In the aerospace industry, applications for polymer coatings are increasing. They are now used for thermal control on aerospace structures and for protective insulating layers on optical and microelectronic components. However, the effectiveness of polymer coatings depends strongly on their microstructure and adhesion to the substrates. Currently, no technique exists to adequately monitor the quality of these coatings. We have adapted positron lifetime spectroscopy to investigate the quality of thin coatings. Results of measurements on thin (25-micron) polyurethane coatings on aluminum and steel substrates have been compared with measurements on thicker (0.2-cm) self-standing polyurethane discs. In all cases, we find positron lifetime groups centered around 560 psec, which corresponds to the presence of 0.9-A(exp 3) free-volume cells. However, the number of these free-volume cells in thin coatings is larger than in thick discs. This suggests that some of these cells may be located in the interfacial regions between the coatings and the substrates. These results and their structural implications are discussed in this report

Eftekhari, Abe

Singh, Jag J.

Sprinkle, Danny R.

English

NASA Technical Reports Server

NASA Technical Memorandum 4421
Positron Lifetime Spectroscopy
for Investigation of
Thin Polymer Coatings
Jag J. Singh
Langley Research Center
Hampton, Virginia
Abe Eftekhari
Hampton University
Hampton, Virginia
Danny R. Sprinkle
Langley Research Center
Hampton, Virginia
National Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Program
1993
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Abstract
In the aerospace industry, applications for polymer coatings are
increasing. They are now used for thermal control on aerospace
structures and for protective insulating layers on optical and micro-
electronic components. However, the effectiveness of polymer coat-
ings depends strongly on their microstructure and adhesion to the
substrates. Currently, no technique exists to adequately monitor the
quality of these coatings. We have adapted positron lifetime spec-
troscopy to investigate the quality of thin coatings. Results of mea-
surements on thin (25-#m) polyurethane coatings on aluminum and
steel substrates have been compared with measurements on thicker
(0.2-era) self-standing polyurethane discs. In all cases, we find
positron lifetime groups centered around 560 psec, which corresponds
to the presence of 0.9-4 3 free-volume cells. However, the number of
these free-volume cells in thin coatings is larger than in thick discs.
This suggests that some of these cells may be located in the inter-
facial regions between the coatings and the substrates. These results
and their structural implications are discussed in this report.
Introduction
Thin polymer films are used extensively in electro-
optical, pharmaceutical, aerospace, construction, and
transportation industries. These polymers can be
used as thin films and membranes or as thin coatings
on appropriate substrates. In both cases, their effec-
tiveness strongly depends on their molecular archi-
tecture. In the case of self-standing films, molecular
architecture can involve the film's crystalline struc-
ture as well as its microvoid distribution function. In
the case of coatings, greater emphasis is placed on the
investigation of the bond between the coating and the
substrate. Thus, a sensitive, nonintrusive technique
is needed to investigate the morphological properties
of thin films and thin coatings. We have developed a
low-energy positron flux generator ideally suited for
such studies. (See refs. 1 and 2.) This generator has
been used to investigate free-volume characteristics
of polyurethane coatings on aluminum and steel sub-
strates. The results of this investigation are discussed
in the following sections of this report.
Experimental Procedure
A slow positron flux generator has been developed
for microstructural characterization of thin polymer
films and coatings. Figure 1 shows the generator as-
sembly used for measuring free-volume characteris-
tics in thin self-standing films. Figure 2 shows a
modified version of the positron generator used for
investigation of thin polymer coatings on metal sub-
strates and their bond integrity. In both these appli-
cations, the main objective is to measure the positron
lifetime spectrum in the film or coating without any
interference from the moderator or substrate strips,
respectively. A brief description of this measurement
technique for thin film coatings follows.
Aluminized
Na 22 radioisotope ---.. , Mylar f!lm.
Folded end __r"_l-_) (AI outslae_
Incident _"- Sealed
Slow positron energetic ., /., end
electron positron ,." / f
L_ _ _ ,,,z" /.,/" I Tungsten
Test _ _e// _o_yOd;i_t_rne )
film I_'_ / e./- ]-C--'_ ' B as
, vo,tage
/',-Annihilation; _ll (_-)
radiation
Aluminized Mylar
Na"z'zsource foil
Figure 1. Low-energy positron flux generator assembly.
A 200-#C Na 22 source deposited on a 2.5-#m-thick
aluminized Du Pont Mylar film is sandwiched be-
tween two 2.54-cm by 2.54-cm tungsten strips that
are 0.0127 cm thick. Two 2.54-cm by 2.54-cm
polyurethane film coatings on metallic substrates in-
sulate the tungsten moderator strips from the alu-
minized Mylar source holder. The aluminized side
of the source foil is in contact with the metallic sub-
strate on which the polyurethane coating is applied.
A potential difference of ±60 V is applied between
the tungsten strips and the aluminized source foil.
Thermalized positrons diffusing out of the moderator
/-0.005
0.0127 / 0.007 cm
source
Metal substrate
Polymer coating
Tungsten moderator
Figure 2. Slow positron beam generator.
strips are attracted to the metMlic substrates when
they are at a negative potential. These positrons
have to drift through the polyurethane coatings to
reach the metallic substrates. They will, in all like-
lihood, annihilate in the polyurethane coatings be-
fore reaching the substrates. On the other hand, the
positrons diffusing out of the moderator strips are
forced back into the strips when the substrates are at
a positive potential. Thus, more positrons annihilate
in the tungsten moderator strips when the coating
substrates are positive than when they are negative.
Careful measurements with well-characterized
Du Pont Teflon films have shown that the excess
of positrons stopping in the moderator strips when
the coating backings are at a positive potential is
4 percent. (See refs. 1 and 3.) Thus, subtraction of
96 percent of the positive substrate bias lifetime spec-
trum from the negative substrate bias lifetime spec-
trum should give the lifetime spectrum exclusively
due to the positrons annihilating in the polyurethane
coatings.
The lifetime spectra were acquired by using a
standard fast-fast coincidence counting system with
a resolution of 250 psec. These lifetime spectra werc
then analyzed by standard deconvolution techniques.
Lifetimes in excess of 500 psec are assumed to be
associated with orthopositronium (O-psec) annihila-
tions. With the previously established relationship
between O-psec Ijfetimeand free-volume ce!l radius
(refs. 4 and 5), the average volumes of the cells can
be readily calculated.
Experimental Results
To assess the quality of thin polyurethane film
coatings, we decided to first measure the lifetime
spectrum in a thick polyurethane disc. Any de-
viation from the thick disc spectrum was inter-
preted as an indication of defects (microvoids) in tile
thin films or film-substrate intcrfacial regions. Fig-
ure 3 shows a typical positron lifetime spectrum in
a 0.254-cm-thick polyurethane disc. This spectrum
was analyzed by using the POSFIT-EXTENDED
program (ref. 6). A two-component analysis gave
the best least squares fit to the data, and table I
summarizes the results.
105
"_ 104 " ;, rl = 345 + 1 psec
c _ _2 = 556 + 31 psec
e-. _
103 - _.
I,
\.
_ °'.,%,.
8 102 - "_:',":'., ." <....... ",:, - ..v=_=--..
........ .._........ ".._%_:
101 ............
100 150 200 250 300 350 400 450 500
Channel number (1 channel = 65 psec)
Figure 3. Typical positron lifetinm spectrum in thick poly-
urethane disc.
Table I. Summary of Positron Lifetime Values in Thick
Polyurethane Disc and Transparent Polyurethane
Coatings
Lifetime, Intensity, Lifetime, Intensity,
_'1, I1, r2, I2,
Test sample psec percent psec percent
345 + 1 85 ± 1 556 + 31 15 ± 1Thick
polyurethane
disc
Polyurethane
coating
on steel
Polyurethane
coating
on aluminum
192 + 4
231 ± 4
74+ 1
74+ 1
572 + 35
562 ± 35
26± 1
264- ]
Polyurethane coatings on aluminum and steel
substrates were analyzed with the slow positron flux
generator shown in figure 2. The efficiency of the gen-
erators shown in figures 1 and 2 is 4 percent (refs. 1
and 2). Figure 4 illustrates typical positron life-
time results in polyurethane coatings. Figures 4(a)
106
105
==104
¢,.
103
tit
1r-
102
101
100
106 _
150 200 250 300 350 400 450
Channel number (1 channel = 59 psec)
(a) Source bias = -60 V.
5OO
"6 105
t'-
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,_ 1o4
103
t'-
'-i
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o 102
• 104
I:
t-
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103
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?, 102
o
II
_Z+-----_---_ --_---=-2 ...... _
=__,'-.
101
100
+ _
i i i • , t, £
150 200 250 300 350 400 450
Channel number (1 channel = 59 psec)
(b) Source bias = +60 V.
105 -
r"
101
101
5OO
'_1= 192 _+4psec --
"c2 572 + 35psec
-- +
\
,: •_.:.¢.._+.....
l i i i l ..... j ....... + _,t___
150 200 250 300 350 400 450 500
Channel number (1 channel = 59 psec)
(c) Difference spectrum.
Figure 4. Typical positron lifetime spectrum in coated steel.
and 4(b) show the spectra observed with T60-V bias,
respectively, on the source foil. Figure 4(c) shows the
difference between the -60-V bias spectrum and 96
percent of the +60-V bias spectrum. This difference
spectrum represents the true positron lifetime spec-
trum in the coatings and the coating-steel interfacial
region. Once again, a two-component analysis gave
the best least squares fit to the data. Lifetime re-
sults in coatings on aluminum and steel substrates
are included in table I for comparison.
Discussion
Prom the data summarized in table I, the follow-
ing differences between the lifetime spectra of the disc
and the coatings are noted:
1. The lifetime of the shorter-lived component
(q) is longer in the thick disc than in the thin
coatings.
2. The intensity (/2) of the longer life component
(r2) is higher in the thin coatings than in the
thick disc.
The first difference implies that the morphologies
of the thin polyurethane coatings slightly differ from
that of the thick polyurethane disc. However, the
second difference implies that more microvoids are
present in the thin film coatings than in the thick
disc. Some of these mierovoids may be located in
the interracial regions of the coating-substrate bonds.
However, their volume is only _0.9 /_3 which is
too small to significantly impact the coating bond
strength. Microvoids of _> 2 A 3 are needed to
significantly impact the bond strength.
Lifetime measurements were also made in poly-
urethane coatings of a different variety on steel sub-
strates. These polyurethane coatings were translu-
cent, whereas the coatings used in the previous
samples (table I) were transparent. Table II summa-
rizes the results for the translucent coatings. Note,
the mierostruetures of the two types of polyurethane
coatings on steel substrates are almost identical. The
intensity of the longer life component in translucent
coatings is slightly larger than it is in the transpar-
ent coatings. This difference is more indicative of
differences in the morphologies of the two types of
polyurethanes than the quality of eoatings-substrate
bond.
Table II. Summary of Positron Lifetime Values in Thin
Translucent Polyurethane Film Coatings
Lifetime, Intensity, Lifetime, Intensity,
71, I1, 7"2, [2,
Test sample psec percent psee percent
Polyurethane 168 4- 1 70 4- 1 570 4- 31 30 4= 1
coating
on steel
Concluding Remarks
A slow positron flux generator has been devel-
oped to monitor the quality of thin polymer coatings
onmetalsubstrates.Thisgeneratorhasbeenused
to monitorthe"quality of coatings and their bonds
to the substrates for a number of polyurethane coat-
ings on aluminum and steel substrates. A compari-
son of positron lifetimes in thin coatings with those
observed in thick polyurethane discs indicates that
their morphologies arc rather different. Addition-
ally, the presence of more 0.9-/1,3 free-volume cells
(microvoids) in thin film coatings than in thick discs
suggests that some of these microvoids may be lo-
cated in the interracial regions of the coatings. Even
though these results do not necessarily mean that
the coatings or their bonds to the substrates are of
good quality, they do demonstrate that positron life-
time spectroscopy can provide very useful informa-
tion about the quality of the coatings and their ad-
herence to the substrates.
NASA Langley Research Center
Hampton, VA 23681-0001
December 7, 1992
References
1. singh, Jag J.; Eftekhari, Abe; and St. Clair, Terry L.:
Low Energy Positron Flux Generator for Lifetime Studies
2.
3.
4.
5.
in Thin Films. Nucl. Instrum. _ Methods Phys. Res.,
vol. B53, Mar. 1991, pp. 342 348.
Singh, Jag J.; Eftekhari, Abe; and St. Clair, Terry L.:
Low-Energy Positron Flux Generator for Microstructural
Characterization of Thin Films. NASA TP-3074, 1991.
Singh, J. J.; and Eftekhari, A.: Free Vohlme Model
for Molecular Weights of Polymers. NucI. Instrum.
Methods Phys. Res., vol. B63, 1992, pp. 477 .!83. :
Nakanishi, H.; and .lean, Y. C.: Positrons and Positro-
nium in Liquids. Positron and Positronium Chemistry,
D. M. Schrader and Y. C. Jean, eds., Elsevier Science
Publ. Co. Inc., 1988, pp. 159 192.
Singh, Jag J.; Eftekhari, Abe; Upchurch, Billy T.; and
Burns, Karen S.: An Investigation of Microstructural
Characterization of Contact-Lense Polymers. NASA
TP-3034, 1990.
Kirkegaard, Peter; Eldrup, Morten; Mogensen, Ole E.;
and Pedersen, Nieis J.: Program System for Analy-
sing Positron Lifetime Spectra and Angular Correla-
tion Curves. Comput. Phys. Commun., vol. 23, no. 3,
July 11, 1981, pp. 307 335.
i
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Positron Lifetime Spectroscopy for Investigation of Thin Polymer
Coatings WU 141-20-06-10
6. AUTHOR(S)
Jag J. Singh, Abe Eftekhari, and Danny R. Sprinkle
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
NASA Langley Research Center
Hampton, VA 23681-0001
g. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
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L-17165
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Singh and Sprinkle: Langley Research Center, Hampton, VA; Eftekhari: Hampton University, Hampton, VA.
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_. ABSTRACT (tv_aximum 200 words)
In the aerospace industry, applications for polymer coatings are increasing. They are now used for thermal
: control on aerospace structures and for protective insulating layers on optical and microelectronic components.
However, the effectiveness of polymer coatings depends strongly on their microstructure and adhesion to the
: substrates. Currently, no technique exists to adequately monitor the quality of these coatings. _re have
adapted positron lifetime spectroscopy to investigate the quality of thin coatings. Results of measurements on
thin (25-pro) polyurethane coatings on aluminum and steel substrates have been compared with measurements
on thicker (0.2-cm) self-standing polyurethane discs. In all cases, we find positron lifetime groups centered
: around 560 psec, which corresponds to the presence of 0.9-A 3 free-volume cells. However, the number of these
free-volume cells in thin coatings is larger than in thick discs. This suggests that some of these cells may be
located in the interracial regions between the coatings and the substrates. These results and their structural
implications are discussed in this report.
14. SUBJECT TERMS
Positron lifetime spectroscopy; Positronium formation; Free-volume cell radius;
Thin polymer coatings; Metallic substrates; Slow positron generator; Negative work
function moderators -,
ii
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Positron lifetime spectroscopy for investigation of thin polymer coatings

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