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Phase transitions, optical, dielectric and viscoelastic
properties of colloidal suspensions of BaTiO3nanoparticles and cyanobiphenyl liquid crystals
a

b

c

d

a

M.V. Rasna , L. Cmok , D.R. Evans , A. Mertelj & Surajit Dharaa

School of Physics, University of Hyderabad, Hyderabad, Indiab

Department of Physics, University of Ljubljana, Ljubljana, Sloveniac

Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson
Air Force Base, OH, USAdClick for updates

J. Stefan Institute, Ljubljana, Slovenia
Published online: 08 Apr 2015.

To cite this article: M.V. Rasna, L. Cmok, D.R. Evans, A. Mertelj & Surajit Dhara (2015) Phase transitions, optical, dielectricand viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals, Liquid Crystals,
42:7, 1059-1067, DOI: 10.1080/02678292.2015.1021719
To link to this article: http://dx.doi.org/10.1080/02678292.2015.1021719

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Liquid Crystals, 2015
Vol. 42, No. 7, 1059–1067, http://dx.doi.org/10.1080/02678292.2015.1021719

Phase transitions, optical, dielectric and viscoelastic properties of colloidal suspensions of
BaTiO3 nanoparticles and cyanobiphenyl liquid crystals
M.V. Rasnaa, L. Cmokb, D.R. Evansc, A. Merteljd and Surajit Dharaa*aSchool of Physics, University of Hyderabad, Hyderabad, India; bDepartment of Physics, University of Ljubljana, Ljubljana,
Slovenia; cAir Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH,
USA; dJ. Stefan Institute, Ljubljana, Slovenia

Downloaded by [University of Hyderabad] at 07:11 22 August 2015

(Received 6 January 2015; accepted 18 February 2015)
We report experimental studies on the phase transitions and physical properties of colloidal suspensions of
BaTiO3 nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4ʹ-cyanobiphenyl and 4-octyl-4ʹ-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectricanisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational
viscosity decrease considerably. The experimental results are discussed based on the possible contribution of
BaTiO3 nanoparticles and free surfactant molecules in the suspensions.
Keywords: nanoparticles; liquid crystals; phase transitions; viscoelastic properties

1. Introduction
The study on the colloidal suspension of nanoparticles
in liquid crystals is interesting from both the fundamental and technological points of view. The effect ofdispersion of nanoparticles on various physical properties depend on an important length scale, called theextrapolation length (ξ), defined as the ratio between
the typical elastic constant K and the surface anchoringenergy W. When the size of the nanoparticle, R, ismuch smaller than ξ, the molecules are not strongly
pinned at the nanoparticles and there is no long-range
elastic distortion. If the initial liquid crystal orientation
is monodomain, it is likely to remain as such with noapparent evidence of dissolved nanoparticles.
Although the molecular orientation does not changemuch, the small particles affect significantly the anisotropic physical properties that depends on the size,
shape, concentration and properties of the nanoparticles.[1–6] There are several reports on the various physical measurements on liquid crystal nanocomposites
based on ferromagnetic, ferroelectric, metal and inorganic nanoparticles.[7–15] The colloidal dispersion ofthese nanoparticles in liquid crystals combines the physical properties of nanoparticles and orientational
order of liquid crystals. It has been reported that theseveral liquid crystal nanocomposites are useful for
display applications.[16–21]
In this article, we present experimental results on
the phase transitions and physical properties of colloidal suspension of two liquid crystals, namely,
4-pentyl-4ʹ-cyanobiphenyl (5CB)- and 4-octyl-4ʹ*Corresponding author. Email: sdsp@uohyd.ernet.in© 2015 Taylor & Francis

cyanobiphenyl (8CB)-doped BaTiO3 nanoparticles.
We show from the thermal measurements that thenematic (N) temperature range of both the compounds have decreased while the smectic-A (SmA)
temperature range is increased in 8CB. We measured
birefringence, dielectric, curvature elastic constants
and rotational viscosity at various temperatures byusing an electrooptic technique. We show that birefringence, dielectric anisotropy and splay elastic constant are unchanged, whereas the bend elastic
constant and rotational viscosity decrease.
2. Experimental
Indium-tin-oxide-coated (ITO) glass plates with circularly patterned electrode were used to make experimental cells. The ITO plates were spin-coated withpolyimide AL-1254 and cured at 180°C for 1 h andrubbed antiparallel way for homogeneous alignmentof the director (the average alignment direction of themolecules). For homeotropic alignment, plates were
coated with JALS-204 and cured at 200°C for 1 h.
Empty cells of different gaps in the range 5–13 µm
were fabricated using appropriate spacers. Using animpedance analyser (Novocontrol, Alpha-A), the perpendicular and parallel components of the dielectric
constant were measured in a planar and homeotropic
cells, respectively. All the measurements were performed at a frequency 1 KHz by applying a verysmall voltage (much less than the Freedericksz’s
threshold voltage) across the cell.

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Birefringence and elastic constants were measured
using an electrooptic technique. The sample retardation was measured by using a helium–neon laser(λ = 632.8 nm), photoelastic modulator (PEM) anda lock-in amplifier.[22, 23] Using an LCR meter(Inductance-Capacitance-Resistance), sinusoidal voltage of frequency 1 KHz and amplitude up to 20 V
was applied. The phase retardation and sample capacitance were measured simultaneously as a functionof temperature and voltage. A photodetector collected the light transmitted through the liquid crystal,and lock-in amplifier measured the first and second
harmonics of the ac signal. The first and second
harmonics signals (1f and 2f signals) measured by
the lock-in amplifier are given by I1f ¼
I0 J1 ðA0 Þ sinð!ϕÞ and I2f ¼ I0 J2 ðA0 Þ cosð!ϕÞ whereJ1 ðA0 Þ and J2 ðA0 Þ are first- and second-order Bessel
coefficients, I0 is the transmitted intensity and !ϕ isthe optical phase shift and A0 ¼ 2:405 is the retardation amplitude of the PEM. The birefringence of thesample is given by [24]
!n ¼

!
"
I1f $ J2 ðA0 Þ
λλ!ϕ ¼
arctanI2f $ J1 ðA0 Þ
2πd
2πd

(1)

The splay elastic constant is obtained directlyfrom the Freedericksz #threshold voltage (Vth) and isgiven by K11 ¼ !o !! Vth =πÞ2 . At strong surface

anchoring condition, the voltages above the thresholdvoltage (Vth) and the retardation ð!ϕÞ are given by
the parametric equations: [25, 26]
ne d!ϕ ¼ 2π2ðλπ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
32
ð1þγ sin2 ϕm sin2 ψ Þð1þκ sin2 ϕm sin2 ψ Þdψ
6ð1&sin2 ϕm sin2 ψ Þð1þν sin2 ϕm sin2 ψ Þ
n' 7
67$ 6ð 0π rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi & 7;
4 2 ð1þγ sin2 ϕm sin2 ψ Þð1þκ sin2 ϕm sin2 ψ Þ
ne 5dψð1&sin2 ϕm sin2 ψ Þ
0

(2)

and
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ γ sin2 ϕm
ð π sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2
1 þ κ sin ϕm
&
'&
' dψ
$
2
1 þ γ sin ϕm sin2 ψ 1 & sin2 ϕm sin2 ψ
0

V
2
¼
Vth
π

(3)

where d is the cell thickness, ϕm is the tilt angle at the
middle of the cell andother terms of the reduced
# the
( )
!
quantities are γ ¼ k !? & 1, κ ¼ ðK33 =K11 Þ & 1,&'
ν ¼ n2e =n2' & 1, sin2 ϕ ¼ sin ϕm sin ψ. The sample
retardation at higher voltage is fitted to the above
equations by an iterative procedure to get the bendelastic constant K33.[27, 28] It may be mentioned that
it is ideal to measure the elastic constants by using
magnetic field instead of electric field for such nanocomposite, especially where the ionic conductivity islarge. However, comparatively, the enhancement ofconductivity in our sample was not so high and in
addition we did not observe any agglomeration of thenanoparticle under applied electric field. Hence, it
was reasonable to use the electric field technique.

3. Results
3.1. Sample observation and thermal measurements
The liquid crystals used for the experiments are 8CB
and 5CB. These compounds were obtained from
Sigma-Aldrich and used without further purification.
8CB and 5CB exhibit the following phase transitions:Cr. 22°C SmA 32.5°C N 40 °C I and Cr. 22.5 °C N
35 °C I, respectively. The BaTiO3 nanoparticles wereobtained by milling micron sized particles together
with heptane and oleic acid (surfactant).[29] The
details of the particle harvesting is discussed in [30].
The average size of the nanoparticles measured bytransmission electron microscope is approximately
12 nm. It has been reported that the ferroelectricity
is retained in these nanoparticles due to the compressive and tensile surface strain. The ferroelectric nanoparticles were suspended in liquid crystals and thesolvent was evaporated. Two different concentrationsof BaTiO3 nanoparticles, namely, 0.2 and 0.4 wt% of
both the samples, were prepared. Physical observations indicate that the dispersions are stable andhomogeneous. Figure 1(a) shows that the suspension
with 0.4 wt% of nanoparticle also appears like pureliquid crystal with no evidence of dispersion. We
show the textures obtained under polarising optical
microscope for 5CB + 0.4 wt% BaTiO3 nanocomposite in Figures 1(b) and (c), respectively. It is noticed
that the director is uniformly oriented with no evidence of agglomeration of nanoparticles. The measurements were restricted to 0.4 wt% of nanoparticles
as the agglomeration-free stable suspension (over a
few months) was achieved approximately up to thisconcentration.
Figure 2 shows the differential scanning calorimetry(DSC) measurements of all the samples. It is observedthat the nematic to isotropic transition temperature(TNI) of 5CB liquid crystal decreases with increasing
concentration of nanoparticles (Figure 2(a)). In particular, it decreases by about 2.5°C compared to the pristine
sample although the enthalpy remains almostLiquid Crystals

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Figure 1. (a) (colour online) Physical appearance of a colloidal suspension of 5CB (with 0.4 wt% BaTiO3). Photomicrographof textures under crossed polarisers with rubbing direction (b) 45° with respect to the polariser and (c) parallel to the polariser.

Figure 2. (colour online) Heat flow versus temperature on heating and cooling of (a) 5CB and (b) 8CB and various colloidal
suspensions. The inset shows an enlarged part of SmA to N and N to I phase transitions.

unchanged ð!HNI ’ 3 J=gmÞ. The DSC data of 8CB
samples are presented in Figure 2(b). It is observed that
TNI decreases and the N to SmA transition temperature(TNA) increases with increasing concentration of

nanoparticles. The SmA to crystallisation temperatureis also decreased compared to the pristine sample. For
example, in suspension with 0.4 wt% of BaTiO3 in 8CB,
TNI is decreased by 2°C whereas TNA is increased by

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M.V. Rasna et al.

Table 1. Nematic and SmA temperature range andenthalpy of N–I and N–SmA transitions of 8CB and thecolloidal suspensions.
Cooling

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BaTiO3 (wt%)∆HIN (J/gm)∆HNA (J/gm)
N range (°C)
SmA range (°C)

0
3.3
0.41
6.9
36.3

0.2
0.4
3.1
3.0
0.48 0.66
5.5
4.8
39.2 39.3

Heating
0
3.4
0.47
7.1
11.4

0.2
0.4
3.0
2.9
0.56 0.75
5.7
4.9
12.3 12.4

1.5°C. Thus, the nematic temperature range is shortened and SmA temperature range is widened withincreasing concentration of BaTiO3 nanoparticles. We
also measured the enthalpy of the respective phase transitions of 8CB and they are listed in Table 1. It isobserved that the enthalpy (∆HNI) of the NI transition
decreases in both heating and cooling experiments compared to the pristine sample. On the other hand, theenthalpy (∆HNA) of the N–SmA transition increases
with the concentration of BaTiO3 nanoparticles. For
example, ∆HNA of pristine sample is 0.41 J/gm whereas
in suspension with 0.4 wt%, it is 0.66 J/gm and this isabout 60% larger than the pristine sample.
3.2. Birefringence and dielectric measurements
We measured birefringence (!n ¼ ne & no ) anddielectric anisotropy (!! ¼ !k & !? ) of all the samples as a function of temperature. The variations of!n and !! of 5CB as a function of relative temperature (T – TNI) are shown in Figure 3. Both !nand !ε of 5CB remain almost unchanged comparedto the pristine sample. The temperature variationsof !n and !! of 8CB are shown in Figure 4. It isobserved that !n remains almost unaffected, and in
addition, the enhancement of TNA is reflected inboth the measurements in accordance with DSC
measurements (Figure 2(b)). The small jump of !nat the N–SmA transition is about 0.007 and comparable to the value obtained by direct intensity
measurement technique.[31] The perpendicularcomponent of the dielectric constant !? in thenematic phase of 8CB liquid crystal remains almost& 'unchanged, whereas the parallel component !kshows a small decreasing tendency with increased
concentration of nanoparticles. For example, at
T & TNI ¼ &4' C, in the suspension with 0.4 wt%
nanoparticles, !! is decreased by 10%.
3.3. Elastic constant measurements
Figures 5 and 6 show the temperature variation of
splay (K11) and bend (K33) elastic constants of bothFigure 3. (colour online) Relative temperature& ' dependenceof (a) birefringence ð!nÞ and (b) parallel !k and perpendicular ð!? Þ components of dielectric constant of pure 5CB
and the colloidal suspensions.

the samples and various suspensions. In the pristine
sample, both K11 and K33 are comparable to thepreviously reported values.[32, 33] When they arecompared with relative temperature, it is observedthat there is no significant change in K11 (Figures 5(a) and 6(a)), whereas K33 decreases considerably(Figures 5(b) and 6(b)). For example, in both samples, at 0.4 wt% of nanoparticles, it decreases
almost 20% compared to the respective pristine
samples.

3.4. Rotational viscosity measurements
Rotational viscosity was measured by using a phasedecay-time measurement technique.[34] First ac voltage-dependent (frequency 1 KHz) transmitted intensity in a planar aligned cell was measured. A small
voltage (Vb) corresponding to first minima or maxima
of the transmitted intensity was applied, and at time
t ¼ 0, the voltage Vb was removed and the relaxationtransmission intensity change was measured. The
time-dependent transmitted intensity at a particulartemperature is given by I ¼ I0 sin2 ½!tot & δðtÞ)=2,
where I0 is the maximum intensity and !tot is theDownloaded by [University of Hyderabad] at 07:11 22 August 2015

Liquid Crystals

1063

Figure 4. (colour online) Relative temperature& ' dependenceof (a) birefringence ð!nÞ and (b) parallel !k and perpendicular ð!? Þ components of dielectric constant of pure 8CB
and the colloidal suspensions.

Figure 5. (colour online) Relative temperature variation of(a) splay (K11) and (b) bend (K33) elastic constants for pure
5CB and various colloidal suspensions. The verticallydownward arrow indicates the decreasing behaviour of K33.

total phase difference. The optical phase differenceδðtÞ ¼ δ0 expð&2t=τ 0 Þ, where δ0 is the total phase difference under bias voltage Vb. The slope of the plotlnðδ0 =δðt ÞÞ with time (t) gives the relaxation time ðτ 0 Þ.
Rotational viscosity is given by γ1 ¼ π2 K11 τ 0 =d 2 ,where d is the cell thickness. The variation of γ1 ofboth 5CB and 8CB liquid crystals at various concentrations of nanoparticles is shown in Figure 7.
Rotational viscosity increases in all the samples withdecreasing temperature as expected. More importantly, at a relative temperature, it decreases withincreasing concentration of nanoparticles. For example, in 8CB at T & TNI ¼ &2 ' C, γ1 decreases from0.04 to 0.03 Pa s. This is about 25% reduction compared to the pristine sample. It may be mentionedthat γ1 is a derived quantity and depends on K11.
The reduction in γ1 is entirely due to the decrease of
the relaxation time as K11 remains almost unchanged.

[37, 38] In case of ferroelectric nanoparticles, such as
Sn2P2S6, it is reported that TNI can increase or
decrease.[39] Gupta et al. showed that TNI decreases
in 4-n-octyloxy-4ʹ-cyanobiphenyl liquid crystal dopedwith BaTiO3 nanoparticles.[40] In colloidal systems
with BaTiO3 nanoparticles, in a negative dielectricanisotropy nematic liquid crystal mixture, TNI isreported to get enhanced.[41] From the temperaturedependent birefringence measurements, Mertelj et al.
reported that TNI decreases whereas TNA increases
with concentration of BaTiO3 in 8CB liquid crystal.
[31] The decrease of TNI was attributed to the contribution of the free oleic acid molecules. The increaseof TNA was argued due to the increase of de Gennes
coupling coefficient C in the relation δS ¼ χ ðT ÞCjψj2 ,where δS is the increase in the nematic order parameter, ψ is the SmA order parameter and χ(T) is the
response function.[31] This could be a pointer to thefact that the oleic acid molecules act like a mesogenic
dopant in 8CB liquid crystal. The typical length of afully stretched oleic acid molecule is about 19.7 Å,which is almost comparable to the typical length of a
8CB molecule (22.1 Å). However, in the low melting
form of oleic acid, a cis-double bond introduces apronounced bend in the alkyl chain.[42] Thus, the

4. Discussion
There are several studies on the N–I and N–SmA
phase transitions in nanoparticle-dispersed liquid
crystals.[35–37] It has been found that in aerosil-dispersed liquid crystals, both TNI and TNA decrease.

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M.V. Rasna et al.

Figure 6. (colour online) Relative temperature variation of(a) splay (K11) and (b) bend (K33) elastic constants for pure
8CB and various colloidal suspensions. The vertically downward arrow indicates the decreasing behaviour of K33.

Figure 7. (colour online) Relative temperature variation ofrotational viscosity (γ1) of (a) 5CB and (b) 8CB and various
colloidal suspensions. The vertically downward arrow indicates the decreasing behaviour of γ1.

oleic acid molecules may act as a dopant similar tothat of a bent-shaped molecule. Another interesting
aspect is the significant change in the enthalpy valuesof the respective transitions. The decrease of enthalpy
corresponding to the N–I transition (see Table 1) suggests that the first-order N–I transition is weakened
due to inclusions. On the other hand, the enthalpy of
the N–SmA transition is increased suggesting that thisweakly first-order transition is becoming relatively
stronger with the inclusions. Thus, the increase of
TNA is consistent with enhanced enthalpy of the N–
SmA transition.
There are several reports on the phase transitions
and physical properties of the binary mixture of rodlike and bent-core liquid crystals. Here, we cite aparticular example that reports on the effect of doping of bent-core molecules in rod-like nematic system.
Pratibha et al. reported that in binary mixtures ofrod-like and bent-core liquid crystals, TNI decreases
whereas TNA increases with the concentration of bentcore molecules.[43] In the same system, we reported
that K11 remains almost constant whereas K33
decreases with increasing concentration of bent-coremolecules.[23] The shift of N–I and N–SmA phasetransition temperatures and the variation of the physical properties in our present system as discussed aresomewhat similar to that reported in [23, 43]. In themean field theory, Kii / S 2 / !n2 , !n and S being
the birefringence and orientational order parameter,
respectively. In the present systems, no significant
change in !nð/ S Þ is observed (Figures 3(a) and
4(a)) except the order parameter increases more steeply with decreasing temperature in 8CB. Hence, elastic constants are expected to remain unaffected and it
is seen only in case of K11 (Figures 5(a) and 6(a)).
However, it is known that K33 may decrease in thebinary system of rod and bent-core molecules due tothe shape of the bent-core molecules keeping theorientational order the same.[23] This is due to thestrong coupling of bent shape with the bend
distortion.
The small concentration of BaTiO3 nanoparticles
does not significantly alter the dielectric constants andhence the anisotropy, except a tendency to decrease& 'the parallel component of the dielectric constant !k .
This suggests that either the ferroelectricity of theparticles is lost or the effective polarisation of thenanoparticles is screened by the ionic impurities. The
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Liquid Crystals
reported by several authors.[16, 44, 45] We have not
performed any additional measurements such as thepolarisation current to test their ferroelectric properties in the suspension. Moreover, the concentration of
nanoparticles is too low to be isolated from the contribution of ionic polarisation current. The decreasing
tendency of !k suggests the lowering of effective dipole
moment in the corresponding direction and this could
be due to the transverse dipole moments of the oleic
acid molecules. This is expected when the bow axes ofthe oleic acid molecules are parallel to the long axesof the 8CB molecules. We conjecture that the reduction in K33 is due to the effective shape (bent shape) of
the oleic acid molecules. However, it has to be verified by independent experiments by adding oleic acidin 8CB liquid crystal. Several experiments are planned
for the future to study the impact of oleic acid alone
on various physical properties of liquid crystals. The
decrease of γ1 in various nematic liquid crystal nanocomposites has been reported previously.[5, 17] This
could be due to the trapping of some mobile ions,
reducing the free ions concentrations and hence theinternal friction of the nematic medium as recently
reported by Basu et al. [44].

5. Conclusion
To summarise, we have performed DSC, birefringence, dielectric and viscoelastic measurements ofcolloidal systems of BaTiO3 nanoparticles and two
liquid crystals (5CB and 8CB). We found that apartfrom the nanoparticles, the free oleic acid molecules
also significantly affect the phase transition and physical properties. The DSC data suggest that the temperature range of nematic is decreased whereas thetemperature range of SmA is increased with increasing concentration of nanoparticles. In addition, theN–I transitions become relatively weaker and the N–
SmA transitions become relatively stronger with concentration of BaTiO3 nanoparticles. The birefringence, dielectric anisotropy and splay elastic
constant remains almost unchanged. We provided apossible explanation on the reduction of bend elastic
constant based on the effective shape of the free oleic
acid molecules.

Acknowledgement
We acknowledge experimental support of Dr. S. Maqbool
Ahmed, Principal Scientific Officer, CIL, UoH.

1065

Disclosure statement
No potential conflict of interest was reported by theauthors.

Funding
We gratefully acknowledge the support from the UPE-II,
DST(SR/NM/NS-134/2010),
CSIR(03(1207)/12/EMR-II)
and DST-PURSE. MVR acknowledges UGC-BSR for
fellowship.

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