Mol. Cryst. Liq. Cryst., Vol. 547: pp. 39=[1729]–45=[1735], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.572507

Rheological Properties of a Chiral Liquid Crystal
Exhibiting Type-II Character
M. RAJESWARI,1 J. ANANTHAIAH,1R. DABROWSKI,2 V. S. S. SASTRY,1 S. DHARA,1 AND
B. K. SADASHIVA3
1

School of Physics, University of Hyderabad, Hyderabad, India
Military University of Technology, Warsaw, Poland
3Raman Research Institute, Bangalore, India
2We measured rheological properties of cholesteryl nonanoate and its mixture with
40 -octyloxy-4-cyanobiphenyl. In both systems, wider temperature range of bluephase under shear with anomalous increase in viscosity associated with non- Newtonian flow behavior has been observed. Shear thinning followed by Newtonian flow isfound in cholesteric phase and non-Newtonian flow behavior at high shear rates isseen in smectic-A phase. In cholesteryl nonanoate a small peak in the viscosity atcholesteric to smectic-A transition along with a shear rate dependent viscosity athigher shear rate is attributed to the type-II character of the compound. We find thatthe type-II character of the compound is further enhanced by mixing a small amount(20 wt%) of 40 -octyloxy-4-cyanobiphenyl in cholesteryl nonanoate. No significant
change in the temperature dependent rheological behavior is observed at the twistgrain-boundary-A to smectic-A transition. It is suggested that the small anomalous
peak in the viscosity just above the twist-grain-boundary-A phase is due the existence of chiral line liquid phase.
Keywords Shear; type-II liquid crystals; viscosity

1. Introduction
Chiral liquid crystals have been studied since the time of their discovery. Since then alarge number of chiral liquid crystals exhibiting cholesteric (N ) phase are synthesized and characterized. The electrooptic properties of this phase have been studied
extensively and in some cases they are exploited to make display devices. N tosmectic-A (SmA) phase transitions in such materials have remarkable similarity withother systems in condensed matter like superconductor to normal metal and are
fundamentally important. A formal similarity was first pointed out by de Gennes[1]. He predicted that like superconductors SmA liquid crystals can also be classified
as type-I and type-II depending on their response to bending or twisting stress.
Renn and Lubensky [2] showed that in such intermediate phase of highly chiral
Address correspondence to S. Dhara, School of Physics, University of Hyderabad,Hyderabad 500046, India. Tel.: þ 91-40-66794331 5684; Fax: þ91-40-23010227; E-mail:sdsp@uohyd.ernet.in

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materials smectic blocks are separated by twist grain boundaries (TGB) andexperimentally they were discovered by Goodby et al. in a highly chiral compound[3]. Subsequently, a variety of such TGB phases like TGBA, TGBC, UTGBC
(undulated-twist-grain-boundary-C ) are also reported in many other compounds
including mixtures of chiral and nonchiral compounds [4–6].
In this context, some of the studies suggested the presence of a TGBA phase incholesteryl nonanoate, though thermodynamically stable phase is not observed
through optical polarizing microscopy or by calorimetric measurements. Nastishin
et al. [7] suggested the existence of chiral line liquid (N L) and TGBA phase in thiscompound from the defect studies, and this was supplemented by the rheologicalmeasurements by Asnacois et al. [8].
In this paper we report the measurement of shear viscosity (g), storage (G0 ) andloss (G00 ) modulus, as functions of shear rate and temperature in cholesteryl nonanoate (CN) and its mixture with 40 -octyloxy-4-cyanobiphenyl (8OCB). We report: (i)anomalous viscoelastic behavior of the blue phases, (ii) widening of the blue phase
under shear and (iii) anomalous viscosity at N -SmA transition.

2. Experimental Methods
Cholesteryl nonanoate (CN) and 40 -octyloxy-4-cyanobiphenyl (8OCB) were synthesized in our laboratory, and have the following phase sequences observed on cooling:
I 90.4 C N 73.4 C SmA (CN) and I 79.5 C N 66.5 C SmA 54.5 C Cr. (8OCB).
80 wt% of CN and 20 wt% of 8OCB were dissolved in chloroform, and sonicatedfor several hours, and evaporated slowly to prepare the mixture. The measurementof transition temperatures and the heat of transitions were made by using a differential scanning calorimeter (DSC) (Perkin-Elmer), which was calibrated against theIndium standard. For each DSC measurement 6 mg of sample was used with a scanning rate 5 C=min. Textures were observed by using an optical polarizing
microscope (Olympus BX-51) fitted with a digital camera and Mettler heating stage.
Rheological measurements were performed using a Rheometer (Anton Paar MCR
501) with plate–cone geometry (diameter 25 mm, cone angle 1o and with a minimum
gap of 50 mm between the plate and the truncated cone).
The temperature of the sample was controlled within an accuracy of 0.02 C by a
Peltier temperature controller fitted with a hood for the uniformity of the sample
temperature, and no measurable temperature gradient was observed. Calibrationof the temperature controller was checked by measuring rheological properties ofsome standard liquid crystalline materials. All the measurements were made on
cooling the sample from the isotropic phase.

3. Results and Discussion
The storage (G 0 ) and loss (G00 ) moduli were measured as a function of temperatureand these are shown along with the calorimetric data in Figure 1(a).
Two clear peaks between I to N except the blue phases and N to SmA transitions are detected in the DSC measurements. In the isotropic phase, G 0 < G00 and bothincreases sharply as the temperature is lowered from the isotropic phase. In the cholesteric phase G 0 > G00 and they increase sharply at N -SmA transition. Between thephases I and N both G 0 and G00 exhibit broad peaks superimposed with multiple narrow peaks. Though the exact transition temperatures from I to BP and to N could

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Figure 1. (a) Variation of storage modulus, G 0 (open circles) and loss modulus, G00 (open triangles) at strain amplitude c ¼ 5% and x ¼ 1 rad=s and differential scanning calorimetric scan(solid line) of pure cholesteryl nonanoate (CN) as a function of temperature. (b) Variation ofshear viscosity (g) of CN as a function of temperature at various shear rate (c_ ).

not be detected from the present rheometric measurements nevertheless it is clearthat under shear the temperature range between I and N is broadened comparedwith the sharper transition (0.7 C) as seen in the present DSC data or reported
earlier [9]. The finer structure in G 0 and G00 data within this temperature region
may be a pointer to the existence of multiple blue phases bound by the I and N
phases. The overall broadening without any fine structures of the blue phaseswith increasing shear rate was also reported by Negita et al. from the viscosity
measurements [10].
Measurement of storage modulus of the blue phases using a torsional oscillation
method was also reported by Kleiman et al. [9]. Transitions among the blue phases
were detected even though no such broadening was reported. The stiffnesses of the00blue phases are comparable to that of the SmA phase (i.e., GBPs
 GSmA), and is
attributed to presence of many dislocations in both the phases [9].
We show the variation of shear viscosity as a function of temperature at various
shear rate (c_ ) in Figure 1(b). Two distinguishable viscosity peaks between the I to N
and N to SmA transitions are observed. Viscosities (g) of BPs and SmA phases aremuch larger compared with that of N phase. For example, g at 91 C, 80 C and 65 C
are 3 Pa s, 0.1 Pa s and 4.2 Pa s respectively. Further, at higher c_ e.g., c_ ¼15 s1, splitting of I-N peak and a small shifting of the N -SmA viscosity peak towards thelower temperature is observed. Temperature dependent c_ versus shear stress of CN
was also studied by Asnacios et al. [8] and they suggested the possibility of existenceof a TGBA phase between the N to SmA transition and of course no such phase wasobserved through polarizing microscopy.
Furthermore we prepared a mixture of 80 wt% of CN and 20 wt% of 8OCB andinterestingly found that it exhibits the following phase sequences: I 82.4 C N 62.3 CTGBA 61.8 C SmA on cooling, as seen under a polarizing microscope. Textures ofTGBA phase in planar and homeotropic cell are shown in Figure 2. In planar cell,a typical texture of TGBA phase with relatively large pitch is seen (Fig. 2(a)). The
helix axis is perpendicular to the glass plate and different colours appear due to42=[1732]

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Figure 2. Photomicrographs of TGBA phase (a) in planar cell at 62.2 C on cooling (b) inhomeotropic cell at 63.3 C during heating from the SmA phase. (Figure appears in coloronline.)

the coexistence of different twisted states under cooling. In homeotropic cell(Fig. 2(b)) SmA planes are parallel to the substrate and appear dark. The SmA phase
was then heated slowly and bright filaments started growing in the dark background.
Similar textures in homeotropic cell in the case of other compounds showing TGBA
phase were also reported [5].
DSC scan along with the measurements of G0 and G00 of the mixture as a function of temperature are shown in Figure 3(a). Temperature dependence of G 0 and G00
are almost similar to that of the pure compound as discussed earlier. In the DSC
thermogram the blue phases could not be resolved between I to N transition.
Similarly individual peaks of N -TGBA-SmA transition were not detected through
DSC; instead the profile was a single broad peak. Such broad peaks bracketingthe transition N -TGBA-SmA were also reported for pure compounds [11,12]. It is
anticipated that this broad peak is due to the occurrence of a TGBA-like short rangeorder called chiral line liquid (or N L) phase. Further, in a high resolution a.c calorimetric study [11], a slight slope change superimposed on the broad peak wasFigure 3. (a) Variation of storage modulus, G0 (open circles) and loss modulus, G00 (open triangles) at strain amplitude c ¼ 5% and x ¼ 1 rad=s and differential scanning calorimetric scan(solid line) of the mixture (20 wt% 8OCB and 80 wt% CN) as a function of temperature. (b)Variation of shear viscosity (g) of the mixture as a function of temperature at various shear
rate (c_ ). Black bars indicate the temperature range of TGBA phase.

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reported, suggesting a phase sequence: N -N L-TGBA-SmA. Variation of g withtemperature of the mixture is presented in Figure 3(b). The viscosity decreases withincreasing c_ and N exhibits Newtonian and SmA phase exhibits non-Newtonian
flow behavior at higher c_ . Distinguishable viscosity peaks between I to N and N
to SmA transition are observed. Similar temperature dependent viscosity of cholesteryl myristate (CM) with pronounced peaks at the transitions were also reported[13]. However, occurrence of TGBA is not known in CM. In the present mixture
we note that the TGBA appears reasonably below the anomalous viscosity peak withno significant change in the viscosity across N L-TGBA-SmA transitions.
The variation of g with c_ for both the pure and the mixture at various
temperatures is shown in Figure 4. The viscosities of the I phase (91 C in Fig. 4(a)and 84.5 C in Fig. 4(b)) is independent of shear rate showing a Newtonian behavior.
At 86 C (Fig. 4(a)) and at 73.5 C (Fig. 4(b)) the N phase exhibits shear thinningfollowed by a Newtonian flow behavior beyond c_  40 s1. The shear thinning isdue to the strong shear induced alignment of the liquid crystal director. In cholestericliquid crystals in the absence of field the director commonly tend to be parallel tothe shear (flow) direction and hence the measured viscosity represent Miesowicz
viscosity g2 [14].
Viscosities are much higher in the SmA phase in both the samples compared tothe N phase. For example, c_ at 0.1 s1 gSmA =gN   80, (at 86 C and 65 C) andgSmA =gN   310 (73.5 C and 57.5 C) where gSmA and gN  are the viscosities of theSmA and N phase (Fig. 4(a) and Fig. 4(b)), respectively. Further it is noticed thatin the N phase, c_ above which the sample becomes non-Newtonian is decreased as
the temperature is reduced towards the SmA phase. For example, Newtonian flow
is observed above c_  40s1 at 86 C and c_  10s1 at 72.5 C respectively inFigure 4(a). Similar phenomena are observed in Figure 4(b). This is attributed tothe larger pitch at lower temperature. Similar pitch-dependent shear thinning behavior is known for other cholesteric liquid crystals [15].
In the SmA phase viscosity decreases steadily with increasing shear rate showing
non-Newtonian flow behavior up to c_ ¼103 s1 in both the cases. In the blue phases

Figure 4. (a) Variation of shear viscosity of CN as a function of shear rate (c_ ) at varioustemperatures. (b) Variation of shear viscosity of the mixture as a function of shear rate (c_ )at various temperatures. Before every measurement the sample was heated to isotropic phaseand cooled to the desired temperatures.

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(90.2 C in Fig. 4(a) and 82.7 C in Fig. 4(b)) the viscosities decrease rapidly
accompanied by a non-Newtonian behavior, and their values become almost
comparable to the values of viscosity of the N phase indicating that the flow behavior of blue phases is similar to the N phase. An interesting shear rate dependent
viscosity is observed in both the samples at the N -SmA transition temperatures.
At low shear rate it exhibits shear thinning accompanied by the Newtonian behavior
with viscosity similar to the values of SmA phase (72.5 C in Fig. 4(a) and 63.5 C inFig. 4(b)), suggesting the existence of an intermediate phase that possesses higher
viscosities like SmA and flow behavior similar to N phase. This intermediate phase
was identified as twist grain boundary (TGBA) by Nastishin et al. from the observation of defects in freely suspended droplets [7]. This observation was also supported by other rheological measurements [8]. However, in our mixture we clearlyobserve that the viscosity peak appears at the N -SmA transition (Fig. 3(b)) andTGBA phase appears at lower temperature with a very narrow (0.5o) range. We
believe that the peak of the viscosity is associated with the fluctuations of theTGBA-like short range order, identified as N L. No significant difference in theviscosities and mechanical properties (G0 and G00 ) are observed between the TGBA
and SmA phases suggesting that these two phases are almost similar from the viscoelastic point of view.

4. Conclusions and Perspectives
In conclusion, we measured viscoelastic properties of cholesteryl nonanoate (CN)and its mixture with 40 -octyloxy-4-cyanobiphenyl (8OCB). Our measurements show
the broadening of the blue phases under shear. We find that in the mixture type-II
character is enhanced. It appears that the flow behavior of BPs is similar to thatof the N phase. The anomalous increase in viscosity near N to SmA transition isdue to the presence of the N L phase. Viscoelastic properties of SmA and TGBA
seem to be similar. These results suggest that work on compounds exhibiting large
temperature range of TGBA phase can be very rewarding.

Acknowledgment
We thank Mrs. K. N. Vasudha, from Raman Research Institute, Bangalore forhelping us to do DSC measurements. MR thanks UGC-CAS fellowship.

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