10 Far-Ultraviolet Degradation of Selected Polymers Mark R. Adams and Andrew Garton Downloaded by UNIV OF MONTANA on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch010
1
Polymer Program and Chemistry Department, Box U-136, University of Connecticut, Storrs, CT 06269-3136
The far-UV
(100-250 nm) photodegradations
A polycarbonate ene) (FEP), The
(PC),
in vacuum of
and a polyimide-block-polysiloxane
results of controlled far-UV
irradiation
(PISX) are
reviewed.
in vacuum are
compared
with the results of exposure in a National Aeronautics ministration
(NASA)
low
source of both far-UV
Earth
radiation
orbit
(LEO)
Long
(LDEF)
nm rapidly
mission. Far-UV
at >190
simulation
Duration
facility
(a
atomic oxy-
Exposure
Facility
decarboxylated
the PC
efficiency of about 0.07 and lead to extensive
mass loss and the production tionality
and Space Ad-
and high-kinetic-energy
gen), and exposure on the NASA surface with quantum
bisphenol
poly(tetrafluoroethylene-co-hexafluoro-propyl-
of a cross-linked skin. The chemical func-
in the surface of PISX was also rapidly
changed.
FEP was
little affected at >190 nm because of its weak absorption at these wavelengths. However, fluorinated
radiation from an argon plasma (100-150 nm) de-
the FEP surface. The effects of atomic oxygen
the effects of far-UV
in the LEO simulation facility.
istry of FEP facing
the ram direction
dominated
in the LDEF
overwhelmed
The surface chemmission was also
by the effects of atomic oxygen, but specimens in the wake
direction showed effects consistent with extensive
photodegradation.
MOST ALL STUDY OF POLYMER PHOTODEGRADATION has been limited to wavelengths >250 nm (I), probably because far-UV radiation forms a negli gible component of terrestrial sunlight and so there is little practical concern. However, spacecraft experience the full solar spectrum, unshielded by the ozone layer. The continuum of solar radiation has significant intensity even below 200 nm (2), and plasma lines occur at lower wavelengths, notably the Current address: Franklin International, 2020 Bruck Street, Columbus, OH 43207 0065-2393/96/0249-0139$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
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140
POLYMER DURABILITY
emission from atomic hydrogen at 121 nm. Examples where far-UV degra dation have been considered include studies of far-UV lithography (3, 4), and the response of materials that may be relevant to spacecraft applications, such as fluoropolymers (5, 6). However, important considerations such as the effect of wavelength on the quantum yields and reaction pathways are essentially unknown. We described previously (7, 8) a systematic study of far-UV (180-250 nm) irradiation of bisphenol A polycarbonate (PC), both in vacuum and i n air; we demonstrated that decarboxylation is an efficient process (quantum yields about 0.07), but chain scission processes and mass loss occur in preference to the photo-Fries processes that are observed at mid-UV wavelengths. F a r - U V processes are also highly surface specific, and shielding by a layer of crosslinked material reduces quantum yields at longer irradiation times in vacuum. The purpose of this chapter is to review our experience of far-UV degradation of three very different polymers (PC, poly(tetrafluoroethylene-co-hexafluoropropylene) [FEP], and a polyimide-Mocfc-polysiloxane [PISX]) and to relate this experience to the effect of the spacecraft environment, real or simulated, on these polymers.
Experimental Details The P C was a commercial product (Lexan, General Electric) with a numberaverage molecular weight (M ) of 15,300 g/mol and a weight-average molecular weight of 28,700 g/mol. Specimens were examined after the commercial stabilizer package was removed by reprecipitation of the polymer. However, the presence of stabilizer did not affect the far-UV processes (7). Films of PC were cast from dichloromethane solution. The PISX (see structure in Chart I) was a block copoly mer of poly(dimethyl siloxane) (PDMS, M = 3000 g/mol) and a polyimide that was supplied by Jeffrey W. Gilman of Phillips Laboratories, Edwards Air Force Base, CA. The PISX was supplied as a powder in fully imidized form and was n
n
Polycarbonate (PC)
l
O
Ο
Fluorinated ethylene-propylene (FEP)
0
χ Ο
Ο
CH, Y J
Polymer endcapped with phthalimide
Polyimide-siloxane (PISX) Chart I. Polymer structures.
In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF MONTANA on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch010
10.
Far-UV Degradation of Selected Polymers
ADAMS & GARTON
141
spin-cast at 2000 rpm from a 10% solution in dimethylacetamide on to silicon wafers in a Class 100 clean room. The PISX was annealed (to bring the siloxane component to the surface) by heating to 230 °C for 24 h. The final film thicknesses were 150-1200 nm, as measured by IR spectroscopy, and were chosen depending on the anticipated level of chemical modification and the analytical technique to be employed The F E P was a thermal control blanket (Sheldahl Inc.) consisting of a 25-μηι F E P film backed with thin vacuum-deposited layers of inconel and silver. The F E P was used as received except for cases where transmission spec troscopic measurements (IR and UV) were desired, when the metallic layers were removed by mild abrasion and washing with acetone. The chemical structures of the polymers are shown in Chart I. Controlled irradiation at 180-250 nm was carried out in a stainless steel vac uum chamber with a Suprasil grade quartz window. A vacuum of 250 nm (i.e., in the mid-UV and beyond). The typical solar irradiance (2) is also shown in Figure 1 for comparison, plotted on the same intensity axis as that of the deuterium lamp. The integrated far-UV intensity of the deuterium lamp for the chosen optical configuration was therefore approximately two "far-UV suns". 2
. t t
10
_i
I
ι
I
I
ι — l _
ι
• « » » l_
Deuterium Lamp Ar Plasma
6H
4H
Solar Irradiance
ECR Source
Solar Lyman Line
,
100
,
,
,
J
125
,
,
1
1
J
150
1
1
1
1
J S
175
1
1
1
J
200
1
1
I
I
I
I
225
I
I
250
Wavelength (nm) Figure 1. Far-UV spectral distributions of light sources used in this study.
In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
142
POLYMER DURABILITY
Far-UV irradiation at shorter wavelengths (predominantly 150 nm) was car ried out by using a low-pressure argon plasma. The specimen, in a Petri dish covered with a calcium fluoride window, was placed just below the glow region of 50 W radio-frequency-generated argon plasma. Such an arrangement exposes the specimen to the far-UV component of the plasma without bombardment by the high energy ions in the plasma. No information is available on the radiation intensity. Specimens of PC, PISX, and F E P were exposed in the National Aeronautics and Space Administration (NASA) low Earth orbit (LEO) simulation facility at NASA Lewis, courtesy of Bruce Banks and Sharon Rutledge. This facility consisted of an electron cyclotron resonance (ECR) source containing low pressure oxygen. The E C R source produced atomic oxygen and far-UV radiation at about 130 nm. The chamber pressure was 0.027 Pa and the atomic flux was 2 X 10 atoms/ (cm · s). No information is available on the intensity of the far-UV radiation. Specimens of F E P were also obtained from the NASA Long Duration Ex posure Facility ( L D E F ) space experiment, courtesy of Philip Young of NASA Langley. The F E P specimens were located such that they experienced the ex tremes of possible exposure environments. Two specimens were taken from op posite sides of the spacecraft, but the sides had experienced nearly identical conditions (about 90° from the ram direction, 3 Χ 10 atoms/cm of atomic oxy gen, 7000 U V h). Two other specimens were taken from ram and wake directions. They had experienced 11,200 h and 9 Χ 10 atoms/cm (ram) and 10,500 h and 9 Χ 10 atoms/cm (wake). Because far-UV degradation is predominately a surface phenomenon (see dis cussion in subsequent section), greater emphasis was placed on careful surface analysis or the examination of ultrathin film specimens. Transmission IR spectra were obtained on thin (
