Light-control of liquid crystal alignment from vertical to planarRasha Ata Alla, Gurumurthy Hegde, and Lachezar Komitov Citation: Applied Physics Letters 102, 233505 (2013); doi: 10.1063/1.4808383 View online: http://dx.doi.org/10.1063/1.4808383 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/23?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crystal-liquid crystal binary phase diagrams J. Chem. Phys. 124, 224902 (2006); 10.1063/1.2200688 Electrically commanded surfaces for nematic liquid crystal displays Appl. Phys. Lett. 86, 023502 (2005); 10.1063/1.1849844 Polarization-independent electrically tunable phase grating fabricated from ultraviolet-curable liquid crystals J. Appl. Phys. 96, 5909 (2004); 10.1063/1.1805181 Surface-mediated light-controlled Friedericksz transition in a nematic liquid crystal cell J. Appl. Phys. 90, 5963 (2001); 10.1063/1.1418421 Electrically controlled light scattering from thermoreversible liquid-crystal gels J. Appl. Phys. 88, 161 (2000); 10.1063/1.373636

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Light-control of liquid crystal alignment from vertical to planar

Rasha Ata Alla,1 Gurumurthy Hegde,1,2 and Lachezar Komitov1,a)

1Liquid Crystal Group, Department of Physics, Gothenburg University, SE-41296 Gothenburg, Sweden2Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300 Gambang, Malaysia

(Received 8 February 2013; accepted 17 May 2013; published online 12 June 2013)

Continuous or discontinuous transition from vertical to planar alignment was found in nematic cells,

containing polyimide alignment layer exposed to linear polarized UV light, depending on whether

the alignment layer was mechanically rubbed or not prior the illumination. The polyimide material

is designed originally to promote vertical alignment when deposited onto the inner cell substrates’

surface. The mechanism behind these observations is complicated. It includes inclination of the

alkyl side chains of the polyimide alignment layer towards the rubbing direction and the bond

breaking process due to UV illumination which are considered as the key factors for occurrence of

such alignment transition. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4808383]

The alignment of liquid crystals is of vital importance

for their application in displays and photonic devices. In the

absence of external fields, such as electric or magnetic field,

the alignment of the liquid crystal, confined in conventional

sandwich cells, is defined by the solid surface/liquid crystal

interactions. Due to these interactions, the liquid crystal mol-

ecules may adopt perpendicular, parallel, or tilted orientation

with respect to the solid substrate they are in contact with.

Which kind of alignment the liquid crystal will adopt

depends primarily on the character of the surface/liquid crys-

tal interactions and their balance. In some cases, this balance

can be changed by temperature or light resulting in an align-

ment transition from vertical to planar, for instance.1,2 To

control the alignment of the liquid crystal and in particular

the inclination (pretilt angle) of the liquid crystal molecules

at the solid surface/liquid crystal interface is very important

for the performance of the liquid crystal displays (LCDs).

Defect-free alignment and high quality performances of

LCDs, such as fast response times, high contrast, and a wide

viewing angle can be achieved only by strict control of the

pretilt and anchoring conditions in the LCDs. Various

attempts have been made to control the liquid crystal align-

ment from vertical (VA) to planar alignment (PA). For

instance, different alignment layers prepared from either one

polyimide or a mixture of two polyimides, one promoting

VA and the other promoting PA, were reported in the litera-

ture where the pretilt can be controlled by using the rubbing

technique and/or varying either the relative concentration of

the polyimides or their curing time.3–6 Photoalignment tech-

nique, as a non-contact alignment method, has been also

used in different approaches such as single (or double) polar-

ized (or non-polarized) UV light exposure of different mate-

rials by different incident angles. As a result, wide range of

pretilt angles was accomplished by changing either the rela-

tive ratios of the alignment materials or by changing the con-

ditions of the exposure with the UV light.7,8 We have

previously studied theoretically and experimentally the con-

trol of the pretilt in nematic cells.1,9 We showed that by

proper nano-engineering of the alignment materials, it is

possible to achieve an efficient control of the pretilt angle

from 0� to 90�.2

In this work, we present the result of our investigations

on the light-control of the pretilt in a nematic cell and the

factors which are of importance for realizing such a control.

The experiments were carried out with conventional

sandwich cells consisting of two parallel glass substrates

forming a gap of about 4 lm. Onto the glass substrates which

are precoated with a transparent conductive film made of

ITO (Indium Tin Oxide) was deposited thin alignment film

of polyimide SE-1211 (Nissan Chemicals Industries Ltd,

Tokyo, Japan) by using the following procedure. The polya-

mic acid SE-1211, for promoting a VA of nematics, was first

dissolved in N-Methyl-2-Pyrrolidone (NMP) with concentra-

tion of 5% by weight. This solution was spin coated onto the

ITO film at 500 rpm for 10 s and then at 2000 rpm for 20 s.

The substrates were prebaked at 100 �C for 3 min to evapo-

rate the solvent and then baked at 190 �C for 30 min for com-

plete imidization, as prescribed by the manufacturer. Before

assembling, the glass substrates of our experimental cells

were divided into two sets. The substrates of the first set

were illuminated only by linear polarized UV light source

(6 mW/cm2) [standard USHIO light exposure equipment

with deep medium pressure mercury lamp] at normal inci-

dence varying the exposure time. Before illuminating, the

substrates of the second set were first rubbed unidirectionally

using Velvet cloth valve (Yoshikawa Chemical Company,

YA-20-R, Filament density 24 000/cm2) in order to induce a

small pretilt of the standing up alkyl chains of the polyimide

and thus to break the symmetry of the vertical alignment.

The mechanical rubbed substrates were divided also into two

categories—exposed to weak and strong rubbing, respec-

tively. After rubbing, the substrates were illuminated by line-

arly polarized UV light along the direction perpendicular to

the rubbing. The substrates were then assembled in standard

sandwich cells divided into two categories, with weak and

strong rubbing, respectively. The rubbing directions on

the substrates were oriented to be anti-parallel. The empty

cells of the three sets were filled with the nematic liquid

crystal MLC 6608 (De< 0) (Merck KGaA, Germany) in the

isotropic phase by means of capillary force. The pretilt angle

in the cells of the sets as a function of the illumination time

a)Author to whom correspondence should be addressed. Electronic mail:

komitov@physics.gu.se

0003-6951/2013/102(23)/233505/4/$30.00 VC 2013 AIP Publishing LLC102, 233505-1

APPLIED PHYSICS LETTERS 102, 233505 (2013)

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was estimated by means of Meuller matrix measurement

instrument.10

The imidized polyimide SE-1211 we used as liquid crys-

tal alignment layer has a relatively rigid backbone, which

alone promotes a planar alignment of the nematic liquid

crystals. The polyimide contains also alkyl side chains in its

structure oriented perpendicular to the polyimide backbone,

thus being parallel to the substrate normal. The alkyl side

chains are in fact responsible for the vertical alignment

which this polyimide promotes.11,12

It is known that UV illumination of the polyimides pro-

moting VA results in bond breaking and cleavage of both the

aromatic and the imide rings which in turn increases the polar-

ity of the alignment layer and hence the surface tension which

on the other hand leads to uniform planar alignment after suita-

ble illumination time by linear polarized UV light.13,14

The alignment layer in the cells made from SE-1211

was found to promote VA, due to the alkyl side chains, even

after UV exposure for 90 min. However, after a longer illu-

mination by UV light for 100 min, for istance, the promoted

alignment was not anymore VA but PA, i.e., a transition

from VA to PA took place at a longer exposure time of this

alignment layer. This alignment transition was found to be

rather sharp, i.e., of the first order (cf. Fig. 1). This

is because, at enough high UV dose, the bonds C-O, C¼O,

C-N, C-C, and C-H in the polyimide structure break and

cleave away the aromatic and the imide rings as well as the

alkyl side chains. This decomposition process, however,

resulted in the release and formation of more polar groups

which increased the surface polarity and tension.13,14 Due to

the increase of the substrates surface tension to value higher

than the liquid crystal surface tension, the liquid crystal mol-

ecules preferred to align parallel to the surface, i.e., the sur-

face/liquid crystal interactions now favored PA instead of

VA.15 Moreover, the UV illumination induced also a cleav-

age of the alkyl chains of the polyimide, responsible for the

VA of the nematic liquid crystal in the cells, thus facilitating

also the light-induced alignment transition from VA to PA

due also to reduction of the steric interactions between the

liquid crystal molecules and the alkyl side chains.2

Light-induced alignment transition from VA to PA,

which was observed at certain exposure time, appeared to be

of first order since, as it can be seen from Fig. 1, the value of

the pretilt changed abruptly from 90� to 0� as a function of

the exposure time. It should be mentioned here that the

decomposition of the alignment layer under the illumination

with the linear polarized UV light resulted in not only transi-

tion from VA to PA but also a preferred direction of the light

induced PA, which was found to be perpendicular to the

polarization direction of the UV light.

In the rubbed cells, on the other hand, the pretilt angle

showed a continuous change from 87.3� to 0� with increasing

the UV exposure time from 0 to 110 min (cf. Figs. 2 and 3).

The character of the alignment transition was found to depend

on the rubbing strength, as it can be seen in Figs. 2 and 3.

It could be very sharp (weak first order), at weak rubbing, or

quite smooth (typical second order), at strong rubbing. It

should be noted here that the polarization direction of the UV

light was oriented perpendicular to the rubbing direction so

that the preferred direction of alignment induced by the rub-

bing and the UV light, respectively, coincided.

Let us now analyze what happened in the cells with an

alignment layer which is exposed to a linear polarized UV light

but is not mechanically treated (rubbed) prior the exposure.

As mentioned above, the polyimide used in our study contains

a rigid backbone, promoting PA, and alkyl side chains,

FIG. 1. The pretilt angle changes abruptly from 89:8� to 0� with the exposure

time. The circles represent the experimental data, and the solid line represents

the fitted curve of Eq. (3) with hVA ¼ 89:8�, p¼ 0.00542, and u¼ 1.

FIG. 2. Pretilt angle vs UV exposure time at weak rubbing condition. The

circles represent the experimental data, and the solid line represents the fitted

curve of Eq. (3) with hVA ¼ 87:25�, p¼ 0.0089, and u¼ 0.25.

FIG. 3. Pretilt angle vs. UV exposure time at strong rubbing condition.

The circles represent the experimental data, and the solid line represents

the simulated curve of Eq. (3) with hVA ¼ 70:85þ 0:00379t 2 , hPA ¼ 1

þ ½1=ð1� 0:000131t 2Þ �, p¼ 0.0091, and u¼ 0.376.

233505-2 Alla, Hegde, and Komitov Appl. Phys. Lett. 102, 233505 (2013)

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promoting VA. Hence, the alignment layer made from the pol-

yimide has, generally speaking, two distinct anchoring compo-

nents each of them promoting alone VA or PA. The relative

magnitude of the components, as we shall see, defines whether

the alignment layer will promote VA or PA of the nematic liq-

uid crystal. Obviously, in our case, the polyimide has stronger

VA component than PA one and therefore promotes VA before

the UV illumination. It has been already pointed out that the

illumination with linear polarized UV light is cutting the alkyl

chains and their surface density decreases with the exposure

time. As a result, the strength of the vertical anchoring compo-

nent also decreases. At the same time, the UV illumination

cause destruction of the backbone which in turn generate ani-

sotropy in the surface properties of the alignment film impos-

ing a preferred direction of the PA component, which in this

case is perpendicular to the polarization of the UV light. The

balance between vertical and planar anchoring will define

whether the liquid crystal will adopt VA or PA.

In the set of rubbed cells, the rubbing process aligns the

backbone along the rubbing direction and thus introduces a pre-

ferred direction of the PA.16 In addition, the rubbing of the pol-

yimide alignment layer made of SE-1211 generates certain

pretilt of the liquid crystal director from VA due to the mechan-

ically induced tilt of the alkyl chains.17 The illumination of the

rubbed alignment layer is made in such way that the polariza-

tion of the UV light is oriented perpendicular to the rubbing

direction so that the preferred direction of the PA promoted by

the backbone after the UV illumination coincides with the rub-

bing direction, i.e., with the tilt plane of the alkyl side chains. In

this case, the balance between the vertical anchoring, promot-

ing VA with certain pretilt, and the planar anchoring, practi-

cally without pretilt, will define the behavior of the pretilt of

the liquid crystal as a function of the exposure time.

Let us now analyze these two cases of light-induced

alignment transitions.

(a) First order—observed in the cells containing non-

rubbed alignment layer exposed to a linear polarized

UV light, and

(b) Second order—observed in the cells containing rubbed

alignment layer exposed to a linear polarized UV light

In the case of non-rubbed alignment layer illuminated only

with linear polarized UV light, two anchoring components are

present, vertical and planar, designated as WVA and WPA,

respectively. The surface energy of a nematic layer in contact

with the polymer alignment layer deposited on the inner sub-

strates of the confining substrates of the cell will read as1

f ¼ � 1

2wVAcos2ðh� hVAÞ þ wPA cos2ðh� hPAÞ� �

; (1)

where wVA and wPA are the anchoring strength due to the

VA and PA components, respectively. VA and PA compo-

nents are assumed to promote alignment with pretilt hVA and

hPA, respectively. The anchoring strength is given as

wPA ¼ cPuPA ¼ cuPA; wVA ¼ cVuVA ¼ ð1� cÞuPA ; (2)

where u ¼ uPA

uVA, cP and cV are the density of the centers re-

sponsible for PA and VA, respectively, and uPA and uVA are

connected with the interactions responsible for the alignment

promoted by the corresponding center. We have considered

that the density of the PA component is increasing linearly

with exposure time, i.e., c ¼ pt.

As seen, in the case of VA and PA without pretilt, i.e., the

case of unrubbed cells, the transition from VA to PA is discon-

tinuous, i.e., of first order, as experimentally found (cf. Fig. 1).

On the contrary, in the case of rubbed cell, the VA

component has a pretilt hVA 6¼ 0, due to the rubbing, while

planar anchoring is with zero pretilt, i.e., hPA¼ 0, due to the

destructive effect of the UV light.

For the continuous transition, i.e., of second order, we

can consider the general case to calculate the easy axis direc-

tion he and the effective anchoring strength we given by

he ¼ arccos

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiwVA cos2ðhVAÞ þ we þ wPA cos2ðhPAÞ

2we

s(3)

and

we ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiwVA

2 þ 2wVAwPA cos2½ðhVA � hPAÞ� þ wPA2

p; (4)

respectively.

The dependence of he for the first order transition is

shown in Fig. 1 when hVA ¼ 89:8�, hPA ¼ 0�, the fitting pa-

rameters are p¼ 0.00542 and u¼ 1. For the weak rubbing

case, as we see in Fig. 2, hVA ¼ 87:25�, hPA ¼ 0� , the fitting

parameters are p¼ 0.0089 and u¼ 0.25. For the strong rub-

bing, we have three different regions (Fig. 3).

In the first region, at short exposure times, the pretilt,

generated by the rubbing process, initially increased with the

exposure time and then started to decrease. The initial

increase of the pretilt is due to the decrease of the density of

alkyl chain since some of them are cut at certain UV light ex-

posure level. It is known by the experimental result that a

looser packing of the alkyl side chains usually gives good

VA since it allows a higher degree of penetration (interdigi-

tation) of the liquid crystal molecules in between the alkyl

chains.18,19 However, further decrease of their surface den-

sity reduces the strength of VA component which in turn

results in a reduction of the VA pretilt angle.

In the second region, with pretilt transition in the range

68.60� to 30.17�, the competition between the vertical and

planar components let the pretilt angle to change continu-

ously with the exposure time. In this region, we have consid-

ered that both hVA and hPA are changing with the exposure

time. From the fitting of Eq. (3) to the experimental data, we

have found that

hVA ¼ 70:85þ 0:00379t 2; hPA ¼ 1þ ½1=ð1� 0:000131t 2Þ �;p¼ 0:0091 and u¼ 0:376:

In the third region of the curve, further increase of the

exposure time resulted in increasing density of the polar

groups on the surface which led to the value of substrates

surface tension higher than the one of the liquid crystal. The

anchoring strength competition of VA/PA component will,

after all, give rise to PA with preferred direction parallel to

the rubbing direction and perpendicular to the polarization

direction of the UV light.

233505-3 Alla, Hegde, and Komitov Appl. Phys. Lett. 102, 233505 (2013)

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It should be noted here that in Ref. 20 several examples

of discontinuous (first order) and continuous (second order)

change of the pretilt angle are reviewed. However, for none

of them it is clearly shown that a non-zero pretilt of the VA

and/or PA anchoring component is the necessary condition

for a continuos control of the pretilt angle (i.e., of second

order transition from VA to PA) whereas the lack of pretilt

of both alignment components result in an abrupt (first order)

transition from VA to PA.20

In conclusion, our study showed that UV linear polar-

ized light illuminating thin layer made of the polyimide

SE1211 had at the same time photo-destructive as well as

photo-aligning effect. Illuminating the polyimide alignment

layer by linear polarized UV light cleaved away the alkyl

side chains, reducing thus the steric interactions between liq-

uid crystal molecules and the alkyl side chains which caused

an abrupt light-induced transition from VA to PA at enough

long exposure time. Long exposure time caused also decom-

position of the polyimide backbone which in turn imposed a

preferred direction of the light-induced PA perpendicular to

the light polarization.

Rubbing process of the imidized polyimide layer before

UV illumination generates a small pretilt of the alkyl side

chain with respect to the confining substrates as well as

aligning the backbone along the rubbing direction. Strong

rubbing before illumination induced a continuous control of

the pretilt angle (second order alignment transition) in the

range of 87.25� to 0� under illumination by linear polarized

UV light with different exposure times since the longer is the

exposure the more cleavage of the bonds took place. When

the rubbing is weak, a sudden increase of the pretilt angle is

detected after relatively short exposure time since cutting off

the side chains leads to improvement of the VA at certain ex-

posure time before the VA component started to decline with

the exposure time. After the light-induced alignment transi-

tion from VA to uniform PA, the preferred direction of PA

was found to be perpendicular to the light polarization direc-

tion due to the destructive effect of the linear polarized UV

light on the polyimide backbone.

1L. Komitov, G. Barbero, I. Dahl, B. Helgee, and N. Olsson, Liq. Cryst. 36,

747 (2009).2L. Komitov, Thin Solid Films 516, 2639 (2008).3K. E. Vaughn, M. Sousa, D. Kang, and Ch. Rosenblatt, Appl. Phys. Lett.

90, 194102 (2007).4J.-H. Lee, D. Kang, C. M. Clarke, and C. Rosenblatt, J. Appl. Phys. 105,

023508 (2009).5F. S. Yeung, J. Y. Ho, Y. W. Li, F. C. Xie, O. K. Tsui, P. Sheng, and H. S.

Kwok, Appl. Phys. Lett. 88, 051910 (2006).6Y.-J. Lee, J. S. Gwag, Y.-K. Kim, S. I. Jo, S.-G. Kang, Y. R. Park, and

J.-H. Kim, Appl. Phys. Lett. 94, 041113 (2009).7M. Nishikawa and J. L. West, Jpn. J. Appl. Phys., Part 1 38, 5183 (1999).8D. Ahn, Y.-C. Jeong, S. Lee, J. Lee, Y. Heo, and J.-K. Park, Opt. Express

17, 16603 (2009).9L. Komitov, J. Soc. Inform. Dis. 16, 919 (2008).

10Ingolf Dahl, Measure. Sci. Technol. 12, 1938 (2001).11H. D. Jayathilake, M. H. Zhu, Ch. Rosenblatt, A. N. Bordenyuk, C.

Weeraman, and A. V. Benderskii, J. Chem. Phys. 125, 064706 (2006).12G. Carbone and C. Rosenblatt, Phys. Rev. Lett. 94, 057802 (2005).13S. Gong, J. Kanicki, L. Ma, and J. Z. Z. Zhong, Jpn. J. Appl. Phys., Part 1

38, 5996 (1999).14K.-W. Lee, A. Lien, J. H. Stathis, and S.-H. Paek, Jpn. J. Appl. Phys., Part 1

36, 3591 (1997).15F. J. Kahn, G. N. Taylor, and H. Schonhorn, Proc. IEEE 61, 823 (1973).16M. Oh-e, D. Kim, and Y. R. Shen, Analy. Sci. 17 Supplement, i805

(2001).17Z. Huang and Ch. Rosenblatt, Appl. Phys. Lett. 86, 011908 (2005).18B. Jerome, Rep. Prog. Phys. 54, 391 (1991).19K. Hiltorp and H. Stegemeyer, Ber. Bunsenges. Phys. Chem. 85, 582

(1981).20V. Chigrinov, V. Kozenkov, and H. S. Kwok, Photoalignment of Liquid

Crystalline Materials: Physics and Applications (John Wiley and Sons,

Ltd., England, 2008).

233505-4 Alla, Hegde, and Komitov Appl. Phys. Lett. 102, 233505 (2013)

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