APPLIED PHYSICS LETTERS 103, 141910 (2013)

Possible enhancement of physical properties of nematic liquid crystals
by doping of conducting polymer nanofibres
R. Manda,1 V. Dasari,2 P. Sathyanarayana,2 M. V. Rasna,2 P. Paik,1 and Surajit Dhara2,a)1School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India
School of Physics, University of Hyderabad, Hyderabad 500046, India

2(Received 1 July 2013; accepted 17 September 2013; published online 2 October 2013)
We report on the preparation and physical characterization of the colloidal suspension ofconducting polyaniline (PANI) nanofibres and a nematic liquid crystal (5CB). The ac electricalconductivity anisotropy increases significantly and the rotational viscosity decreases withincreasing wt. % of PANI nanofibres, while other physical properties such as birefringence,dielectric anisotropy, splay, and bend elastic constants are changed moderately. The highconductivity anisotropy of liquid crystal nano-composites is very useful for magnetically steered
C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824030]
liquid crystal-nanofibre switch. VNanoparticle doped liquid crystals (LCs) have drawn alot of attention over the last decade. There are many fundamental studies on the LC nano-composites, typical examples
are aerosil,1 metal nanoparticles,2–5 and carbon nanotubes(CNTs)6–8 dispersed liquid crystals. In many of these LC
nano-composites, it has been reported that the physical andelectrooptical properties are enhanced which improves theliquid crystal display performances.9,10 So far there is only
one report on the dispersion of conducting nanofibres in liquid crystal,11 in which substantial decreases in the thresholdvoltage were reported. Apart from the high conductivity,
these fibres are very useful for various applications such as
in flexible electronic devices and antistatic coatings.12,13 In
the bulk or in the dispersed state in liquids, these nanofibres
are, in general, not orientationally ordered. There are also
some reports that elongated nanoparticles such as
CNTs,14–16 gold nano-rods,3 and self-assembled chains ofquantum dots17 are weakly coupled with the orientationalorder of the nematic liquid crystals. However, detailed physical characterization of these nano-composites is rare. In thispaper, we report on the measurement of physical properties
in dilute dispersion of conducting nanofibres of polyaniline(PANI) doped in a nematic liquid crystal. Our results indicate that the long axes of the nanofibres are oriented along
the liquid crystal director (the average alignment direction ofthe molecules) as a result the ac conductivity along the director has increased significantly than the perpendicular direction, while the birefringence, dielectric anisotropy, andelastic constants are moderately modified. The increase ofconductivity along the director is expected to come from theelectronic conduction of the nanofibres.
In the present work, the experimental cells were madeof two indium tin oxide (ITO) coated glass plates with circularly patterned electrodes. The plates were spin coated withpolyimide (AL-1254) and cured at 180 8C for 1 h and rubbed
antiparallely for planar or homogeneous alignment of thesample. For homeotropic alignment, ITO plates were spincoated with polyimide (JALS-204) and cured at 200 8C fora)

Author to whom correspondence should be addressed. Electronic mail:sdsp@uohyd.ernet.in.

0003-6951/2013/103(14)/141910/4/$30.00

1 h. The perpendicular and parallel components of the static
dielectric constant and conductivity were measured in a planar and homeotropic cells, respectively, using an impedance
analyser (Novocontrol, Alpha-A) at a voltage below theFreedericksz’ threshold voltage. The birefringence, elasticconstants, and rotational viscosity are measured following
the procedure reported in our previous studies.18–21 The
structure of the polymer and the method of preparation ofnanofibres are presented briefly in the supplementary materials.22 A typical field emission scanning electron microscope(FE-SEM) image of the nanofibres is shown in Fig. 1(a).
The nanofibres are solid and mostly straight. The length of
the nanofibre varies from 1.5 to 11 lm, while the diameter ofthe fibre varies from 100 to 250 nm.22 We used 4-n-pentyl40 -cyanobiphenyl (5CB) liquid crystal. It has the following
phase transitions Cr. 22 8C N 35 8C Iso. The colloidal dispersion of PANI in 5CB was prepared by using chloroform as aFIG. 1. (a) FE-SEM image of the conducting nanofibres (without liquidcrystal). (b) Liquid crystal nano-composites (5CB-PANI) at various wt. % ofPANI nanofibres.

103, 141910-1

C 2013 AIP Publishing LLC
V141910-2

Manda et al.

solvent. The mixtures were sonicated for 30 min and the solvent was evaporated slowly (12 h) using a vacuum oven. The
picture of bulk samples at various concentrations of nanofibres (PANI) is shown in Fig. 1(b). The dispersions are stable,
homogeneous, and becoming darker as the concentration isincreased. We found from the optical microscope observation that beyond 0.2 wt. %, the nanofibres tend to agglomerate and defect free uniform alignment of liquid crystal
director cannot be obtained. Hence, we restricted measurements within 0.2 wt. % of PANI nanofibres.
We measured the dielectric properties of the composites
as a function of frequency in the nematic phase. The real andimaginary parts of the dielectric constant (!0 and !00 ) as afunction of frequency in the range of 10!3 ! 106 Hz for thepure 5CB and sample with 0.1 wt. % PANI nanofibres areshown in Fig. 2. It is noted that both the dielectric properties
are enhanced in the doped samples compared to the pristine
sample especially at low frequency range and the frequency
for the ionic relaxation has increased slightly. There are different mechanism for dielectric response and the electricalconduction in organic and polymeric materials.23,24 The
dielectric spectra in the low frequency range (10!3 –10 Hz)
describe the near electrode processes such as electric double
layer and formation of space charge polarization.25–29 In
the present data, the dielectric relaxation due to double
layer (10!3 ! 10!1 Hz) and space charge polarisation(10!1 ! 10 Hz) are seen clearly in both the planar and homeotropic cells. Nematic liquid crystal exhibits finite conductivity due to the ionic impurities that depends on the density ofions, temperature, and the frequency of the applied field.
Usually, the ionic conductivity along the director is different
than the perpendicular direction and liquid crystal exhibits
finite conductivity anisotropy Dr (¼ rjj ! r? ), where thesubscripts refer to the direction in relation to the director.
Since PANI has a large conductivity, we estimated the ac
conductivity of composites by measuring the imaginary component of the dielectric constant. The ac conductivity (r) iscalculated using the relation, rjj;? ðf Þ ¼ 2pf !o !00jj;? , where !o isFIG. 2. Variation of (a) real and (b) imaginary parts of the dielectric constant(!0 and !00 ) as a function of frequency. The solid and open symbols correspondto data in homeotropic and planar cells, respectively. The black and red colors
represent data for pure 5CB and sample with 0.1 wt. % of PANI nanofibres.

Appl. Phys. Lett. 103, 141910 (2013)

the permittivity of the free space and f is the frequency of theac voltage. In order to bring out the effect of PANI nanofibres
on the estimated ac conductivity, we carried out temperaturedependent measurement of the same at a fixed frequency(100 Hz), which is in the non-relaxation frequency range andreasonable away from the frequency range of space chargeand electric double layer effects. In all the dielectric measurements, the applied voltage was chosen 0.2 V. This voltage ismuch lower than the Freedericksz threshold voltage (0.62 V
at 31 8C) that causes elastic distortion. In Fig. 3(a), we show
the variation of conductivity in homeotropic (rjj ) and planar(r? ) cells at various concentrations of PANI nanofibres.
Some important features are noted in Fig. 3(a). The nematicisotropic phase transition temperature (TNI) slightly decreases
with increasing concentration of PANI and the conductivity
anisotropy vanishes above TNI. At a fixed temperature,
the estimated ac conductivity anisotropy (Dr ¼ rjj ! r? )increases with the increasing wt. % of PANI nanofibres. Thetemperature dependent slope of the ac conductivity in boththe cells increases with the PANI nanofibre concentration.
This clearly indicates that the estimated conductivity in thenano-composites is contributed by electron conduction. InFig. 3(b), we plot the variation of ac conductivity in both thecells as a function of concentration of PANI nanofibres. It isFIG. 3. (a) Temperature variation of estimated conductivity in homeotropic(rjj ) and planar (r? ) cells at a frequency 100 Hz and voltage 0.2 V. The solidand open symbols correspond to date in homeotropic and planar cells,respectively. (b) Variation of estimated conductivity in both the cells withthe concentration of PANI. (Inset) Variation of the relative enhancement,
Dr5CBþPANI =Dr5CB with the wt. % of PANI nanofibres at 30 8C. The dottedlines serve only as a guide to the eye and no curve fitting has been performed. Schematic representation of the alignment of the nanofibres in thenematic liquid crystal.

141910-3

Manda et al.

Appl. Phys. Lett. 103, 141910 (2013)

TABLE I. Concentration dependent physical properties of the nanocomposites. Except c1 (measured at 27 & C) all are measured at 31 & C.
PANI (wt. %)

00.05

0.1

0.2

Dnð¼ ne ! no ÞD!ð¼ !jj ! !? ÞK11 (pN)
K33 (pN)
c1 (mPa s)

0.147
9.6
3.7
5.1
70

0.149
9.7
3.6
5.3
64

0.144
10.1
3.7
5.4
57

0.148
10.5
3.8
5.6
53

noticed that both rjj and r? increase with the concentrationof PANI but the relative rate in rjj is much larger than that inr? . To show the relative magnitude of enhancement, we plot
the variation of Drð5CBþPANIÞ =Dr5CB as a function of wt. % ofPANI nanofibres in the inset of Fig. 3(b). It is foundthat the ratio increases rapidly, for example, at 0.2 wt. %,Drð5CBþPANIÞ =Dr5CB ’ 12. Such a large enhancement of conductivity anisotropy clearly indicates that the nanofibres areoriented along the liquid crystal director and the enhancementof ac conductivity is due to the electronic conduction of thenanofibres. A schematic representation of the orientation ofPANI nanofibres in liquid crystal is also shown in the inset ofFig. 3(b). The estimated conductivity in pure 5CB liquid crystal in the frequency range of 1-103 Hz is due to the presenceof impurity ions in the sample and the anisotropy in theconductivity is related to the anisotropy in diffusion of ions,
i.e., the ions can move easily along the director than the perpendicular direction. Usually, the adsorbed molecules of theliquid crystals form a Helmholtz layer which prevents or limits the charge transfer between the electrode and the ions. The
highly resistive electric double layers also prevent chargetransfer. The pronounced enhancement of rjj in our composites can be explained due to the charge transfer through theelectrode double layers in which the nanofibres shunt theresistive double electric layer. Hence, nanofibres provide a
medium for electron transfer between the ions and electrode.
Similar near electrode process was reported in case of
liquid crystal doped with multi-wall carbon nanotubes(MWCNTs).30
In order to observe the effects of PANI nanofibres on
the other physical properties, we measured birefringence(Dn ¼ ne ! no ), dielectric anisotropy (D! ¼ !jj ! !? ) andsplay, bend elastic constants (K11 and K33), and rotational
viscosity (c1 ). The brief description about the method of themeasurements is presented in the supplementary materials.22
In Table I, we show the variation of these physical properties
with the concentration of PANI nanofibres at a fixed temperature for four samples. It is observed that Dn and K11 remain
almost constant. Other properties such as D! and K33 appear
to show a small (’ 10%) but systematic increase with theincreasing concentration of PANI nanofibres. However, it ishard to conclude about their dependence on the concentration unless composites with higher concentration of PANI
can be made. Interestingly, we also find that the rotational
viscosity decreases with increasing concentration of PANI
nanofibres. For instance, at 27 8C, c1 for pure 5CB is about
70 mPa s and reduces to 53 mPa s in the nano-composite(0.2 wt. %) and this reduction is about 24%. The phenomena
could be related to the decrease of TNI with increasing

concentration of PANI in the sense that the easier the anisotropic particles can rotate, the less the amount of thermalenergy is required to heat the system up to the isotropic liquid state. However, further studies are needed in this direction to fully understand the origin of decrease of rotational
viscosity of the nano-composites. More details study towards
the composites with higher concentration and their physicalproperties will be reported elsewhere.
In conclusion, we prepared colloidal suspension of
nanofibres of conducting polymers (PANI) in 5CB liquid
crystal and measured various physical properties. The nanofibres are oriented along the liquid crystal director and as aresult the ac conductivity anisotropy has increased significantly due to the contribution from the electronic conductivity along the long axes of the nanofibres. The rotational
viscosity tends to decrease while birefringence, dielectric anisotropy, both splay and bend elastic constants do not change
significantly with the concentration of PANI nanofibres. As
we have mentioned that in 5CB uniform dispersion of nanofibres was not possible beyond 0.2 wt. % nevertheless weanticipate that the conductivity anisotropy in the LC-nanocomposites can further be increased in high concentration byappropriate choice of liquid crystals. These high conductivity
anisotropies of liquid crystal nano-composites are useful forvarious applications such as magnetically steered liquid
crystal-nanotube switch,31 in which the conductivity can beswitched from low to high value by external magnetic fields.
We gratefully acknowledge the support from the DST(SR/NM/NS-134/2010), CSIR (03(1207)/12/EMR-II), andDST-PURSE. V.D and P.S. acknowledge UGC for fellowship.

1G. S. Iannacchione, Fluid Phase Equilib. 222, 177 (2004).
E. Ouskova, O. Buchanev, V. Reshetnyak, Y. Reznikov, and H. Kresse,Liq. Cryst. 30, 1235 (2003).
3
S. Sridevi, S. K. Prasad, G. G. Nair, V. D’Britto, and B. L. V. Prasad,Appl. Phys. Lett. 97, 151913 (2010).
4
S. K. Prasad, K. L. Sandhya, G. G. Nair, U. S. Hiremath, C. V.
Yelamaggad, and Sampath, Liq. Cryst. 33, 1121 (2006).
5
S. Kobayashi, T. Miyama, N. Nishida, Y. Saakai, H. Shiraki, Y. Shiraishi,and N. Toshima, J. Disp. Technol. 2, 121 (2006).
6I. Dierking, G. Scalia, and P. Morales, J. Appl. Phys. 97, 044309 (2005).
7R. Basu and G. S. Iannacchione, Appl. Phys. Lett. 95, 173113 (2009).
8J. P. F. Lagerwall and G. Scalia, J. Mater. Chem. 18, 2890 (2008).
9
H. Y. Chen, W. Lee, and N. Clark, Appl. Phys. Lett. 90, 033510 (2007).
10
S. Y. Jeon, S. H. Shin, S. J. Jeong, S. H. Lee, S. H. Jeong, Y. H. Lee, H. C.
Choi, and K. J. Kim, Appl. Phys. Lett. 90, 121901 (2007).
11
S. Ghosh, P. Nayek, S. K. Roy, R. Gangopadhyay, M. R. Molla, and R.
Dabrowski, Appl. Phys. Lett. 96, 073101 (2010).
12
L. A. A. Pettersson, S. Ghosh, and O. Ingans, Org. Electron. 3, 143 (2002).
13
D. Hohnholz, H. Okuzaki, and A. G. Mac Diarmid, Adv. Funct. Mater. 15,51 (2005).
14
R. Basu and G. I. Iannacchione, Phys. Rev. E 81, 051705 (2010).
15
M. Rahman and W. Lee, J. Phys. D: Appl. Phys. 42, 063001 (2009).
16
M. D. Lynch and D. L. Patrick, Nano Lett. 2, 1197 (2002).
17
R. Basu and G. I. Iannacchione, Phys. Rev. E 80, 010701(R) (2009).
18
P. Sathyanarayana, V. S. R. Jampani, M. Skarabot, I. Musevic, K. V. Le,
H. Takezoe, and S. Dhara, Phys. Rev. E 85, 011702 (2012).
19
P. Sathyanarayana, B. K Sadashiva, and S. Dhara, Soft Matter 7, 8556
(2011).
20
P. Sathyanarayana, M. Mathews, Q. Li, V. S. S. Sastry, B. Kundu, K. V.
Le, H. Takezoe, and S. Dhara Phys. Rev. E 81, 010702(R) (2010).
21
P. Sathyanarayana, M. C. Varia, A. K. Prajapati, B. Kundu, V. S. S.
Sastry, and S. Dhara, Phys. Rev. E 82, 050701(R) (2010).
22
See supplementary material at http://dx.doi.org/10.1063/1.4824030 for thebrief description of the length distribution of PANI nanofibres and the2