Polymer Durability and Radiation Effects - American Chemical Society

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Chapter 11

Aspects of the Radiation Chemistry of Some Transparent Polyimides

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David J . T. Hill Department of Chemistry, The University of Queensland, Brisbane 4072, Australia

The sensitivities of a series of transparent polyimides to simulated space conditions for GEO and LEO have been assessed. The effects of UV, VUV, atomic and molecular oxygen, gamma and electron beam radiation on the polymers have been reviewed. The changes in the visible spectra, the radicals formed, the changes in the surfaces of polyimide films under vacuum and in the presence of oxygen and, in the case of high-energy irradiation, the structural changes in the polyimide have been considered.

Aromatic polyimides are well recognized to have excellent chemical stability, so they have found applications in extremely harsh and corrosive environments. For example, they are used as insulators in nuclear facilities and as thermal blankets on spacecraft where the materials may be exposed to highenergy radiation in the presence of oxygen. The best known of the commercial polyimides is Kapton, which is marketed by Du Pont. The chemical structure of Kapton is shown in Figure 1, along with that of Ultem, another commercial polyimide marketed by General Electric. In Kapton and Ultem the carbonyl groups of the imides are directly attached to phenyl rings and they are strongly electron withdrawing, so the adjacent aromatic groups tend become positively charged. On the other hand, the nitrogen atoms in the imides are electron donating, so the aromatic groups attached to these atoms tend to become negatively charged. Where the two imide rings in the polyimide repeat units are attached to the same aromatic ring, 116

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Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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as in Kapton, the charge separation along the polymer chain can be significant. Then strong donor-acceptor complexes may be formed between adjacent polymer chains (1). These donor-acceptor complexes in the case of Kapton absorb visible radiation, as shown in Figure 2, and are responsible for its intense brown colour.

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Ultem on the other hand has the imide groups attached to different phenyl rings in the repeat unit, so the charge separation in Ultem is lower than in Kapton. In addition, because of the kinked nature of the repeat units in Ultem, adjacent chains are unable to approach one another as closely as they can in Kapton. So in Ultem donor-acceptor complex formation is not as efficient as in Kapton, and Ultem absorbs less strongly in the visible region, as can be seen in Figure 2. In space the solar energy maximum emission from the sun occurs at approximately 550 nm (2). Thus for some space applications, polymers are required that have a high transmittance at 550 nm, so there is a need for polyimides more transparent than Kapton throughout the visible region. Several polyimides with low absorption at 550 nm have been prepared by St Clair (1,3) by utilizing dianhydride monomers with the anhydride units on different phenyl rings and with bulky electron withdrawing groups on the phenyl rings in the diamine units so as to reduce charge separation. They have also incorporated meta linkages between phenyl rings to generate chain kinks to

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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minimize chain alignment and hence disrupt donor-acceptor complex formation. Some examples of the use of these principles are demonstrated in the polyimides shown in Figure 3 that have excellent transparency, particularly at 550 nm. In geosynchronous earth orbits (GEO) a space vehicle is exposed to intense UV and VUV radiation from the sun, as well as high-energy cosmic radiation and the electron fields which surround the earth. Depending on the location of a space vehicle, the dose rates of the high-energy radiation can range from approximately 0.5 to 2 MGy per year (4). In low earth orbits (LEO) the dose rates for high-energy radiation are lower, being of the order of approximately 0.5 kGy per year, but there is a high atomic oxygen flux (5), as well as the cosmic and UV and VUV radiation in the range 100 - 400 nm. The atomic oxygen flux impacting the surface of a spacecraft depends on its altitude and orientation. For example, the leading surface of a spacecraft experiences a higher oxygen atom flux than does the trailing surface. The velocity of the spacecraft as well as the angle of incidence determines the impact energies of the oxygen atoms.

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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ODPA/ODA

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ODPA/DAB

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Figure 3. Chemical structures of some transparent polyimides.

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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In addition, the spacecraft in GEO and LEO experience fluctuating temperatures as they pass through the dark and light regions of their orbits. These temperatures can range from below 200 Κ to above 400 K. So, the polymers used in space, and particularly those used on the outer surfaces of space vehicles, must be able to withstand extremely harsh environments. Therefore, before any polymer is used in a space, it is important to assess its sensitivity to space conditions. We have previously reported on the sensitivity of several transparent polyimides to conditions that are similar to those found in GEO and LEO (6-13). In this paper some of the results of these studies are reviewed using examples from the series of polymers presented in Figure 3.

Experimental Samples The polyimides shown in Figure 3 were prepared at the NASA-Langley Laboratory by Dr John Connell and are all very pale yellow in colour. Typical visible spectra of «50 μηι films of some polyimides are presented in Figure 4 as examples. The glass transition temperatures, Tg, of the polyimides are high, all being above 450 K.

UV Photolysis The UV photolysis studies were performed under vacuum or in air using an unfiltered high-pressure mercury/xenon lamp purchased from Oriel. The films were located 30 cm from the lamp and separated from it by a heat filter. The output from the lamp was 9.1 mW cm" . 2

VUV Photolysis The VUV radiation and oxygen atom flux were generated in an oxygen plasma powered by a radio-frequency source (14). The charged species in the plasma were eliminated using an earthed grid and VUV only, atomic and molecular oxygen only or VUV as well as atomic and molecular oxygen could be made incident on a sample via an appropriate choice of the protective baffle.

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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