UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using electron spin resonance (ESR) spectroscopy, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The ESR study revealed the formation of a terminal polymer radical. The stability of this radical was investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF2 in the polymer surface decreased with exposure time and the percentage of CF, CF3, and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen identified a rough surface with undulations similar to sand dunes

George, Graeme A.

Hill, David J. T.

Odonnell, James H.

Pomery, Peter J.

Rasoul, Firas A.

English

NASA Technical Reports Server

N93-28261
A STUDY OF THE UV AND VUV DEGRADATION OF FEP
Graeme A. George, David J.T. Hill.
James H. O'Donnell, Peter J. Pomery and
Firas A. Rasoul"
Polymer Materials and Radiation Group
The University of Queensland
Australia 4072
Phone:07-365 3686, Fax:07- 365 4299
SUMMARY
UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer
was studied using ESR, XPS and SEM. The ESR study revealed the formation of a
terminal polymer radical. The stability of this radical has been investigated under
different environments. An XPS study of FEP film exposed to VUV and atomic
oxygen showed that oxidation takes place on the polymer surface. The study revealed
also that the percentage of CF z in the polymer surface decreased with exposure time
and the percentage of CF, CF 3 and carbon attached to oxygen increased. SEM
micrographs of FEP film exposed to VUV and atomic oxygen identified a rough
surface with undulations similar to sand dunes.
INTRODUCTION
Spacecraft in low orbit are subjected to significant levels of high energy
radiation, including UV and VUV wavelengths. The effects of UV radiation are
enhanced over those at the surface of the earth, where the only incident wavelengths
are greater than 290 nm (1). In low earth orbit the incident UV wavelengths extend
below 290 nm into the VUV region, where the Lyman-a emissions of atomic
hydrogen occur at 121 nm (2). In addition to electromagnetic radiation, in low earth
orbit polymer materials may also be subjected to atomic oxygen particle radiation,
which will result in direct oxidation of the polymer (1).
Thus, polymeric materials for space applications must exhibit a resistance to
radiation damage of this type. One class of materials which have this characteristic
are the fluorinated ethylene-propylene copolymers (FEP).
FEP is produced commercially by the copolymerisation of tetra/luoroethylene and
perfluoropropylene in approximately 6:1 mole ratio. The polymer is an insoluble,
semi-crystalline thermoplastic (crystallinity 50-60%) with a glass transition
temperature (Tg) of approximately -10" C (1).
The extent of photodegradation of a polymer will be determined by the absorption
867
https://ntrs.nasa.gov/search.jsp?R=19930019072 2020-03-17T05:50:44+00:00Z
characteristicsof the material. Optically,
FEP is transparent in the visible region
of the spectrum, but the absorbance
begins to rise slowly below 300 nm. A
complete absorption spectrum of FEP is
not available in the literature, but the
absorption spectrum of
polytetrafluoroethylene (FFFE) has
been reported,(see Fig.l), and that of
FEP would be similar. The spectrum of
PTFE is characterised by a strong
absorption at 161 nm, which has been
assigned to a transition from the
HOMO levels for the C-C and C-F
bonds to a conduction band (3). This
peak has a long tail extending into the
UV region. Below 130 rim, the
absorbance of FEP again increases, due
to the onset of the continuous
absorption region of the material.
o_
I I 1 I
100 125 150 175 200 225
Wavelength (rim)
K,Seki et ol Phyg/co Scfipto. 41,167.1990.
Figure 1: VUV absorption
spectrum of FIFE.
Photoinduced degradation can only occur at wavelengths where there is a
significant overlap between the solar spectrum and the absorption of the material (4).
In the VUV region there is a significant overlap between the absorption spectrum of
FEP and Lyman-a line of atomic hydrogen and the continuous solar emission at
wavelengths above 150 nm (2).
Two reports published recently by NASA have dealt with studies of the effect of 5
years and 9 months exposure of FEP thermal blankets to the Low-Earth-Orbit
environment (LEO) on the Long Duration Exposure Facility (LDEF)(1 and 5).
Stiegman et al.(1) investigated the synergistic effect of VUV and atomic oxygen on
the FEP surface. They concluded that samples which received only VUV radiation
possessed a hard embrittled surface layer, that was absent in the samples which were
exposed to VUV and atomic oxygen, and also in the unexposed control samples.
Stiegman suggested that this surface layer resulted from a "synergistic" effect between
VUV and atomic oxygen. Young and Slemp (5) concluded from the XPS, FYIR and
thermal analyses that there was no significant change at the molecular level for FEP
thermal blankets exposed to LEO. However, various microscopic analyses revealed a
roughening of the surface due to atomic oxygen erosion, which resulted in some
materials changing from specular reflectors to diffuse reflectors (5).
As part of a materials evaluation program for space applications, we have studied
the effects of UV-VUV induced degradation processes in FEP virgin polymer
(FEP100) and unetched commercial films. The studies involve the photo-generation
of radical species in the polymer matrices as the initial steps in the degradation
process. It is the aim of this paper to obtain molecular level information for the initial
stages of photo-degradation processes by using ESR spectroscopy and also to assess
the effect of VUV radiation and atomic oxygen on FEP surfaces using Scanning
Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS).
868
EXPERIMENTAL
Samplesof FEP commercial films and FEP100 virgin polymer beads were
irradiated in vacuum in Spectrosil-A grade quartz ESR tubes (i.d. 3.0 ram), at room
temperature usinig a high power xenon lamp (wavelength > 200 nm). The radical
concentrations,[R-], were determined by ESR spectroscopy (Bruker ER-200D),
utilising a strong pitch standard reference (3.0 x 10TM spin/cm). Spectra were
determined at room temperature using Klystron frequency of 9.25 GHz, microwave
power of 27db and magnetic field strength of 0.33 Tesla.
For surface analysis, ( XPS and
SEM), samples of FEP films were
exposed to VUV radiation and
atomic oxygen, which are generated
using an oxygen plasma source
designed specially for this purpose.
The plasma tube was equipped with
an earthed fine metal grid, which
allows the passage of VUV radiation
and atomic oxygen only, and prevents
the plasma from reaching the
polymer. Furthermore, the source was
fitted with a removable MgF z window
so that the polymer films could be
separated from the flow of the atomic
"-.--. t0mln
80 rain Ill " t_-004140 I
(...-- le0 rain
I I I
3200 3350 3400
Magnetic field (G)
Figure 2: ESR spectra of FEP100 polymer
exposed to UV radiation and air.
oxygen stream, to allow exposure to VUV radiation only. A Perldn-Elmer model 560
XPS/SAM/SIMS multi-technique surface analysis system and a JEOL model JSM
6400F Scanning Electron Microscope were used to evaluate the effects of VUV
radiation and the synergistic effects of VUV and atomic oxygen on the FEP films.
RESULTS AND DISCUSSION
ESR Studies:
Samples of FEP film and virgin polymer beads were irradiated in vacuum by a
high power xenon lamp (Intensity = 9.1 mW/cn_ ), using quartz tubes at room
temperature.
The ESR spectra of the radicals formed on exposure of the virgin FEP polymer
beads (FEP100; obtained from DuPont Australia) to UV radiation (10-180 rain.) in
vacuum at room temperature are shown in Fig.2. The spectra are characterised by a
triplet with a splitting of 17-18 G and g = 2.004. This triplet is identical to that
observed by Kim and Liang (6), who assigned it to the chain scission radical I.
869
I
-C-
-SF )oCF 2 -CF2 -CF-CF 2 -
I II HI
There was no evidence for the
formation of radicals of the type II and
III, which is also in agreement with Kim
and Liang for irradiation with a xenon
lamp.
The concentration of the chain
scission radical increases with exposure
time up to approximately 180 minutes,
as shown in Fig.3. For exposure times
beyond approximately 200 minutes, the
radical concentration started to decline.
This decrease in radical concentration
4
o
o_
o
2
,e
i i i i i i
O_ t I i i I I
0 50 I O0 150 200 250 300 350
Exposure Time (rain)
Figure 3: The effect of exposure time on
radical concentration for FEP100.
could be due to a decrease in the concentration of the species responsible for radical
formation.
Samples of irradiated virgin polymer (FEP100) stored in the dark under vacuum
show a rapid decrease in the radical concentration due to recombination. After 24 h
in the dark at room temperature all the chain seission radicals had decayed.
Samples of virgin polymer exposed to UV radiation under vacuum and then
exposed to air have an ESR spectrum with a centre field shifted by 19 G to lower
field (g = 2.016). The original triplet spectrum found for irradiation under vacuum is
transformed into a broad singlet with a A _ of 9.3 G.
This spectrum can be
assigned to the peroxy radical IV
(6). A similar peroxy radical is
formed on UV exposure of the
virgin polymer in air.
-CF2OO" 15 mtn-
IV
The ESR spectra of the FEP
polymer film shows that samples
irradiated for 15 min. produce a
I I 1 I |
3200 3250 3300 3350 3400
Magnetic field (G)
F'_u'e 4: ESR spectra of FEP filmexposed to UV
radiation.
radical characterised by a triplet
structure with an 18 G splitting (see Fig.4). This triplet is identical to that found for
the virgin polymer, and is due to the chain scission radical I. On the other hand, the
ESR spectra of the same polymer film irradiated for 1-60 h produces a completely
870
different spectral shape. The outer peaks of the triplet merge into the central peak.
This behaviour could be explained by photodegradation of the FEP to form the
chain scission radical, followed by reaction of this radical with an impurity present in
the polymer film to form a new and relatively stable carbon centred radical. This
impurity in the FEP polymer film could be for example a processing aid used in
preparation of the film. The processing aid could be an antioxidant or stabiliser.
12
Furthermore, it was observed from
this experiment that there was a fast
build up of radical concentration in the
first few minutes (2-15 rain) of
irradiation and then a sharp drop as the
irradiation continued to about 75 min.,
see Fig.5. This decrease in the radical
concentration coincided with the
transition from the triplet ESR spectrum
to a broad singlet. After approximately
75 minutes, the radical concentration
then started to build up to higher values
with increasing UV irradiation time (see
Fig.5). This behaviour might be related
to a time delay between the generation
of the first radical species and the
activation of the radical scavenger.
I I I i I
10
0
v 8
6
= j o/
8 6q
,O_'_O_
•4 6 J--
0 i
0 2 4
Oq i I I I t
0 10 20 30 40 50 60
Exposure Time (it)
Figure 5: The effect of UV exposure time
on radical concentration of FEP film.
0
v
o
0
7
6
5
4
3' I
0 1O0 200 300 400
Ageing Time at 60°C (h)
Figure 6: The effect of ageing at 60" C on
radical concentration of FEP film exposed
to UV radiation.
On ageing, the time dependence of
the radical concentration at 60" C
exhibited a relatively sharp drop (55%)
in the radical concentration up to 100 h
(see Fig.6). However, longer ageing to
about 400 h produced no significant
further change in the radical
concentration. A similar trend was
observed for the room temperature
ageing of the UV irradiated polymer
film, however the decay rate in the
radical concentration was, as expected,
slower than that for 60"c ageing (30%
upto 100 h). This behaviour suggests
that some of the secondary radicals are
restricted from decomposition by their
location in the polymer matrix.
The radicals responsible for the
broad singlet species (g = 2.004) formed
in the FEP polymer film were stable on exposure to air. Thus, their behaviour
contrasts with that observed for the radicals formed by irradiation of virgin polymer,
which reacted readily with oxygen to form peroxy radicals (g = 2.016).
871
SurfaceAnalysis:
Surfaceanalysesof FEP films were
conducted using XPS spectroscopy and
SEM.
The XPS multiplex spectrum at 25
eV pass energy of unirradiated FEP film
for the C(ls) and F(ls) regions!is
shown in Fig.7. Using a set of standard
sensitivity factors(7) the ratio of C:F was
found to be 1:2, as expected for FEP.
Thus, there is no significant
contamination of the surface layer of the
film, but a close examination of the
C(ls) peak reveals a small tail in the
lower binding energy region, containing
approximately 3.6 percent of the total
peak area. The reason for this tail is
unclear, but possible contributing
sources could include crosslinks, end
groups, branches and oxidation of the
polymer, although there was no evidence
of a significant O(ls) peak. Curve fitting
of the carbon peak yielded the analysis
2
Q)
_=
0
t¢
O 6
c.
i
I I I l I
294 292 290 288 286
I
C ls
I
284 282
i i i i i i
I i i i i I
694 692 690 688 686 684 682
Figure 7:
FEP film.
Binding Energy (eV)
XPS spectrum of unirradiated
shown in Table 1. In this simulation, two minor peaks at 286.6 and 288.3 eV were
used to account for the tail, but they are unassigned.
Table 1: Percent composition of the FEP film from curve fitting of surface analysis
data.
Binding
Energy(eV)/
Chem.Bond
285.0/C-H
286.6/C-C,C-O
288.3
289.5/CF-CFo
Unexposed
FEP
(%)
287.8/c=o
1.2
2.4
8.5
FEP Exposed
to VUV &
AO for 15
rain. (%)
0.2
1.2
2.5
9.5
FEP Exposed
to VUV &
AO for 75
rain. (%)
0.8
1.4
2.7
9.9
FEP
Exposed to
UV rad.
then air (%)
1.5
13.8
5.7
2.7
7.1
291.2/CF 2 79.5 76.0 73.8 64.0
293.3/CF 3 8.4 9.6 9.8 5.7
1.0 1.0
872
The curve fitting analysis is consistent
with a tetrafluoroethylene :
periluoropropylene ratio in the polymer
film of 4.2:1. This ratio in the surface is
lower than that generally reported for
the composition of FEP copolymers,
which is 6:1.
Exposure of FEP film to VUV
radiation in the presence of atomic
oxygen results in the oxidation of the
polymer surface. This is revealed in the
XPS spectrum, as demonstrated in Fig.8
by the presence of an oxygen peak.
Furthermore, it was observed that the
oxygen peak is very broad, which
suggests the possible presence of various
oxygen species. It is also revealed by an
increase in the tail of the carbon peak.
The curve fitting analysis of the carbon
peak is given in Table 1. This increase
in the area of the tail has been
accounted for by inclusion of an
additional peak at 287.8 eV, which has
been assigned tentatively to formation of
a carbonyl moeity. (The intensities of
the unassigned peaks at 286.6 and 288.3
remain constant within experimental
error). The important trends observed in
Table 1 with increasing irradiation time
are: an increase in the percentage of
carbon attached to oxygen, a decrease in
the percentage of CF z and an increase
in the percentage of CF and CF]. This
suggests that oxidation takes place at the
fluorinated ethylene units.
I. 6
"t:l 4
_3 2
E
L
O
:z o
i ! i I i i i
i J i t J I i
296 294 292 290 288 286 284
J i I i i i i
i i J i i I i
694 692 6go 688 686 684 682
.L /%
0 1 I " i I I I "I - _"I _
540 538 536 534 532 530 528
Binding Energy (eV)
Figure 8: XPS spectrum of FEP film
exposed to VUV and atomic oxygen.
2.0
4
1.9
0
1.8
p,,
The changes in the F(ls):C(ls) and
O(ls):C(ls) ratios with increasing _ 1.7
irradiation time and atomic oxygen flux
are shown in Table 2 and in Figs. 9 and
10, respectively. As the irradiation time 16
increases, the fluorine to carbon ratio
decreases and the oxygen to carbon 15
ratio increases. At longer irradiation
times, the rate of change in these ratios
decreases, and the ratios appear to
approach constant values. This
observation would be consistent with a
I I I I
40 80 120 1 60 200
Exposure Time (min)
Figure 9: The change of F(ls)/C(ls) ratio
in FEP film exposed to VUV and atomic
oxygen.
873
gradual erosion of the polymer surfaceby the radiation, with a concurrent loss of
small molecule products.
0.04
The oxygen atom flux was varied by
changing the oxygen pressure in the
plasma tube and by introduction of a .o 0.03
MgF 2 window between the plasma and
the polymer film. As shown in Fig. 10, ._
the rate of oxidation of the polymer .7. o02
tO
surface increases with the atomic oxygen /
flux. -_
O
The XPS carbon spectrum (Fig. 11)
of FEP polymer film irradiated by UV
(xenon lamp) in vacuum and exposed to
air immediately after irradiation showed
about 1% oxygen atom concentration in
the surface. This was attributed to the
formation of C=O. Furthermore,
additional peaks were observed in the
C-C (286.6 eV)(8) and C-H (285.0 eV)
binding energy regions of the XPS
0.01
0.00:
0 200
• (2) ...e
• 0 _
/ ./
-J
//-
(a)
A-
._ i .I,r'li I I
40 80 120 160
Exposure Time (rain)
Figure 10: The effect of high(l),
medium(2), and zero(3) oxygen atom flux
on the surface of FEP film exposed to
VUV radiation.
spectrum, which might be assigned to the formation of crosslinks between polymer
chains and to the presence of hydrocarbon contamination, which might have resulted
from the increased susceptibility of the oxidised polymer surface to laboratory
contamination. A similar small hydrocarbon peak was observed in the FEP films
exposed to VUV and atomic oxygen (see Table 1).
Compared with the films irradiated using VUV, the films irradiated with UV then
exposed to air have a lower surface F(ls)/C(ls) ratio and a higher concentration of
crosslinks. The O(ls) peak also appears narrower, suggesting a different distribution
of oxidation products, which arise via peroxide formation in this case.
Table 2: XPS results for FEP polymer films exposed to simulated and actual space
environment.
Peaks
Cls(eV)
AC (%)
Fls(eV)
AC (%)
01s(eV)
AC (%)
I
a: Data obtai
FEP
Type A
291.1
34
688.6
66
0
md from b
FEP
(vUV &
AO)
291.1
35.2
688.6
_.1
534.4
0.7
ASA teclmi
AC: Atomic concentration (%).
FEP(UV>
200 m'n)
and air
291.6
40
688.8
59
532
1.0
al report no.:
Control
8
290.9
31.6
689
65.6
AIO
Opaque
290.9
28.5
688.5
69.8
530.9
1.3
C8
•Opaque
290.9
30.4
688.8
66.2
<0.5
04096 (5).
874
The surface analysis of the FEP
polymer film obtained in the
6
simulated space environment used
in the experiments described here
are compared in Table 2 with 4
those reported by Young and
Slemp (5) for FEP polymer films 2
exposed to the space environment
in the LDEF experiments, o
SEM micrographs of FEP film _
6
before and after exposure to VUV 0_
and atomic oxygen showed a
4
dramatic change in the polymer -o
surface (see Figs. 12). These ._
micrographs show the extensive _ 2
loss of polymer material during the _.
O 0VUV and atomic oxygen z
irradiation of the film. The texture
of the surface becomes rough with
regular undulations similar to sand 6
dunes. Thus, as the polymer erodes
and "new" surfaces are exposed,
the chemical composition of the
surface would be expected to 2
approach a constant average value;
as found by the XPS analysis, o
C ls
I I I I 1 I I I
296 294 292 290 288 286 284 282
1 i I i i i i i
F ls
I I I I I I I I
696 694- 692 690 688 686 684 682
i i i i i i
0 ls
538 536 534 532 530 528 526
Binding Energy (eV)
Figure 11: XPS spectrum of FEP film exposed
to UV radiation and air.
Figure 12: SEM micrograph of FEP film before and after exposure to VUV
and atomic oxygen.
875
REFERENCES
1- A.E. Stiegman, D.E. Brinza, M.S. Anderson, T.K. Minton, and E.G. Hang, Jet
Propulsion Laboratory Publication, 91-10, California Institute of Technology;
Pasadena, California, USA.
2- D. Heath, and M. Thekaekara, The Solar Output and its Variation, Colorado
Associated University Press, Boulder, CO, 1977, p. 193.
3- K. Seki, H. Tanaka, T. Ohta, Y. Aold, and A. lmamura, Physica Scripta; 1990, 41,
167.
4- NJ. Turro, Modem Molecular Photochemistry, Benjamin/Chammings Publishing,
Menlo Park, CA, 1979.
5- P.R. Young and W.S. Slemp, NASA Technical Memorandum 104096, Dec.1991.
6- S.S. Kim and R.H. Liang, Polym. Prepr.(Am.Chem.Soc., Div. Polym. Chem), 1990,
31, 389.
7- RJ. Ward and BJ. Wood, A comparison of experimental and theoretically derived
sensitivity factor for XPS. Surface and Interface Analysis, 1992, 18, 679.
8- D.R. Wheeler and S.V. Pepper, J. Vac. Sci. Technol. 1990, A8(6), 4046.
Acknowledgement
We would like to acknowledge support of this work by the National Aeronautics and
Space Administration (NASA), langley Research Center.
876


A study of the UV and VUV degradation of FEP

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