Langmuir Blodgett films of arachidic acid and

N4 molecules: characterization and use in

homeotropic alignment of N4

John Collins* Denis Funfschilling** Michael Dennin

Department of Physics and Astronomy, University of California at Irvine, Irvine,California 92697-4575

*current address: Department of Biomedical Engineering University of Californiaat Irvine, Irvine, California 92697-4575

** current address: Department of Physics and Astronomy, University ofCalifornia at Santa Barbara, Santa Barbara, California

Abstract

We study the behavior of a mixed Langmuir monolayer consisting of a fatty acidand a nematic liquid crystal. We demonstrate that the mixed monolayer success-fully transfers as a Langmuir-Blodgett film and characterize the transferred filmusing UV spectroscopy. An important application of Langmuir-Blodgett films is inthe alignment of liquid crystals for electro-optical applications, such as displays.We show that including the liquid crystal in the Langmuir-Blodgett film produceshomeotropic alignment for a system which fails to align by other standard tech-niques.

Key words: Langmuir Blodgett, homeotropic alignment, liquid crystal,electroconvection

1 Introduction

The importance of liquid crystals in a wide range of applications relies onthe ability to produce liquid crystal devices with macroscopically uniformalignment [1]. The two basic types of alignment are planar and homeotropic[2]. In planar alignment, the director is aligned parallel to the boundaries ofinterest. In homeotropic alignment, the director is aligned perpendicular to theboundary. (The director is the axis along which the molecules of the nematicliquid crystal are aligned on average.) In addition to these two basic types ofalignments, there are a number of variations, depending on the application. In

Preprint submitted to Elsevier Science 4 May 2005

general, all of the alignment techniques are based on some thin film technology.For planar alignment, one is generally interested in obtaining alignment in aparticular direction in the plane. Therefore, planar alignment techniques ofteninvolve treating a thin film so that steric effects play a role, such as rubbinga polymer to induce “grooves”. The grooves select a direction of interest.Homeotropic alignment techniques tend to be more straightforward. Usually,a simple surfactant coating is used that induces the liquid crystal to prefer aperpendicular alignment. A challenge in any alignment technique is the factthat there are usually both chemical and steric effects, and no one techniqueis guaranteed to work for all liquid crystals [2].

One motivation for this work was our need to produce homeotropic align-ment in a particular liquid crystal: N4. Several techniques exist for producinghomeotropic cells [2]. Most of the common techniques are based on coatingthe glass surface with a surfactant. Typically, the slide is immersed in a so-lution of the surfactant and then dried or baked to remove any solvent andfix the surfactant to the surface. Common surfactants are lecithin (egg yolk),DMOAP (dimethyloctadecyl[3-trimethoxysilyl)-propyl] ammonium chloride),and N-methyl-3-aminopropyltrimethoxysilane [MPA]. As these technique failedto align N4, an alternative approach was needed.

A promising development in homeotropic alignment is the use of techniquesbased on Langmuir-Blodgett deposition (LB film) of fatty acid films [3–6,?].Successful alignment of standard liquid crystals has been achieved with LBfilms of pure fatty acids [3,5] and fatty acids mixed with a liquid crystal (5CB)[4]. The later experiments motivated our approach of using a mixed film offatty acids and N4. The mixed film increases the interaction between the LBfilm and the liquid crystal by taking advantage of the aligning properties ofthe nematic itself. The liquid crystal molecules that are trapped inside theLB film are oriented in the same direction as the fatty acid, i.e. perpendicularto the surface of the substrate. The expectation is that this orientation istransmitted to the bulk liquid crystal in the cell.

Because a LB film is a layer by layer transfer of material from a Langmuirmonolayer to a solid substrate, the experiments reported on in this paper canbe divided into three steps. First, we characterized the mixed monolayer ofN4 and a fatty acid. (A Langmuir monolayer is a single layer of moleculesat the air-water interface.) Second, we confirm that the monolayer can betransferred to the appropriate solid substrate, which in this case is glass thatis coated with a transparent conductor. Finally, we test the alignment andelectro-optical response of the a liquid cell that is made from the treatedglass.

On the technology side, homeotropic alignment has been identified as a poten-tially useful arrangement for displays (see for example, [7–9]). This is true for

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the case of so-called vertical cells and mixed alignment displays. The interestin homeotropic displays stems from the potential advantages in terms of speedof response and viewing angle. For N4, our main interest stems from the roleliquid crystals have played in the study of pattern formation in anisotropic sys-tems [10,11]. The general problem of pattern formation refers to the study oftransitions in system subjected to external driving [12]. Traditionally, a systemis prepared in a spatially uniform state. Above a critical value of the driving,transitions to states with periodic structures (patterns) are observed. Amongthe advantages of using liquid crystals is the ability to control the alignmentto select varying degrees of anisotropy. One common pattern forming systemis electroconvection in nematic liquid crystals [10,13,14]. This utilizes mate-rials with a negative dielectric anisotropy in a planar alignment between twoparallel glass plates. (The dielectric anisotropy is the difference between thedielectric constant when the material is aligned parallel to an electric field andwhen it is aligned perpendicular to an electric field.) When driven with an acelectric voltage, there is a transition from the uniform conducting state to aperiodic convecting state.

An interesting variation on electroconvection is to start with a homeotropicsample. In this case, a material with a negative dielectric anisotropy typi-cally undergoes the Freédericksz transition at some initial critical voltage, asit prefers to align perpendicular to the electric field. This is a transition fromhomeotropic alignment to planar alignment. Then, at a higher critical volt-age, there is a transition to electroconvection [15–17]. Therefore, the abilityto produce homeotropic samples of nematic liquid crystals with negative di-electric anisotropy for studies of electroconvection is useful. Though a detailedstudy of electroconvection in homeotropic N4 is outside the scope of this pa-per, it is this example of electro-optical response on which we report in thispaper as a test of the alignment of the liquid crystal samples. (There havealso been studies of homeotropic samples in which there is a direct transitionto electroconvection, e.g. see [18], but these cases involve a positive dielectricanisotropy.)

2 Materials and characterization techniques

We used arachidic acid (C20) obtained from Sigma-Aldrich with a quoted pu-rity of ≥ 99%. It was used without further purification. The C20 was dissolvedin spectral grade chloroform obtained from EM Science at a concentration of1 mg/ml. The liquid crystal N4 is a eutectic mixture of the two isomers of 4-methoxy-4’-n-butylazoxybenzene (CH3O− C6H4 − NON− C6H4 − C4H9 andCH3OC6H4 − NNO− C6H4 − C4H9). It was obtained from EM Industries (aMerck company), now EMD Chemicals Inc. [19]. The N4 was used withoutfurther purification.

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The monolayers were formed on an ionic subphase composed of 1 mMCaCl2solution in purified water. The water was deionized milli-Q water with a re-sistivity of 18.2 MΩcm. Two different methods were used to include the N4in the C20 monolayers. First, a C20 monolayer was formed by placing dropsof the C20/chloroform solution on the subphase. Subsequently, a solution of3.95 × 10−3 mol/l of N4 in chloroform was added to the surface. The secondmethod involved placing different mixtures of C20 and N4 (described later)directly on the surface.

The most basic characterization of Langmuir monolayers is the measurementof the surface pressure versus area isotherms (or isotherms, for short) [20,21].For monolayers, the surface pressure (Π) is defined as the surface tension ofpure water (γw) minus the surface tension of the water-monolayer system (γ),i.e. Π = γw−γ. We made isotherm measurements using a KSV 5000 LB troughof maximum area 150× 475 mm2. The monolayer is compressed by moving asingle barrier at a rate of 10 mm/min (or 15 cm2/min). The surface pressureis monitored during the compression. Phase transitions within the monolayerare identified by the presence of plateaus (first order transitions) or kinks(second order transitions) in the isotherm. The isotherms were measured at atemperature of 22 ◦C.

In addition to surface-pressure area isotherms, we also characterized the com-position of the LB films using UV-visible absorption spectroscopy recorded ona Hewlett-Packard 8453 diode array spectrophotometer. The LB films are de-posited on a CaF2 substrate for this characterization. The LB films were madeby passing the substrate through the Langmuir monolayer in a vertical orien-tation at a constant rate of 10 mm/min. The number of layers transferred tothe substrate was set by the number of times the substrate was passed throughthe monolayer, with one layer transferred during each lowering and raising ofthe substrate through the monolayer.

3 Results and Discussion

Figure 1 shows the isotherms for the pure C20 system in which 50 µl of the1 mg/ml C20 solution was placed on the trough. Figure 1 also shows the C20systems subjected to the addition of N4 in 10 µl increments of the 3.95 ×10−3 mol/l N4 solution each time. The pure C20 isotherms are consistent withthe behavior expected from previous measurements of this system. Because weremain below the collapse pressure, the measured isotherm is reproducible incompression/expansion, as can be seen by the overlap of the solid and dashedline. We are able to identify the three phases of C20 that are expected at roomtemperature. Up to an area per molecule of 20.5 Å2/molecule, the phase isgaseous. At this point (the “liftoff point”), we observed the initial rise in the

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isotherm that corresponds to the L2 phase of the monolayer. This exists untilthe observed kink at a surface pressure of 22.5 mN/m. This corresponds tothe transition to the L′2 phase. As expected, there are no observed plateaus inthe isotherm.

As the N4 is added to the monolayer with the C20 in the gas phase, it isexpected that the N4 inserts into the monolayer. This expectation is con-firmed by the changes in the liftoff point and the existence of a plateau in theisotherm at 12 mN/m. The isotherms are still being plotted as a function ofthe area/molecule for the number of C20 molecules present in the monolayer.Therefore, the fact that the liftoff point occurs at a higher apparent area permolecule is due to the space occupied by the N4 molecules.

Using the isotherms in Fig. 1, one can compute the effective liftoff area as afunction of the amount of N4 added to the system. Using the fact that theareas are in terms of the initial amount of C20, one can estimate the areaper molecule occupied by N4 at liftoff based on the shift. For example, forthe 10 µl addition, there are 2.4 × 1016 N4 molecules. The additional areaat liftoff is 50.3 cm2. This corresponds to each N4 molecule occupying anarea of approximately 21.0 ± 0.1 Å2. Similar calculations for the 20 µl and30 µl additions also yield an area of 21± 0.1 Å2. This suggests that the N4 isoccupying a similar area as the C20 molecules at liftoff, and one can expect avertical orientation for the N4 molecules in the L2 phase of the monolayer.

The next issue is the nature of the plateau. It is known that other liquid crystalsystems are able to form multilayers at the air-water interface under compres-sion. The plateau region is indicative of the formation of a multilayer of N4.The fact that the isotherms ultimately converge on the pure C20 isothermsuggests that the N4 is able to form a complete layer on top of the C20 mono-layer. The subsequent expansion illustrates that most of the N4 is recovered,as the isotherms returns to the compression isotherm at high areas. Thoughnot directly related to the focus of this paper, this collapse mechanisms isinteresting and requires further study.

An alternate method of formation for the monolayer is to make a mixture ofC20 and N4 and directly form a Langmuir monolayer from the mixture. Theresults for isotherms from this solution are given in Fig. 2. Mixtures by molepercent of 43%-57%, 60%-40% and 69%-31% of N4 and C20 respectively arepremixed before introduction on the trough. A plateau region at approximately13 mN/m is observed for these systems. In this case, the shift in liftoff area isalso consistent with the amount of N4 present. However, it is interesting thatin this case, the expansion curves do not ever recover the compression curves.This suggests a loss of material from this system. It will be interesting toexplore the differences between the two film formation mechanisms in futurework. However, for the purposes of obtaining LB films with N4 molecules

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orientated perpendicular to the surface, both types of film suggest that thebest conditions for transfer are just before the plateau region. In this region,the N4 is still fully incorporated into the C20 monolayer, and it is orientedperpendicular to the surface.

To test that the N4 remains incorporated during the formation of the LB film,the monolayer is transferred at 10 mN/m on to a CaF2 substrate. A UV-visibleabsorption spectrum of LB films with different numbers of layers is carried out.A typical result is presented in Fig. 3. For comparison, we show the spectrafor the plain substrate and bulk N4. The peak for the N4 molecules is clearlyobservable at 350 nm in all of the LB examples, with the expected increase inamplitude of the peak as the number of layers is increased.

To test for homeotropic alignment, a liquid crystal cell was made using thefollowing procedure. An indium-tin oxide (ITO) coated glass slide was washedsequentially in water, in NH3 solution, and in an ultrasonic bath with a so-lution of 350 ml distilled water, 50 ml sodium hydroxide, and 40 ml liquinox.We selected the mixture of 43% N4 and 57% C20 diluted in chloroform andformed a monolayer that was compressed to 10 mN/m. The pressure was heldconstant while the ITO glass is coated with 10 layers of the Langmuir mono-layers. After the coating of the surface, the glass is baked in an oven at atemperature of 50 ◦C. A 25 µm mylar spacer is placed between the two ITOglasses. Two opposite sides of the cell are sealed with epoxy. The cell is filledwith N4 by capillary action. The two remaining sides of the cell are sealed with5 minute epoxy. The alignment was checked by observing the samples betweencrossed polarizers. This confirmed the general homeotropic alignment. As theultimate goal is to produce samples useful for the study of electroconvectionin homeotropic samples, we further tested the alignment by measuring theresponse of the N4 samples to an applied ac electric field as a function of bothapplied voltage and frequency.

As discussed, N4 has a negative dielectric anisotropy. Therefore, at each fre-quency, we expect to observe the Freédericksz transition followed by electro-convection. To measure the response, the applied ac voltage was increased insteps of 0.05 V. A waiting time of 120 s was used at each step. The tempera-ture was maintained at 30 ◦C. The system was placed between cross polariz-ers and imaged from above. In the undistorted state, the image is uniformlyblack. Above the Freédericksz transition, as the director tilts, the total inten-sity of the image increased continuously as a function of the applied voltage.This increase in intensity can be used to determine the onset voltage for theFreédericksz transition. Finally, at the critical value for electroconvection, oneobserves periodic patterns in the image. For the purposes of this paper, wemade a quantitative measure of the Freédericksz transition as a test of thealignment characterization and report on some qualitatively features of thesubsequent electroconvection. A more detailed study of the electroconvection

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will be the subject of future work.

The response to the applied electric field is summarized in Fig. 4. As expected,the Freédericksz transition is independent of the applied frequency, but theonset to convection is frequency dependent. For strong alignment, one expectsthe Freédericksz transition to occur at VcF = (K33/(²a²o))

1/2, where ²a isthe difference between the dielectric constant parallel and perpendicular tothe electric field (²a = ²|| − ²⊥), K33 is the bend elastic constant, and ²o =8.85×10−12 C2/Nm2. For our measured value of VcF = 9.3 V and the reportedvalue of ²a = 0.2 [19], we get K33 = 15.5× 10−12 N, consistent with previouslyreported values of K33 [22].

It is worth briefly commenting on the nature of the patterns that are observedabove the transition to electroconvection. Typical images are summarized inFig. 5 and Fig. 6. All of the images in Figs. 5 and 6 cover 3.05× 3.05 mm2. Itshould be noted that both figures focus on the case for higher frequencies. Forfrequencies lower than 1500 Hz, the initial electroconvection state is chaotic,and single images are not particularly useful. Figure 5 illustrates the twomain transitions. Figure 5a is below the Freédericksz transition; hence, it isuniformly dark. Figure 5b illustrates a state above the Freédericksz, but itis below the transition to electroconvection. Finally, Fig. 5c shows the initialstate above the transition to electronconvection for high values of the appliedfrequency. In this regime, the orientation of the rolls is essentially uniformthroughout the cell, and independent of the underlying Freédericksz domain.(The Freédericksz domains are distinguished by different degrees of overallintensity due to the crossed polarizers.) Figure 6 illustrates the secondarytransition exhibited by the convecting state as the voltage is increased. InFig. 6a, the the orientation of the rolls has begun to align with the Freéderickszdomains. As the voltage is increased, even more “solid looking” boundariesdevelop between the roll orientations and the domains of electroconvectionshrink in size (Figs 6b-d). It is interesting to note that this series of patternsis different from that reported in Ref. [15] for a homeotropic MBBA cell. Thequantitative characterization of these transitions is beyond the scope of thispaper and will be the subject of future work.

In summary, we have demonstrated that one can form monolayers of the liq-uid crystal N4 within a C20 monolayer, and we have characterized the phasebehavior of this mixed system at a single temperature. This system can betransferred to ITO coated substrates using standard LB techniques. The re-sulting surface provides homeotropic alignment for N4 in a standard electro-convection cell when other standard alignment techniques failed. The cellswere tested both by imaging the initial state with cross polarizers and bymeasuring the response to an applied electric field.

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4 Acknowledgments

This work was supported by NSF grant DMR-9975479 and PRF 39070-AC9.The authors thank Wytze Van Der Veer for his help in using the UCI LaserFacility.

References

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[10] L. Kramer, W. Pesch, Annu. Rev. Fluid Mech. 27 (1995) 515.

[11] A. Buka, L. Kramer (Eds.), Pattern Formation in Liquid Crystals, Springer-Verlag, New York, 1996.

[12] M. C. Cross, P. C. Hohenberg, Rev. Mod. Phys. 65 (1993) 851.

[13] E. Bodenschatz, W. Zimmermann, L. Kramer, J. Phys. (France) 49 (1988) 1875.

[14] I. Rehberg, B. L. Winkler, M. de la Torre Juárez, S. Rasenat, W. Schöpf,Festkörperprobleme 29 (1989) 35.

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[19] EMD Chemicals Inc., Hawthorne, New York, 10532-2156.

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[22] J. S. Martin, S. Garg, Private communication.

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20 25 30 350

5

10

15

20

25

30

(mN

/m)

Area/molecule (Å2/molecule)

Fig. 1. Plot of isotherms for C20 (solid black curve), and C20 with the additionof N4 (dashed curves). Reading from left to right, each dashed curve represents anaddition of 10 µl of the N4 solution, as described in the text. All curves are plottedas a function of the area per molecule for the C20 molecules. Each isotherm showsa single compression and expansion. For the pure C20 case, the compression andexpansion are essentially identical. For the cases with N4, the direction of each curveis indicated by the arrows.

5 10 15 20 25 300

5

10

15

20

25

30

(mN

/m)

Area/molecule (Å2/molecule)

Fig. 2. Isotherms for three examples of the C20/N4 mixtures. The mixtures are (bymole percent) 43% (solid curve), 60% (dashed curve), and 69% (dotted curve) N4.

10

200 300 400 500 6000.10

0.12

0.14

0.16

0.18

0.20

(nm)

Opt

ical

Den

sity

(a.u

)

0.02

0.04

0.06

0.08

0.10

0.12

0.14 Optical D

ensity (a.u.)

Fig. 3. Ultraviolet-visible absorption spectra for bulk N4 (dashed line), blank sub-strate (dotted line), C20/N4 film transferred at 10 mN/m (lower solid line) andC20/N4 film transferred at 20 mN/m. The optical density is given in arbitraryunits, as the relative intensity is the important factor. The bulk film is relative tothe left axis, with all other cases relative to the right axis. The key feature is theN4 peak at 350 nm that is visible in both LB films.

0 2000 4000 6000 80000

5

10

15

20

25

Vc (V

)

frequency (Hz)

Fig. 4. Critical voltage (Vc) for the onset of the Freédericksz transition (squares)and electroconvection (circles) as a function of applied frequency. As expected, theFreédericksz transition is independent of frequency, and the transition to electro-convection increases with increasing frequency.

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(a) (b) (c)

Fig. 5. Three images of the sample covering an area of 3.05× 3.05 mm2. The scalebar represents 1 mm. The images were taken with an applied frequency of 5000 Hzand a voltage of (a) 9.05 V (b) 9.55 V and (c) 14.5 V.

(a) (b)

(c) (d)

Fig. 6. Four images of the sample covering an area of 3.05 × 3.05 mm2. The scalebar represents 1 mm. The images were taken with an applied frequency of 5000 Hzand a voltage of (a) 14.55 V (b) 14.60 V (c) 14.79 V and (d) 14.95 V.

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