Microstructural changes in Polyetherether-ketone (PEEK) films were investigated in the temperature ranges of 23 to -196 C, using Positron Lifetime Spectroscopy (PLS) technique. It was determined that the total free volume decreases by about 46 percent in amorphous PEEK samples and about 36 percent in semicrystalline PEEK samples when they are cooled down from room temperature to liquid nitrogen (LN2) temperature. If this trend in reduction in free volume with decreasing temperature continues, as expected, it is surmised that PEEK will be able to withstand cooling down to liquid hydrogen (LH2) temperature without any detrimental effect on its diffusivity for liquid hydrogen

Eftekhari, Abe

Singh, Jag J.

Sprinkle, Danny R.

St.clair, Terry L.

English

NASA Technical Reports Server

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_Technical
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Investigation of
Microstructural Changes
in Polyetherether-Ketone
Films at Cryogenic
Temperatures by Positron
Lifetime Spectroscopy
Jag J. Singh,
Abe Eftekhari,
Terry L. St. Clair,
and Danny R. Sprinkle
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MICROSTRUCTURAL CHANGES IN NQI-L_21o
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NASA
Technical
Paper
3064
1991
Nationat Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Division
Investigation of
Microstructural Changes
in Polyetherether-Ketone
Films at Cryogenic
Temperatures by Positron
Lifetime Spectroscopy
Jag J. Singh
Langley Research Center
Hampton, Virginia
Abe Eftekhari
Analytical Services & Materials, Inc.
Hampton, Virginia
Terry L. St. Clair and Danny R. Sprinkle
Langley Research Center
Hampton, Virginia
The use of trademarks or names of manufacturers in this
report is for accurate reporting and does not constitute an
official endorsement, either expressed or implied, of such
products or manufacturers by the National Aeronautics and
Space Administration.
Abstract
Microstructural changes in polyetherether-ketone
(PEEK) films have been investigated in the temper-
ature range of 23°C to -196°C by using positron
lifetime spectroscopy. It has been determined that
the total free volume decreases by about 46 per-
cent in amorphous PEEK samples and about 36 per-
cent in semicrystalline PEEK samples when they
are cooled from room temperature to the tempera-
ture of liquid nitrogen. If this trend in reduction
of free volume with decreasing temperature contin-
ues, as expected, PEEK should be able to withstand
cooling to the temperature of liquid hydrogen with-
out any detrimental effect on its diffusivity for liquid
hydrogen.
Introduction
Polyetherether-ketone (PEEK) is a high-temper-
ature thermoplastic polymer with excellent proper-
ties for a wide range of demanding applications.
When heated above its melting temperature Tm and
then rapidly quenched to ambient or lower tempera-
tures, the polymer can be isolated in its amorphous
form, which has a glass transition temperature Tg
of 143°C. Once the amorphous form of the material
is exposed to a temperature between Tg and Tin, it
rapidly transforms to a semicrystalline form, which
has a greatly suppressed glass transition tempera-
ture. The chemical structure of PEEK is shown in
figure 1.
Polyetherether-ketone is a strong candidate for
the matrix of the fiber-reinforced composites under
consideration for the slush hydrogen fuel tank for the
National Aero-Space Plane (NASP). However, there
is some concern that PEEK might undergo molecu-
lar rearrangements under cryogenic conditions which
may result in fuel leakage. Positron lifetime spec-
troscopy (PLS) has been used to investigate morpho-
logical features of PEEK as a function of temperature
in the range of 77K to 300K, and the results of these
measurements are described herein.
Symbols
A bar over a symbol indicates an average value.
C
F
Ii, I2, Ia
LN2
microstructural constant
= .f2+f3
free-volume fractions associated with
second and third lifetime components
intensities of first, second, and third
lifetime components
liquid nitrogen
PEEK
T.
Tm
v:2,v:a
rl, r2, r3
polyetherether-ketone
glass transition temperature, °C
melting temperature, °C
free-volume cell size for second and
third lifetime components
lifetimes of first, second, and third
components
Experimental Procedure
Ten-mil-thick PEEK films were obtained from
ICI Films, a business subsidiary of ICI Americas,
Inc. The as-received films were unoriented, noncrys-
talline, and transparent. The semicrystalline variety
of these films was prepared by heating them to 160°C
for 1 hour and allowing them to cool to room temper-
ature. The presence of crystallinity was evidenced by
the onset of opacity in the films.
The Na 22 positron lifetimes in the test films
were measured in order to infer the concentration
and size distributions of free-volume cells in them.
(See refs. 1 and 2.) Since the single-film thickness
(0.25 mm thick) was inadequate to stop the majority
of Na 22 positrons, the films were folded three times to
make test coupons that were 0.75 mm thick, 2.5 cm
long, and 2.5 cm wide. The Na 22 source foil was
sandwiched between two test coupons, and positron
lifetime spectra in them were measured by using
the standard fast-fast coincidence measurement tech-
nique (ref. 3). However, before starting extensive life-
time measurements in the test coupons, spectra were
also measured in single 0.25-mm-thick films by using
a recently developed (refs. 4 and 5) slow positron
beam generator. These spectra were then com-
pared with the lifetime spectra in the 0.75-mm-thick
coupons. The results are summarized in table I. It
is obvious that the results are in very good agree-
ment. This indicates that the three films in the test
coupons are in intimate contact with each other. (Or,
at least, the annihilation lifetimes in the noncontact
islands (free-volume cells) at the interface are outside
the experimental time window of 50 nsec.) After this
test, all measurements were made with three-fold test
coupons in standard PLS geometry for thick targets,
since this arrangement permits easier sample temper-
ature control.
Positron lifetime measurements in both the amor-
phous and semicrystalline PEEK test coupons were
made at a series of decreasing temperatures, begin-
ning with the ambient temperature.
Amorphous Films
The PLS measurements in as-received PEEK film
samples were made at a steady-state temperature of
23°C(roomtemperature),-12°C, -40°C, -62°C,
and- 196°C.The-12°C and-40°C measurements
weremadebyusingthesystemillustratedin figure2,
while the -62°C and -196°C measurementswere
madewith thesystemillustratedin figure3. There-
sultsin amorphousfilmsaresummarizedin tableII.
SemicrystallineFilms
Thesemicrystallinefilmswerepreparedbyheat-
ingtheamorphousfilmsto 160°Candthenallowing
themto coolto roomtemperature.Noattemptwas
madeto releaseany thermalstressthat mayhave
beeninducedin thetest samplesto bettersimulate
realserviceconditions.Lifetimemeasurementswere
madeatroomtemperaturebeforeheatingthefilmsto
160°Candaftercoolingthemto roomtemperature.
Followingroom-temperaturemeasurements,lifetime
measurementsweremadeat thesametemperatures
asamorphousfilms. Theseresultsaresummarized
in tableIII.
On the basisof lifetime data, the free-volume
fraction (ref. 6) associatedwith microvoidsin the
PEEK sampleshasbeencalculated. The results
aresummarizedin tableIV. It is obviousthat the
total free-volumefractiondecreaseswith decreasing
temperaturein bothphasesof thePEEKsamples.
Results and Discussion
The microstructuresof the two phases,aswell
as the effectsof temperatureon them, arediffer-
ent. For example,the sizesof the two amorphous
samplemicrovoidpopulationgroupsaverageabout3 3 2 39.9× 10- nm and8.3× 10- nm at roomtem-
perature,whereasthe correspondingsizesin the
semicrystallinesamplesare 1.7x 10-3 nm3 and
8.8× 10-2 nm3. As thesampletemperatureis low-
eredfrom roomtemperature(23°C)to LN2 tem-
perature(-196°C), the smallerholesizeincreases
from9.9x 10-3 nm3to 1.2x 10-2 nm3in theamor-
phoussample,andit goesfrom 1.7× 10-3 nm3 to
8.9× 10-3 nm3 in the semicrystallinesample.The
largermicrovoidsremainunchangedin sizewithde-
creasingtemperaturein both phases,thoughtheir
concentrationsaredrasticallyreduced.Thetotal free
volumeassociatedwith both groupsof microvoids
in both typesof samplesof PEEKhasbeencalcu-
lated. It goesdownfrom 0.50percentto 0.26per-
centin the amorphousamplesandfrom 0.44per-
centto 0.28percentin the semicrystallinesamples
asthe sampletemperaturesareloweredfromroom
temperatureto LN2temperature.Theseresultsare
illustratedin figures4 to 9.
Severalinterestingconclusionscanbedrawnfrom
thesefigures:(1)Theconcentrationof bothtypesof
microvoidsdecreaseswithdecreasingtemperaturein
bothphasesof PEEK; (2) Thefree-volumefraction
associatedwith largermicrovoidsdecreases,andthe
fractionassociatedwithsmallermicrovoidsincreases,
as the sampletemperatureis lowered;(3) There
is a net decreasein total free-volumefractionwith
decreasingtemperaturein both phasesof PEEK.
The last two conclusionsindicatethat someof the
largermicrovoidsdisappearaltogether,whereasa
smallerfractionof themreappearas intermediate-
sizedmicrovoids.
Fromthe precedingdiscussion,it appearsthat
PEEK undergoesconformationalchangeswhenits
temperatureis lowered.However,theseconforma-
tionalchangesappearto reducethefree-volumefrac-
tion in thematerialandthereforemakeit le,_slikely
to sufferdeleteriouseffectson its diffusivityfor hy-
drogenwhencoolingto LH2temperature.
Concluding Remarks
The morphologyof amorphousand semicrys-
talline polyetherether-ketone(PEEK) sampleshas
beeninvestigatedat a numberof temperaturesin
the rangeof 23°Cto -196°C.Free-volumefraction
in PEEKdecreasesasits temperatureis loweredto
that of liquid nitrogen(LN2). Consequently, PEEK
should not suffer any adverse effects on its diffusivity
for hydrogen when exposed to LH2 temperature if, as
expected, the trend in reduction in free volurne with
decreasing temperature continues.
NASA Langley Research Center
Hampton, VA 23665-5225
February 7, 1991
References
1. Eldrup, Morten: On Positron Studies of Molecular Crys-
tals. Positron Annihilation, Paul G. Coleman, Suresh
C. Sharma, and Leonard M. Diana, eds., North-Holland
Publ. Co., 1982, pp. 753 762.
2. Nakanishi, H.; and Jean, Y. C.: Positrons and Positro-
nium in Liquids. Positron and Positronium CKemistry,
D. M. Schrader and Y. C. Jean, eds., Elsevier Science
Publ. Co., Inc., 1988, pp. 159 192.
3. Singh, Jag J.; Mall, Gerald H.; and Sprinkle, Danny R.:
A_mlysis of Positron Lifetime Spectra in Polymers. NASA
TP-2853, 1988.
4. Singh, Jag J.; Eftekhari, Abe; and St. Clair, Terry L.:
A Slow Positron Beam Generator for Lifetime Studies.
NASA TM-101590, 1989.
5. Singh, Jag J.; Eftekhari, Abe; and St. Clair, Terry L.:
Low Energy Positron Flux Generator for Lifetime Studies
in Thin Films. Nucl. Instrum. _ Methods Phys. Res.,
vol. B53, Mar. 1991.
6. Nakanishi, H.; Jean, Y. C.; Smith, E. G.; and Sandreczki,
T. C.: Positronium Formation at Free-Volume Sites in the
Amorphous Regions of Semicrystalline PEEK. J. Polymer
Sci., Pt. B: Polymer Phys., vol. 27, no. 7, 1989, pp. 1419
1424.
TableI. Comparisonof LifetimeData in SingleandThree-FoldPEEKFilms
at RoomTemperature
Single-layerfilm; positron Triple-layerfilm; conventional
beammeasurement lifetimemeasurement
T1//1, T2/I2, r3/I3, T1/I1, T2/I2, 7"3/]3,
psee/percent psee/percent psec/pereent psec/percent psec/percent psee/pereent
274+6/524-2 561+23/25±3 1841=t=21/23±2 268+8/51+1 569+19/26=1=2 1823±27/23+2
Table II. Summary of Results in Amorphous PEEK at Various Temperatures
T1/I1, T2/I2, T3/I3 b,
Temperature a, °C psec/percent psec/percent psec/percent
278±11/53±3 589i39/25±223
-12
-40
-62
-196
312±8/68±2
344±9/76±2
381±8/75±2
334±8/79±2
825±32/17±2
858±25/10±2
850±40/10±2
879&38/11±2
1854±30/22±2
1854±30/15±1
1854+30/14±1
1854±30/15±1
1854±30/10+1
aTemperature errors are <_l°C.
bOrtho-positronium lifetime has been held constant at room-temperature average value
of 1854±30 psec, since its value at different temperatures changes by less than experimental
error.
Table III. Summary of Results in Semicrystalline PEEK at Various Temperatures
Temperature a, °C
23
-14
-35
-60
-196
rl/Ii,
psec/percent
290±5/57±2
275+9/65±2
310±12/67±2
316+10/70+2
328±9/77±2
I2,
psec/percent
616±22/25±2
596±33/21±2
643±38/20±2
675±38/17±2
803±28/13±2
/ I3b,
psec/percent
1914±19/18±1
1914±19/14±1
1914±19/13±1
1914±19/13±1
1914±19/10±1
aTemperature errors are <l°C.
bOrtho-positronium lifetime has been held constant at room-temperature average value
of 1914±19 psec, since its value at different temperatures changes by less than experimental
error.
TableIV. Comparisonof MicrovoidSizesof AmorphousandSemicrystallineSamplesof
PEEK at Various Temperatures
[Errors in f2, f3, and F are <5 percent]
Temperature,
°C
Small microvoids
T2/I2, f2a,
psec/percent percent
Large microvoids
_/5, _,
psec/percent percent
Total free volume c,
F, percent
Amorphous sample
23
-12
-40
-62
-196
589/25
825/17
858/10
850/10
879/11
0.01
.04
.04
.03
.04
1854/22
1854/15
1854/14
1854/15
1854/10
0.49
.35
.33
.34
.23
0.50
.39
.37
.37
.27
23
-14
-35
-60
-196
Semicrystalline sample
616/25 ] 0.01
596/21 .01
643/20 .01
75/17 . 2
803/13 .04
1914/18
1914/14
1914/13
1914/13
1914/10
0.43
.34
.32
.32
.24
0.44
.35
.33
.34
.28
aFree-volume fraction f2 = CI2V.f2
bFree-volume fraction f3 = C[3Vf3
CThe constant C has been obtained by equating F = ./'2 + f3 to the reported value of saturation
moisture pickup in PEEK (amorphous).
OFigure 1. Chemical structure of PEEK.
LN2 (or dry ice in acetone)
,rofoam
cylinder
chamber
Source/film/ extension
thermocouple
Figure 2.
I
Detector 1 Detector 2 t
Schematic diagram of sample assembly for lifetime measurements at -12°C and -40°C.
LN2 (or dry ice in acetone)
_-- Styrofoam
cylinder
:hamber
Sou rce/film
assembly
I n Ii!Detector 1 Detector 2
Figure 3. Schematic diagram of sample assembly for lifetime measurements at -62°C and -196'_C.
6
1.4 m
1.2
.8
_2,
nsec
.6
.4
.2 - _ Amorphous
Semicrystalline
0 I ! I I I I
50 0 -50 -100 -150 -200 -250
Temperature, °C
Figure 4. Second-component lifetime versus temperature in PEEK samples.
50-
40
30-
12,
percent
20-
10-
0
50
Figure 5.
Amorphous
¢ Semicrystalline
T
1 I I I I I
0 -50 -100 -150 -200 -250
Temperature, °C
Second-component intensity versus temperature in PEEK samPles.
7
50-
4O
3O
13,
percent
20-
10-
0
5O
_3, psec Vf, nm 3
Amorphous 1854 8.3 x 10-2
Semicrystalline 1914 8.8 x 10-2
I I I I I l
0 -50 -100 -150 -200 -250
Temperature, °C
Figure 6. Third-component intensity versus temperature in PEEK samples.
1.25
1.00
.75 -
percent
.50 -
.25
0
5O
Large microvoids, f3
Small microvoids, f2
l ! I I I
0 -50 -100 -150 -200
Temperature,°C
Figure 7. Microvoid fraction versus temperature in amorphous samples.
.125
.100
- .075
f2,
percent
- .050
- .025
0
-250
1.25- - .125
1.00
.75
f3,
percent
.50
.25
Large microvoids, f3
Small microvoids, f2
0 I I I I I
50 0 -50 -100 -150 -200
Temperature, °C
-.100
-.075
f2,
percent
-.050
-.025
0
-250
Figure 8. Microvoid fraction versus temperature in semicrystalline samples.
1.25 -
1.00
.75 -
F,
percent
.50 -
.25 -
0
5O
Figure 9.
Amorphous
Semicrystalline
I I I I I
0 -50 -100 -150 -200
Temperature, °C
Total free-volume fraction versus temperature in PEEK samples.
I
-250
9
Report Documentation Page
NalionalAeronaulic$a_
Space Adm;ni_trat_on
13. Recipient's Catalog No.1. Report No. [ 2. Government Accession No.
NASA TP-3064 J
4. Title and Subtitle
Investigation of Microstructural Changes in Polyetherether-Ketone
Films at Cryogenic Temperatures by Positron Lifetime
Spectroscopy
7. Author(s)
Jag J. Singh, Abe Eftekhari, Terry L. St. Clair,
and Danny R. Sprinkle
9. Performing Organization Name and Address
NASA Langley Research Center
Hampton, VA 23665-5225
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, DC 20546-0001
5. Report Date
March 1991
6. Performing Organization Code
8. Performing Organization Report No.
L-16841
10. Work Unit No.
506-43-21-05
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Paper
14. Sponsoring Agency Code
15. Supplementary Notes
Jag J. Singh, Terry L. St. Clair, and Danny R. Sprinkle: Langley Research Center, Hampton,
Virginia.
Abe Eftekhari: Analytical Services & Materials, Inc., Hampton, Virginia.
16. Abstract
Microstructural changes in polyetherether-ketone (PEEK) films have been investigated in the
temperature range of 23°C to -196°C by using positron lifetime spectroscopy. It has been
determined that the total free volume decreases by about 46 percent in amorphous PEEK
samples and about 36 percent in semicrystalline PEEK samples when they are cooled from room
temperature to the temperature of liquid nitrogen. If this trend in reduction of free volume with
decreasing temperature continues, as expected, PEEK should be able to withstand cooling to
the temperature of liquid hydrogen without any detrimental effect on its diffusivity for liquid
hydrogen.
17. Key Words (Suggested by Author(s))
Polyetherether-ketone (PEEK)
Cryogenic temperatures
Microstructural changes
Positron lifetime spectroscopy
Free-volume fraction
Liquid hydrogen diffusion
19. Security Classif. (of this report)
Unclassified
18. Distribution Statement
Unclassified Unlimited
Subject Category 24
20. UnclassifiedSecurityClassif. (of this page) 21. NO.l2 of Pages 22. A02Price
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NASA-Langley, 1991


Investigation of microstructural changes in polyetherether-ketone films at cryogenic temperatures by positron lifetime spectroscopy

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