Chapter 21
Molecular-Level Response of Selected Polymeric Materials to the Low Earth Orbit Environment Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch021
1
Philip Young, Emilie J . Siochi , and Wayne S. Slemp Langley Research Center, National Aeronautics and Space Administration, Hampton, VA 23681-0001
The NASA Long Duration Exposure Facility (LDEF) enabled the exposure of a wide variety of materials to the low Earth orbit (LEO) environment. This paper provides a summary of research conducted at the Langley Research Center into the response of selected LDEF polymers to this environment. Materials examined include graphite fiber reinforced epoxy, polysulfone, and addition polyimide matrix composites, films of FEP Teflon, Kapton, and several experimental high performance polyimides, and films of more traditional polymers such as poly(vinyl toluene) and polystyrene. Exposure duration was either 10 months or 5.8 years. Flight and control specimens were characterized by a number of analytical techniques including ultraviolet-visible and infrared spectroscopy, thermal analysis, scanning electron and scanning tunneling microscopy, x-ray photoelectron spectroscopy, and, in some instances, selected solution property measurements. Characterized effects were found to be primarily surface phenomena. These effects included atomic oxygen-induced erosion of unprotected surfaces and ultraviolet-induced discoloration and changes in selected molecular level parameters. No gross changes in molecular structure or glass transition temperature were noted. The National Aeronautics and Space Administration Long Duration Exposure Facility (LDEF) provided a novel opportunity for the aerospace community to examine the effects of long term low Earth orbit (LEO) exposure on a variety of materials. The 11-ton satellite depicted in Figure 1 was returned to Earth by the Space Shuttle Columbia in January 1990 after 69 months in orbit. It contained 57 experiments to assess the effects of the space environment on materials, living matter, and various space systems (1). The saga of this remarkable vehicle is continuing to unfold through a series of symposia, workshops, and journal articles (2-7). Perhaps as much as 90% of ourfirst-handknowledge of LEO space environmental effects rests with the L D E F and its contents (8). The Langley Research Center actively pursued the chemical characterization of polymeric materials which flew on L D E F (9-22). The present paper summarizes 1
Current address: Lockheed Engineering and Sciences Company, Hampton, VA 23666 0097-6156/96/0620-0264$14.25/0 © 1996 American Chemical Society
In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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21. YOUNG ETAL.
Molecular-Level Response to the LEO Environment
almost 5 years of LDEF-related polymer research at this facility. It represents the collective efforts of a number of individuals and organizations in both assembling and analyzing a broad variety of control and exposed specimens. Table I lists L D E F polymeric materials assembled for analysis. These materials were provided by several Principal Investigators and, depending on L D E F row and tray location, experienced somewhat different environments. Specimens were exposed for either 10 months or 5.8 years as noted. Materials ranged from early 1980 state-of-the-art graphite fiber reinforced polymer matrix composites, to space films and coatings, high performance polymer films, and more traditional polymers. Representative data obtained on these materials is given in this paper. A more complete data presentation can often be found in accompanying referenced reports. The intent of this activity is to increase our fundamental understanding of space environmental effects on polymeric materials and to develop benchmarks to enhance our methodology for the ground base simulation of those effects so that polymer performance in space can be more reliably predicted.
Experimental Most materials identified in Table I were originally obtained from commercial sources. The fabrication, quality control, specimen preparation, and baseline testing of Langley-supplied P1700/C6000,934/T300, and 5208/T300 composite materials is discussed in references 23 and 24. Polyimide-polysiloxane copolymer films were synthesized under N A S A Grant NAG-1-343 with Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Several high performance polyimides films were synthesized in-house (25-27). As noted in the text, some specimens were exposed for only 10 months while other materials received the full 5.8-year exposure. Specimens exposed for 10 months were inside an Experimental Exposure Control Canister (EECC) (1). The E E C C was closed when L D E F was launched. It opened 1 month after deployment and closed 10 months later. Various environmental exposure parameters are included with Figures 1 and 2.
Instrumental Methods of Analysis.
Thermal analyses were conducted using a DuPont 9900 Computer/Thermal Analyzer to process data from a DuPont 943 Thermomechanical Analyzer operating in the expansion mode. The glass transition temperature (Tg) was obtained by noting the point of inflection from the thermogram baseline. Ultraviolet-Visible (UV-VIS) transmission spectra were scanned on a Perkin-Elmer Lambda 4A Spectrophotometer. Infrared spectra were recorded on a Nicolet 60SX Fourier Transform Infrared System (FTIR) using a diffuse reflectance technique (28). X-ray Photoelectron Spectroscopy (XPS) measurements were conducted under N A S A Grant NAG-1-1186 at the Virginia Tech Surface Analysis Laboratory, VPI&SU, Blacksburg, V A . Measurements were made on a PerkinElmer PHI 5300 Spectrometer equipped with a Mg Koc x-ray source (1253.6 eV), operating at 15 kV/120mA. Scanning Tunneling Microscopy (STM) was performed in air on a NanoScope H instrument (Digital Instruments, Inc., Santa Barbara, CA) using a tungsten tip and G-Head accessory. Specimens were prepared by coating with 5-7 nm of gold-palladium using a Hummer IV sputtering system (Anatech, Ltd., Alexandria, VA). Transmission Electon Microscopy (TEM) analyses were conducted under N A S A Contract NAS1-19656 at the Virginia Institute of Marine Science, Gloucester Point, V A . A Cambridge StereoScan 150 (Cambridge Instruments, Deerfield, IL) equipped with an E D A X SI50 detecting unit (EDAX International, Inc., Prarie View, IL) performed Scanning Electron Microscopy (SEM) analyses. Various photographic techniques were used to document specimen appearance.
In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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IRRADIATION OF POLYMERS
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266
Figure 1.
The LDEF and flight orientation (Reproduced from reference 20.). TABLE I. LDEF POLYMERIC MATERIALS
Composites: P1700/C6000 Polysulfone 934/T300 £ poxy 5208/T300 Epoxy PMR-15/C6000 Polyimide LARC-160/C6000 Polyimide
Films: F E P Teflon Silvered FEP Teflon Kynar Fluorocarbon P1700 Polysulfone ^Kapton Polyimide
e
traditional Polymers: Polystyrene Polyvinyl toluene Polytetrafluoroethylene Polymethylmethacrylate Nylon Polyethylene terephthalate
a
a
a
b
b
High Performance Polymers: Polyimide-Polysiloxane Copolymer BTDA-ODA Polyimide BTDA-ODA-AI^ Doped polyimide 6F-DDSO2 Soluble polyimide 6F-BDAF Soluble polyimide PMDA-DAF Polyimide
a
c
a
a
9
Polyetheretherketone (PEEK)
9
Various Silicones Polyurethane
9
Source:
a
W. Slemp, PI, Expts. A0134/S0010 (B9). R. Vyhnal, PI, Expt. A0175 (A1 and A7). LDEF MSIG (various LDEF locations). J. Whiteside, PI, Expt. A0133 (H7). W. Slemp and A. St. Clair, PI, Expt. S0010 (B9). J. Gregory, PI, Expt. A0114 (C9/C3). s A. Whitaker, PI, Expt. A0171 (A8). b
c
d 6 f
Reproduced from reference 7. In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Molecular-Level Response to the LEO Environment
YOUNG ET AL.
(a) Atomic Oxygen Fluence Summary
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Yaw: 8.1 degrees Pitch: 0.8 degress Roll: 0 degrees
1.33E+217.22E+19
L84E+08 5.45E+2T* Ram direction: 3.39E+2-T 9.09D+21 atoms per sq cm 1.16E+2T
9.60E+12 I4E+19
7.33E+16
(b) Equivalent Sun Hours Summary 6.900
Yaw: 8.1 degrees Pitch: 0.8 degress Roll: 0 degrees
7,000
Z-Axls (Ram — vector) 8.1 degrees
Equivalent sun hours Summation: Solar form factor x Hours + Earth form factor x Albedo x Hours
(c) Integrated 5.8-Year Parameters
(d) Selected 10-Month Parameters 20
2
Atomic Oxygen: 2.6 x 10 atoms/cm Thermal Cycles: UV Radiation: -2300 hrs -34,000 (-20to160°F, ±20°) Thermal Cycles: Particulate Radiation: -4900 (-20to140 F) e-andp : 2.5x10 rad cosmic:
