Positron lifetimes in the isotropic phases of chiral nematic liquid crystal formulations and their mixtures up to the racemic level were measured. The lifetime spectra for all liquid crystal systems were analyzed into three components. Although the individual spectra in the left- and right-handed components are identical, their racemic mixtures exhibit much larger orthopositronium lifetimes; these larger lifetimes indicate the presence of larger microvoids. This result is consistent with the reportedly higher thermodynamic stability and color play range in the racemic mixtures of chiral nematic liquid crystals

Eftekhari, Abe

Parmar, Devendra S.

Singh, Jag J.

English

NASA Technical Reports Server

_Technical
per
--October 1991
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Positron Lifetime
Measurements in
Chiral Nematic
Liquid Crystals
Jag J.Singh,
Abe Eftekhari,
and Devendra S. Parmar
(NASA-TP-3122) POSITRON LIFETIME
MEASUREMENTS IN CHIRAL NEMATIC LIQUID
CRYSTALS (NASA) 14 p CSCI 20L
N92-10677
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NASA
Technical
Paper
3122
1991
- IM/k.._A
National Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Program
Positron Lifetime
Measurements in
Chiral Nematic
Liquid Crystals
Jag J. Singh
Langley Research Center
Hampton, Virginia
Abe Eftekhari
Hampton University
Hampton, Virginia
Devendra S. Parmar
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 endorsemcnt, either expressed or implied, of such
products or manufacturers by the National Aeronautics and
Space Administration.
Abstract
Positron lifetimes in the isotropic phases of chiral
nematic liquid crystal formulations and their mix-
turcs up to the raeemie level have been measured.
Tile lifetime spectra for all liquid crystal systems
wcrc analyzed into three components. Although thc
individual spectra in the left- and right-hande_t com-
ponents arc identical, their racemic mixtures exhibit
much larger orthopositronium lifetimes, which indi-
cates tile presence of larger microvoids. This result
is consistcnt with the reportedly higher thermody-
namic stability and color play range in the racemic
mixturcs of chiral ncmatic liquid crystals.
Introduction
Liquid crystals are an interesting phase of mat-
tcr. They retain some of the features of the solid
state well beyond their melting points. They are used
extensively in display devices, image converters, opti-
cal switching elements, and thermal indicators. They
have also bccn used in aerodynamic studies as indi-
cators of flow transition and flow separation on test
model surfaces. Unfortunately, most of the uncneap-
sulated chiral nematic liquid crystals applied on test
surfaces erode away under the influence of the ap-
plied shear stress. Encapsulation of liquid crystals
in appropriate matrices, on the other hand, reduces
their sensitivity to the external influences. As a re-
sult, there is a strong need for the development of
liquid crystal systems that will have strong steric in-
teractions with the model surface and will not be
washed away under the impact of tangential forces
on the model surfaces. In an effort to address this
problem, the microstruetural characteristics of sev-
eral monomer liquid crystal systems were evaluated.
Positrons were used as probes for measuring mi-
crocavity dimensions in TM74A amt TM74A* mix-
tures supplied by BDH, 1 and chiral ncmatic (right-
handed) and cholesteryl ester (left-handed) liquid
crystals supplied by Hallerest. 2 The results of this
study are described in the following sections.
Experimental Procedure
Preparation of Liquid Crystal Systems
Two types of liquid crystal systems were inves-
tigated: (1) TM74A, a low-temperature color play
formulation, and its racemized analogue, TM74B;
(2) BN/R20CIW (right-handed chiral nematic) and
CN/R20CIW (left-handed cholesteryl ester) liquid
crystals. The phase transition temperatures of these
1 BDH Ltd., Poole, BH12 4NN, England.
2 Hallcrest, Glenview, IL.
systems are summarized in tables I and II. Figures 1
and 2 are typical phase diagrams for mixtures of pure
chiral nematie systems.
Different mixtures of left- and right-handed sys-
tems were prepared by mixing them at room temper-
ature. Each mixture was then heated to about 80°C
(in the ]sotropic liquid phase), held there for 15 min,
and kept well stirred. The mixture was then allowed
to cool down slowly to room temperature and was
kept overnight before making the positron lifetime
measurements. Positron lifetime measurements were
made in TM74A samples before and after heating
them to 80°C. All lifetime components were equal
within experimental errors. Thus, heating the con-
stituent formulations to 80°C for producing stable
mixtures of liquid crystalline systems does not affect
their intrinsic structures.
Positron Lifetime Measurements
Figure 3 shows the target chamber for holding
the fl+ source and the liquid crystal sample. A 50-
pC Na 22 source was sealed between two 2.54-pm-
thick Kapton a films. The source bag was then sand-
wiched between two 2.24-cm x 2.00-cm x 0.025-cm
aluminum wafers. The wafers have 0.45-cm-diameter
holes to permit positrons to escape into the liquid
crystal mixture that surrounds the source assem-
bly. Positron lifetime measurements were made at
room temperature by using a fast-fast coincidence
measurement system. (See ref. 1.) The coinci-
dence system time resolution is _225 psec. Fig-
ure 4 shows a typical lifetime spectrum in TM74A
liquid crystal formulations. Lifetime spectra were
analyzed by using PAPLS (ref. 2) and POSFIT-
EXTENDED (ref. 3) computer programs. In all
eases, 3-component analyses gave the best fits to the
experimental spectra.
Results
Positron lifetime measurements were made in
mixtures of TM74A and TM74B liquid crystals at
room temperature (23°C). As can be seen from the
phase transition temperatures summarized in ta-
ble I, the test mixtures were in the isotropic liquid
phase. The composition of the mixtures ranged from
100-percent (pure) TM74A to 100-percent (pure)
TM74B. The results of the 3-component lifetime
spectral analysis are summarized in table III and il-
lustrated in figures 5 to 10. It is evident that the
first-component lifetime T1 and its intensity I1 are in-
dependent of the ratio of TM74A to TM74A* in the
3 Kapton polyimide resin, manufactured by E. I. du Pont
de Nemours X: Co., Inc.
mixture. The second-componentlifetimer2, how-
ever, increases as we go from pure TM74A to the
racemic mixture, TM74B, though its intensity/2 re-
mains essentially constant. The second-component
lifetime is related to the depth of the defect where
the positron was trapped before its annihilation with
a free electron. Deeper defects lead to larger values
of r2. Just as with 7-2 and /2, the third-component
lifetime ra increases as we go from pure TM74A to
the racemic mixture, TM74B, but its intensity Ia re-
mains essentially constant.
The third-component lifetime r3 is related to the
size of the microvoid, where orthopositronium atoms
are trapped, by the following equation (ref. 4):
1 -1 R 1
2ra RO + 27 sin rr_0° (1)
where
ra third-component lifetime, nsec
R mierovoid radius, nm
R 0 (R + 0.1656), nm
The microvoid volume V/ is given by _rrR a.
The values of Vf for different concentrations of
TM74A in tile TM74A + TM74B mixture have
been calculated using equation (1). Thc results
are summarized in table III and are illustrated in
figmrc 11. It is evident that the racemic mix-
ture (TM74B) has much larger microvoids than the
pure TM74A formulation. This result indicates that
the racemic mixtures have larger microvoids than
their constituent formulations. This fact was fur-
ther verified by making positron lifetime measure-
mcnts in BN/R20CIW (right-handed liquid crystal),
CN/R20CIW (left-handed liquid crystal), and their
racemized mixture. As indicated in table II, all these
systems are in cholesteric liquid crystalline phase at
the room temperature where positron lifetime mea-
surements were made. The positron lifetime results
are summarized in table IV. It is evident that while
the left- and right-handed formulations have similar
spectra, their racemic mixture has longer lifetimes
for the third (ra) and second (r2) components.
Discussion
The positron lifetime measurements in mixtures
of variable relative concentrations of TM74A and
TM74B chiral nematic liquid crystals indicate that
the second-component lifetime r2 increases from
438 :k 11 psec to 592 ± 22 pscc, whereas the third-
component lifetime r3 increases from 2732 4- 18 psec
to 3121 4- 41 psec, as one goes from a pure formu-
lation to its racemized analogue. The increase in
2
r2 implies deeper positron defects in racemized mix-
tures. The increase in ra, on the other hand, in-
dicates that microvoids are larger in the racemized
nfixture. These results are quite consistent with the
reportedly higher thermodynamic stability and color
play ranges in racemized mixtures of such formu-
lations (ref. 5). Because of the asymmetry of the
chiral molecules, the steric interactions in racemized
states are stronger than in the nonracemized states.
These stronger steric interactions among the compo-
nent molecular systems lead to a higher degree of en-
tanglements which result in larger microvoids in the
raccmized mixtures. The entangled systems also re-
quire more thermal energy to unwind the helix; this
increases their color play ranges and the transition
temperatures.
Conclusions
From the positron lifetime results discussed in
this report, the following general conclusions can be
drawn:
1. The racemic liquid crystal formulations have
deeper defects than their constituent systems.
2. The racemic formulations have almost 25-
percent larger microvoids than the nonraccm-
ized liquid crystal components.
These results are consistent with the higher ther-
modynamic stability and color play ranges observed
in the racemized nfixtures of cholesteric liquid crystal
formulations.
NASA Langley Research Center
tlampton, VA 23665-5225
August 23, 1991
References
1. Hardy, W. H., II; and Lynn, K. G.: A New Approach to
Timing: The Fast-Fast System. IEEE Trans. Nuel. Sci.,
vol. NS-23, no. 1, Feb. 1976, pp. 229-233.
2. Singh, Jag J.; Mall, Gerald H.; and Sprinkle, Danny R.:
Analysis of Positron Lifetime Spectra in Polymers. NASA
TP-2853, 1988.
3. Kirkegaard, P.: Positronfit Extended: A New Version of a
Program for Analyzing Positron Lifetime Spectra. Corn-
put. Phys. Commun., vol. 7, no. 7, July 1974, pp. 401 409.
4. Nakanishi, H.; and Jean, Y. C.: Positrons and Positro-
nium in Liquids. Positron and Positronium Chemi.stry,
D. M. Schrader and Y. C. Jean, eds., Elsevier Science
Publ. Co., Inc., 1988, pp. 159 192.
5. Gray, G. W.; and McDonnell, D. G.: Some Cholesteric
Derivatives of S-(+)-4-(2'-Methylbutyl)phenol. Mol.
Cryst. 8dLiq. Cryst., vol. 48, no. 1 2, 1978, pp. 37 52.
TableI. Summaryof PhaseTransitionTemperaturesof TM74AandTM74B1Systelns
SA = SmecticA ]Ch= CholestericI = Isotropicliquid
Liquidcrystalsystem
TM74A
TM74B
Transitiontemperature,°C
SA-Ch
32.6
32.3
Ch-I
15.9
16.2
150-50mixtureof TM74Aandits complexconjugateTM74A*.
TableII. Summaryof PhaseTransitionTemperaturesof BN/R20CIW(Right-Handed)
andCN/R20CIW(Left-Handed)LiquidCrystals
SA= SmecticA ]Ch = Cholesteric
I = Isotropic liquid
Liquid crystal system
Transition temperature, °C
SA-Ch Ch-I
BN/R20CIW 18.5 41.5
CN/R20CIW 16.0 43.0
3
TableIII. PositronLifetimeParametersin Mixturesof TM74AandTM74B
ri = Lifetime of ith component ]
Ii = Intensity of ith component
V/ = Microvoid volume
Percent
TM74A T 1 , psec
242 4. 1350.00
58.19 250 + 5
62.72 268 ± 2
71.2 [ 264 ± 5
74.36 266 4. 10
] 277± 11:77.40
80.16 271 + 5
83.85
87.38
90.50
93.18
95.15
-ioo.0
I1,
percent
44.3 + 5.0
45.2 4. 5.0
51.5 ± 5.0
59.8 4- 2.0
52.4 4. 4.0
54.7 ± 5.0
Y2, psec
592 i 22
568 4. 9
568 + 30
5124.12
528 4. 27
509 + 34
I2_
percent w3, psec
39.5 + 5.0 3121 + 41
38.3 4. 9.0 3097 4. 15
32.5 4- 5.0 3072 4- 44
33.7
31.7
29.2
51.7 4. 2.0 482 4- 14 31.9
454 4. 33
445 _ 42
455 4. 23
442 ± 10
I 273 4. 10 55.4 4- 4.0
[ 282-1-12 _ 57.5 4- 6.0
247 4- 14 44.1 4. 5.0
260 4- 15 49.7 4- 2.0
233 4- 17 36.7 4. 6.0
259 ± 5 47.0 4- 2.0
28.4
26.9
39.7
34.5
percent
4- 2.0 2994 ± 18
4- 4.0 2953 4- 41
4- 5.0 2906 ± 41
4. 2.0 2912 4- 19
4- 4.O 2905 4- 47
+ 5.0 2818 ± 52
4. 5.0 2861 ± 41
4. 2.0 2708 ± 17
+ 5.0 2649 4- 38
± 2.0 2732 4. 18
16.3 ± 1.0
16.5 ± 1.0
16.1 ± 1.0
16.5 + 1.0
213.2
209.1
207.7
198.3
15.9 4- 1.0 193.2
16.1 ± 1.'0 188.1
16.4 4. 1.0 189.3
16.1 4- 1.0 188.0
15.6 4- 1.0
16.2 4- 1.0
15.8 4- 1.0
178.1
184.2
167.3
457 4- 19 46.5 16.8 4- 1.0 160.3
438 ± 11 36.9 16.1 4- 1.0 169.6
*Vf has been calculated from the microvoid radius R in equation (1).
Table IV. Positron Lifetime Parameters in BN/R20CIW and CN/R20CIW Liquid Crystals
Ti = Lifetime of ith component ]I i Intensity of ith co ponent
100 percent BN/R20CIW
100 percent CN/R20CIW
Nematic (60 percent BN + 40 percent CN)
I1,
T1, psec percent "r2, psec
264:t:4 48.44-2.0 5124-8
272±6 49.0±3.0 512±12
269±9 53.8 4. 4.0 559±30
I2, 13,
percent _-3,psec pcrcent
34.5 4- 2.0 [3i89 _13 17.1 4- 1.0
33.4 ± 3.0 [3223 ± 15 17.6 4- 1.0
28.3 ± 4.0 13265 4- 42 17.9 ± 1.0
6O
50
40
30
20
Temperature,
°C
10
-10
-20
i
Cholesteric phase
Color play
, Smectic phase
Red
-30 I I I I I I I I
TM74A 100 g0 80 70 60 50 40 30 20 10 0
0 10 20 30 40 50 60 70 80 90 100 TM75A
Percent composition
Figure 1. Phase diagram for mixtures of TM74A and TM75A. (TM74A and TM75A are low- and high-
temperature color play formulations, respectively. Their chemical compositions are identical.)
6
5
4
Color play t
range, oC 3
2
1
0 I ! I i i ! I
TM74A 100 90 80 70 60 50 40 30
0 10 20 30 40 50 60 70
Percent composition
I I I
20 10 0
80 90 100 TM75A
Figure 2. Color play range for mixtures of TM74A and TM75A. (TM74A and TM75A are low- and
high-temperature color play formulations, respectively. Their chemical compositions are identical.)
1__ Quartz cuvette[ luminum holderI I I I / (2.24 cm x 2.00 cm
4.5 cm _1 r / Kapton window
J v _ (Thickness=0.0085mm,
Diameter = 4.50 mm)
_ Na 22 source
•---2..0 crn--_
Figure 3. Schematic of liquid crystal target and source assembly.
105
F
I
10 4 ' st-component lifetime)
d-component lifetime)103 .......
" \ _onent "fetime)
....,.,-,-"To \ \ -_-,',.v. .....
" _-- Background " "" ..... """_
102 , I 1 I t L__._
1O0 150 200 250 300 350 400
Channel number ( 1 channel = 65 psec)
Figure 4. Typical positron lifetime spectrum in TM74A liquid crystal.
Number of
events
6
First-
component
lifetime,"Cl,
psec
35O
300
250
200
150
100
50
0 I I I [ I I I I I I I
45 50 55 60 65 70 75 80 85 90 95 100
Percent TM74A
Figure 5. First-component lifctime versus percent of TM74A in mixture of TM74A and TM74A*.
First-
component
intensity, 11
percent '
100
80 -
60 -
40 - t
20 -
0 I u
45 50 55
I I I I I 1 1 I I
60 65 70 75 80 85 90 g5 100
Percent TM74A
Figure 6. First-component intensity versus percent of TM74A in mixture of TM74A and TM74A*.
7'
Second-
component
lifetime,_ 2,
psec
35O
3OO
25O
2OO
150
100
5O
0
45
I I I I I I I I I I I
50 55 60 65 70 75 80 85 90 95 100
Percent TM74A
Figure 7. Second-component lifetime versus percent of TM74A in mixture of TM74A and TM74A*.
Second-
component
intensity, I2,
pement
100
8O
60
4O
2O
+
0 | i I ! ! ! ! I ! ; I 1
45 50 55 60 65 70 75 80 85 g0 95 100
Percent TM74A
Figure 8. Second-component intensity versus percent of TMT4A in mixture of TMT4A and TMT4A*.
8
Third-
component
lifetime, "c3,
psec
4000 r
3800
3600
3400
3200
3000
2800
2600
2400
I IIF
45
I 1 I I I I I I I I I
50 55 60 65 70 75 80 85 90 95 100
Percent TM74A
Figure 9. Third-component lifetime versus percent of TM74A in mixture of TM74A and T]k[74A*.
Third-
component
intensity, 13,
percent
100 -
80-
60-
40-
20-
Figure 10. Third-component intensity versus percent of TM74A in mixture of TM74A and TM74A*.
0 I I I 1 I I I I 1 I I
45 50 55 60 65 70 75 80 85 90 95 100
Percent TM74A
Microvoid
volume,
Vf, A3
24O
22O
2OO
180
160
140
120
IO0
45
I I I I l I I ] I I I
50 55 60 65 70 75 80 85 90 95 100
Percent TM74A
Figure 11. Microvoid volume versus percent of TM74A in mixture of TM74A and TM74A*.
10


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I October 1991 Technical Paper
4. TITLE AND SUBTITLE
Positron Lifetime Measurements in Chiral Nematic Liquid Crystals
6. AUTHOR(S)
Jag J. Singh, Abe Eftekhari, and Devendra S. Parmar
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
NASA Langley Research Center
Hampton, VA 23665-5225
9. SPONSORING/MONITORING AGENCY_NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546-0001
S. FUNDING NUMBERS
WU 506-43-21-05
8. PERFORMING ORGANIZATION
REPORT NUMBER
L-16948
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
NASA TP-3122
11. SUPPLEMENTARY NOTES
Singh and Parmar: Langley Research Center, Hampton, VA; Eftekhari: Hampton University, Hampton, VA.
12a. DISTRIBUTION/AVAILABILITY STATEMENT
Unclassified-Unlimited
Subject Category 76
12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 words)
Positron lifetimes in the isotroplc phases of chiral nematic liquid crystal formulations and their mixtures up to
the racemic level have been measured. The lifetime spectra for all liquid crystal systems were analyzed into
three components. Although the individual spectra in the left- and right-handed components are identical, their
racemic mixtures exhibit much larger orthopositronium lifetimes; these larger lifetimes indicate the presence
of larger microvoids. This result is consistent with the reportedly higher thermodynamic stability and color
play range in the racemic mixtures of chiral nematic liquid crystals.
|7 I
14. SUBJECT TERMS
Positron annihilation; Positron lifetime; Mierovoids; Free-volume cell size; Chiral
nematic liquid crystals; Cholesteric phase; Color play range; Racemic mixture
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Positron lifetime measurements in chiral nematic liquid crystals

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