Publications

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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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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.

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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.

We report an evaporation-induced self-assembly procedure to prepare poly(bridged silsesquioxane) thin-film and particulate mesophases that incorporate organic moieties (1-3) into periodic, mesostructured frameworks as molecularly dispersed bridging ligands. Capacitance-voltage measurements along with a variety of structural characterization procedures were performed to begin to elucidate structure-property relationships of this new class of surfactant-templated mesophases. We observed a consistent trend of increasing modulus and hardness and decreasing dielectric constant with substitution of the bridged silsesquioxane (≡Si-(CH2)2-Si≡) for siloxane (≡Si-O-Si≡) in the framework. This preliminary evidence suggests that the introduction of integral organic groups into the frameworks of mesoporous materials can result in synergistic properties, promising an unprecedented ability to tune properties and function.

Self-assembled monolayers (SAMS) are commonly produced by immersing substrates in organic solutions containing trichlorosilane coupling agents. Unfortunately, such deposition solutions can also form alternate structures including inverse micelles and lamellar phases. The formation of alternate phases is one reason for the sensitivity of SAM depositions to factors such as the water content of the deposition solvent. If such phases are present, the performance of thin films used for applications such as minimization of friction and stiction in micromachines can be seriously compromised. Inverse micelle formation has been studied in detail for depositions involve 1H-, 1H-, 2H-, 2H-perfluorodecyltrichlorosilane (FDTS) in isooctane. Nuclear magnetic resonance experiments have been used to monitor the kinetics of hydrolysis and condensation reactions between water and FDTS. Light scattering experiments show that when hydrolyzed FDTS concentrations reach a critical concentration, there is a burst of nucleation to form high concentrations of spherical agglomerates. Atomic force microscopy results show that the agglomerates then deposit on substrate surfaces. Deposition conditions leading to monolayer formation involve using deposition times that are short relative to the induction time for agglomeration. After deposition, inverse micelles can be converted into lamellar or monolayer structures with appropriate heat treatments if surface concentrations are relatively low.

The {gamma}-irradiated-oxidation of pentacontane (C{sub 50}H{sub 102}) and the polymer polyisoprene was investigated as a function of oxidation level using {sup 17}O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using {sup 17}O labeled O{sub 2} gas during the {gamma}-irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the {sup 17}O 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 {sup 17}O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches.

Oligomethylhydridosiloxane and tis copolymer with dimethylsiloxane undergo redistribution chemistry with catalytic tetrabutylammonium hydroxide (TBAH) to produce methylsilane and polymethylsilsesquioxanes. The rate and extent of redistribution reaction can be controlled by the amount of TBAH added, as well as use of solvent. The extent reaction can be followed by both infrared radiation (IR) and solid state NMR spectroscopy, following the disappearance of the SiH in the starting oligosiloxane.

Polysilsesquioxane foams and gels of the formula (RSiO1.5)n were produced via the catalytic an stoichiometric redistribution of organohydridosiloxanes. The extent of reaction was followed by both infrared (IR) and solid state NMR spectroscopy, following the disappearance of the SiH in the starting oligosiloxane.

{sup 13}C-enriched polyethylene was subjected to {gamma}-irradiation in the presence of air at 25 and 80 C for total doses ranging from 71 to 355 kGy. Significant quantities of hydroperoxides were detected in the 25 C irradiated sample by {sup 13}C magic angle spinning NMR spectroscopy. This method of detection was performed on the solid polymer and required no chemical derivatization or addition of solvent. The chemical stability and subsequent products of the hydroperoxide species were studied by annealing the irradiated samples in air at temperatures ranging from 22 to 110 C. A time-temperature superposition analysis provided an activation energy of 108 kJ/mol for the hydroperoxide decomposition process. The primary products of hydroperoxide decomposition were ketones and secondary alcohols with lesser amounts of acids and esters. EPR measurements suggest that the reactive hydroperoxide species reside in the amorphous phase of polyethylene, consistent with degradation occurring in the amorphous phase.

Long range substituent effects on the ²⁹Si NMR chemical shifts in a series of alkylene and arylene-bridged triethoxysilanes were observed over as many as 11 bonds. The hydrolysis reaction of an ethoxide caused the resonance of the silicon on the opposing end of the bridging unit to move downfield. The alkylene bridging units ranged from ethylene to octylene while the arylene bridging units included phenyl and biphenyl. Resonance assignments were confirmed by the absence of these shifts for the triethoxysilyl in l-triphenylsilyl-2-triethoxysilylethane. The magnitude of the downfield shift decreased as the length of the bridging unit between silicon atoms increased. Transmission of the substituent effect along a polyethylene chain was successfully modeled by a through-bond mechanism with an attenuation factor of 1.88 for each methylene unit.

The porosities of three mesoporous silica materials were characterized with {sup 129}Xe NMR spectroscopy. The materials were synthesized by a sol-gel process with r = 0, 25, and 70% methanol by weight in an aqueous cetyltrimethylammonium bromide solution. Temperature dependent chemical shifts and spin lattice relaxation times reveal that xenon does not penetrate the pores of the largely disordered (r= 70%) silica. For both r = 0 and 25%, temperature dependent resonances corresponding to physisorbed xenon were observed. An additional resonance for the r = 25% sample was attributed to xenon between the disordered cylindrical pores. 2D NMR exchange experiments corroborate the spin lattice relaxation data which show that xenon is in rapid exchange between the adsorbed and the gas phase.

A fundamental understanding of aging in an organic material requires that one understand how aging affects the chemical structure of a material, and how these chemical changes are related to the material`s macroscopic properties. This level of understanding is usually achieved by examining the material on a variety of length scales ranging from atomic to meso-scale to macroscopic. The authors are developing and applying several {sup 13}C nuclear magnetic resonance (NMR) spectroscopy experiments to characterize the aging process of organic materials over a broad range of length scales. Examples of studies which range from atomic to macroscopic will be presented.

The hydrolysis and self- and cross-condensation kinetics of the hybrid sol tetraethoxysilane and ethyltriethoxysilane were investigated by high resolution {sup 29}Si NMR spectroscopy. A kinetic model in which hydrolysis is reversible and condensation is irreversible was developed. The authors found excellent agreement between the product distributions measured by {sup 29}Si NMR spectroscopy and calculated by the model. The cross-condensation rates for each of the sols were intermediate to the condensation rates of the individual components. Calculations show that for these sols, the concentration of cross-condensed species is a weak function of the relative rates of self-condensation.

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.

The chemical synthesis of advanced ceramic and glass materials by the sol-gel process has become an area of increasing activity in the field of material science. The sol-gel process provides a means to prepare homogeneous, high purity materials with tailored chemical and physical properties. This paper surveyed the nuclear magnetic resonance (NMR) studies of silicon-based sol-gel kinetics. A review of the various models which have been used to analyze the chemical kinetics of various sol-gel systems was presented. The utility of NMR spectroscopy was demonstrated in investigating the influence that various reaction conditions have on the reaction pathways by which sol-gel derived materials are synthesized. By observing in a direct fashion the chemical pathway of the sol-gel, it is often possible to relate the final properties of the material to the formulation and reaction conditions of the sol-gel. The study of reaction kinetics by NMR is expected to play an increasingly important role in understanding sol-gel processing and material properties. 15 refs. (DP)

This paper surveys a few of the current issues in sol-gel reaction kinetics. Many times seemingly modest changes in reactants or reaction conditions can lead to substantial differences in the overall reaction rates and pathways. For example, qualitative features of the reaction kinetics can depend on catalyst concentration. At very high acid-catalyst concentrations, reverse are significant for TMOS sol-gels, while for moderate acid-catalyst concentrations, reverse reactions are substantially reduced. The reaction kinetics are substantially reduced. The reaction kinetics of two similar tetraalkoxysilanes: tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), can be markedly different under identical reaction conditions. Under acid-catalyzed reaction conditions, a TMOS sol-gel undergoes both water-and alcohol-producing condensation reactions while a TEOS sol-gel undergoes only water-producing condensation. The early time hydrolysis and condensation reactions of a TMOS sol-gel are statistical in nature and can be quantitatively described by a few simple reaction rate constants while the reaction behavior of a TEOS sol-gel is markedly nonstatistical. A comprehensive theory of sol-gel kinetics must address diverse experimental findings. 9 refs., 3 figs., 1 tab.