LIGHT POLARIZATION FROM FILMS OF LYOTROPIC NEMATIC LIQUID CRYSTALS

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LIGHT POLARIZATION FROM FILMS OF LYOTROPIC NEMATIC LIQUID CRYSTALS

John F. Dreyer

To cite this version:

John F. Dreyer. LIGHT POLARIZATION FROM FILMS OF LYOTROPIC NEMATIC LIQUID CRYSTALS. Journal de Physique Colloques, 1969, 30 (C4), pp.C4-114-C4-116.

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JOURNAL DE PHYSIQUE Colloque C 4 , supplément au no 11-12, Tome 30, Nov.-Déc. 1969, page C 4

- 114

LIGHT POLARIZATION FROM FILMS OF LYOTROPIC NEMATIC LIQUID CRY STALS

John F. DREYER President of Polacoat, Inc.

Résumé -

On peut produire des films polariseurs par évaporation contrôlée d'un nématique lyotropique sur une surface orientée

les molécules s'alignent avec l'axe

parallèle

la direc- tion de polissage de la surface.

Abstract. - Light

polarizing films are produced by controlled evaporation of a lyotropic nematic dichroic liquid crystal upon ail oriented base. The molecules align themselves with their

axes parallel in the direction of the surface orientation. This orientation is maintained upon evaporation of the solvent.

It has been known for over one hundred and fifty years that crystals grown in contact with another crystal of different chemical constitution can be oriented by the contact [Il. In 1913, M. Mauguin reported that Iiquid crystals could also be oriented by contact

Hans Zocher, using Methylene Blue, showed in 1925 that rubbing the surface of glass produced a dichroic orientation with evaporated dyes [3]. He made an extensive study of the rubbing phenomena and reco- gnized its potential for production of light polarizing films. He did not expand this phenomena into other dye classes.

Paul Gaubert in 1933 recorded oriented lyotropic iiquid crystal films of other dyes [4].

In 1937, P. Chatelain reported the orientation of thermotropic liquid crystals on a rubbed surface [5].

That liquid crystals could be produced by solvent as well as by temperature had been known for some time.

In fact, an early recognition of the mylene form of lyotropic character was in 1854 [6]. The thermotropic melt type liquid crystals were not recognized until

1888 [7].

Consequently, the background for producing com- mercial light polarizing films was well established prior to my first patent application in 1941 [a].

There are a number of classes of dyes and hundreds of commercial dyes which will produce these dichroic films on a rubbed surface. These classes include the following with an example using the first edition colour index number

Class - Nitro Dis Azo Tris Azo Stilbene Pyrazonle

Triphenylmethane Quinoline Thiazole Azine Oxazine Thiazine Anthraquinone

Number -

1 O 246 533 620 639 658 808 814 842 911 925 1 034

Example - Naphthol Yellow S Naphthol Blue Black B Benzo Fast Blue 4 G L Stilbene Yellow T P Ero Flavine

Setoglaucine

Pinacyanol

Solantine Yellow FF Safranine

New Methylene Blue N Toluidine Blue O Alizarin Red S

Dyes having various colors make it possible to combine dyes of compatible constitution to produce deeper colors such as brown and gray.

It is to be noted that some of the above compounds are not long chain para compounds. There are several common characteristics, the most important are that they are al1 essentially flat molecules and consist of benzene or naphthalene rings surrounded with solubilizing sulfonic or methyl groups and a hydrogen bonding hydroxyl or amine group.

Using a monoazo dye structure such as of the dye Amaranth W colour Index No. 184, one can readily Vary the position of the various solubilizing sulfonic groups on the molecule. One finds that when stearic hindrance is produced causing the molecule to twist

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1969427

LIGHT POLARIZATION FROM FILMS OF LYOTROPIC NEMATIC C 4 - 1 1 5

that the intensity of the color is reduced, and dichroic the uniform orienting effect is reduced and the familiar films can no longer be formed 191. Schlieren type pattern is produced (Fig. 2).

The process for the production of the light polarizing films consists of rubbing a glass or plastic surface substrate with a water slurry of a rouge in the direction or pattern of the desired polarization, then washing off the rouge and while the surface is still clean applying a solution of a pure dye or dye mixture in a solvent.

The coating is carefully dried. During the drying pro- cess, observation tvith a light polarizing analyzer shows that the filin while it is still wet goes from an isotropic to an anisotropic state. Further drying reduces the coating to a dry, yet still oriented film.

This film should not be greater than twenty microns thick to be efficiently oriented throughout. It is subject to deterioration by rubbing and can be protected by a lacquer coating or by laminating with plastic to make a durable light polarizing filter.

With plastic the choice of substrate is important.

Cross linked plastics such as phenol formaIdehyde do not take satisfactory orientation by rubbing.

Some of the dichroic films produced, if exposed to water vapor, will go into the crystalline state (Fig. 1).

By treating the surface of a glass substrate with strong acid prior to application of the dye solution,

FIG. 2.

-

FIG. 1.

-

-

C 4 - 116 JOHN F. DREYER

This pattern of lines moves with rotation of the

polarizing analyzer. By evaporating a thick film rapidly, the common nematic Thread type disconti- nuity lines are formed which do not move on rotating the polarizing analyzer (Fig.

Brilliant Yellow (Color Index no

forms a positive dichroic film while Methylene Blue and Ama- ranth form negative dichroic films, with the negative dichroism, the direction of the vibration of the electric vector of the absorbed light is perpendicular to the original surface rubbing direction.

Methylene Blue is trichroic. Solid crystals of the dye are a blue color for light vibrating on one axis, a pink on another perpendicular axis and colorless on the third perpendicular axis.

A dichroic film of Methylene Blue when made as above and viewed with a polarizing analyzer can be seen to appear a reddish blue on one axis and colorless on the other axis. For it to appear colorless on one axis the molecules must be lined up with al1 the short color-free axes parallel.

G . Friedel

[IO] has said that the nematic phase consists of molecules with one axis parallel but he did not claim that this axis had to always be the long axis of the molecule. Here is evidence that with nematic liquid crystals the molecules lie with their Aat sides together and with their short not their long axis paraliel

DISCUSSION

F. C. FRANK. -

1 agree with the possibility in principle that there may be nematic phases in which plate-like molecules have their shortest axes preferen- tially aligned to the nematic axis

but 1 do not think you have given an indisputable example of this.

Firstly,

wasformed. The molecules are aligned on a solid sur face, and it was not unambiguously demonstrated that they became aligned while still in the liquid.

Secondly, 1 remember being told, about 20 years ago, by Theo Forster (because he had made a theory of why they do it) that plate-like dyestuff molecules, at a certain concentration in solution, aggregate into

long columns like piles of pennies. Then, if a nematic liquid crystal phase was formed, the bodies beco- ming nematically aligned could be Forster aggregates instead of single molecules, so that the nematic axis corresponded to the longest axis after all.

J. F. DREYER. -1 don't know how you would define a liquid crystal.

F. C . FRANK.

It should be both liquid and ani- sotropic.

J. F. DREYER. - Then described the preparation with the appearance of two boundary lines as the methanol solution of dyestuff dried out on the sup- porting film, previously rubbed with rouge slurry

the film was isotropic up to the first boundary line

there it became anisotropic but was still a liqujd which could be smeared with a finger.

F.

C.

Good.

J. F. DREYER. - AS to the second point, 1 shall be giving further details at a forthcoming American Che- mical Society meeting.

H. KELKER.

What is the

of the birefringe of your nematic phases, ordered parallel to their Aat sides, if you look normally to the layer

1 refer to Zocher's observations on Salvarsen, who found

birefringence in a nematic solution phase, too.

J. F.

DREYER.

The majority shows negative bire- fringence, but positive is also seen

Bibliographie

[l] FRANKENHEIM, Pogg. Ann. 1836, 37, 516.

[2] MAUGUIN (M. Ch.), C . R. Acad. Sci. Paris, 1913, 156, 1246.

[3] ZOCHER (H.), Naturwissenchaften, 1925, 13, 1015, [4] GAUBERT (Paul), C . R. Acad. Sci. Paris, 1933,197,1436.

[5] CHATELAIN (P.), Bull. SOC. Franc. Miner. Crist., 1937, 60, 280.

[6] Arch. Path. Anat. Physiol., 1854, 6, 562.

[7] REINITZER (F.), Moratsh Chem., 1888, 9, 421.

[8] DREYER

(John

[9] DREYER

(John

[IO] FRIEDEL (G.), Ann. Physique, 1922, 18, 273.