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Phase Transitions: A Multinational
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Anchoring transition in a nematic liquid
crystal doped with chiral agents
Tatipamula A. Kumar
a

a b
a

a

a

, Khoa V. Le , Satoshi Aya , SungminabKang , Fumito Araoka , Ken Ishikawa , Surajit Dhara & Hideo
TakezoeaaDepartment of Organic and Polymeric Materials, Tokyo Instituteof Technology, 2-12-1-S8-42 O-okayama, Meguro-ku, Tokyo
152-8552, Japanb

School of Physics, University of Hyderabad, Hyderabad 500046,
India
Version of record first published: 14 Jun 2012.

To cite this article: Tatipamula A. Kumar, Khoa V. Le, Satoshi Aya, Sungmin Kang, Fumito Araoka,
Ken Ishikawa, Surajit Dhara & Hideo Takezoe (2012): Anchoring transition in a nematic liquid crystal
doped with chiral agents, Phase Transitions: A Multinational Journal, 85:10, 888-899
To link to this article: http://dx.doi.org/10.1080/01411594.2012.692092

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Phase Transitions
Vol. 85, No. 10, October 2012, 888–899

Anchoring transition in a nematic liquid crystal doped with chiral agents
Tatipamula A. Kumarab, Khoa V. Lea, Satoshi Ayaa, Sungmin Kanga,
Fumito Araokaa, Ken Ishikawaa, Surajit Dharab and Hideo Takezoea*aDownloaded by [University of Hyderabad] at 21:07 21 September 2012

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-S8-42
O-okayama, Meguro-ku, Tokyo 152-8552, Japan; bSchool of Physics,
University of Hyderabad, Hyderabad 500046, India(Received 27 March 2012; final version received 2 May 2012)
Anchoring transition in cholesteric liquid crystals (CLCs) was studied in thisarticle. The orientational change is not simple in CLCs; instead of theorientational change of the helical axis, a variety of molecular orientations incells such as planar, Grandjean, fingerprint, and homeotropic orientations wereobserved depending on chiral dopant, its concentration and temperature. It is
proposed that the competition between helix forming power and preferential
surface anchoring must be the origin of this phenomenon.
Keywords: anchoring transition; cholesteric liquid crystal; chiral dopant; surface
interaction

1. Introduction
The molecular alignment of liquid crystals (LCs) on treated surfaces is of great importancefor the basic understanding of the interfacial phenomena as well as technologicalapplications. The bulk LC and the interfacial properties strongly influence the electrooptic properties of liquid crystal displays (LCDs). Generally, two different categoriesof alignment layers are used in LCDs, i.e., parallel (homogeneous or planar) and vertical(homeotropic) alignment of the director (the average molecular direction). Apart from
LCDs, depending on the strength of the anchoring energy, LCs can also exhibit a richvariety of behaviors such as memory effects, surface melting, orientational wetting, etc.
Recent reports show that the liquid crystal CCN-47 (Merck Japan Ltd., Aikoh-gun,
Japan) can show spontaneous discontinuous orientational change (anchoring transition)
within the nematic (N) LC phase on amorphous perfluoropolymer (CYTOP, Asahi Glass
Co. Ltd., Tokyo, Japan) surface [1]. The same authors also demonstrated a novel bistable
memory device writable by a laser beam [2,3] or applying an electric (E) field [3]. The
device utilizes the hysteresis of a temperature-driven or an E-field-driven discontinuous
anchoring transition in a dye-doped NLC. They also showed that the temperature range ofthe hysteresis was lowered down to room temperature using a binary mixture system [2]. It
is noteworthy that CYTOP provides shock-free alignment layer for smectic LCs [4].
Recently, CYTOP was found to give planar alignment also for NLC mixture ZLI-2293(Merck); the anchoring energy of ZLI-2293 on CYTOP is almost eighteen times less than*Corresponding author. Email: takezoe.h.aa@m.titech.ac.jp
ISSN 0141–1594 print/ISSN 1029–0338 online! 2012 Taylor & Francis
http://dx.doi.org/10.1080/01411594.2012.692092
http://www.tandfonline.com

Downloaded by [University of Hyderabad] at 21:07 21 September 2012

Phase Transitions

889

that obtained in polyimide (AL-1254, JSR)-coated cells that are commonly used for planaralignment [5]. It is expected that the low surface anchoring and high transmittance in
CYTOP cells will make CYTOP a suitable aligning agent for LCDs as well as many other
LC devices.
Cholesteric liquid crystals (CLCs) are chiral analogs of NLCs, and the local director
forms a helical structure, which makes this phase more interesting and attractive. It is
known that planar and homeotropic alignment layers give helical axes perpendicularand parallel to the surfaces, respectively. However, the situation of surface orientationof molecules is quite different from that in NLCs. Because of the helical structure, themolecular orientation is not compatible to that at surfaces, if surfaces are treatedhomeotropically. Owing to such a circumstance, a variety of textures emerge in CLCs,i.e., fingerprint, cholesteric finger, Grandjean, and fan-shaped (or focal conic) textures [6].
Then questions arise: (1) Does the helical axis reorient from perpendicular to parallelor remain unchanged, when the favorable surface orientation changes from planar tohomeotropic? (2) Does the opposite change in the helical axis occur from parallel toperpendicular upon surface change from homeotropic to planar? (3) Does the complete
homeotropic state occur on homeotropic surfaces? If it occurs, it could be a new class oftransition; surface-driven cholesteric-N phase transition; (4) Does new type of anchoringtransition, e.g., between fingerprint and fan-shaped textures occur? If CCN-47 andCYTOP are used together with chiral dopants, we can address these questions. In thiscontext, the study on the anchoring transition in CLCs provides us additional aspects
compared with NLC systems.
From an application viewpoint, CLCs have received considerable attention for
applications to optical devices utilizing a spontaneously formed helical structure; onedimensional photonic structure. In this regard, the switching of a helical axis between
normal and tangential to surfaces may provide new applications. For instance, tuning of
beam direction in dye-doped CLC lasers becomes possible. Moreover, continuous tuningof wavelength by external stimuli is possible [7,8], if the molecules are released fromstrongly anchored surfaces, whereas only tuning among discrete wavelengths is possible ina planar-aligned cell configuration with the helical axis oriented perpendicularly to the cell
substrates. In this way, the reorientation of helical axis using the anchoring transition
is useful from the application viewpoint. Here, we report the anchoring transition in CLCs
prepared with several chiral agents.

2. Experimental details
We used amorphous perfluoropolymer, poly[perfluoro-(4-vinyloxy-1-butene)] (CYTOP(CTX-809A), Asahi Glass Co., Ltd. Japan), as an alignment layer for this experiment. TheCYTOP solution was prepared by dissolving CTX-809A with CT-Solv.180 (Asahi GlassCo., Ltd. Japan) at a weight rate of 1 : 2. Then, it was spin-coated on ITO-coated glass
plates using a spin coater. After spin coating, substrates were kept in an oven for curing
at 100! C for half an hour. In order to maintain uniform cell thickness, we used glass beadsof 5 mm as spacers. These spacers were mixed with glue and a line of glue was applied at
the edge of the substrates. Then ultraviolet (UV) polymerization was done by exposing
substrates to UV light. CYTOP-coated unrubbed cells were mainly used to observe
textures. On the other hand, polyimide-coated wedge cells were used to measure the pitchof the cholesteric liquid crystals. The liquid crystal CCN-47 exhibits the following
sequence of phase transitions: Cr 25.6! C SmA 28.2! C N 57.3! C Iso, and has a large

890

T.A. Kumar et al.
CB15

S811

Downloaded by [University of Hyderabad] at 21:07 21 September 2012

d-2

Figure 1. Chemical structures of chiral agents CB 15, S811, and d-2.

negative dielectric anisotropy (D" ¼ #5.7 at 30! C), where Cr, SmA, and Iso stand forcrystal, smectic A, and isotropic phases.
We used several chiral agents, i.e., 4-cyano-40 -(2-methylbutyl)-biphenyl (CB15, Merck),
4-[1-methylheptyloxy]carbonylphenyl-4-(hexyloxy)benzoate (S811, Merck) and an iminebased chiral dopant (d-2) [9]. CB15 has a phase sequence Cr 4! C N* 54! C Iso, and exhibits
right-handed helix. S811 has a phase sequence Cr 47.8! C Iso, and exhibits left-handed
helix. The chemical structures of CB15, S811 and d-2 chiral agents are shown in Figure 1.
We prepared several CLCs by adding chiral agents of various concentrations to CCN-47.
The texture observation was made using a polarizing optical microscope (POM). The
temperature of the sample was controlled by a Mettler hot stage (FP82HT).
3. Results and discussion
3.1. CLCs doped with CB15
We first prepared a CLC by adding 5 wt% of CB15 to CCN-47. Then it was injected into
the CYTOP-coated unrubbed cell of thickness $5 mm. When the sample was cooled from
Iso to cholesteric phase at a rate of 1! C per minute, the fan-shaped texture appeared
at 51.1! C as shown in Figure 2(a). A closer look at Figure 2(a) indicates the existence offringe structures (see inset). When the temperature reached 48.0! C, the fan-shaped texturechanged to a fingerprint texture as shown in Figure 2(b)–(d). We also observed the sample
on heating and found that the texture changed from fingerprint to fan-shaped at 48.3! C
as shown in Figure 2(e) and (f). Thus, the anchoring transition between fingerprint
and fan-shaped textures occurs. Interestingly, however, we did not observe any anchoringtransition to a complete homeotropic orientation in this concentration. This suggests thatthe interfacial forces that are responsible for a conventional anchoring transition areaffected by the addition of chiral dopant. In other words, helix-forming ability is strong
enough compared with the interfacial forces.
To reduce the helix-forming ability, we reduced the concentration of CB15 from 5 to
3 wt%. The textures are shown in Figure 3. When the temperature was reduced to 55.4! Cfrom the Iso phase, only a planar state existed with several cholesteric defects in it asshown in Figure 3(a). As the temperature decreased further to 46.7! C, the fingerprint
texture disappeared and some regions with reduced birefringence were seen, as shownin Figure 3(b), suggesting a tilting of the director. This transition proceeded until itDownloaded by [University of Hyderabad] at 21:07 21 September 2012

Phase Transitions

891(a) 51.1°C(b) 48.0°C(c) 45.0°C(d) 44.2°C(e) 48.3°C(f) 50.3°C

Figure 2. The evolution of textures in CCN-47 doped with 5 wt% of CB15: (a) fan-shaped texture,(b)–(d) fan-shaped to fingerprint on cooling, and (e) and (f) fingerprint to fan-shaped on heating.
Even in (a) fine fringes are seen (inset). Polarizer and analyzer directions are horizontal and vertical.
Blue and orange labels for temperatures are those on cooling and heating, respectively.

reached 45.7! C. On further cooling, some small dark regions were formed at 39.7! C andbecame more prominent when the sample was cooled to 28.7! C as shown in Figure 3(c).
Almost complete homeotropic state with some thin bright lines were observed when thetemperature was reduced to 25! C, as shown in Figure 3(d). On heating, the cholestericfingers appeared from homeotropic domains at 27.3! C, as shown in Figure 4(a), and the
texture remained the same until the temperature reached 51.3! C as shown in Figure 4(b).
At 51.4! C, some bright domains were also seen along with the fringe texture as shown in
Figure 4(c) and (d). Gradually the fringe texture disappeared as the temperature proceeded
towards Iso as shown in Figure 4(e) and (f). The situation is clearer in the sample with 1
wt% of CB15. By decreasing temperature, fringe textures could not be observed. Instead,the anchoring transition could be more clearly observed. Thus, with decreasing the weightfraction of chiral dopant CB15, the anchoring transition becomes prominent, as expected.
3.2. CLCs doped with S811
We further studied a cholesteric liquid crystal prepared by mixing the chiral agent
S811 with CCN-47. The behavior in the sample with 5 wt% of S811 was similar to thatin the samples with 5 wt% of CB15, although the anchoring transition from fan-shaped toDownloaded by [University of Hyderabad] at 21:07 21 September 2012

892

T.A. Kumar et al.
(a) 55.4°C(b) 46.7°C(c) 28.7°C(d) 25.0°C

Figure 3. Photomicrographs in CCN-47 doped with 3 wt% of CB15 on cooling: (a) planar state withsome cholesteric defects, (b) planar to tilt transition, (c) fingerprint texture with some homeotropic
regions, and (d) homeotropic state with a few thin bright lines. Polarizer and analyzer directions arehorizontal and vertical.

(a) 27.3°C(b) 51.3°C(c) 51.4°C(d) 51.5°C(e) 55.1°C(f) 55.3°C

Figure 4. Photomicrographs in CCN-47 doped with 3 wt% of CB15 on heating: (a) the transition
from homeotropic to fingerprint, (b) fingerprint texture, (c) and (d) the transition from fingerprint
to tilted state, and (e) and (f) tilted state with reduced brightness. Polarizer and analyzer directions
are horizontal and vertical.

Downloaded by [University of Hyderabad] at 21:07 21 September 2012

Phase Transitions

893(a) 56.9°C(b) 50.0°C(c) 48.0°C(d) 41.0°C(e) 38.5°C(f) 31.3°C

Figure 5. Photomicrographs in CCN-47 doped with 1 wt% of S811 on cooling: (a) fan-shaped
texture with fringes, (b)–(d) the gradual transition from fan-shaped to fingerprint, and (e) and (f) thetransition from fingerprint to homeotropic state. Polarizer and analyzer directions are horizontal
and vertical.

fingerprint at $31.5! C and to homeotropic at $30! C was observed in the former sample.
Here we only show the behavior of the sample with 1 wt% of S811. Mostly the fan-shaped
texture with fringe texture was observed during the Iso to cholesteric phase transition asshown in Figure 5(a). As the temperature decreased a fingerprint texture was graduallytransformed to a homeotropic texture as shown in Figure 5(b)–(e). When the sample
temperature was reduced further, the fingerprint texture almost disappeared and became
nearly homeotropic as shown in Figure 5(f).
The variation is more clearly seen when the same sample (1 wt%) was filled in CYTOPcoated rubbed cell. With decreasing temperature from Iso to cholesteric, a tilted state
appeared first as shown in Figure 6(a). The overall brightness of the tilted state increased as
the temperature decreased. At around 37.0–35.0! C the tilted state changed to a planar state
as shown in Figure 6(b). When the temperature reached 32.0! C, the anchoring transition
from planar to homeotropic state occurred along the rubbing direction as shown in
Figure 6(c). Here the anchoring transition at both surfaces normally occurred separately,
as indicated by three different brightnesses. With further decreasing temperature thehomeotropic regions grew as shown in Figure 6(d) and (e), and became completely
homeotropic at 30! C as shown in Figure 6(f). On the other hand, the sample
exhibited homeotropic state on heating till the temperature reached 37.8! C as shownDownloaded by [University of Hyderabad] at 21:07 21 September 2012

894

T.A. Kumar et al.
(a) 44.0°C(b) 35.0°C(c) 32.0°C(d) 31.5°C(e) 31.1°C(f) 30.0°C

Figure 6. Photomicrographs in CCN-47 doped with 1 wt% of S811 in a CYTOP-coated rubbed cellon cooling: (a) tilted state, (b) tilted state with increasing brightness, (c)–(e) anchoring transition
from planar to homeotropic, and (f) homeotropic sample. Polarizer and analyzer directions arehorizontal and vertical.

in Figure 7(a). When the temperature reached 38! C, it exhibited anchoring transition from
homeotropic to planar state as shown in Figure 7(b) and (c). As the temperature approached
CLC-N transition, it exhibited a tilted state with reduced brightness as shown in Figure 7(d).
3.3. CLCs doped with d-2
The two chiral dopants CB15 and S811 have similar helical twisting power (HTP) [10,11].
In order to study the effect of the dopant with large twisting power, we chose d-2, which
is known to have a high helical twisting power (HTP ¼ 140 mm#1 in the host 6OCB) [9].
We first prepared a CLC by adding 5 wt% of d-2 with CCN-47 and it was introduced
in a CYTOP-coated unrubbed cell. When the temperature decreased from the Iso phase,
we found Iso to cholesteric phase transition at 58.7! C as shown in Figure 8(a). As thetemperature decreased, a dark bluish color due to selective reflection was observedwith Grandjean steps as shown in Figure 8(b). When the temperature decreased further,
the color shifted from blue to cyan, and the width of Grandjean steps also increased
as shown in Figure 8(c) and (d). With this concentration we did not see the anchoringDownloaded by [University of Hyderabad] at 21:07 21 September 2012

Phase Transitions

895(a) 31.0°C(b) 38.0°C(c) 40.0°C(d) 50.0°C

Figure 7. Photomicrographs in CCN-47 doped with 1 wt% of S811 in a CYTOP-coated rubbed
cell on heating: (a) homeotropic state at 31.0! C, (b) and (c) anchoring transition from homeotropic
to planar state, and (d) tilted state with reduced brightness. Polarizer and analyzer directions arehorizontal and vertical, unless otherwise stated.

(a) 58.7°C(b) 57.1°C(c) 50.0°C(d) 33.0°C

Figure 8. Evolution of textures with selective reflection on cooling in CYTOP-coated rubbed cellwith 5 wt% of d-2: (a) Iso to cholesteric phase transition, (b) selective reflection with Grandjean
steps, (c) selective reflection of blue color, and (d) selective reflection of cyan color. Polarizer andanalyzer directions are horizontal and vertical.

transition in the sample. No anchoring transition was observed in a sample with
0.7 wt% of d-2 either. Then we reduced the concentration of d-2 to 0.5 wt% andmade microscopic observations as shown in Figure 9. As the temperature decreased
from Iso phase, we observed focal conic texture with fingerprints at 58.5! C as shownDownloaded by [University of Hyderabad] at 21:07 21 September 2012

896

T.A. Kumar et al.
(a) 58.5°C(b) 47.2°C(c) 53.0°C(d) 58.2°C

Figure 9. Photomicrographs in CCN-47 with 0.5wt% of d-2: (a) focal conic texture, (b) discontinuous anchoring transition from planar to homeotropic on cooling, (c) discontinuous
anchoring transition from homeotropic to planar on heating, and (d) tilted state. Polarizer andanalyzer directions are horizontal and vertical. Blue and orange labels for temperatures are those on
cooling and heating, respectively.

in Figure 9(a). We also observed a discontinuous anchoring transition fromtilted to homeotropic state at 47.2! C as shown in Figure 9(b). On heating it exhibitedanchoring transition from homeotropic to tilted state at temperature 53.0! C as shownin Figure 9(c) and (d).
We also prepared a contact cell of CLC and NLC (pure CCN-47), and observed the
texture to investigate how much concentration of chiral dopant is needed to have auniform helix with the helical axis along the cell surface normal. For this experiment weused CYTOP-coated rubbed cells and prepared CLC by doping CCN-47 with 0.7 wt% of
d-2. We filled the cell with pure CCN-47 and CLC in their Iso phase. As shown in Figure10(a) and (b), the texture exhibited Grandjean steps as the temperature was decreased from
Iso. A texture under decrossed polarizers is also shown in Figure 10(c). It exhibitedGrandjean steps till it reached 32.2! C, at which we observed discontinuous anchoringtransition on the CCN-47 side and continuous anchoring transition on the CLC side.
While the continuous anchoring transition took place, we also observed fingerprint texture
as shown in Figure 10(d). A similar behavior was observed on heating as shown in
Figure 10(e) and (f).
We also measured the pitch of the mixture with concentrations of 7.5 wt%, 5 wt%
and 0.7 wt% of d-2. We used polyimide-coated wedge cells with a wedge angle ! ¼ 1.54! .
In order to measure a pitch, we took several wedge lines and measured the pitch below$10! C from the Iso-cholesteric phase transition temperature. The pitch of the CLC wasmeasured using the following equation.
P ¼ 2s tan !,Phase Transitions
d-2 0.7%

897

CCN 47

Rubbing direction
P

Downloaded by [University of Hyderabad] at 21:07 21 September 2012(a) 57.0°C(b) 37.0°CAP(c) 37.0°C(e) 39.7°CA(d) 32.2°C(f) 43.0°C

Figure 10. Photomicrographs in a contact cell of pure CCN-47 and CCN-47 with 0.7 wt% of d-2:(a) and (b) Grandjean steps observed with decreasing temperature from the Iso phase, (c) the same asin (b) under slightly decrossed polarizers, (d) discontinuous (NLC side) and continuous (CLC side)anchoring transition, (e) fingerprint texture emerging during the anchoring transition on heating,and (f) after anchoring transition on heating. Polarizer and analyzer directions are horizontal andvertical, unless otherwise stated. Blue and orange labels for temperatures are those on cooling andheating, respectively.

where s is the distance between the wedge lines. The temperature dependence of the pitchobtained is shown in Figure 11(a). The pitch was $0.54 mm at a temperature very close to
the Iso to cholesteric phase transition, increasing to 0.59 mm near room temperature 30! C.
In the same temperature range, the pitch exhibited similar temperature dependence from
4.4 to 5.2 mm in the sample with 0.7 wt% of d-2. We also estimated the HTP of d-2. The
HTP of a chiral dopant is defined as" ¼ ðPcrÞ#1 ,where P is the pitch of CLC, c is the molar fraction of the dopant, and r is theenantiometric excess of the dopant. The variation of the inverse of the pitch as a functionof molar fraction of d-2 is plotted in Figure 11(b). HTP was found to be " $ 70 mm#1,
which was almost a half compared with the value in the host liquid crystal 6OCB [9].
Table 1 is a summary of the result on various chiral dopants.

898

T.A. Kumar et al.

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(a)

(b)

Figure 11. (a) Variation of pitch as a function of temperature for the concentration 0.7 wt% of d-2.
(b) Variation of inverse pitch as a function of molar fraction of dopant.

Table 1. Summary of Iso to N and anchoring transition temperatures in CCN-47 withchiral dopants.
Chiral dopant
(wt%)

Iso-N
(! C)

Anch. Trans.
(! C on cooling)

Anch. Trans.
(! C on heating)

None
CB15 (5 wt%)
CB15 (3 wt%)

57.9
51.4
55.4

S811 (5 wt%)

55.1

S811 (1 wt%)

56.9

54.0 (H to P)$48 (Fin to Fan)
$27 (H to Fin)
51.4 (Fin to T)$32 (H to Fin)
$52 (Fin to Fan)
$38 (H to Fan)

d-2 (5 wt%)
d-2 (0.7 wt%)
d-2 (0.5 wt%)

58.9
58.5
58.5

47.8 (P to H)$44 (Fan to Fin)
$47 (Fan to T)$30 (T to H)$38 (Fan to Fin)
$32 (Fin to H)$48 (Fan to Fin)
$38 (Fin to H)
no ATrno ATr$47 (Fin to H)

no ATrno ATr$53 (H to Fin)

Note: P (planar), H (homeotropic), Fan (fan-shaped), Fin (fingerprint), T (tilted).

Phase Transitions899

4. Conclusions

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We have observed the anchoring transitions in CLCs prepared by several chiral agents
such as CB15, S811, and d-2. Very complicated anchoring transitions were observed
mainly depending on HTP. When HTP was very large, a helical structure with the axis
perpendicular to surfaces appeared with clear selective reflection and Grandjean lines,
and no anchoring transition was observed. In smaller HTP samples, on cooling, a fanshaped texture with fine fringes was transformed to a fingerprint texture and further to ahomeotropic texture. On heating, a cholesteric finger emerged from the homeotropic
texture, and then the anchoring transition to fingerprint and to fan-shaped texture
occurred. Such complicated anchoring transition originates from the competition between
surface interaction and helix-forming power.

Acknowledgements
We acknowledge Merck Japan for supplying a LC material (CCN-47) and chiral dopants (CB15 andS811) and Prof. J. Watanabe and K. Fukuda for supplying a chiral dopant (d-2). The visit of T.A.K.
to Tokyo Tech was supported by the Global COE programs (Education and Research Center for
Material Innovation) at the Tokyo Institute of Technology. He also acknowledges Center for
Advanced Studies, School of Physics for the fellowship. K.V.L. acknowledges the support of theJapan Society for the Promotion of Science (JSPS). S.D. acknowledges DST (SR/NM/NS-134/2010)
and UPE-II, University of Hyderabad for financial support.

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