PHYSICAL REVIEW E 89, 052503 (2014)

Discontinuous anchoring transition and photothermal switching in composites of liquid
crystals and conducting polymer nanofibers
M. V. Rasna,1 K. P. Zuhail,1 R. Manda,2 P. Paik,2 W. Haase,3 and Surajit Dhara1,*
1

School of Physics, University of Hyderabad, Hyderabad 500046, India
School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India
3
Darmstadt University of Technology, Eduard Zintl Institute for Inorganic and Physical Chemistry, Petersenstrasse 20,
Darmstadt 64287, Germany(Received 17 March 2014; revised manuscript received 25 April 2014; published 8 May 2014)2

We prepared nanocomposites of a nematic liquid crystal and nanofibers of a conducting polymer (polyaniline).
All the nanocomposites exhibit a discontinuous surface anchoring transition from planar to homeotropic in thenematic phase on a perfluoropolymer coated surface with a thermal hysteresis (≈5.3 ◦ C). We observe a relatively
large bistable conductivity and demonstrate a light driven switching of conductivity and dielectric constant in dye
doped nanocomposites in the thermal hysteresis (bistable) region. The experimental results have been explained
based on the reorientation of the nanofibers driven by the anchoring transition of the nematic liquid crystal.
We show a significant enhancement of the bistable temperature range (≈13 ◦ C) by an appropriate choice ofcompound in the binary system.
DOI: 10.1103/PhysRevE.89.052503

PACS number(s): 61.30.Hn, 61.30.Eb, 42.70.Df

I. INTRODUCTION

In liquid crystal displays (LCDs) appropriate alignment
layers are used on the substrates for the uniform and defect freealignment of the director (average orientational direction of themolecules [1]). Based on the surface alignment layers liquid
crystals (LCs) can show two stable molecular orientations,
namely, planar (long axes parallel to the substrate) andhomeotropic (long axes perpendicular to the substrate) [2].
These states are stable, and the switching between these
two states in the cell is realized by applying electric or
magnetic fields [3]. LCDs are monostable optical devices
in the sense that the “on” state is maintained as long as
the field is kept on, requiring a continuous power supply.
Optical bistable devices are those in which two states withdifferent transmissions can be obtained at the same appliedfield and temperature. There are several reports on thebistable orientation of nematic liquid crystals [4–13]. In thecase of bistable electrical devices two states with different
conductivities can be obtained simultaneously. These devices
are ideal for switching and memory applications and have been
demonstrated in both inorganic and organic materials [14–19].
It is known that liquid crystals can be used as a templateto achieve self-assembly of carbon nanotubes [20–22], gold
nanorods [23], quantum dots [24], and nanofibers [25]. In
the case of elongated particles the long axes of the particles
are orientationally ordered along the LC director. The degreeof orientational ordering of the nanoparticles in the LCs cansignificantly change the anisotropic physical properties. By
applying external fields the orientation of the LC molecules
and hence the nanoparticles can be changed. However, they
are also monostable because of the monostablity of thehost medium (liquid crystal). In previous studies we havereported a discontinuous anchoring transition in a nematic
liquid crystal from planar to homeotropic with a large thermal
hysteresis on perfluoropolymer treated substrates [26,27].

*

Corresponding author: sdsp@uohyd.ernet.in

1539-3755/2014/89(5)/052503(5)Recently, the heat capacity and thermal diffusivity change ofthe discontinuous anchoring transition have also been reported[28,29]. We demonstrated the optical bistability and memory
effects driven by laser light [30–33]. However, the possibilityof achieving the bistable orientation of elongated nanoparticles and switching the orientation via the external field or
temperature has remained unexplored. In this paper, we reporton the liquid crystal conducting polymer nanofiber (PANI)
composites that exhibit a discontinuous anchoring transition
from planar to homeotropic and vice versa with a large thermal
hysteresis in perfluoropolymer treated cells. We show that theanchoring transition is retained in the nanocomposite without
affecting the thermal hysteresis (i.e., bistable temperature
range). The orientational coupling of the PANI nanofibers
in the liquid crystal medium is examined by measuring the
low frequency dielectric constant and conductivity. Exploiting
the enhanced conductivity in the bistable region, we demonstrate an electrical conductivity switching in the dye doped
sample driven by light. Finally, we show that using theappropriate choice of compound from the homologous series,
the bistable temperature range can be enhanced by more thandouble.
II. EXPERIMENT

Indium tin oxide coated (ITO) glass plates with a circularly
patterned electrode are used to make experimental cells.
The glass substrates are spin coated with the perfluoropolymer poly[perfluoro(4-vinyloxy-1-butene)], known as CYTOP(Asahi Glass Co. Ltd.), and cured for 30 min at 100 ◦ C. Empty
cells with 13.5 µm spacing were fabricated using two suchglass plates. The liquid crystal used in the experiment is
4# -butyl-4-heptyl-bicyclohexyl-4-carbonitrile (CCN-47) andwas obtained from Merck. It exhibits the following phasetransitions: Cr 25.6 ◦ C SmA 28.4 ◦ C N 58.2 ◦ C I. It showsa discontinuous planar to homeotropic anchoring transition
with thermal hysteresis (≈5.3 ◦ C) in CYTOP coated cells.
The chemical structures of CYTOP and CCN-47 are shownin Fig. 1(a). Various physical characterizations of this liquid052503-1©2014 American Physical Society

PHYSICAL REVIEW E 89, 052503 (2014)RASNA, ZUHAIL, MANDA, PAIK, HAASE, AND DHARA

FIG. 1. (Color online) (a) Chemical structures of CCN-47 andCYTOP. (b) FESEM image of conducting PANI nanofibers. Photomicrograph of the textures showing the anchoring transition in
CCN-47 under crossed polarizers: (c) CCN-47 at 58 ◦ C, (d) CCN-47
at 47.6 ◦ C, and (e) CCN-47 at 46 ◦ C.

crystal have already been reported [34–36]. The method of
preparation of polyaniline (PANI) nanofibers has also beenreported recently [37]. The length and diameter of the PANI
nanofibers range from 1.5 to 11 µm and 100 to 250 nm,
respectively [Fig. 1(b)]. PANI nanofibers are dispersed inchloroform and sonicated for 30 min for uniform suspension.
Different concentrations of suspensions are added to theliquid crystal and mixed thoroughly by physical mixing,
and then the solvent is evaporated very slowly (12 h). We
found that the colloidal suspension of the nanofibers in
the liquid crystal can be obtained with a maximum of 1.5
wt %, beyond which they tend to agglomerate. Hence werestricted our measurements to within 1.5 wt % of PANI
nanofibers in the liquid crystal. It may be mentioned that
this concentration is about 10 times larger than that reported
previously in the case of pentyl cyanobiphenyl (5CB) [25].
After empty capacitance measurement, the sample was filledin the cell through capillary action. The dielectric properties
were measured using an impedance analyzer (Novocontrol
Alpha-A). All measurements were performed at a frequencyof 100 Hz by applying 0.3 V (much less than the Fréedericksz
threshold voltage) across the cell. The ac conductivity is##
calculated using the relation σ||,⊥ (f ) = 2πf #o #||,⊥ where #ois the permittivity of the free space and f is the frequencyof the applied field. The subscripts refer to the directions inrelation to the director. Considering the variation in the cell
thickness and the standard deviation of the conductivity anddielectric data, our results are accurate within 3%. It may bementioned that the conductivity anisotropy $σ (=σ|| − σ⊥ ) of
CCN-47 changes sign from positive to negative around 3 kHz
frequency. The space charge polarization and electric double
layer effects are observed below 10 Hz; hence we calculated
conductivity from the imaginary part of the dielectric constantat 100 Hz in all the measurements.
III. RESULTS AND DISCUSSION

We show the textures observed under a polarizing optical
microscope at various temperatures of CCN-47 in a CYTOP

FIG. 2. (Color online) Temperature variation of the effective
dielectric constant (#eff ) and the effective conductivity (σeff ) in a
CYTOP coated cell (inset). Applied voltage: 0.3 V, frequency:
100 Hz. Cell thickness is 13 µm. Molecular orientations at different
temperatures are also schematically depicted.

coated cell in Fig. 1. In the nematic phase a typical Schlieren
texture with a few integer and mostly half defects are observed[Fig. 1(c)]. On cooling below 47.6 ◦ C, a few small, dark regions
grow abruptly and randomly [Fig. 1(d)]. The dark regions are
domains [two dark spots seen in Fig. 1(d)] with homeotropic
orientation of the molecules [26]. On further cooling, thewhole sample becomes completely dark, suggesting a perfecthomeotropic alignment of the molecules in the entire sample.
This transition is discontinuous, and the width of the coexistingregion is <1 ◦ C. To investigate the change of molecularorientation from isotropic to planar and then to homeotropic,
we measured the dielectric constant and conductivity of thesample as a function of temperature. The variation of both

FIG. 3. (Color online) Time dependence of the dielectric constant and conductivity in the bistable region at 48 ◦ C in CYTOP coatedcells in a pure liquid crystal and in a nanocomposite with 0.2 wt %PANI nanofiber. The subscripts parallel (||) and perpendicular (⊥)
correspond to the quantities measured in the homeotropic and planarstates, respectively.

052503-2

PHYSICAL REVIEW E 89, 052503 (2014)

DISCONTINUOUS ANCHORING TRANSITION AND . . .

the effective dielectric constant and conductivity in CYTOP
coated cells is shown in Fig. 2. Both the effective dielectric
constant (#eff ) and the effective conductivity (σeff ) show
a thermal hysteresis in the nematic phase. Comparing thedielectric constant reported in the literature for both planar andhomeotropic cells [36], we find that in the hysteresis region(between 53.8 ◦ C and 47.8 ◦ C) #eff ≈ #⊥ while cooling (stateα) and #eff ≈ #|| while heating (state β). These two states arestable with time. This is further verified from microscopeobservations and by measuring the dielectric constant andconductivity of the respective states with time at a fixed
temperature. Figure 3 shows the variation of the conductivity
and dielectric constant as a function of time in a pure liquid
crystal as well as in a nanocomposite (with 0.2 wt % PANI).
It is observed that the dielectric constants (both #|| and #⊥ )remained almost constant, while the conductivities (both σ||
and σ⊥ ) increased with time and saturated after 3 to 4 days.
Hence all our measurements are made after 5 days. A similar
behavior of the conductivity was also observed in carbon
nanotube dispersed liquid crystals, and it was attributed tothe time dependent percolation of the nanotubes [38].
Figure 4(a) shows the temperature variation of #eff for all the
nanocomposites. The nematic to isotropic (TNI ) transition andthe anchoring transition temperatures are slightly increased.
For example, TNI increases about 1.4 ◦ C when the concentration of PANI is increased to 1.5 wt %. All the nanocomposites
exhibit thermal hysteresis, and the temperature range of the

FIG. 4. (Color online) (a) Temperature variation of the effective dielectric constant (#eff ) for various weight percent of PANI
nanofibers. Only five different concentrations are presented for clarity.
In the bistable region, #eff ≈ #|| in the homeotropic state, and #eff ≈ #⊥in the planar state. (b) Variation of both dielectric constants in thebistable region at a fixed temperature (48.5 ◦ C). The inset shows thevariation of dielectric anisotropy $# with various weight percent of
PANI nanofiber.

hysteresis remains almost unaffected. In Fig. 4(b) the variationof #|| and #⊥ with the concentration of PANI nanofibers at
a fixed temperature (48.5 ◦ C) is shown. It is observed that#⊥ remains almost constant (≈8.5), while #|| increases withthe concentration of PANI nanofibers. For example, #|| has
increased from 4.3 to 5.5, showing an enhancement of about
20%. The dielectric anisotropy ($# = #|| − #⊥ ) in the bistableregion is negative and has increased from −4.2 to −3 (inset).
This suggests that the nanofibers are dielectrically anisotropic
and the perpendicular component of the dielectric constant of
the nanofibers is comparable to that of the liquid crystal i.e.,#⊥PANI ≈ #⊥LC , and the parallel component must be larger thanthat of a pure liquid crystal (i.e., #||PANI > #||LC ).
The effect of the orientational coupling of nanofibers can befurther investigated by measuring the effective conductivity of
the nanocomposites since the nanofibres are made of conducting polymer (PANI). Figure 5(a) shows the temperature variation of the effective conductivity of various nanocomposites.
The conductivity of the nanocomposites in the isotropic phaseincreases with the weight percent of the PANI nanofibers as
expected. Figure 5(b) shows the variation of conductivity (σ||and σ⊥ ) with concentration at a fixed temperature (48.5 ◦ C).
The conductivity anisotropy (the difference in conductivities
between states α and β) at 100 Hz is positive and increases withthe concentration of PANI nanofibers. For example, the conductivity anisotropies in pristine CCN-47 and in a nanocomposite (1.5 wt %) are ≈0.5 × 10−9 and ≈10.5 × 10−9 S/m,respectively (see the inset). Thus the conductivity anisotropy

FIG. 5. (Color online) (a) Temperature variation of the effective
conductivity (σeff ) in the bistable regime for various weight percent of
PANI in the nanocomposites. In the bistable region, σeff ≈ σ|| in the
homeotropic state, σeff ≈ σ⊥ in the planar state. (b) Variation of the
conductivity in the bistable region at a fixed temperature (48.5 ◦ C).
The inset shows the variation of the conductivity anisotropy $σ withvarious weight percent of PANI nanofiber.

052503-3

PHYSICAL REVIEW E 89, 052503 (2014)RASNA, ZUHAIL, MANDA, PAIK, HAASE, AND DHARA

FIG. 6. (Color online) Diagram of the experimental setup forphotothermal switching.

is enhanced nearly 20 times. Previously, we reported that the
conductivity anisotropy is increased by about 10 times just by
doping 0.2 wt % of PANI nanofibers in a 5CB liquid crystal[25]. We explained that such a large enhancement in σ|| is due tothe contribution of both the ionic and electronic conductivities.
Although the absolute values of both conductivities (σ||and σ⊥ ) in the nanocomposites are much lower than the
conductivity of pure PANI nanofibers [37], the relative
enhancement of $σ in the composites is large comparedto that of the pristine liquid crystal. Hence these nanocomposites can be used as biconducting materials in which two
conductivities can be obtained simultaneously, and switching between them is possible by an appropriate external
stimulus. Here we demonstrate a light driven switchingof conductivity by doping a small amount (0.5 wt %) oflaser dye [4-dicyanomethylene-2-methylene-2-methyl-6-(pdimethylaminostyryl)-4H-pyran (DCM)]. A schematic diagram of the experimental setup is shown in Fig. 6. We useda tunable Ar-ion laser (130 mW) whose wavelength range
falls in the absorption range of DCM. The light beam was
expanded to illuminate the cell by an appropriate combinationof lenses. First, the sample was cooled below 47 ◦ C to obtaina homeotropic state (state β in Fig. 2); then it was heated up to53 ◦ C, which is just about 1 ◦ C below the homeotropic to planaranchoring transition temperature. The cell is exposed to laser
light for a few seconds (2–4 s), and the dye molecules absorb
the light energy; as a result the temperature of the exposedarea is increased. If the local temperature is greater than theanchoring transition temperature (homeotropic to planar), themolecules in the exposed area change orientation from thehomeotropic to the planar state (state β to state α in Fig.
2, inset), and consequently, the conductivity decreases. In
order to measure the response time, we measured the timedependent dielectric constant and conductivity for a duration
before and after the irradiation using the impedance analyzer.
The variation of the normalized dielectric constant [#(t)/#|| ]and conductivity [σ (t)/σ|| ] with time is shown in Fig. 7.
After switching off the laser, the normalized conductivity
quickly decreases to a lower value, while the normalized
dielectric constant increases to a higher value, as expected.
The representative models of the molecule and nanofiber
orientation in two states are also shown in Fig. 7 (inset).
The response time, i.e., the time within which the normalized
conductivity decreases from the high to low value, is about
25 s. This is rather a slow response as the change in the

FIG. 7. (Color online) Time dependence of the normalized dielectric constant [#(t)/#|| ; open red circles] and conductivity [σ (t)/σ|| ;solid blue circles] in the bistable region under the action of laser. The
duration of the laser irradiation is indicated between two verticaldotted lines. The schematic orientation of liquid crystal molecules
and the nanofibers is also shown.

molecular orientation is a photothermal process and switchingtime is limited by the heat transfer rate. It may be mentionedthat the conductivity decay has two components, namely, thecontributions from the director reorientation and also from thegrowth of the planar regions with time (after switching off
the laser). At this stage we do not have much control over thelatter process, which is expected to make the response slow.
For practical applications and also for fundamental studies
it is necessary to have a wider range of bistability at lower
temperature. In previous studies we reported that the anchoringtransition temperatures can be lowered to room temperature ina binary mixture with positive dielectric anisotropy material[30]. However, there was no change in the bistable temperature
range. Here we prepared a binary mixture of CCN-47 andCCN-55. The phase transition temperatures of CCN-55 are
I 66 ◦ C N 30 ◦ C smectic-B (Sm-B), and it (CCN-55) is ahomologue of CCN-47. CCN-55 also exhibits a discontinuous
anchoring transition on CYTOP coated substrates with abistable range of about 9 ◦ C. Although the bistable range iswider than that of CCN-47, the main disadvantage is that theplanar to homeotropic transition occurs in the Sm-B phase.
Hence we prepare binary mixtures of these two compounds.
Figure 8 shows the phase diagram with the bistable temperature

FIG. 8. (Color online) Phase diagram of a binary mixture composed of CCN-47 and CCN-55. The shaded area represents thebistable region where both the homeotropic and planar orientations
are stable.

052503-4

PHYSICAL REVIEW E 89, 052503 (2014)

DISCONTINUOUS ANCHORING TRANSITION AND . . .

range. It is observed that the largest bistable temperature
range (≈13 ◦ C) is obtained in the mixture with 62 wt % of
CCN-55. This is about 2.5 times larger than the bistable rangeof pure CCN-47. In addition, the bistability occurs in thenematic phase near room temperature. Finally, we would liketo mention that the photothermal switching is an experimental
demonstration of reorientation of PANI nanofibers by nematic
director reorientation. The absolute values of the conductivities
have to be increased several orders of magnitude for any
practical application.

nematic director. In the nanocomposites, both the dielectricanisotropy and conductivity anisotropy are increased, but in thelatter one the enhancement is significantly large. The bistableorientation of the nanofibers is achieved by the temperaturedriven anchoring transition of the host liquid crystal. In thebistable region we demonstrated a photothermal switching of
the nanofibers from vertical to planar. The bistable temperature
range in the nematic phase is enhanced 2.5 times in a binary
mixture.
ACKNOWLEDGMENTS

IV. CONCLUSION

In conclusion, we prepared liquid crystal conducting
polymer nanocomposites, and our results suggest that thepolyaniline nanofibers are aligned in the direction of theWe gratefully acknowledge financial support from DST(SR/NM/NS-134/2010), CSIR [03(1207/12/EMR-II)], UPEII (UoH), and Ashai Glass, Japan, for CYTOP. We thankDr. V. S. R. Jampani for useful discussions.

[1] P. G. de Gennes, The Physics of Liquid Crystals (Clarendon,
Oxford, 1993).
[2] K. Takatoh, M. Hasegawa, M. Koden, N. Itoh, R. Hasegawa,and M. Sakamoto, Alignment Technologies and Applications ofLiquid Crystals (Taylor and Francis, London and New York,
2005).
[3] Liquid Crystals: Applications and Uses, edited by B. Bahadur(World Scientific, Singapore, 1990), Vol. 1.
[4] J. Niitsuma, M. Yoneya, and H. Yokoyama, Appl. Phys. Lett.
92, 241120 (2008).
[5] J. S. Gwag, J. H. Kim, M. Yoney, and H. Yokoyama, Appl. Phys.
Lett. 92, 153110 (2008).
[6] G. D. Boyd, J. Cheng, and P. D. T. Ngo, Appl. Phys. Lett. 36,
556 (1980).
[7] D. W. Berreman and W. R. Heffner, J. Appl. Phys. 52, 3032
(1981).
[8] H. Ong, R. B. Meyer, and A. Hurd, J. Appl. Phys. 55, 2809
(1984).
[9] R. Barberi, M. Boix, and G. Durand, Appl. Phys. Lett. 55, 2506
(1989).
[10] J. H. Kim, M. Yoneya, J. Yamamoto, and H. Yokoyama, Appl.
Phys. Lett. 78, 3055 (2001).
[11] P. C. Schuddeboom and B. Jerome, Europhys. Lett. 39, 515(1997).
[12] M. Yoneya, J. H. Kim, and H. Yokoyama, Appl. Phys. Lett. 80,
374 (2002).
[13] D. Andrienko, A. Dyadyusha, Y. Kurioz, V. Reshetnyak, and Y.
Reznikov, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 321,
299 (1998).
[14] I. B. Altfeder and D. M. Chen, Phys. Rev. Lett. 84, 1284
(2000).
[15] H. Carchano, R. Lacoste, and Y. Segui, Appl. Phys. Lett. 15,414 (1971).
[16] R. S. Potember, T. O. Poehler, and D. O. Cowan, Appl. Phys.
Lett. 34, 405 (1979).
[17] J. Miller, Angew. Chem. 42, 27 (2003).
[18] B. Y. Yang, J. Ouyang, L. Ma, R. J. Tseng, and C. W. Chu, Adv.
Funct. Mater. 16, 1001 (2006).

[19] L. P. Ma, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 2997
(2002).
[20] R. Basu and G. S. Iannacchione, Phys. Rev. E 81, 051705 (2010).
[21] M. Rahman and W. Lee, J. Phys. D 42, 063001 (2009).
[22] M. D. Lynch and D. L. Patrick, Nano. Lett. 2, 1197 (2002).
[23] S. Sridevi, S. K. Prasad, G. G. Nair, V. D’Britto, and B. L. V.
Prasad, Appl. Phys. Lett. 97, 151913 (2010).
[24] R. Basu and G. S. Iannacchione, Phys. Rev. E 80, 010701(R)(2009).
[25] R. Manda, V. Dasari, P. Sathyanarayana, M. V. Rasna, P. Paik,and S. Dhara, Appl. Phys. Lett. 103, 141910 (2013).
[26] S. Dhara, J. K. Kim, S. M. Jeong, R. Kogo, F. Araoka, K.
Ishikawa, and H. Takezoe, Phys. Rev. E 79, 060701(R) (2009).
[27] T. A. Kumar, P. Sathyanarayana, V. S. S. Sastry, H. Takezoe,
N. V. Madhusudana, and S. Dhara, Phys. Rev. E 82, 011701(2010).
[28] S. Aya, Y. Sasaki, D. Pociecha, F. Araoka, E. Gorecka,
K. Ema, I. Musevic, H. Orihara, K. Ishikawa, and H.
Takezoe, Phys. Rev. E 89, 022512 (2014).
[29] M. Uehara, S. Aya, F. Araoka, K. Ishikawa, H. Takezoe, and J. Morikawa, Chem. Phys. Chem. (2014),
doi:10.1002/cphc.201300975.
[30] J. K. Kim, F. Araoka, S. M. Jeong, S. Dhara, K. Ishikawa, and
H. Takezoe, Appl. Phys. Lett. 95, 063505 (2009).
[31] J. K. Kim, K. V. Le, S. Dhara, F. Araoka, K. Ishikawa, and H.
Takezoe, J. Appl. Phys. 107, 123108 (2010).
[32] T. A. Kumar, K. V. Le, S. Aya, S. Kang, F. Araoka, K. Ishikawa,S. Dhara, and H. Takezoe, Phase Trans. 85, 888 (2012).
[33] V. S. R. Jampani, M. Skarabot, H. Takezoe, I. Musevic, and S.
Dhara, Opt. Express 21, 724 (2013).
[34] S. Dhara and N. V. Madhusudana, Europhys. Lett. 67, 411(2004).
[35] S. Dhara and N. V. Madhusudana, Europhys. J. E 13, 401 (2004).
[36] S. Dhara and N. V. Madhusudana, Phase Trans. 81, 561 (2008).
[37] P. Paik, R. Manda, C. Amgoth, K. S. Kumar, RSC Adv. 4, 12342(2014).
[38] A. I. Goncharuk, N. I. Lebovka, L. N. Lisetski, and S. S.
Minenko, J. Phys. D 42, 165411 (2009).

052503-5