Dynamics of polymer-dispersed cholesteric liquid crystals H.-S. Kitzerow, J. Rand, P. Crooker
To cite this version: H.-S. Kitzerow, J. Rand, P. Crooker. Dynamics of polymer-dispersed cholesteric liquid crystals. Journal de Physique II, EDP Sciences, 1992, 2 (2), pp.227-234. <10.1051/jp2:1992126>.
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Phys.
J.
France
II
(1992)
2
227-234
1992,
FEBRUARY
PAGE
227
Classification
Physics
Abstracts
61.30
61.40K
78.20J
Dynamics H.-S.
(*), J.
Kitzerow
Department 96822,
HI
polymer-dispersed
of
Physics
of
Rand
and
liquid
crystals
Crooker
P. P.
University
Astronomy,
and
cholesteric
of
Hawaii,
2505
Correa
Road,
Honolulu,
U.S.A.
(Received
accepted
1991,
August
13
October
22
1991)
les propri£t4s £lectro-optiques des cristaux liquides cholest4riques polymbre. L'application d'un champ 41ectrique fait [email protected] une un l'amplitude du d'apparition de cette r4flection tone r£flection de Bragg. La vitesse augmente avec diminution diambtre gouttelettes. du des champ £lectrique, la temp6rature et avec la avec L'interruption du champ dlectrique cause deux rdactions trbs diffdrentes, ddpendant du voltage et
R4sum4.
Nous
dispers6s
du
diambtre
£tud14
avons
dans
matdriau
des
gouttes.
liquid crystal cholesteric studied the electrooptic properties of droplets of dispersed in a polymer. Application of AC electric fields to the droplets leads to a conversion of When the field is suddenly the droplet from a nonreflecting to a selectively reflecting appearance. strongly with increasing field strengtli, witli switched times found to decrease are on, the switching both witli decreasing drop size. For tile switching off process, increasing and temperature different tliat relaxation switching times and microscopic indicate two textures processes can depending on the applied voltage and on tile drop size, occur, We
Abstract.
have
Introduction.
1.
for suitable which have proved to be liquid crystals (PDLC), Polymer-dispersed nematic electrooptic display applications, have been extensively investigated during the last decade have recently invented a Crooker extended by and Yang [2], who ill. This idea has been device (PDCLC). This cholesteric liquid crystals display using polymer-dispersed new unlike reflectivity. nematic of different However, provides switching between two states
displays,
PDLC the
In
PDCLC
anisotropy
and
known
exhibit
the
to
there
is
also
a
a
strong
color
effect,
dielectric (I.e., cholesteric) materials with negative sufficiently substances is small, these used. pitch p are selective reflection in the visible wavelength range [3] according to
display, chiral high chirality are
nematic
If the
equation A
(*) StraBe
Permanent
des
17.
address:
Juni135,
1000
=
Iwan-N.-Stranski-Institut, Berlin 12, Gennany.
(1)
np
Technische
Universitit
Berlin,
Sekr.
ERll,
228
JOURNAL
refractive index), provided a mean sample is observed along the helix droplets in a polymer matrix, anchoring This sudace droplet sudace. orientation
(n is
that
the
axis.
causes
Thus,
field.
director
However,
for
in
reorientation
a
absence
the
with
materials of
of
a
negative
the
PHYSIQUE
DE
helix
the
If
causes
a
II
N° 2
is
stntcture
cholesteric
planar
uniformly
materials
director
are
orientation
leads, in tum, to a complicated field, such droplets do not show
anisotropy,
dielectric
cholesteric
structure
to
a
application
uniform
oriented
and
dispersed
everywhere configuration selective of
at
the
of
the
reflection. electric
an
of
orientation
that small
as
the
field
helix
axis
direction. Consequently, the display shows a bright selective reflection if along the field sufficiently high voltages are applied. configurations occurring in cholesteric director droplets have been studied earlier by The microscopic investigations of dispersed chiral liquid crystals exhibiting large pitch [4-10]. The characteristic indicates configuration known observation of fingerprint the textures a as Frank-Pryce model [4, 5]. Application of both magnetic [7, 8] and electric fields [9, 10] has influence reorientation of the field inside the of the revealed director the droplets under a field. focuses determination This experimental of the dynamic behaviour of the on paper electrooptic switching in PDCLC samples. Using the same material reported in reference as [2], we investigate the dependence of the switching times on applied voltage, temperature and droplet size. In sections 3.1-3, we describe of the intensity of reflected light measurements time if step voltages are applied to the sample. Section 3.4 contains complemenversus some observations. tary microscopic
2.
Experiment.
The
U.S.A.),
28.0 fb
of
investigation
under
material
Aldrich,
the
22.0 fb wide
consists
chiral
temperature
of
50.0 9b
monomer
CE2
range
nematic
(by weight) poly-(vinylbutyral) (PVB Great Britain) and Dntg House, 2806 mixture ZLI (Merck, Germany)
(British
exhibiting negative dielectric anisotropy. Samples were prepared by first mixing the together with components diameter spheres with a of 31.2 ~Lm either 9.7 ~Lm or added to were thickness. A glass slide with an electrically coated conducting ITO layer
chloroform.
Glass
sample covered by the was chloroform mixture and the allowed Then, the sample was hours. evaporate for several to was heated to 130 °C in order to complete the evaporation of the solvent and the sample was ITO-coated covered by the second glass slide. In order to obtain drops of uniform size, the cooled sample down from 130°C 90°C with cooling ranging from to rate was a 3 °C/mn to 0.01°C/mn. of cholesteric observed The droplets was about at appearance 105 °C. Depending on the cooling rate, drop diameters between about 4 ~Lm (for fast cooling) obtained. and about 32 ~Lm (for slow cooling) were The controlled PID thermostat temperature of the sample was regulated using a pulsewidth (Instec/Apple He) with an than better ±0.01K. sample was The observed in accuracy reflection using a Zeiss Universal microscope with polarizers. A Jarrel crossed Ash monochromator and photomultiplier connected the microscope tube in order to to were reflected the intensity spectntm or the intensity at constant wavelength either measure versus field strength or time. I kHz altemating voltages were used throughout reported voltages are
3.
define
the
values.
rms
Results.
Under
the
influence
of
ac
fields,
electric
given by equation (I). Typically,
we
our
observed
a
samples selectively spectral halfwidth
reflect between
at
40
the and
wavelength 50
nnJ
for
DYNAMICS
N° 2
229
PDCLC
OF
reflection peak. The spectra have been previously published [2, III. The dependence of reflectivity on the field strength shows a threshold behaviour. Above the threshold Eo, the reflectivity increases reaching 3Eo. The continuously until saturation E=2 at threshold voltage Eo was found to decrease with increasing temperature from 28 V/9.7 ~Lm 67.3 °C to 20 V/9.7 ~Lm at 95.0 °C for c~m~~j 22.0 fb CE2. The cholesteric pitch of our at and thus the materialselective reflection wavelengthwith increasing also decreases Reported intensities respective wavelength of maximum measured temperature. at the were reflectivity. the
the
=
3.I
EFFECT
measured
after
t
I(t)
FIELD
fit
well
In order to investigate behaviour, the dynamic we intensity I at the selective reflection wavelength versus voltage is suddenly applied to the sample (Fig. I, inset). We find of two exponentials : sum STRENGTH.
reflected
constant
a
is
oF
.the
ac
by
a
(t)
I The
two
switching
times
rj
=
and
figure I. Both time constants strengths between 20 V/9.7 ~Lm by the relation
lo r~
+
AI [x e~
are
decrease and 60
shown
observed
the
nematic
x) e~
(l
that
(2)
~'~~]
r
(3)
E,
cc
relaxation times has also been straight line in figure I. The appearance of two of the field in reorientation director droplets [12] where they are due to disclinations sudace, respectively. of the Chiral the bulk of the droplet and drop movement at identified have unique droplets undergo a texture change, however, and two not yet we as
in
+
time
plotted versus the rms value of the applied voltage in with increasing voltage. For field exponentially V/9.7 ~Lm, this dependence is approximately described log
by
~'~~
have
processes.
io3
io2 n
~
~s
v~
~
~Ql
Q
$
D
Tz~~
~
'
my
)
1~0
aov
~l,on -zo
10~l
o
zo,o
0
constants
for
each
eo
20 E
Switch-on Fig. I. functions exponential
«
~~"
60
40
80
/9.7/tm)
(V
field strength for T 95 °C, A 452 nm. I (t) is fit by versus (inset, solid lines), so that the behaviour of the sample is described voltage. Solid lines are straight line fits to equation (4). time
=
=
a
sum
by
of two
two
time
230
JOURNAL
The
results
for
time
versus
We
in
switch-off times
in
value,
switching
the
initial
however, inset). For
note,
lines
for
time.
E
50
~
order
figure 2 irrespective
of
in
the
lower
voltage
V/9.7
~Lm,
I(t)
to
N°
2.
The
~Lm,
35
V/9.7
range is reasonably found exhibit to
was
switching for
II
figure
25 V/9.7 =
the compare therefore the times
are
shown
process are of E
voltages
only
that
In
off
tum-off
PHYSIQUE
DE
times
shows
~Lm,
and
the
50
intensity V/9.7
~Lm.
by exponentials (solid negative the curvature at different voltages, the switch-off 10 fb of its original to retum to
for
reflectivity
the
inset
2
fit
shape.
curve
io3 50
I
v
(
~
j 4
I
l
~
z5
V
0~
I
~ -20
0
20
~
t
40
60
80
(s)
101 20
0
60
40
(v)
v intensity Fig. 2. Switch-off times v~~~ (where the 95 °C, A 452nm. strengtli for T In contrast to exponentials only for E 50 V/9.7 ~m (inset). For E
retums
to
the
10 fb
switch-on
=
~
~
switch-off
3.2 E
50
V/9.7
~m,
of
its
value) versus (Fig. I), I(t) is negative curvature
initial
process has I (t
field fit
by
at
the
time.
TEMPERATURE
Temperature
EFFECTS.-
increases
the
switching
(measured
times
at
within the V/9.7 ~Lm) by about two orders of magnitude as the temperature is reduced cholesteric (Fig. 3). The dependence of the switch-on time temperature temperature range 50
=
constants
r~,
and
r~
~~
(Eq. (2))
is
well
described
by
an
Arrhenius
equation
according
to
the
~~
relation In where
activation
the
obtain
we
In rj, ~~ When
energy activation
the
and the
temperature.
In r~, ~~, field is For
exhibits r~~~ similar that to
r~~~,
T~
'
w
2.72
10~
~
(r~ ~~)
cc
e~/kT,
(4)
e~ depends on the applied e~/k=27.8K energies and
electric
field
strength.
e~/k=25.9K
from
From the
figure 3a, slopes of
respectively.
off, we find different shapes for I(t), depending on the curve temperatures, I (t) is fit by a single exponential, with a time constant Arrhenius (Fig. 3b, solid line) with dependence slope temperature an a higher (Tm95°C~ obtained for and For temperatures rj,~~ r~,~~. switched
lower
K~ ~), I
(t) is
not
exponential
and
the
relaxation
times
can
be
very
slow.
From
N°
DYNAMICS
2
OF
231
PDCLC
10~
(a)
Tz,on u
~~3
a
Ti,an
~
i~~ z j /ioi loo
D
u
(b)Q I I I I
I
10~ l I
~s
W
I
~'
z~
l I
lo i
I I
2.7
Z-B
2.9
3
1/T (1/1000 K) Fig. v2
3.
plot
Awhenius
corresponding
for
these
defined
as
text.
experiments
relaxation may
in
not
and
from
mechanisms be
an
such
parameters,
are
oft fits
(t) to
to
double
equation (4)
in
effect,
field
Eo,
the
but
(a)
Switch-on
3. I,
slow rather
we
conclude
mechanism due
to
the
DROP
Different
EFFECTS.
SIZE
cooling the sample for field (measured
130°C
from
to
90°C
and
process : v~ Switch-off
that
~~
times
eye.
different
two
high temperatures dependence of other
at
temperature.
on
for the sample same cooling Time rates. 150V/31.2 ~m) tend increase with increasing drop to D for both the switch-on and the switch-off (Fig. 4). These dependencies processes ( as might be expected from follow obvious mathematical rifle, such as r~, any cc D diffusive For the switch-off I (t) again shows time dependences, two process. process, slow mechanism occurring for 32 droplets. very ~Lm
3.3
drop
;
~m).
exponential (Eq. (2)). (b) dashed line is guide to the
section
of
occurrence
thermal
V/9.7
50 =
a
reported
results The
threihold
the
(E
constants
fits
the
exist.
intrinsically as
time
square Solid lines
on
v~j,
tile
least
to
sizes
were
with
obtained
cell
different
by
constants
diameter do
not
purely
a
~~
3.4
MICROSCOPIC
reflection axis
originates
oriented
along
OBSERVATIONS.
from the
a
uniformly
field
direction
-Microscopic oriented region (Fig. 5a).
The
observations in the
reveal
of the
center
diameter
of
these
that
droplet regions
the
with
the
selective
with
its
grows
helix with
JOURNAL
232
PHYSIQUE
DE
N°
II
,,~
io4 ,'
2
Toff
,
,'
io3
,"
R" , , ,
io z
~
w
, , '
-
~
,,
10~
,
~
,' ,' ,
~
iQ0
10
0
Fig.
Influence
4.-
V/31.2
150
E
of
the
drop
diameter
30
20
D
40
~~lm)
D
the
on
switching
times
T~,~~,
v~,~~
and
v~~~
for
~Lm.
=
investigations on with microscopic observation is in agreement increasing field strength. This droplets with a pitch of several ~Lm [9, 10]. cholesteric large droplets, the dynamics of the reflectivity change corresponds to a reversible For small growth and shrinkage of the oriented region in the drop center. The material at the periphery observed that high oriented by the field. For large droplets, we however, of the drop is not, droplets the region. For these surface entire drop, including the field strengths can orient the minutes several quite few seconds is stable from selectively reflecting up to a appearance (depending on the drop size and the field strength) after the voltage is switched off. After this of the drop while it is still selectively in the time, a non-reflecting region center appears after metastable configuration 5b). This (Fig. droplet periphery reflecting at the some : seems reorganization and complete undergoes determined time, the droplet randomly texture a I
Our
size
demonstrate on
texture.
This
slow
curvature
just
after
relaxation the
switch-off
process time.
field
corresponds
to
those
cases
conclusions.
and
results
droplet
negative
(t) has
Summary
4.
sphentlitic
the
to
retums
where
the
the
dynamic
influence
of
the
parameters
behavior
of
the
electric
field
strength,
effect
in
temperature cholesteric
and
droplets
spherical cavities in a polymer matrix. Previous investigations [9] on the configuration within larger such droplets have shown that the director pitch is cholesteric described by the sphentlite model [4] which is droplets with large characterized by planar anchoring at the droplet sudace. In these previous studies, the same compositions) were used as in the system reported here and the ratio materials (but different similar (D/p 50 ~Lm/5 ~Lm) to that of our between the drop diameter D and the pitch p was 5 ~Lm/300 nnJ). Therefore, the director configuration in our droplets is most samples (D/p described model. In with previous investigations [9], the likely by the agreement same reflection occurring in our samples under the influence of electric fields is due to the selective of each droplet, of a helicoidal with the helix axis parallel in the center structure appearance, diameter field. On increasing voltage, the of this reoriented region grows continuously to the with increasing field strength. confined
static
in
small
behaviour
of
=
=
N°
DYNAMICS
2
OF
233
PDCLC
a)
$#
«
W
b) (a) Textures of relatively small droplets at moderate field strength : the central regions exhibit off. (b) which become continuously smaller when switched the voltage is planar texture loo V/31.2 ~m). Some of of large drops about 10 min after switching off a high voltage (E Texture relaxation non-planar oriented region appears in the center, inverse the larger drops show process an a direction. axis oriented parallel to the former field while at the outside of the drop the helix remains
Fig. 5.
uniform
a
=
We
have
shown
high voltages, can
the
be
well
time JOURNAL
that
high
DE
by
described
is
constants PHYSIQIJE
II
switch-on
the
double
a
described T
2, N' 2,
by
FEBRUARY
polymer-dispersed droplet diameters. The exponential and the function, times
and
temperature,
an 1992
for
small
Arrhenius
relation.
However,
droplets is reduced by time intensity curve versus dependence of temperature for the switch-off process,
chiral
ii
234
JOURNAL
intensity
both
relaxation
spherulitic high voltages the
is
removed.
that
in
induce
can
this
first
by
affected
can
the
from
configuration
reorientation
field
and
can
initiate
which from
starts
persisting boundary layer
a
sudace
the
at
the
the
relaxation,
the
the
of
is
2
N°
microscopic investigations small droplets and small
inward
director
a
case
case
occur.
the
II
For
proceeds
texture
In
the
and
measurements
mechanisms
PHYSIQUE
DE
that
indicate
voltages droplet.
metastable
large
For
when
while
in
the
second
We
field
conclude
interface case
to
droplets,
extemal
the
droplet. center polymerfliquid-crystal of the
different
two
reorientation
the
high
is
not
field
volume of the drop. This leads to an director field in the entire strengths. field high electric indicate results that small droplet sizes and high With respect to display applications, our Conceming the applied field strength, compromises have favour fast switching. temperatures but too high voltages found : high voltages hasten the switching to be process, can on cause PDCLC displays interesting On hand, be long switch-off times. the other storage may as very
strengths
devices
change
cause
extremely
due
of the
for
relaxation
slow
their
to
slow
very
relaxation
at
low
temperatures.
Acknowledgments. H.-S.
K.
Hawaii
like
would
for
their
thank
to
very
kind
the Department hospitality. This
of
Physics
research
and was
Astronomy at the University of partially supported by Research
Corporation.
References
ill
DOANE
J. W.,
Cryst.
165
GOLEMME
(1988)
A.,
WEST
J.
L.,
WHITEHEAD
J. B.
and
WU
B.-G.,
Mol.
Cryst. Liq.
511-532.
2529-31. [2] CROOKER P. P. and YANG D. K., Appl. Phys. Lett. 57 (1990) 219-226. [3] DE VRIES H. L., Acta Crystallog. 4 (1951) Faraday Soc. 25 (1958) 29-42. [4] ROBINSON C., WARD J. C. and BEEVERS R. B., Disc. 467-494. [5] RoBiNsoN C., Mol. Cryst. 1 (1988) [6] CANDAU S., LE ROY P. and DEBEAUVAIS F., Mol. Cryst. Liq. Cryst. 23 (1973) 283-297. 1599-1607. [7] PATEL D. L. and DU PRE D. B., J. Polymer. Sci. 18 (1980) 1899-1923. [8] BOULIGAND Y. and LIVOLANT F., J. Phys. 45 (1984) 245-51. [9] YANG D. K. and CROOKER P. P., Liq. Cryst. 9 (1991), [10] KITzEROW H.-S. and CROOKER P. P., Liq. Cryst., in press. Ferroelectrics 122 (1991) 183-196. H.-S. [ll] KITzEROW and CROOKER P. P., [12] DRzAIC P. S., Liq. Cryst. 3 (1988) 1543-59.
