The free volume and gas permeability in several contact lens specimens were measured as part of a Space Commercialization Program. Free volume was measured using positron lifetime spectroscopy, while permeability for O2, N2, CO2 gases was measured using mass spectrometry and polarography. Permeability for all gases increases with the mean free volume cell size in the test samples. As might be expected, the specimens with the highest free volume fraction also exhibit the lowest Rockwell Hardness Number. An interesting corollary is the finding that the presence of fluorine atoms in the lens chemical structure inhibits filling up of their free volume cells. This is expected to allow the lenses to breathe freely while in actual use

Burns, Karen S.

Eftekhari, Abe

Singh, Jag J.

Upchurch, Billy T.

English

NASA Technical Reports Server

NASA
Paper
An Investigation
of Microstructural
Characteristics of
Contact-Lens Polymers
Jag J. Singh,
Abe Eftekhari,
Billy T. Upchurch,
and Karen S. Bur//s
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NASA
Technical
Paper
3034
1990
National Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Division
An Investigation
of Microstructural
Characteristics of
Contact-Lens Polymers
Jag J. Singh
Langley Research Center
Hampton, Virginia
Abe Eftekhari
Analytical Services & Materials, Inc.
Hampton, Virginia
Billy T. Upchurch
Langley Research Center
Hampton, Virginia
Karen S. Burns
Old Dominion University
Norfolk, 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 tile National Aeronautics and
Space Administration.
Abstract
The free volume and gas permeability in several
contact-lens samples have been measured as part of a
space commercialization program. Free volume was
measured by using positron lifetime spectroscopy,
while permeability for 02, N2, and CO2 gases was
measured by using mass spectrometry and polaro-
graphy. Permeability for all gases increases with
the mean free-volume cell size in the test samples.
As might be expected, the samples with the highest
free-volume fraction also exhibit the lowest Rockwell
hardness number. An interesting corollary of this
study is the finding that the presence of fluorine
atoms in the lens chemical structure inhibits the
filling up of the free-volume cells. This is expected
to allow the lenses to breathe freely while in actual
use.
Introduction
Contact lenses are widely used by a large segment
of the U.S. population, particularly the younger gen-
eration. Primary qualifications of a good contact-
lens polymer are softness, wettability, and high
permeability for water and for N2, O2, and CO2
gases. The ability to develop good contact-lens ma-
terials demands an understanding of the relation-
ships between the chemical structures of the poly-
mers and their gas permeabilities and other physical
properties.
Careful studies of gas transport in polymers below
their glass transition temperatures (ref. 1) indicate
that the usual Henry's law has to bc modified as
follows:
( bp ) (la)C = KDp + CH
where
C
KD
P
CH
concentration of penetrant gas
dissolved in polymer, ca, a (STP)
cm3 polymer
solubility coefficient for pcnctrant
ema (STP)
gas, cm a polymer • atm
gas pressure at solution equilibrium,
atm
Langmuir mode concentration of
sorbed gas or gas population in
microvoids, cma (STP)
cm 3 polymer
gas affinity parameter, arm- 1
The first term in equation (la) represents the
usual Henry mode of sorption in solids. The second
term is attributed to sorption in microvoids normally
present in all glassy polymers. This modified form of
Henry's law is known as the dual sorption model,
because there are two different modes of sorption in
the glassy polymers. Equation (la) can be simplified
as follows:
C = KDp + CHbp
= (K D + CHb)p
= K'DP
(lb)
Similarly, the generalized form of Fick's law in glassy
polymers (ref. 2) takes the following form:
where
N
DD
cD
DH
Cu
r, dCD D dCH
N=--,_D_Tx H _ (2a)
rate of gas transfer per unit area
Fick's diffusion coefficient
Henry's concentration of sorbed gas
diffusion coefficient for gas trapped
in microvoids (D H < DD)
gas population in microvoids (CH <
CD)
Equation (2a) can be simplified as follows:
ec'
=-- D-_X
(2b)
The molecules sorbed by the Henry's law mode
and the Langmuir mechanism are in equilibrium with
each other. The Langmuir molecules have less dif-
fusional mobility than tile Henry molecules. Since
permeability P is the product of the solubility co-
efficient K D and the diffusion coefficient D, it is ex-
pected that P in glassy polymers will change with the
free volumes. We have measured free volumes in sev-
eral contact-lens samples using positron annihilation
spectroscopy. (See ref. 3.) %re have also measured
permeabilities for O2, N2, and CO2 gases in these
samples. The results and their interpretations are
discussed in this paper.
Symbols
b gas affinity parameter, atm -1
C concentration of penetrant gas dissolved
in polymer, cm a (STP)
cm a polymer
CD Henry's concentration of sorbed gas
CH Langmuir mode concentration of sorbed
gas or gas population in microvoids,
cm 3 (STP)
cm 3 polymer
C S structural constant
D diffusion coefficient
DD Fick's diffusion coefficient
D H diffusion coefficient for gas trapped in
microvoids, D H < D D
f free-volume fraction, percent
I 3 relative intensity of longest component
lifetime, percent
K D solubility coefficient for penetrant gas,
cm 3 (STP)
cm 3 polymer . atm
N rate of gas transfer per unit area
P permeability (KDD),
cm (test gas at STP) cm 2] × 10-!1
cm a polymer . mm Hg s_c-e ]
p gas pressure at solution equilibrium, atm
R microvoid radius, nm
R0 R + 0.1656, nm
Vf microvoid volume or free-volume, A 3
v/o saturation moisture content by volume
percent
w/o saturation moisture content by weight
pcrcent
a Rockwell hardness number
r3 longest component lifetime
Experimental Procedures
Contact-Lens Samples
The test samples were copolymers of silicone
methacrylate and methyl methacrylate monomers
crosslinked by a difunctional monomer. Their phys-
ical properties are summarized in table I. The last R
two columns list the respective saturation moisture R0
content by weight percent (w/o) and volume percefi_
(v/o). These values were obtained by immersing the
samples in distilled water at 80°C till their weight
became constant. The first three samples had in-
creasing amounts of silicone methacrylate monomer
but no fluorine. The last three samples had increas-
ing amounts of silicone methacrylate as well as flu-
orinated acrylate monomers. These samples were
provided by Paragon Optical. The samples for gas
permeability measurements were fabricated to match
the commercial contact-lens geometry. Mass spec-
trometry and polarography were both used to mea-
sure gas permeabilities in these samples. (See ref. 4.)
Positron Lifetime Measurements
Positron lifetime measurements were made by us-
ing a standard fast-fast coincidence measurement
technique. Samples for positron annihilation spec-
troscopy (PAS) measurements were obtained in the
form of 1-cm-diameter rods from which 2-mm-thick
discs were cut for positron lifetime measurements.
A 50-#C Na 22 positron source was sandwiched be-
tween the test discs, and the spectra were accu-
mulated for 6 to 8 hours. This counting time
produced total counts of about 2 x 106 in each
spectrum. The time resolution of the lifetime system
was 250 pscc. Figure 1 shows a typical lifetime spec-
trum in contact-lens samples. The lifetime spectra
were analyzed using the computer program (ref. 5)
for analyzing positron lifetime spectra (PAPLS) and
the POSFIT-EXTENDED program (ref. 6).
Experimental Results
The permeability measurements for O2, N2, and
CO2 gases in various test samples were measured
at 21°C and 35°C. The results obtained by mass
spectrometry are summarized in table II.
The positron lifetime spectra were analyzed into
three components, from 0.2 to 5.0 nsec, for each sam-
ple. The results are summarized in table III. Only
the longest component lifetimes T3 and their rela-
tive intensity values/3 are listed, since only the or-
thopositronium quench rates relate directly to the
free-volume cell sizes (rcf. 7), as seen from the fol-
lowing equation:
where
T3
2T3 _ + _ sin- (3)
longest component lifetime, nsec
microvoid radius, nm
(R + 0.1656), nm
The microvoid volume is given by _rR 3.
The free-volume fraction f can be calculated as
follows:
f = CsI3Vf (4)
where
cs
13
yl
structural constant
intensity of longest component
lifetime, percent
microvoid volume
The structural constant has been calculated by
equating saturation moisture content in volume per-
cent of Paraperm 02 sample with its free-volume frac-
tion, that is,
Cs(27.8)132 = 1.71 ]
CS = 4.66 x 10 -4
(5)
The free-volume fraction in the remaining contact-
lens samples has been calculated by assuming that
the value of the structural constant Cs is the same
for all the samples.
From the data summarized in tables II and III,
it is evident that permeability values and the free-
volume cell sizes in the test samples are strongly in-
terrelated. This, of course, is not surprising in view
of the arguments supporting equations (la) and (2a).
The results for oxygen are illustrated in figure 2.
Even though the presence of microvoids affects both
the solubility coefficient K D and the diffusion coef-
ficient D, the latter coefficient is more directly af-
fected. A comparison of the data summarized in ta-
bles I and III brings out an interesting feature. If
we look at the ratio of free-volume fraction to sat-
uration moisture content, it increases from 1.0 to
2.7 as we go from sample 1 to sample 6. This re-
sult is quite consistent with the chemical structure
of the strongly cross-linked silicone acrylate copoly-
mers. The silicone content increased in samples 1 to
6, but samples 4 to 6 also had increasing amounts of
fluoroacrylates. Thus, while sample 1 had no fluo-
rine, the fluorine content of sample 6 was reasonably
high (approximately 7 percent by weight). The pres-
ence of fluorine in one of the monomers making up
the contact lens apparently interferes with the entry
of moisture into the free-volume cells. (See ref. 3.)
The Rockwell hardness numbers (ref. 8) of the
contact-lens samples were also measured. The results
are summarized in table IV and are illustrated in
figure 3. It is apparent that ct decreases linearly
with increased free-volume fraction. As expected,
sample 6, which has the highest free-volume fraction,
also has the lowest Rockwell hardness number.
Concluding Remarks
The following conclusions can be drawn from the
results presented herein:
1. The free-volume fraction in contact-lens sam-
ples containing fluorine atoms is the highest, but they
pick up the least amount of moisture. This implies
that the free-volume cells in these samples, which
play an important role in gas and vapor transport
through the contact lens, will not be flooded with
the eye fluids when in use. This process will facili-
tate "breathing" by, as well as cleansing of, the lenses
during use.
2. The fluorine-containing lens samples are the
softest, as would be anticipated on the basis of their
high free-volume fraction.
3. The permeability of the lens samples increascs
with the free-volume cell sizes. This increase con-
firms the validity of the dual sorption model in glassy
polymers below their transition temperatures.
NASA Langley Research Center
Hampton, VA 23665-5225
October 12, 1990
References
1. Paul, D. R.: Gas Sorption and Transport in Glassy
Polymers. Ber. Bunsenges. Phys. Chem., vol. 83, no. 4,
1979, pp. 294 302.
2. Petropoulos, J. H.: Quantitative Analysis of Gaseous
Diffusion in Glassy Polymers. J. Polymer Sci.: Pt. A-
2, vol. 8, no. 10, Oct. 1970, pp. 1797 1801.
3. Singh, Jag J.; Eftekhari, Abe; and St. Clair, Terry L.: A
Study of Physical Properties of ODPA-p-PDA Polyimide
Films. NASA TM-102625, 1990.
4. Burns, Karen S.: Gas Pcrmeability Measurements on
Small Polymer Specimens. M.S. Thesis, Old Dominion
Univ., Dec. 1987.
5. Singh, Jag J.; Mall, Gerald H.; and Sprinkle, Danny R.:
Analysis of Positron Lifetime Spectra in Polymers. NASA
TP-2853, 1988.
6. Kirkegaard, P.: Positronfit Extended: A New Version of a
Program for Analysing Positron Lifetime Spectra. Corn-
put. Phys. Commun., vol. 7, no. 7, July 1974, pp. 401 409.
7. 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, vol. 27, 1989, pp. 1419 1424.
8. Standard Test Method for Rockwell Hardness of Plastics
and Electrical Insulating Materials. ASTM Designation:
D 785-89. Volume 08.01 of 1990 Annual Book of ASTM
Standards, 1990, pp. 257 261.
3
Sample
TableI. Summaryof Contact-LensSampleProperties
Description
Paraperm02
Paraperm02+
ParapermEW
Fluoroperm30
Fluoroperm60
Fluoroperm90
Density,
g/cm3
1.12
1.09
1.07
1.14
1.15
1.10
Saturationmoisture
Refractive
index
1,480
1.475
1.475
1.475
1.473
1.471
content,
Weight
1.55
1.69
1.40
1.12
0.86
0.87
percent by--
Volume
1.71
1.81
1.48
1.26
0.98
0.95
Table II. Permeability of 02, N2, and CO2 Gases in Contact-Lens Samples
Sample
1
2
3
4
5
6
Permeability, P, of test gas at
35°C
02
15.8 ± 0.9
31.4 4- 1.1
45.3 i 0.9
40.2 + 2.0
60.0 + 2.0
68.3 4- 1.1
21°C
N2
9.3 4- 0.6
10.3 4- 1.8
15.5 4- 1.1
(a)
(a)
32.0 4- 1.0
CO2
95.0 4- 3.6
257.5 4- 2.8
348.5 + 6.8
(a)
(_)
498.5 4- 7.2
02
32.8 4- 2.2
64.6 4- 3.1
76.9 i 1.6
41.4 4- 2.0
63.4 4- 2.0
109.0 4- 1.1
N2
17.9 4- 2.3
31.5 4- 1.7
37.0 4- 2.3
(a)
(a)
55.2 4- 2.6
CO2
129.0 ± 3.2
272.0 4- 4.6
414.5 4- 6.0
(a)
(a)
561.5 4- 14.6
aOnly 02 permeability data are available.
TableIII. Summaryof PositronLifetimeResultsin Contact-LensSamples
Sample
Positroncomponentparameters
Lifetime,7-3,
psec
2364± 16
2477± 18
2583± 20
2586± 17
2711 ± 17
2795 ± 24
Intensity, /3,
percent
27.8
30.0
27.8
28.7
29.8
30.5
V t-, A 3
132
144
155
155
169
178
Free-volume
fraction, f,
percent
1.71
2.01
2.01
2.07
2.34
2.53
Table IV. Summary of Free-Volume Fraction and Rockwell Hardness Numbers
for Contact-Lens Samples
Free-volume fraction, Rockwell hardness number, a
Sample f, percent c_
1.71
2.01
2.01
2.07
2.34
2.53
107.5 4- 0.8
(b)
93.3 ± 1.0
99.0 =k .5
90.6 ± .5
79.5 4- 1.4
J
aScale: L
Indenter diameter: 6.3500 ± 0.0025 mm
Minor load: 10 kg
Major load: 60 kg
bSample 2 was not available in appropriate size for hardness testing.
5
Counts
10 6
10 5
104
Background
Short-life component, '_ I
Intermediate-life component, "c 2
10 3 1 l J
50 70 90 110
Long-life component,'_ 3
Zero time, T O
L+____ J t ] .... _ . I
130 150 170 190 210 230 250
Channel number (1 channel = 66.2 psec)
270 29O
Figure 1. Typical spectrum in polyacrylate polymers.
P70 m
60--
50--
40--
30--
20
10--
0
120
P 1.44Vf 1
I I I I I I I I I I 1
125 130 135 140 145 150 .155 160 165 170 175
Free-volume size, A3
]
180
Figure 2. Permeability of 02 versus free-volume size.
o_
110
100
9O
80 -
7O
1.5
[]
[] []
[]
I I I I I
1.7 1.9 2.1 2.3 2.5
f, percent
I
2.7
Figure 3. Rockwell hardness number versus free-volume fraction.
8
Report DocumentationPage
Nal onaf Aeronaullcs and
Soace AdmmlsIrallon
1. Report No. 1 2. Government Accession No.
NASA TP-3034
4. Title and Subtitle
An Investigation of Microstructural Characteristics of
Contact-Lens Polymers
7. Author(s)
Jag J. Singh, Abe Eftekhari, Billy T. Upchureh,
and Karen S. Burns
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
3. Recipient's Catalog No.
5. Report Date
December 1990
6. Performing Organization Code
8, Performing Organization Report No.
L-16817
10. Work Unit No.
412-20-26-01
1. Contract or Grant No.
13. Type of Report and Period Covered
Technical Paper
14. Sponsoring Agency Code
15. Supplementary Notes
Jag J. Singh and Billy T. Upchurch: Langley Research Center, Hampton, Virginia.
Abe Effekhari: Analytical Services & Materials, Inc., Hampton, Virginia.
Karen S. Burns: Old Dominion University, Norfolk, Virginia.
16. Abstract
The free volume and gas permeability in several contact-lens samples have been measured as
part of a space commercialization program. Free volume was measured by using positron
lifetime spectroscopy, while permeability for O2, N2, and CO2 gases was measured by using mass
spectroscopy and polarography. Permeability for all gases increases with the mean free-volume
cell size in the test samples. As might be expected, the samples with the highest free-vohlme
fraction also exhibit the lowest Rockwell hardness number. An interesting corollary of this study
is the finding that the presence of fluorine atoms in the lens chemical structure inhibits the filling
up of the free-volume cells. This is expected to allow the lenses to breathe freely while in actual
use.
17. Key Words (Suggested td Authors(s))
Contact lens
Permeability
Positron annihilation spectroscopy
Positron lifetime
Free-volume cell size
Free-volume fraction
18. Distribution Statement
Unclassified Unlimited
Subject Category 24
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified 21. No. of Pages 22. Price9 A02
NASA FORM 1626 OCT 86
For sale by the National Technical Information Service, Springfield, Virginia 22161-2171
NASA Langley, 1990



An investigation of microstructural characteristics of contact-lens polymers

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