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Submitted on 1 Jan 1992

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Surface induced transitions in the nematic phase of 4-n octyloxybenzoic acid

M. Petrov, A. Braslau, A. Levelut, Geoffroy Durand

To cite this version:

M. Petrov, A. Braslau, A. Levelut, Geoffroy Durand. Surface induced transitions in the nematic phase of 4-n octyloxybenzoic acid. Journal de Physique II, EDP Sciences, 1992, 2 (5), pp.1159-1193.

�10.1051/jp2:1992194�. �jpa-00247700�

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Classification

Physics Abstracts

61.30 68.42 61.10 64.70M

Surface induced transitions in the nematic phase of 4-n octyloxybenzoic acid

M. Petrov (*), A. Braslau (**), A. M. Levelut and G. Durand

Laboratoire de Physique des Solides, (LA2) Universit6 de Paris-Sud, 91405 Orsay, France (Received 28 May 1991, revised 15 October 199J, accepted 9January 1992)

Rdsum£. L'acide octyloxybenzdique (ainsi que l'heptyl et le nonyl) pr6sente des phases cristal

liquide n6matique et smectique C. En utilisant la microscopie optique en lumibre polarisde et la diffraction des rayons X, on mesure l'orientation et l'ordre pr~s de la surface d'un monocristal

n6matique orients par des lames de verre £vapordes SiO. En temp6rature ddcroissante, on observe une rotation spontan6e et la bifurcation de l'orientation de surface, une instabilit6 texturale induite par la surface, et la croissance de couches et de b£tonnets smectiques A prbs de la surface. Ces transitions pourraient dtre expliqu6es par une augmentation de la concentration des monom~res h la surface. La baisse corr616e de l'ordre ndmatique permettrait la croissance de

surface du smectique A plut6t que C. La rotation de l'orientation de surface et l'instabilit£ de

texture pourraient dtre les premiers exemples des instabilit6s flexo61ectriques de surface

r£cemment pr6dites.

Abstract. The heptyl-, octyl- and nonyl-benzoic acids present dimerized nematic and smectic C liquid-crystal phases. Using polarized optical microscopy and X-ray scatterig, we have measured the orientational and positional order close to the surface of the nematic single-crystal, oriented between SiO evaporation-coated glass plates. Decreasing the temperature, we observe a spontaneous twist and bifurcation of the surface orientation, a surface induced texture instability

and the growth of a surface layer with «batonnets» of smecticA. These transitions are

tentatively explained by an increase of the surface monomer concentration. The correlated decrease of surface nematic order would allow for the growth of surface smectic A rather than C.

The surface twist and textural instability could be the first examples of recently predicted surface flexoelectric instabilities.

1. Introduction.

The physical properties of the nematic phase preceding a smecticA or C phase are significantly different from those of a normal (classical) nematic. This pre-smectic phase is

(*) Present address : Institute of Solid-State Physics, Bulgarian Academy of Sciences, Lenin Blvd. 72, 1184 Sofia, Bulgary.

(**) Present address : Service de Physique de l'Etat Condens6, Orme des Merisiers, Centre d'Etudes de Saday, 91191 Gif sur Yvette Cedex, France.

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l160 JOURNAL DE PHYSIQUE II 5

characterized by local (short-range) smectic order fluctuations. The correlation length of these smectic fluctuations increases in the low temperature nematic region, as demonstrated

by X-ray investigations [I].

Such a nematic phase (preceding a smectic C phase) is found for the homologous series 4-n-

heptyl-, octyl- and nonyl-oxybenzoic acids (HOBA, OOBA and NOBA). These substances, however, possess a peculiarity different from other nematics having short-range smectic order in that the molecules are cyclic dimers [2] ; open dimers and monomers (in small percentages)

can also exist in addition to the cyclic dimers in the nematic phases of HOBA, OOBA and NOBA [2, 3]. The short-range smectic order and the thermodynamical equilibrium between

cyclic and open dimers and monomers obviously have a strong influence on the microstructure

as well as on the macroscopical properties of the nematic phases in these substances. As a result, some anomalies in the temperature dependence of ~rj/~r~ for thick OOBA samples.

aligned by a magnetic field, have been reported [4] (~rjj and ~r~ are the electrical conductivities parallel and perpendicular to the nematic director n, respectively). In addition, for very thin samples cells (< 50 ~Lm) of HOBA, OOBA and NOBA in the nematic phase aligned by the walls, it has been demonstrated using textural analysis and depolarized light scattering that a definite temperature exists at which the texture and the depolarized,

scattered light intensity change sharply [5, 6]. This temperature depends on the boundary

conditions and divides the nematic phase into high and low temperature regions with different

macroscopic properties. Recently, the existence of this transition in the nematic phase for the

4-n-alkyloxybenzoic acids has been independently confirmed [7]. For certain boundary conditions, the described phenomenon is enhanced, and a strong textural transition is

observed. Up until now, however, the evolution of the textures has only been studied

qualitatively, due to the difficulty in controlling the boundary conditions for the texture orientation. The purpose of the present investigation, with good control on the boundary conditions using SiO oblique evaporation at different angles of incidence to the substrate's normal, is to characterize this phenomenon quantitatively.

2. Optical properties of textures.

In this section we first describe the methods used to determine the orientation of various textures of OOBA in samples held between glass plates that were, in most cases, treated by

SiO evaporation [8]. We report the observed surface orientation, its temperature dependence

and some other related phenomena such as surface instabilities and the surface-induced

growth of smectic-like fingers or whiskers («batonnets»). A tentative analysis of these observations is finally presented.

2. I EXPERIMENTAL METHODS.

2.I.I Sample preparation. The sample consists of a drop of OOBA (4-n-octyloxybenzoic

acid) sandwiched between two SiO-evaporated glass plates. The phase transition temperatures of this compound are reported to be [4, 9] :

Crystal = Sc = N =

101.I 'C 108 'C 147 °C

In fact, the observed temperatures can vary widely from these values for instance, we found that the N-I transition may be observed at any temperature ranging from 142 °C to 146 "C, the

Sc-N transition anywhere from 102 °C to 106 °C and the Crystal-Sc transition from 90 °C to

loo °C. The reasons for these discrepancies are multiple : firstly, the absolute calibration of

the Mettler FP-5 oven used in this study is no more accurate than I °C, although a

temperature resolution of better than 0.I °C can be obtained. Secondly, the sample may not

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be completely pure ; we do observe some effect of aging. The main reason is probably that all the transitions are strongly first-order and, thus, hysteretic. At the N-I transition, for instance, we observe the coexistence of the nematic and isotropic phases over an unusually

wide temperature range of

~

lo °C. This coexistence is not stable : after one day at constant temperature, a sample initially with N-I coexistence displays only the N phase. One

explanation for this observation might be that impurities present in the material segregate in

the I Phase. After cooling, these impurities should reach a uniform concentration by

diffusion; taking a typical diffusion coefficient of D~10~~cm~/s, it would take

10i s ( l day) to re-equalize the concentration between domains located I mm apart. Note that these impurities cannot be monomers or dimers of OOBA, since chemical equilibrium

could be reached within molecular times. Effects related to the change in surface concentration of monomers shall be described below. In all that follows, unless otherwise

speciiied, we shall describe the behavior oi uniform phases.

The boundary glass plates which hold our sample were treated by oblique evaporation of

SiO, a treatment which yields an oblique director orientation [8]. The evaporation angles

(seefig. I) were a

= 66°, 70°, 75°, 80° and 86° with SiO nominal thicknesses of

=18.3nm, 19.3nm, 19.4nm, 19.lnm and 13.4nm, respectively. The evaporation

direction was noted on each plate. Samples either with parallel expected surface orientations e and e' (the « homogeneous » samples), as shown in figure 2a, or with antiparallel surface

orientations e and e' (the « inhomogeneous » samples), as shown in figure 2b, were prepared.

Since we were interested in the thickness (plate separation d~ dependence of the texture

orientation, we prepared not only uniform thickness samples in the range

10 ~Lm w dw loo ~Lm using Mylar spacers, but also wedge samples (see Figs. 3a and 3b), where d varies continuously from zero to the spacer (stainless steel cylindrical wire) thickness of h

=

50, loo or 200 ~Lm. The thickness d is then calculated using d

= h,x/L and by

Z

a p

6

X SiO

Fig. I.- Oblique evaporation of SiO on the glass plates. P and a are the plane and angle of evaporation, respectively j e is the evaporation direction and 8 is the thickness of the obliquely evaporated layer.

~l-,

I

-,

d' e

e -,

d d ~

~ d

a) b)

Fig. 2. Uniform thickness sample. e and e' are the evaporation directions and d is the cell thickness.

(a) Homogeneous orientation and (b) non-homogeneous orientation.

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l162 JOURNAL DE PHYSIQUE II 5

d d

d h d h

d' x x d'

L L

a) b)

Fig. 3. (a) Homogeneous wedge sample and (b) non-homogeneous wedge sample. d is the sample

thickness at an arbitrary position, x and L are the distances of the observation point and the wire spacer to the wedge comer and h is the maximal thickness of the sample corresponding to the wire diameter.

measuring the relative distance x/L of the point of observation x to the wedge comer

(L is the total distance to the wire spacer). A steel spring, placed as shown in figure 3, was

used to mechanically squeeze the sample so that d be reasonably well defined over the total lateral size (~ l cm ). To fill the swnple, the empty cell of treated glass plates was placed in the Mettler FP-5 hot stage and the temperature set in the range of the isotropic phase. The sample crystallites were melted along the open sides of the sandwich and drawn in between the glass plates by capillary action. We systematically verified that the entire sandwich was filled with the isotropic liquid crystal sample.

2.1.2 Orientation analysis methods. To analyse the orientation textures (defined by the usual director n with n~

= I), one must determine the orientations of n on the two boundary surfaces, with the geometry defined in figure 4. The sample is illuminated with linearly polarized light, propagating normal to the glass plates (see Fig. 5) in a Leitz polarizing microscope. Because d and the birefringence An

= n~ n~ (n~ and n~ are the extraordinary

and ordinary indices, respectively) are large~ we can assume that we are always in the

Mauguin wave-guide regime, I.e. the horizontal projections ~b and ~b' of n are the surface

optical eigen-axes (see Figs. 4 and 6a) at the input and the output surfaces. ~b and

~b' can be determined by the measurement of the azimuthal rotations of the polarizer and

analyzer of the microscope that result in optical extinction. Since, in practice, only the sample

rotation stage and the analyzer allow accurate azimuthal angular measurements, it was found

Z A

~

ll'

~ / ~tP/

~ ~

P light

Fig. 4. Fig. 5.

Fig. 4. The geometry of the nematic director n at the boundary surfaces. e is the mean angle of the director n with respect to the vertical axis Z and ~b is the azimuth of n.

Fig. 5. The propagation of linearly polarized light in the sample. P is the polarizer and A is the

analyzer.

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~

,

il

~ 0

A

a) b)

Fig. 6. -(a) Horizontal projections ~b and ~b' of the optical eigenaxis at the input and the output surfaces. (b) Absolute angle (90° + 4 + ~b') between the analyzer and the polarizer in the extinction

position.

more convenient to iix the polarizer and to rotate the sample by ~b and then to turn the

analyzer by ~b + ~b' (see Fig. 6b). In the extinct (crossed) position, the absolute angle

between the polarizer and analyzer is thus 90° +

~b + ~b'.

The mean angle o of the director n with respect to the vertical z-axis can, in principle, be

obtained by measuring the birefringence. Assuming that the director n has a uniform

orientation o in the cell, the ordinary ray « sees » the optical index n~ the extraordinary ray

« sees » an optical index n' given by :

n'~ ~

=

n)~sin~ o

+ nj ~cos~ o

The measured birefringence An (o)

=

n' n~ gives o if n~ and n~ are known, which is not the

case for OOBA. However, since most nematic liquid crystals have an ordinary index n~ ~ l.5, we measured An (90°)

= n~ n~ and used n~ =

1.5 to calculate n~, and, hence, to

determine o from An. This is equivalent to assuming the small birefringence law :

An (o )

~

An (90°) sin~ o. To

measure An, we rotated the polarizer and analyzer by 45° away

from the extinction directions previously determined (see Fig. 7a) and illuminated the sample

with a Hg green (A

= 0.546 ~Lm) filtered light. Fringes parallel to the wedge comer were

observed (see Fig. 7b). The local texture birefringence is deduced from the local distance f between two adjacent fringes as An (o

= (A/h) (Lli ). A rotating compensator is then used

f~

h

-,

x

~ L p

I A

a)

Fig. 7. (a) The birefrigence An measurement method. The polarizer P and analyzer A are each rotated by 45° with respect to the extinction positions. f is the interfringe distance. (b) Fringes parallel to the wedge comer for a homogeneous sample with a

=

86°. The magnification is x 125.

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l164 JOURNAL DE PHYSIQUE II 5

~

Fig, 7. Continued.

to determine, from the shift direction of the fringes, the absolute orientation of the long

molecular axis, defined as the n~ optical field direction.

In some cases, to check the relative orientation of n compared to the evaporation direction,

we tilted the sample by a few 10 or 15° about the X-axis (see Fig. 4). From the increase of the decrease in the absolute birefringence, one can distinguish between the two possible director

orientations n and n' of same n~ component, with the two azimuths ~b and ~b + gr (see Fig. 8) which give the same birefringence as for normal incidence.

z

fi' d

P

~~~ Y

x

Fig. 8. Method to determine the relative orientation of n compared to the evaporation direction, as

described in the text,

2.2 TEXTURE DEscRiPTioN IN OOBA. OOBA presents a wide variety of unusual textures

in its nematic phase. We first describe these textures and their hysteretic behavior. More

quantitative measurements are presented after.

2.2. I Texture observations. We first prepared a sample in between two glass plates, coated

by SiO evaporated at a

=

86° and

=

13.4 nm, with parallel evaporation directions, as in

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figure 3b. The sample was a wedge cell with a h

=

50 ~Lm spacer. The nematic phase was homogeneous with its director n tilted along the evaporation direction, I.e. ~b

= ~b'

= 0 (see

below for the determination of o). We call this the normal tilted texture (NIT) which appears uniformly black when observed between the crossed polarizers along X and Y. Upon cooling,

at a temperature T* =125°C (for an observation point with a sample thickness of

d~ 30 ~Lm), we observe the rapid onset of a nematic scattering texture (NST), shown in

figure 9. This new texture is completely different and quite unusual : the nematic phase is

highly distorted with a characteristic length in the range of ~Lm or below~ beyond the limit of resolution of the microscope. Keeping T below 125 °C, but without entering the lower- temperature Sc phase, the NST is stable. To erase this NST, one has to heat the sample to a

temperature T+ =133.5 °C, well above T*. Above T+~ one restores the original uniform NTT with n oblique, tilted along the evaporation direction. Cooling~ one again finds the onset of the NST at T*. This cycling between the NTT and the NST can be repeated many times.

The hysteretic behavior indicates that the NST results from the NTT as a first-order transition, and T* depends on the thickness d. We shall discuss this thickness effect later.

Upon further cooling of the NST, one observes the normal N-Sc transition at

T =105 °C as a small, yet unambiguous rearrangement of the NST, becoming a smectic

scattering texture (SST). Heating again, one observes the melting of the SST to the NST at T =106 °C, I.e., a very small change in textural aspect. Absolutely nothing happens at T

= T*, and one must again heat up to T+ to re-observe the vanishing of the NST and then onset of the transmitting, non-scattering NTT.

However, the passage into the bulk smectic C phase has changed the nature of the surface layer. The novelty is that, now, upon cooling below a temperature T~ = 128 °C, this nematic texture is constituted of twisted domains (see Fig. lo), with twists ~b and ~b', the magnitudes

of which are temperature dependent. These domains can present very well-defined edges

separating regions of constant ~b and ~b', or may show a more-or-less continuous variation of

~b. The smallest size of these domains is in the range of ~Lm, and the largest size is a few tens of

Fig. 9. Two domains ( + + and ) of the nematic scattering texture (NST). In this homogeneous sample, d

= 30 ~m, T

= 122 °C and a

= 86°. The magnification is x 125.

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1166 JOURNAL DE PHYSIQUE II 5

> ,~

' ~

©, S

"

"

.~ '

i

~'(

4d~

'

Fig. 10. The twisted nematic texture (TNT), In this homogeneous sample, d = 20 ~m, T

= 130.5 °C and a

= 86°. The magnification is x 125.

~Lm. Sometimes, a granular aspect is observed, uniformly or in isolated patches, as if the domains were constituted of smaller units. For all of these domains, o is the same ; n remains

oblique and only ~b changes.

Upon cooling this novel twisted nematic texture (TNT), one observes an increase in the twist angle ~b. Then, at T*

=

125 "C, we re-observe the onset of the NST, which can be once

again cleared by heating up to T+, yielding the TNT, as before nothing special changes by cycling back through the Sc phase. In all cases, remaining above T*, o decreases with

increasing temperature, and one eventually finds the normal tilted texture NTT. The

measurement of ~b, ~b' and o versus T in the NIT and the TNT will be described later.

The texture transition sequence can be schematized as follows :

SST = Ns~/-TNT=

T ~m~ ~ ~

NTT

C 4, T

where the alTows indicate reversible and ilTeversible paths and with no implication of unique

transition temperatures upon cooling or heating.

2.2.2 Textures and boundary plate nature. To try to elucidate the origin of these textural

changes, we studied samples sandwiched between glass plates with various coatings. Using the

same glass plates without an SiO coating, we obtained the same change of texture, with the difference that the nematic alignment was not well defined. We again observed the onset of the NST close to T*, however appearing to be more progressive in temperature.

Altemately, we used indium tin oxide (ITO) coated Baltracon glass plates (Balzers, surface

resistance ~100Q) and reproduced the same SiO evaporation la = 86°, =13.4 nm) as

previously. As one cools the sample down from the isotropic phase, a homogeneous~ uniform NTT is built. Cooling further, we do not observe anything like the NST and the texture remains perfectly black between crossed polarizers. The smectic C phase appears as relatively large single-crystals, with the director titled at random to each side of the evaporation plane

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