Positron lifetime measurements were made in two thermoplastic polyimide materials recently developed at Langley. The long component lifetime values in polyimidesulfone samples are 847 + or - 81 Ps (dry) and 764 + or - 91 Ps (saturated). The corresponding values in LARC thermoplastic imides are 1080 + or - 139 Ps (dry) and 711 + or - 96 Ps (saturated). Clearly, the presence of moisture has greater effect on positron lifetime in LARC thermoplastic imides than in the case of polyimidesulfones. This result is consistent with the photomicrographic observations made on frozen water saturated specimens of these materials

Holt, W. H.

Mock, W., Jr.

Singh, J. J.

Stclair, T. L.

English

NASA Technical Reports Server

NASA Technical Memorandum 84532
I
NASA-TM-84532 19820023570
POSITRONLIFETIMESTUDIESINTHERMOPLASTIC
POLYIMIDETESTSPECIMENS
JAGJ,SINGH, TERRY L. ST. CLAIR, WILLIAM H,HOLT,
AND WILLIS MOCK,JR.
JULY1982 _ '_'° ":";"__'
._, .-,r_ Ib_f
l LANGLEY[?r-ST;,E;r:_j CENTER
H.-.... xU,., ','1!',3:i'!:A
National Aeronautics and
Space Administration
LangleyResearchCenter
Hampton,Virginia 23665
https://ntrs.nasa.gov/search.jsp?R=19820023570 2020-03-21T07:51:38+00:00Z

POSITRON LIFETIME STUDIES IN THERMOPLASTIC POLYIMIDE
TEST SPECIMENS
BY
Jag J. Singh and Terry L. St. Clair
NASA Langley Research Center
Hampton, VA
and
William H. Holt and Willis Mock, Jr.
Naval Surface Weapons Center
Dahlgren Laboratory
Dahlgren, VA
ABSTRACT
Positron lifetime measurements have been made in two thermoplastic
polyimide materials recently developed at Langley. The long component life-
time values in polyimidesulfone samples are 847 _ 81 Ps (dry) and 764 _ 91 Ps
(saturated). The corresponding values in LARC-thermoplastic imides are
1080 + 139 Ps (dry)and 711 + 96 Ps (saturated). Clearly, the presence of
moisture has greater effect on positron lifetime in LARC-thermoplastic in[ides
than in the case of polyimidesulfones. This result is consistent with the
photomicrographic observations made on frozen water-saturated specimens
of these materials.
INTRODUCTION
We have been investigating the dependence of positron lifetime on the
moisture content of polymeric materials for several years. I-3 Generally, the
introduction of moisture in the test specimens causes a noticeable reduction
in the positron lifetime. This phenomenon, presumably, results from the
fact that the positrons have a tendency to be localized in regions with
lower than average ionic density--such as voids or free volume cells. 4 Some
of the so-localized positrons form positronium (Ps) atoms there. As the
water enters the medium, it forces the positronium wave function to spill
over on to the regions of normal molecular electrons, thereby facilitating
their pickoff annihilation. This effect is quite pronounced in epoxies and
polyamides studied. I-3 However, it is not so apparent in the polyimide
-- iY/L
specimens investigated mainly due to very low intensity of the long-life
component in their positron annihilation spectra. Low intensity of the long-
life component suggests low free volume, i.e., more closely linked molecular
chains characteristic of high strength polymers. NASA scientists have been
studying polyimide and graphite-fiber reinforced polyimide composites for high
temperature airframe applications for several years. Polyimides are also prime
candidates for future use in the Shuttle program. Recently, two promising
thermoplastic polyimide materials have been developed at Langley.5, 6 They are
polyimidesulfone and LARC-thermoplastic imide (TPI). These newly-developed
polymers are similar in many respects. They are almost identical in chemical
structure as seen in figure i, have similar adhesive strengths and behave
similarly on aging. However, it is easier to process polyimidesulfones than
LARC-TPI's. Furthermore, polyimidesulfones foam during processing whereas
LARC-TPI's do not. The nature of the foam is such that it appears to be due
to water or some other solvent coming out rather than the sulfone group. This
is rather puzzling since water is expected to be hydrogen-bonded in the
polyimidesulfone. It was therefore decided to investigate if the positron
annihilation behavior in these two materials is different. Positron lifetime
measurements were made in both specimens in dry and fully water-saturated
states. These measurements, as well as the other phenomena observed when
the water-saturated polyimide specimens were frozen, are described in the
following sections:.
EXPERIMENTAL PROCEDURE
a. Specimen Preparation.-The polyimide specimens were prepared in the
form of thin circular discs summarized in Table I.
Table I
Geometrical Details of the Test Specimen_
Spec. Geometrical Specimen
No. Material No. Dimensions Volume
1 Polyimidesulfone 1 Diameter = 2.864 cm
Thickness = 0.148 cm 0.954 cc
2 Diameter = 2.865 cm
Thickness = 0.135 cm 0.871 cc
2 LARC-TPI 1 Diameter = 2.869 cm
Thickness = 0.156 cm 1.009 cc
2 Diameter = 2.873 cm
Thickness = 0.149 0.966 cc
The specimens were weighed in the as-received condition (i.e., ambient state)
and then desiccated for 307 hours at ll0°C and 102 hours at 150°C. The
desiccated specimen weights were measured. The specimens were then saturated
by boiling them in distilled water at lO0°C for 162.83 hours and reweighed.
The specimen weights in various states are summarized in Table II.
Table II
Summary of Specimen Weights in Various States
Spec Weight of the Specimen, mg Moisture Content (w/o)
No. Material No. Dry Ambient !Saturated Ambient Saturation
I Polyimide- i 1327.55 1341.10 1367.13 1.02 2.98
sulfone
2 1225.87 1239.24 1262.50 1.09 2.99
2 LARC-TPI i 1373.17 1393.00 1412.22 1.44 2.84
2 1309.93 1332.94 1348.23 1.76 2.92
It is apparent from these results that the saturation moisture contents of the
two polyimides are about the same. However, the ambient moisture contents
of the specimens after they had been left in the laboratory air for about
1 month after their preparation are distinctly different, being much higher
in LARC-TPI than in polyimidesulfone. It would thus appear that though the
free volumes of the sample are roughly equal, the diffusion coefficient for
moisture penetration in thermoplastic imides is considerably larger than in
polyimidesulfone. We shall refer to this observation again, later in the
discussion of the results.
b. Positron Lifetime Measurement.-A 10-microcurie Na 22 positron source
was sandwiched between the two coupons of the test material. The source-test
coupon sandwich was wrapped in a thin (25.4 _aluminum foil and sealed with
vinyl tape to prevent moisture pickup or loss during the positron lifetime
measurement. The positron lifetime was then measured as usual by determining
the time interval between the detection of the 1.28-MeV gamma ray emitted
almost simultaneously with the emission of the positron and the detection of
one of the two 0.511-MeV annihilation photons.2, 7 The positron lifetimes were
recorded by a spectrometer using a fast-slow coincidence method described
elsewhere." The lifetime resolution of the system was approximately
450 picoseconds. Figure 2 shows a schematic diagram of the experimental
system used in the lifetime measurements.
EXPERIMENTAL RESULTS
Positron lifetimes were measured in totally dry and _ully water-saturated
states of the test specimens. Figure 3 shows the positron lifetime spectrum
3
in dry and saturated polyimidesulfone specimens. Figure 4 shows similar
results for LARC-TPI specimens. It is noted that neither material exhibits
a strong long-life component, although its presence is unmistakable.
Figure 5 shows a comparison between the lifetime spectra in water-saturated
states of the two materials. The results are summarized in Table III.
Table III
Summary of Positron Lifetime Results in
the Test Materials
No. Material Status Positron Lifetime Values
'I(_i)(*) {**)
(Long Component Lifetime7
i Polyimidesulfone Dry 2.5% 847 _ 81 Ps
Saturated 2.9% 764 + 91 Ps
2 LARC-TPI Dry 1.0% 1080 + 139 Ps
Saturated 4.0% 711 + 96 Ps
(*)I1 and 12 refer to the areas under the short-lifetime and long-lifetime
components, respectively. It is apparent from the I2/I 1 values listed
that the long-life component is very weak in both materials, thus making
the long-lifetime values subject to rather large errors (see last column).
(**)Long component refers to the component with long lifetime. It is this
component which responds sensitively to the changes in the properties of
the medium produced by diffusing water. 2
While preparing for positron lifetime measurements in water-saturated
states of the test materials, an interesting (and highly relevant) phenomenon
was observed. After saturation with water, the specimens were stored overnight
in a freezer for positron lifetime measurements the next day. Storage in the
freezer was effected to avoid moisture loss during the overnight wait. Next
morning, it was observed that the polyimidesulfone specimens had developed
severe internal crazing (or cracks) whereas the LARC-TPI specimens did not
appear to have suffered any visible effects. The same type of phenomenon
was observed during a repeat test. The internal crazing/stressing in the
polyimidesulfones persisted even after the specimens had been desiccated for
over i00 hours at 120°C. The implications of the phenomenon in polyimi-
desulfones will be discussed in the following section.
DISCUSSIONS
On the basis of the data shown in Table II, it is noted that even though
the polyimidesulfone and LARC-TPI have almost the same saturation moisture
content, their ambient moisture levels are markedly different. For example,
- the ambient moisture level in LARC-TPI is almost 50% higher than in the
polyimidesulfone. These data indicate that the total free volume fractions
in the two materials are the same but the moisture diffusion constant in
LARC-TPI is considerably higher. A logical extension of this indication could
be that there are fewer, but larger, free volume cells in the polyimidesulfone
and more numerous, but smaller, free Volume cells in LARC-TPI. These infer-
ences are consistent with the observed effects of moisture on the positron
lifetimes in the two types of test materials (See Table IIl). It is noted
that the moisture-induced reduction in positron lifetime is larger in LARC-TPI
than in polyimidesulfone. This result would be expected if the LARC-TPI coupons
had a larger number of smaller-sized free volume cells. On filling of these
cells with water, the positronium atoms would be forced against the cell walls.
This wouldenhance the positronium wavefunction overlap with the surrounding
molecular electrons. Because of the larger surface area presented by the
smaller voids in the LARC-TPI, the effects of the overlap would be more pro-
nounced in LARC-TPI than in polyimidesulfone. That this inference is valid is
further supported by the accidental observations that the LARC-TPI did not
experience as severe internal stressing as polyimidesulfone when water-saturated
samples of these materials were frozen. Because of the expansion of water on
freezing, the internal stressing effects would bemore marked in polyimidesul-
fone with larger voids than LARC-TPI with smaller voids. The presence of
larger free volume cavities in polyimidesulfone could be the result of foaming
observed during the processing of these materials.
" CONCLUDING REMARKS
As a result of positron lifetime studies in two Langley-developed thermo-
plastic polyimides, it is concluded that the polyimidesulfones have fewer but
larger free volume cells in them. These voids are presumably produced when
the moisture/solvent leaves the matrix during the processing cycles. LARC
thermoplastic polyimides, on the other hand, have more numerous but smaller-
sized voids in them thereby facilitating moisture diffusion. Because of the
presence of larger-sized voids in polyimidesullone, this material would be
more susceptible to internal stressing on freezing of water-saturated
components.
5
REFERENCES
i. J. J. Singh, W. H. Holt, W. Mock, Jr., and R. D. Buckingham: Positron
Annihilation Studies of Moisture in Graphite-Reinforced Composites.
Ceramic Engineering and Science Proceedings, Vol. l, No. 7-8(A),
473-480, 1980.
2. J. J. Singh, W. H. Holt, and W. Mock, Jr.: Moisture Determination in
Composite Materials Using Positron Lifetime Technique. NASA TP-1681,
1980.
3. J. J. Singh, W. H. Holt, and W. Mock, Jr.: Moisture Dependence of
Positron Annihilation Rates in Molecular Substances. Nuclear Instruments
and Methods (to be published).
4. R.M. Nieminen and J. Oliva: Theory of Positronium Formation and Positron
Emission at Metal Surfaces. Physical Rev. 22(B), 2226, 1980.
5. A. K. St. Clair and T. L. St. Clair: Multipurpose Thermoplastic Polyimide.
SAMPE Quarterly, Vol. 12, No. I, October 1981, pp. 20-25.
6. T. L. St. Clair and D. A. Yamaki: A Thermoplastic Polyimidesulfone.
Proceedings of First Technical Conference on Polyimides, November 1982
(In press).
7. W. H. Holt and W. Mock, Jr.: Positron Lifetime Techniques for Nondestruc-
tive Evaluation of Materials. NSWC/DL-TR-3573, U. S. Navy,
December 1976 (Available from DTIC as AD A038 305).
0 0 0 0 \
ii n
o o
LARC -TPI
" " 0 )n
o o
POLYIMIDESULFONE
FIGUREI-THERMOPLASTIC POLYMIDESTRUCTURE
I' High Voltage Supply _r _
ScintiIIators _ .
!
Lvo t  oI . !
_Oivi ._ Multiplier Multiplier Divider
Pre r--_ Pre
m
Amplifier
Amplifier Sourceand Sample .,
I
TimingDiscriminatorI [TimingDiscriminator
[AmplifierI _ IAmplifier1
_r
StandardVariableDelay
Time to J
i . STOP PulseHeight • START
Single I v Conv rter ["
ChannelI
Analyzer]
O.51MeV 1.28
Slow
(" "_ Coincidence
r Unit
IGatei_
Multichannel J
Analyzer I
Figure-2.Fast-slowcoincidencesystemfor determiningpositronlifetimespectra.
io'-
0
IO5 __
0
r,
o o DRY SPECIMEN
I04 _ • • SATURATED SPECIMEN
l T2 (DRY)-- (847-* 81) ps
(.D T 2 (SAT.) = (764 ± 91) pst--
Z
LLI o
> •
i,i 103 _
Z
0 SHORT LIFETIME
<:_ o _,.r COMPONENT
"J3:zZ<:[l02 _ /_>_• _# _'1=(373 * 20)ps
Z •
o ,i, •
,," •_ %°0° • " o' ••0 • • #o _ • o_-o ° °° "
e o _oo_ _% o o cD_ • 't,o%" 0= °°• a, o • •:o•a_ lOIo_OO oOo0••o•oo • o,o• _ 0_,• o o o •co• o'_•_ •o o o o_, -•B
• e eoe o cO0• qD
• o• o• _ZERO TIME o• m c_ o o•qp o _e o• o _%
_'od'o 1 • "o o• OeolbOOOCOOO ..oabeoOJ_oO_-
•••onto• • o• o oo • •_ _5 _ • •c_oeoom::_c:_oI_s)_G:Ib
O0
I00 I I I, I I I • I o I I I • aIID I
0 20 40 60 80 I00 120 140 160 180 200 220 240
CHANNEL NUMBER (ONE CH.=IIT.65ps)
FIGURE5- POSITRON LIFETIME SPECTRAOBSERVEDIN DRY ANDWATER-
SATURATEDPOLYIMIDESULFONESPECIMENS.
6IO
05 i o DRY SPECIMEN
o • SATURATEDSPECIMEN
Tz (DRY) = (1080_139) ps104 - •
l r2ISAT.) = 1711_ 96) ps
_ o
Z
nil
W 3
I0 -
Z
_o o
i,,t.__------SHORT LIFETIMEj COMPONENT
zZ3::: 102 -- O• i r I =(366±201ps
z 4
.. o• • .o,. 2_#8_ o• o" • ,, •0
a, lOt> o°0_o _o_°a• _o_"_o_.%o0 ,_o.eoo #o°# • °0°% •o
u OOo"_e- • _AJ'P c_oo ,-,,-T-"%_5 • • _41_.oo 'Jo •
o_°Z_'_ _o _'._.oo_o'O_-__.._o -.;._., ,_;%.• , _ _ _o °D o • ,_Oo • _boo• cx:_._ • "-ZERO TiME _ • °°_ '" n °° °:_° _o _o• o o o ._ -o o _o o o oQo
• • • \ • • • _O • • • O0 II
\ 0 O0 0 O0 _0 0(330 CO O0
I OO • \ • • • • O• • •
I0v I I I I I_ I I I I I I I
0 20 40 60 80 I00 120 140 160 180 200 220 240
CHANNEL NUMBER (ONE CH.= 117.65ps)
FIGURE4- POSITRONLIFETIME SPECTRAOBSERVEDIN DRY AND WATER-
SATURATEDTHERMOPLASTICIMIDE(TPI) SPECIMENS
FIGURE 5-COMPARISON BETWEENTHE POSITRONLIFETIME SPECTRAOBSERVED
IN WATER-SATURATED THERMOPLASTIClMIDEAND POLYIMIDE-
SULFONE SPECIMENS.

(a) BEFORE SATURATION(AMBIENT MOISTURE CONTENT)
" (b) AFTER FREEZING (SATURATION MOISTURE CONTENT)
FIGURE 6- TYPICAL PHOTOMICROGRAPHSOF A
POLYIMIDESULFONESPECIMEN.


1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-84532
4. Title and Subtitle 5. RePort .Date
July 1982
Positron Lifetime Studies in Thermoplastic Polyimide
Test Specimens 6. PerformingOrganizationCode
505-33-23-05
3
7. Author(s) 8. Performing Orgamzation Report No.
Jag J. Singh, Terry L. St. Clair, William H. Holt, and
Willis Mock, Jr. 10 Work Unit'No.
9. Performing Organization Name and Address
Langley Research Center '11. Contractor Grant No.
Hampton, VA 23665
13. Typeof RePortand PeriodCovered
12. Sponsoring Agency NameandAddress Technical Memorandum
National Aeronautics and Space Administration 14. ArmyProjectNo.
Washington, DC 20546
15. Supplementary Notes
16. Abstract
Positron lifetime measurements have been made in two thermoplastic polyimide
materials recently developed at Langley. The long-component lifetime values in
polyimidesulfone samples are 847 ! 81 Ps (dry) and 764 + 91 Ps (saturated). The
corresponding values in LARC-thermoplastic imides are iO80 + 139 Ps (dry) and
711 _ 96 Ps (saturated). Clearly, the presence of moisture--has greater effect
on positron lifetime in LARC-thermoplastic imides than in the case of polyimi-
desulfones. This result is consistent with the photomicrographic observations
made on frozen water-saturated specimens of these materials.
17. Key Words(Suggested by Author(s)) 18. Distribution Statement ":
Positron, Positron Lifetime,
Moisture in Polymers, Saturation UNCLASSIFIED - Unlimited
Moisture, Free Volume, Thermoplastic
Polyimides, Polyimidesulfones Subject Category 24
19. Security Ctassif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price"
Unclassified Unclassified 13 A02
For sale by the National Technical Information Service, Springfield, Virginia 22161

!1
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Positron lifetime studies in thermoplastic polyimide test specimens

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