Publications

Abstract not provided.

Piezoelectric polymers based on PVDF are of interest for use in large aperture space-based telescopes similar to the James Web Space Telescope. Dimensional adjustments of polymer films depend on their piezoelectric properties with wireless (electron beam) shape control methods having been successfully demonstrated in the past. Such electron beam controls require a detailed understanding of the piezoelectric material responses. Similarly, space applications demand consistent, predictable, and reliable performance. While PVDF as a generic polymer type is a suitable piezoelectric material, it is also well known that fluorinated polymers are highly radiation-sensitive. Mechanical and other physical properties will suffer under various types of radiation (strong vacuum UV, {gamma}-, X-ray, e-beam, ion-beam) and atomic oxygen exposure. Likewise, extreme temperature fluctuations in space environments will result in annealing effects and cyclic stresses. While the radiative degradation chemistry of polymers is an established field there is little information available on the performance of piezoelectric features in PVDF with respect to their expected changes in these environments. Therefore, understanding such fundamental issues becomes mandatory for the design and deployment of satellite systems utilizing these materials/technology. We have investigated the degradation of PVDF and copolymers under a range of stress environments, and have studied the implications with regard to piezoelectrical properties necessary for reliable operation of thin films in space environments. Initial aging studies using {gamma}- and e-beam irradiation to explore material sensitivities for comparison with expected UV doses have shown complex material changes with lowered Curie temperatures, crystallinity, melting points and significant crosslinking, but little affect on piezoelectric d{sub 33} constants. Similar complexities of the aging processes have been observed in accelerated temperature environments. Overall, the results suggest that poling and polymer orientation are negatively affected by radiation effects and temperature. We have established fundamental correlations between chemical (structural) and physical (morphology) features of various PVDF copolymers and their piezoelectric properties. A frame work for material qualification issues and overall system survivability predictions in low earth orbit conditions has been developed. It will allow for improved material selection, feedback for manufacturing and processing technologies, avenues for material optimization/stabilization strategies and provide the necessary guidance on any alternative materials.

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.

Abstract not provided.

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.

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.

Failure models based on the Palmgren-Miner concept that material damage is cumulative have been derived and used mainly for fatigue life predictions for metals and composite materials. The authors review the principles underlying such models and suggest ways in which they may be best applied to polymeric materials in temperature environments. They first outline expectations when polymer degradation data can be rigorously time-temperature superposed over a given temperature range. For a step change in temperature after damage has occurred at an initial temperature in this range, the authors show that the remaining lifetime at the second temperature should be linearly related to the aging time prior to the step. This predicted linearity implies that it should be possible to estimate the remaining and therefore the service lifetime of polymers by completing the aging at an accelerated temperature. They refer to this generic temperature-step method as the Wear-out approach. They next outline the expectations for Wear-out experiments when time-temperature superposition is invalid. Experimental Wear-out results are then analyzed for one material where time-temperature superposition is valid and for another where evidence suggests it is invalid. In analyzing the data, they introduce a procedure that they refer to as time-degradation superposition. This procedure not only utilizes all of the experimental data instead of a single point from each data set, but also allows them to determine the importance of any interaction effects.

Over the past two decades, Sandia has developed a variety of specialized analytical techniques for evaluating the long-term aging and stability of cable insulation and other related materials. These techniques have been applied to cable reliability studies involving numerous insulation types and environmental factors. This work has allowed the monitoring of the occurrence and progression of cable material deterioration in application environments, and has provided insights into material degradation mechanisms. It has also allowed development of more reliable lifetime prediction methodologies. As a part of the FAA program for intrusive inspection of aircraft wiring, they are beginning to apply a battery of techniques to assessing the condition of cable specimens removed from retired aircraft. It is anticipated that in a future part of this program, they may employ these techniques in conjunction with accelerated aging methodologies and models that the authros have developed and employed in the past to predict cable lifetimes. The types of materials to be assessed include 5 different wire types: polyimide, PVC/Glass/Nylon, extruded XL-polyalkene/PVDF, Poly-X, and XL-ETFE. This presentation provides a brief overview of the main techniques that will be employed in assessing the state of health of aircraft wire insulation. The discussion will be illustrated with data from their prior cable aging studies, highlighting the methods used and their important conclusions. A few of the techniques that they employ are widely used in aging studies on polymers, but others are unique to Sandia. All of their techniques are non-proprietary, and maybe of interest for use by others in terms of application to aircraft wiring analysis. At the end of this report is a list showing some leading references to papers that have been published in the open literature which provide more detailed information on the analytical techniques for elastomer aging studies. The first step in the investigation of aircraft wiring is to evaluate the applicability of their various techniques to aircraft cables, after which they expect to identify a limited subset of techniques which are appropriate for each of the major aircraft wiring types. The techniques of initial interest in the studies of aging aircraft wire are as follows: optical microscopy; mandrel bend test; tensile test/elongation at break; density measurements; modulus profiling/(spatially-resolved micro-hardness); oxygen induction time/oxygen induction temperature (by differential scanning calorimetry); solvent-swelling/gel fraction; infrared spectroscopy (with chemical derivatization as warranted); chemiluminescence; thermo-oxidative wear-out assessment; The first two techniques are the simplest and quickest to apply; those further down the list tend to be more information rich and in some cases more sensitive, but also generally more specialized and more time consuming to run. Accordingly, the procedure will be to apply the simplest tests for purposes of preliminary screening of large numbers of samples. For any given material type, it can be expected that only a limited number of the other techniques will prove to be useful, and therefore, the more specialized techniques will be used on a limited number of selected samples. Samples of aircraft wiring have begun to be released to the authors in late April; they include in this report some limited and preliminary data on these materials.

The thermal degradation of a commercial, stabilized, unfilled neoprene (chloroprene) rubber was investigated at temperatures up to 140 °C. The degradation of this material is dominated by oxidation rather than dehydrochlorination. Important heterogeneous oxidation effects were observed at the various temperatures investigated using infrared micro-spectroscopy and modulus profiling. Intensive degradation-related spectral changes in the IR occurred in the conjugated carbonyl and hydroxyl regions. Quantitative analysis revealed some differences in the development of the IR oxidation profiles, particularly towards the sample surface. These chemical degradation profiles were compared with modulus profiles (mechanical properties). It is concluded that the profile development is fundamentally described by a diffusion-limited autoxidation mechanism. Oxygen consumption measurements showed that the oxidation rates display non-Arrhenius behavior (curvature) at low temperatures. The current results, when compared to those of a previously studied, clay-filled commercial neoprene formulation, indicate that the clay filler acts as an antioxidant, but only at low temperatures.

The so-called Palmgren-Miner concept that degradation is cumulative, and that failure is therefore considered to be the direct result of the accumulation of damage with time, has been known for decades. Cumulative damage models based on this concept have been derived and used mainly for fatigue life predictions for metals and composite materials. The authors review the principles underlying such models and suggest ways in which they may be best applied to polymeric materials in temperature environments. The authors first consider cases where polymer degradation data can be rigorously time-temperature superposed over a given temperature range. For a step change in temperature after damage has occurred at an initial temperature in this range, they show that the remaining lifetime at the second temperature should be linearly related to the aging time prior to the step. This predicted linearity implies that it may be possible to estimate the remaining lifetime of polymeric materials aging under application ambient conditions by completing the aging at an accelerated temperature. They refer to this generic temperature-step method as the Wear-out approach. They then outline the expectations for Wear-out experiments when time-temperature superposition is invalid, specifically describing the two cases where so-called interaction effects are absent and are present. Finally, they present some preliminary results outlining the application of the Wear-out approach to polymers. In analyzing the experimental Wear-out results, they introduce a procedure that they refer to as time-damage superposition. This procedure not only utilizes all of the experimental data instead of a single point from each data set, but also allows them to determine the importance of any interaction effects.

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.

{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.