combinatorial research approach to produce a library of novel monomers and crosslinkers capable of forming porous polymers. The library was screened to determine the physical factors that control porosity, such as porogen loading, polymer-porogen interactions, and polymer crosslink density. We have identified the physical and chemical factors that control the average porosity, pore size, and pore size distribution within epoxy based systems.

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Many weapons components (e.g. firing sets) are encapsulated with blown foams. Foam is a strong lightweight material--good compromise between conflicting needs of structural stability and electronic function. Current foaming processes can lead to unacceptable voids, property variations, cracking, and slipped schedules which is a long-standing issue. Predicting the process is not currently possible because the material is polymerizing and multiphase with changing microstructure. The goals of this project is: (1) Produce uniform encapsulant consistently and improve processability; (2) Eliminate metering issues/voids; (3) Lower residual stresses, exotherm to protect electronics; and (4) Maintain desired properties--lightweight, strong, no delamination/cracking, and ease of removal. The summary of achievements in the first year are: (1) Developed patentable chemical foaming chemistry - TA; (2) Developed persistent non-curing foam for systematic evaluation of fundamental physics of foams--Initial testing of non-curing foam shows that surfactants very important; (3) Identified foam stability strategy using a stacked reaction scheme; (4) Developed foam rheology methodologies and shear apparatuses--Began testing candidates for shear stability; (5) Began development of computational model; and (6) Development of methodology and collection of property measurements/boundary conditions for input to computational model.

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The U.S. is addicted to petroleum--a dependency that periodically shocks the economy, compromises national security, and adversely affects the environment. If liquid fuels remain the main energy source for U.S. transportation for the foreseeable future, the system solution is the production of new liquid fuels that can directly displace diesel and gasoline. This study focuses on advanced concepts for biofuel factory production, describing three design concepts: biopetroleum, biodiesel, and higher alcohols. A general schematic is illustrated for each concept with technical description and analysis for each factory design. Looking beyond current biofuel pursuits by industry, this study explores unconventional feedstocks (e.g., extremophiles), out-of-favor reaction processes (e.g., radiation-induced catalytic cracking), and production of new fuel sources traditionally deemed undesirable (e.g., fusel oils). These concepts lay the foundation and path for future basic science and applied engineering to displace petroleum as a transportation energy source for good.

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The optimization of piezo-electric PVDF polymers for adaptive optics in space environments was discussed. The fundamental correlation between chemical and physical features of various PVDF copolymers and piezoelectric properties was analyzed. Using LEO solar ultraviolet data, total UV energy depositions were estimated as equivalent radiation doses with significant doses predicted for the thin films. A good retention of piezo properties during γ-irradiation was observed despite concurrent polymer damage with crosslinking and morphological changes.

Thin polymer films have been identified as one of the major enabling technologies for future space-based systems. Potential devices include those based on piezoelectric bimorph polymers that deform when a charge is deposited, for example, from an electron gun. The thin-film and lightweight nature of the polymeric devices will allow them to be launched more readily and deployed to operating conditions once in orbit. Until now little work has been done aimed at investigating the performance of piezoelectric properties of PVDF and its copolymers and the prediction of their long-term stability in low Earth orbit (LEO) environmental conditions. In this paper, the piezoelectric properties of PVDF and the copolymers formed from polymerization of vinylidene fluoride and trifluoroethylene (TrFE) or hexafluoropropylene (HFP) have been studied over a broad temperature range simulating that expected in LEO. The temperatures experienced by unprotected polymers on low altitude spacecraft have previously been reported as ranging from approximately -100 C to +130 C as the polymer/spacecraft passes in and out of the Earth's shadow. To examine the effects of temperature on the piezoelectric properties of poled PVDF, P(VDF-TrFE) and P(VDF-HFP) the d{sub 33} piezoelectric coefficients and electric displacement-electric field (D-E) hysteresis loops were measured up to 160 C for the d{sub 33} measurements and from -80 to +110 C for the D-E loops. The room temperature d{sub 33} coefficient of PVDF homopolymer films, annealed for extended periods at 50, 80 and 125 C, dropped rapidly within a few days of heating, then remained unchanged for periods of up to 300 days. In contrast, the TrFE copolymer exhibited greater thermal stability than the homopolymer, with the d{sub 33} remaining almost unchanged from the pre-annealing value after heating at 50, 80 and 125 C. The HFP copolymer exhibited poor retention of d33 at temperatures above 80 C. For all three polymers short term annealing at 160 C reduced the d{sub 33} to zero. The decrease in d{sub 33} for the TrFE copolymer was correlated with the change in Curie temperature upon annealing of the copolymer, as observed by differential scanning calorimetry (DSC). Unlike radiation damage, which may occur from vacuum UV and atomic oxygen in LEO, the depoling of the polymers on exposure to elevated temperatures was attributed to a physical randomization of the morphology rather than a chemical degradation process. In situ D-E loop measurements over the temperature range -80 to +110 C showed that the remnant polarization of the TrFE copolymer was more stable than the PVDF homopolymer. These results suggest that the TrFE copolymer appears to have a better overall performance in thermal extremes compared with PVDF or an HFP copolymer.

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Conventional high-temperature compression stress-relaxation (CSR) experiments (e.g., using a Shawbury-Wallace relaxometer) measure the force periodically at room temperature. In this paper, we first describe modifications that allow the force measurements to be made isothermally and show that such measurements lead to more accurate estimates of sealing force decay. We then use conventional Arrhenius analysis and linear extrapolation of the high-temperature (80--110 C) CSR results for two commercial butyl o-ring materials (Butyl-A and Butyl-B) to show that Butyl-B is predicted to have approximately three times longer lifetime at room temperature (23 C). To test the linear extrapolation assumed by the Arrhenius approach, we conducted ultrasensitive oxygen consumption measurements from 110 C to room temperature for the two butyl materials. The results indicated that linear extrapolation of the high temperature CSR results for Butyl-A was reasonable whereas a significant curvature to a lower activation energy was observed for Butyl-B below 80 C. Using the oxygen consumption results to extrapolate the CSR results from 80 C to 23 C resulted in the conclusion that Butyl-B would actually degrade much faster than Butyl-A at 23 C, the opposite of the earlier conclusion based solely on extrapolation of the high-temperature CSR results. Since samples of both materials that had aged in the field for {approx}20 years at 23 C were available, it was possible to check the predictions using compression set measurements made on the field materials. The comparisons were in accord with the extrapolated predictions made using the ultrasensitive oxygen consumption measurements, underscoring the power of this extrapolation approach.

The γ-irradiated-oxidation of pentacontane (C50H102) and the polymer polyisoprene was investigated as a function of oxidation level using 17O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using 17O labeled O2 gas during the γ-irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the 17O NMR spectra. Production of carboxylic acids is the primary oxygen-containing functionality during the oxidation of pentacontane, while ethers and alcohols are the dominant oxidation product observed for polyisoprene. The formation of ester species during the oxidation process is very minor for both materials, with water also being produced in significant amounts during the radiolytic oxidation of polyisoprene. The ability to focus on the oxidative component of the degradation process using 17O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches. © 2001 Elsevier Science Ltd.

A hydroxy-terminated polybutadiene (HTPB)/isophorone diisocyanate (IPDI) elastomer is commonly used as propellant binder material. The thermal degradation of the binder is believed to be an important parameter governing the performance of the propellant. The aging of these binders can be monitored by mechanical property measurements such as modulus or tensile elongation. These techniques, however, are not easily adapted to binder agents that are dispersed throughout a propellant. In this paper the authors investigated solid state NMR relaxation times as a means to predict the mechanical properties of the binder as a function of aging time. {sup 1}H spin-lattice and spin-spin relaxation times were found to be insensitive to the degree of thermal degradation of the elastomer. Apparently these relaxation times depend on localized motions that are only weakly correlated with mechanical properties. A strong correlation was found between the {sup 13}C cross-polarization (CP) NMR time constant, T{sub cp}, and the tensile elongation at break of the elastomer as a function of aging time. A ramped-amplitude CP experiment was shown to be less sensitive to imperfections in setting critical instrumental parameters for this mobile material.

The authors have shown that the hydroperoxide species in {gamma}-irradiated {sup 13}C-polyethylene can be directly observed by {sup 13}C MAS NMR spectroscopy. The experiment was performed without the need for special sample preparation such as chemical derivatization or dissolution. Annealing experiments were employed to study the thermal decomposition of the hydroperoxide species and to measure an activation energy of 98 kJ/mol. EPR spectroscopy suggests that residual polyenyl and alkylperoxy radicals are predominantly trapped in interracial or crystalline regions, while the peroxy radicals observed after UV-photolysis of hydroperoxides are in amorphous regions.