High Performance Polymers

The creation of large-size space constructions by deploying structures needs a method for making the shell material rigid after unfolding. The best method is a chemical reaction. There is a negative influence of space environment (high vacuum, atomic oxygen flux, cosmic rays, sun irradiations, sharp temperature variations) on a liquid polymer matrix and on the polymerization process. The authors suggest that the space environment can initiate the polymerization reaction, for example a curing reaction of epoxy resin composition. This paper contains the results of experiments on polymerization processes under the action of separate space environment factors: vacuum and plasma action. For the simulation of space plasma a discharge of microwave plasma and radio-frequency plasma were used. Structures of polymer materials cured under plasma and in the usual curing conditions were compared. The results on degradation processes in uncured epoxy resin under plasma action were taken into account for the curing kinetic study. The fornation of new active groups in liquid composition under plasma action and the ability of new groups to take part in the curing reaction was analysed. A process of layer mixing in uncured epoxy resin during plasma action was observed. The mixing of oxidized layers with initial bulk layers of resin leads to an acceleration of the curing reaction. Due to layer mixing the plasma treatment becomes a method for bulk modification of polymers. A method of utilization of negative free space factors for the curing process of composite materials in free space is proposed. The space plasma can be used for the polymerization process of composite materials on Earth orbit. The composite samples of glass fibres impregnated by the epoxy matrix with 5 mm thickness were polymerized in plasma discharge at low pressure without formation of bubbles.

This paper describes a study carried out to evaluate solar absorptance and infrared emissivity degradation of many different thermal control materials (white paints, second-surface mirrors and aluminized films) exposed to geostationnary orbit. For this purpose, a long-term test of about 3 months in constant vacuum was performed reproducing a 3-year space environment exposure including ultraviolet and charged particle effects on North/South panels of a geostationary three-axis stabilized spacecraft. Reflectance spectra were measured in situ in the solar range (250-2500 nm) and the corresponding solar absorptance values were calculated. The test methodology and the obtained degradations of many materials are discussed. Instead of a constant increase with irradiations, oscillations of solar absorptance were obtained for some materials depending on the test sequence. The most important degradations were found in white silicone paints whereas second surface mirrors and aluminium paints remained quite stable.

The erosion of highly-ordered pyrolytic graphite (HOPG) caused by exposure to hyperthermal atomic 0(³P) atoms was explored. Profilometry has revealed that the overall erosion yield is linear with respect to increasing atomic oxygen exposure at a constant sample temperature of 373 K. Atomic force microscopy (AFM) was employed to image the eroded material that contains numerous nanoscale to microscale cylindrical etch pits. As there is also a linear relationship between the etch pit diameter and atomic oxygen fluence, it is proposed that the largest cylinders are nucleated at or near the topmost sheet of the original graphite material. There is a large distribution of depths for cylinders of a chosen diameter. This suggests that the chemical and physical nature of the nucleation event plays a key role in the final depth of the etch pit.

Our current understanding regarding the evaluation of the thermal endurance and UV radiation stability of two different types of commercially available aromatic polyimides at elevated temperatures is presented. Such materials are typically used as thermal control foils. To characterize the thermal endurance a high vacuum thermal ageing campaign was performed at 350 'C up to 4452 h. During that test the mass loss was found to be higher for the Kapton HN (DuPont, USA)-film than for the Upilex S (UBE Industries, Japan)-film material. Further UV/VIS/NIR transmission and thermo-optical measurements revealed the degradation behaviour of both films versus time at 350 'C. It was found that in general the Upilex S-film degrades at a slower rate. To screen the radiation stability an environmental UV test was performed with samples being simultaneously at 100, 230 and 350 'C. After the test it was noted that the Kapton HN-film shows for all three temperatures a higher mass loss than the Upilex S-film. A non-linear correlation of the activation energies for the mass loss of the Kapton-HN film as a function of temperature was also found. Furthermore, a higher loss of transmittance (UVIVIS) was found for the Kapton HN-film and a higher degradation of the a/E ratio when compared with the Upilex-S-film. For the latter material it seems that this degradation is temperature activated.

A quantitative study of the synergistic effect of 5 eV atomic oxygen and 172 nm vacuum ultraviolet radiation exposure was carried out on polyimide samples. The relative intensity of atomic oxygen and ultraviolet was adjusted by rotating the polyimide sample at the crossover point of the beam axes of atomic oxygen and ultraviolet which crossed 900. The erosion rate of polyimide was calculated from the resonant frequency of the quartz crystal microbalance on which the polyimide was spin-coated. It was confirmed that the ultraviolet exposure to the atomic oxygen-exposed polyimide promoted the desorption of carbon oxides from the surface. In contrast, no mass change was detected when ultraviolet irradiated the polyimide surface without atomic oxygen exposure. By changing the relative intensity of atomic oxygen and ultraviolet, increases in erosion rates up to 400% were observed when the relative ultraviolet/atomic oxygen flux intensity reached 10-14 mJ atom-l.

The purpose of this study was to evaluate the thermal endurance of two types of thermal control foils. This is an important parameter for selecting materials for inner solar system missions. Assessing the thermal endurance by isothermal testing in a thermogravimetric analyser (TGA) is a time-consuming method. A quicker method is to perform kinetic modelling on several decomposition curves. Four temperature scans at different heating rates were recorded with the TGA for two thermal control foils, Kapton HN (DuPont, USA) and Upilex S (UBE Industries, Japan). Two methods of kinetic modelling were used to calculate the activation energy, namely the ASTM E 1641 and the approach of model free kinetics (MFK). The first assumes a constant value for the activation energy whereas the latter calculates it as a function of the conversion. With the activation energy it is possible to make a lifetime prediction that indicates the mass loss that occurs at a certain temperature for a certain duration. The lifetime predictions are used to compare the two materials and also the two modelling methods. The modelling is verified by comparing the results to isothermal TGA tests and thermal ageing of samples in high vacuum chambers at 350'C for durations up to half a year and in combination with UV irradiation screening tests. All the experiments performed indicate that Upilex S has a better thermal endurance than Kapton HN. The results of the kinetic modelling were in good agreement with the isothermal experiments in the TGA and in the vacuum facilities. The MFK approach for kinetic modelling was found to be more adequate than the ASTM method.

The present study investigated the interactions of four different types of polyethylene (PE) surfaces with RF oxygen plasma components, simulating the low Earth orbit (LEO) environment. The samples were exposed in the plasma afterglow, with and without the plasma-generated vacuum UV (VUV) radiation. The polymers studied were low-density polyethylene (LDPE), ultra high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), and a composite of highly-oriented fibres of ultra-high molecular weight polyethylene (PolyEitan) having crystalline-to-amorphous phases ratio of 65, 74, 87 and 92%, respectively. Erosion of the LDPE, UHMWPE, and HDPE samples under atomic oxygen (AO) irradiation showed an etching rate dependence on the degree of crystallinity - the higher the crystallinity level the lower the erosion rate. The addition of VUV radiation to the AO flux resulted in a significant increase of the etching rate. An increase of the crystallinity level resulted in a decrease of the total oxygen uptake and an increase of the surface roughness. The composite polyethylene, although highly crystalline, revealed similar erosion rate and chemical composition changes as highly amorphous LDPE. The erosion mechanism of the different PE samples under AO or AO+VUV is discussed in terms of the initial internal structure.

An in-situ, internal space charge measurement system for dielectric materials under irradiation has been developed by using the pulsed electroacoustic (PEA) method. This system can observe space charge profiles in dielectric materials under irradiation, and was applied to an irradiation chamber at the Office National d'Etudes et de Recherches Mrospatiales, Centre de Toulouse (ONERA-CERT), supported by Centre National d'Etudes Spatiales (CNES). Space charge profiles in a several kinds of polymeric materials were clearly observed under and after electron beam irradiations. Negative charges accumulated at the position corresponding to the energy of the electron beam in all the specimens. The in-situ observation of the internal space charge behaviour can give valuable information for the investigation of charging and discharging phenomena and of ageing mechanisms of dielectric materials used in the space.

Solar sailing is a unique form of propulsion in which a spacecraft gains momentum from incident photons. Solar sails are not limited by reaction mass and provide continual acceleration, reduced only by the lifetime of the lightweight film in the space environment and the distance to the Sun. Once thought to be difficult or impossible, solar sailing has come out of science fiction and into the realm of possibility. Any spacecraft using this propulsion method would need to deploy a thin sail that could be as large as many kilometres in extent. The availability of strong, ultra lightweight, and radiation-resistant materials will determine the future of solar sailing. The National Aeronautics and Space Administration's (NASA) Marshall Space Flight Center (MSFC) is concentrating research into the utilization of ultra lightweight materials for spacecraft propulsion. The Space Environmental Effects Team at MSFC is actively characterizing candidate solar sail material to evaluate the thermo-optical and mechanical properties after exposure to space environmental effects. This paper will describe the irradiation of candidate solar sail materials to energetic electrons, in vacuum, in an effort to determine the in-space operational survivability of several candidate sail materials. Results from this research indicate that the candidate sail materials can survive significant doses of electron radiation while under high uniaxial stress.

This paper describes testing to investigate the effects of vacuum ultraviolet (VUV) radiation on Teflon fluorinated ethylene propylene (FEP) film, examining differences in mechanical properties degradation for samples of 50.8 jtm thickness exposed to VUV of various lower cut-off wavelengths. Samples were illuminated in a high vacuum facility by deuterium lamps, which provide radiation in the 115-400 nm wavelength range, but with the highest intensity being below 200 nm. Windows of fused silica, crystalline quartz, and magnesium fluoride provided lower cut-off wavelengths of 155, 140 and 115 nm, respectively. Lamp intensity was measured in the 115-200 nm wavelength range throughout the sample exposures. The determined intensities were used to estimate intensity and incident energy of various wavelength ranges, between 115 and 400 nm. Samples were analysed for tensile strength and elongation at failure. The effects of radiation exposures of different wavelength ranges were compared and discussed in terms of the expected depth to which radiation of various wavelengths is deposited into FEP

Hybrid inorganic/organic polymers have been prepared by copolymerizing a polyimide having the same chemical repeat unit as Kapton with an open-cage polyhedral oligomeric silsesquioxane (POSS). These POSS/polyimide hybrid polymers are Kapton-like polymers containing POSS nanoparticles that are chemically bound into the polymer chain. Samples of these POSS polyimides, as well as polyimide controls, have been exposed to a hyperthermal 0-atom beam that is produced by a laser-detonation source. Exposed and unexposed surfaces have been characterized by surface profilometry, atomic force microscopy, and X-ray photoelectron spectroscopy The data indicate that the POSS-containing polyimides have significantly lower erosion yields than Kapton, because they form a surface SiO₂ layer which passivates the surface and protects the underlying polymer from further 0-atom attack. These results suggest promise for the use of a POSS polyimide polymer as a "drop-in" replacement for Kapton on spacecraft operating in the low-Earth orbital environment.

Ground-based environmental durability tests have indicated that exposing materials in accelerated tests to environmental model predicted spacecraft mission exposures of known degradation sources does not simulate the extent of damage that occurs in the space environment. One approach to overcoming the difficulties in simulating the space environment using ground-based testing is to calibrate the facility using data from actual space-exposed materials to determine exposure levels required to replicate degraded properties observed in space. This paper describes a ground-to-space correlation method that uses a multiple step process to determine the durability of expanded-polytetrafluoroethylene (ePTFE) for International Space Station (ISS) applications based on ground-based X-ray irradiation and heating exposure that simulates bulk embrittlement as occurs in fluorinated ethylene propylene (FEP) thermal insulation covering the Hubble Space Telescope (HST). This method was designed to damage the back surface of equivalent thickness ePTFE to the same amount of scission damage as occurred in HST FEP (based on elongation data) and then correct for differences in ground test ionizing radiation versus space radiation effects, temperature variations, space ionizing radiation environment variations (spacecraft altitude, inclination and duration), and thickness variations. The analysis indicates that after a 10-year mission, the ISS ePTFE will have an extremely embrittled front surface, with surface cracks induced under any given strain, and a very ductile back surface. This study also found that a thermal induced strain of 0.1 will develop in the ePTFE, and under this strain condition, microscopic cracks will start developing very early in the mission at the exposed surface and develop to a depth of approximately 300 am after 10 years.

Most facilities for testing the stability of spacecraft materials under attack by fast atomic oxygen (AO) are based on the generation of hypersonic beams of AO in nozzle sources with the use of continuous electrical discharges or in laser pulse-induced breakdown of the gaseous oxygen. In both cases, the sources of fast AO are simultaneously quite intense sources of vacuum ultraviolet (VUV) radiation. In the present study the intensity of VUV radiation with wavelengths > 115 nm from fast AO source were directly measured in the Central Aerohydrodynamic Institute facility VAT- 103. The intensity of this VUV radiation at the nozzle exit was shown to be almost equal to the intensity of the commercial lamp XeR-2 emitting at 147.0 nm (at its window). This VUV radiation can take part in destruction and erosion of polymer materials simulating the oxydative effects of fast AO. Its contribution to observed mass losses is greatly dependent on the material under test. For polymers consisting of carbon, hydrogen, oxygen and nitrogen the main role in erosion is played by fast AO. The decisive role of VUV radiation is shown in the erosion of fully fluorinated polymers. The mass losses of Teflon and Teflon FEP films in facilities for generation of fast AO are suggested as a means of measuring the VUV radiation dose in these facilities. The main features of mass losses that depend on duration of VUV irradiation are formulated. They are based on the previously proposed model of mass losses from these polymers during VUV photodestruction. The key feature of this model is the limited rate of photofragment evaporation (sublimation) into vacuum.