GB2071131A - Liquid crystal materials suitable for use in twisted nematic displays - Google Patents

Abstract

A nematic liquid crystal material suitable for use in a twisted nematic liquid crystal display comprises a mixture of: (a) at least one nematogenic compound having a positive dielectric anisotropy having a magnitude greater than 3 (e.g. having a terminal cyano group); together with (b) at least one nematogenic compound having a formula <IMAGE> where R<1> is an alkyl group, R<2> is an alkyl or alkoxy group and <IMAGE> is a bicyclo (2,2,2) octane ring. Preferably the compound(s) of formula (I) constitutes between 10 and 90% by weight of the mixture, desirably about 40% by weight.

Description

SPECIFICATION Liquid crystal materials suitable for use in twisted nematic displays and twisted nematic displays incorporating such materials This invention relates to liquid crystal materials suitable for use in twisted nematic displays and to twisted nematic displays incorporating such materials.

Liquid crystal devices are commonly formed by sandwiching a layer typically 7 to 12 ,um thick, of a liquid crystal material between optically transparent electrically insulating substrates, eg glass slides.

Electrode structures on the substrates enable voltages to be applied across the layer and change the molecular arrangement of the liquid crystal molecules in selected regions. This causes an observable effect in those regions which can be used in displays of various types, for example, digital watches, small calculators, voltage meters, etc.

There are several different display effects which may be obtained from electrical re-arrangement of liquid crystal molecules. These depend on the liquid crystal material employed and the initial molecular alignment (if any). This invention is concerned particularly with the twisted nematic effect in which an essentially nematic material is arranged in its OFF state (zero or low applied voltage) in an arrangement in which the molecules gradually twist, usually through 900, from one substrate surface to the other as defined by fixed alignments at these surfaces.

Such an arrangement is optically active, ie rotates the plane of polarized light in this state. An applied voltage greater than a threshold essentially causes the twist to be destroyed so that the molecular arrangement is no longer optically active. Thus, when placed between plane (linear) polarizers a twisted nematic effect device provides one or more optical shutters which are switchable from a light state to a dark state or vice versa depending on whether the polarizers are in a parallel or cross relationship.

Such shutters may in a single device form the component elements of a series of letters, words or numerals, ie in an information display, as defined by the electrode structures on the substrates and the applied voltages. An example of a twisted nematic effect device is further described in UK Patent Number 1,478,592.

Conventionally, liquid crystal materials used in twisted nematic displays are basically nematic mixtures of compounds having a high positive dielectric anisotropy AE, eg I AEl > + 3. The dielectric anisotropy is the difference between the average dielectric constant measured parallel (Ell) to the molecules and the average dielectric constant (El) measured per pendicular to the molecules when aligned together.

The individual compounds in the mixture are normally compounds containing a terminal cyano group in the p-position of a benzene ring. They are also nematogenic, which means that they show nematic liquid crystal properties or tendencies and exhibit a nematic liquid crystal phase when formed into a mixture together with other nematogenic compounds.

For example mixtures of cyanobiphenyls and terphenyls as described in UK Patent No 1,433,130 have found wide use in twisted nematic displays and give an excellent range of properties for many purposes in such displays. One example of such a mixture is the well known material E7 marketed by BDH Chemicals Ltd of Broom Road, Poole, Dorset, England.

One problem with which the present invention is concerned relates to the temperature dependence of operation of a twisted nematic liquid crystal display.

If such a display is designed for optimum performance at a given value of temperature, eg 20"C, it is clearly desirable that the performance should vary as little as possible if the ambient temperature varies from that given value. The temperature dependence of operation may be designated as 1/Vx (AV/QT) where V is the voltage required to achieve the ON state and AV is the change in the voltage V caused by a change in temperature AT. The value of this parameter is determined primarily by the liquid crystal material used in the display.

As noted above, cyanobiphenyls and cyanoterphenyls have found wide use particularly in twisted nematic effect displays. However, mixtures of these materials give typical values of from 0.8 to 0.9% per C" over the temperature range 0 to 400C in twisted nematic displays and this is considered to be rather high.

Another problem with which the present invention is concerned relates to the multiplexibility of a twisted nematic display. Multiplexing involves applying electric potentials to lines of electrodes connected in series, each region or display element, eg each individual shutter in a twisted nematic effect display, being defined by the intersection of the electrodes on the two substrates. Examples of multiplexed displays are the matrix displays described in UK Patent Number 1,458,045.

In multiplexed displays all electrodes in the display receive a non-zero electrical potential, even when they are in the OFF state. Consequently it is difficult to provide sufficient contrast between the ON and OFF states. the contrast can vary with viewing angle, temperature and liquid crystal material as well as applied voltage. As described in UK Patent Application Number 36595/78 and 40580/78 a figure of merit which may be used to define multiplexibility, ie the usefulness of any given material in a multiplexed twisted nematic effect display is the parameter M20. This is the ratio, at 20"C, of voltages V,O(0 ): Vgo(45 ) which are voltages which may be designated as necessary to achieve the ON and OFF states respectively.In fact V,0(0 ) is the minimum voltage required to achieve along an axis normal to the display suL ctrates 10% of the maximum light transmission along that axis for a twisted nematic effect cell: the incident light is plane polarized paral lel to the surface alignment of the liquid crystal molecules upon which the light is first incident and the analyzer is arranged with its polarizing axis per pendicularto that alignment and parallel to the surface alignment of the molecules from which the light emerges.

V90(45 ) is the maximum voltage which gives along an axis at a viewing angle of 45" to the alignment directions on the substrate surfaces 90% of the max imumtransmission alongthatdirectionforthe same cell for which V,0(0 ) is measured. Of the four possible directions which are at 45 to the alignment directions the direction having the lowest value of V90 (450) is chosen.

An ideal material would have M20 equal ofjust greater than unity so that a very small change in applied voltage could be used to switch between the OFF and ON states. In practice, other liquid crystal properties, eg the temperature range of the nematic mesophase, have to be taken into account as well.

Although cyanobiphenyls and terphenyls have found wide use in twisted nematic displays in general it is not easy to achieve multiplexing with them.

For example the material E7 mentioned above has a value of 1.81 for M20 which is considered not to be low enough.

It is the purpose of this invention to provide a liquid crystal material for twisted nematic displays giving a reduced temperature dependence of operation, ie lower values or 1Nx (AV/AT) in twisted nematic displays, or improved multiplexibility in multiplexed twisted nematic displays or both.

According to the present invention in a first aspect a nematic liquid crystal material suitable for use in a twisted nematic liquid crystal display comprises a mixture of: a. at least one nematogenic compound having a positive dielectric anisotropy having a magnitude greater than 3; togetherwith b. at least one nematogenic compound having a formula

where R' is an alkyl group, R2 is an alkyl or alkoxy group and

is a bicyclo (2,2, 2) octane ring.

Preferably the compound(s) of formula (I) constitutes between 10 and 90% by weight of the mixture, desirably about 40% by weight The compound(s) having AEt e| > + 3 may be one or more having a terminal cyano group.

It has been found that the compounds of formula (I) have a low value of! AEl. Consequently when they are mixed together with one or more compounds having AE > +3 the positivity of AE for the mixture is determined mainly by the presence therein of the compound or compounds having|#e| > +3. This means that Ae is not reduced much, if at all, below + 3 by forming the mixture. Also, it has been found, unexpectedly, that the temperature dependence of operation using the mixture is significantly reduced by the presence of the compounds of formula (I). For example values for 1/V (AV/AT) of 0.5% per C and less may be achieved.

Consequently, by adding to a material havingIAJ > +3 a compound of formula (I) the temperature dependence given by the material may be reduced without reducingIAj to an unacceptably low level.

In material embodying the invention the compound(s) of formula (I) preferably have R1 = n-alkyl and RZ = n-alkyl or n-alkoxy. The compound(s) having When #e| > 3 may be one or more of the following (between 10 and 90%, preferably about 50-60%, by weight in total):

where R3 is an n-alkyl or n-alkoxy group and R4 and R5 are n-alkyl groups having less than 10 carbon atoms. One or more of the following additives (up to 20% by weight in total) may also preferably be included in the mixtures:

where R3 and R, have the same definition as R3 and R4, R5r P6 and R9 and R9 have the same definition as R4.

The material may also contain a small amount (eg 1% or less) of an optically active additive.

Materials embodying the present invention also provide values of M20 less than 1.81 in multiplexed twisted nematic displays.

According to the present invention in a second aspect a liquid crystal electro-optical display includes two dielectric substrates at least one of which is transparent, a film of liquid crystal material sandwiched between the substrates, the molecular arrangement in the material being such as to give the twisted nematic effect, and electrodes on the inner surfaces of the substrates for applying electric fields across the liquid crystal material, characterized in that the material is as defined in the first aspect above.

Examples of devices and materials embodying the invention will now be described by way of example only with reference to the accompanying drawings wherein: Figure lisa sectional view of a digital display; Figure 2 is a sectional view of Figure 1; Figure 3 shows a rear electrode configuration for Figure 1; Figure 4 shows a front electrode configuration for Figure 1; Figures 5,6,7 show schematic views of the device of Figures 1 to 4 with typical addressing voltages.

The display of Figures 1 to 4 comprises a cell 1, formed of two, front and back, glass slides 2,3 respectively, spaced 7clam apart by a spacer 4 all held together by an epoxy resin glue. In front of the front glass slide 2 is a front polarizer 5 arranged with its axis of polarization axis horizontal. A reflector7 is arranged behind a back polarizer 6 behind the slide 3.

Electrodes 8,9 of tin oxide typically l00Athick are deposited on the inner faces of the slides 2,3 as a complete layer and etched to the shapes shown in Figures 3,4. The display has seven bars per digit 10 plus a decimal point 11 between each digit. As shown in Figure 3 the rear electrode structure is formed into three electrodes x1, x2, x3. Similarly the front electrode structure is formed into three electrodes per digit and decimal points1, y2, y3..

Examination of the six electrodes per digit shows that each of the eight elements can independently have a voltage applied thereto by application of suit able voltage to appropriate x, y electrodes.

Prior to assembly the electrode clear slides 2, 3 are cleaned then dipped in a solution of 0.2% by weight of polyvinyl alcohol (PVA) in water. When dry, the slides are rubbed in a single direction with a soft tissue then assembled with the rubbing direction orthogonal to one another and parallel to the optical axis of the respective adjacent polarizers. When a nematic liquid crystal material 12 is introduced between the slides 2,3 the molecules at the slide surfaces lie along the respective rubbing directions with a progressive twist between the slides.

When zero voltage is applied to the cell 1 light passes through the front polarizer 5, through the cell 1 (whilst having its plane of polarization rotated 90 ) through its rear polarizer 6 to the reflector 7 where it is reflected back again to an observer, (shown at an angle of 45" to the axis Z normal to axes X and Yin the plane of the slides 2, 3). When a voltage above a threshold value is applied between the electrode 8, 9 the liquid crystal layer 12 loses its rotary power, or optical activity, the molecules being re-arranged to lie perpendicular to the slides 2,3, ie along the axis Z. Thus light at that position does not reach the reflector 7 and does not reflect back to the observer who sees a dark display of one or more bars of a digit 10.

Voltages are applied as follows as shown in Figures 5,6 and 7 for three successive time intervals. An electric potential of 3V/2 is scanned down each x electrode in turn whilst -V/2 is applied to the remaining xelectrodes. Meanwhile3V/2 or V/2 is applied to they electrodes. A coincidence of 3V/2 and - 3V/2 at an intersection results in a voltage 3 V across the liquid crystal layer 12. Elsewhere the voltage is V or -V. Thus by applying - 3V/2 to appropriate y electrodes as 3V12 is scanned down the x electrodes selected intersections are turned ON as indicated by solid circles. The electric voltage V is an ac signal of eg 100 Hz square wave, and the sign indicates the phase.

Each OFF intersection receives Vforthe entire scan period whilst each ON intersection receives 3V for one third of the scan period and V for the rest of the scan period making an rms value of 1.91 V. Thus the material of layer 12 preferably has M20 less than 1.91 to give a reasonable contrast between the ON and OFF states.

For similar displays having more than three rows of electrodes the figure of 1.91 V is reduced accordingly.

This illustrates the importance of the parameter M20.

Suitable nematic mixtures for use to form the layer 12 in the above device are described below.

Small amounts of a cholesteric material may be added to the nematic material to induce a preferred twist in the molecules in the liquid crystal layer. This and the use of appropriate slide surface treatment removes the problems of display patchiness as taught in UK Patent Serial Number 1,472,247 and 1,478,592.

C 15 about 0.1 to 0.5% by weight and CB 15 about 0.01 to 0.05% by weight.

the following mixtures are examples of nematic materials embodying the invention which may be used in the above device. In the properties of the mixture: M20 is defined above; M'20 is a similarly measured ratio but is the voltage required to give 50% transmission at an angle of 10 to the normal to the twisted nematic cell divided by the voltage

this is another figure of merit for the multiplexibility of a material in twisted nematic operation; a typical value for a mixture of cyanobiphenyls alone is 1.36 (for the material E7).

In the following: Compound A is

Compound B is

Compound C is

and Compound D is

N-l is nematicto isotropic transition temperature.

Table 1: Mixture 1 Compound Percentage by weight Compound A 15 Compound B 23 Compound C 10 Compound D 12

Properties of this mixture in a 6 m twisted nematic cell are as follows: N-I = 73.9 C ii V90(45 ) = 1.11 voltsat200C iii M20 = 1.73 iv M201 = 1.32 v 1/V (AV/AT) from 0 # 40 C = 0.41% per C for V90(45 ) 0.46% per C for V50(10 ) and 0.48% per C for V1O(0 )- Table 2:Mixture 2 compound Percentage by weight Compound A 15 Compound B 23 Compound C 10 Compound D 12

Properties of Mixture 2 are as follows: i N-I = 76.4 C ii V90(45 ) = 1.18 volts iii M20 = 1.74 iv M120 = 1.33 v 1/V (#V/#T) from 0 # 40 C = 0.44% per C for V90(45 ) 0.48% per C for V50(10 ) 0.48% per C for V10(0 ) Table 3: Mixture 3 Compound Percentage by weight Compound A 15 Compound B 23 Compound C 10 Compound D 12

Properties of Mixture 3 are as follows: N-I = 80.2 C ii V90(45 ) = 1.17 volts iii M20 = 1.79 iv M120 = 1.34 v 1/V(#V/#T) form 0 # 40 C = 0.32% per C for V90 (45 ) 0.36% per C for V50(10 ) 0.39% per C for V10(0 ) Table 4:Mixture 4 Compound Percentage by weight Compound A 15 Compound B 23 Compound C 10 Compound D 12

Properties of Mixture 4 are as follows: i N-I = 75.6 C ii V90(45 ) = 1.14 volts iii M20 = 1.78 iv M120 = 1.34 v 1/V (#V/#T) from 0 # 40 C = 0.37% per C for V90 (45 0.42% per C for V50(10 ) 0.43% per C for V,O(O ) Table 5: Mixture 5

Compound Percentage by weight n - C3117&num;ff CN 15 n-C59lgHCN CN 20 rL-C7H1mThCN 15 n-C6H13COO C5Hll-n 50 Properties of Mixture 5 are as follows: i N-I = 68.1 C ii 1/V (#V/#T) from 20 # 40 C = 0.40% per C for V90(45 ) 0.48% per C for V50(10 ) 0.38% per Cfor V10(0 ) iii V90(450) = 1.41 iv M20=1.84 v M120=1.38 Table 6: Mixture 6

Compound Percentage by weight n- c7mffo CN 30 n-C5ll11HffO Cm 30 t-C5HgC00+C3H7~0 40 Properties of Mixture 6 are as follows: N-l=74.10C ii 1/V (#V/#T) from 10 # 50 C = 0.42% per C for V90 (450) 0.49% per C for V50(10 ) 0.49% per C for V10(0 ) iii V90(45 ) = 1.36 iv M20=1.95 v M120 = 1.43 Mixture 7 Compound Percentage by weight A 15 B 23 C 10 D 12

i N-I = 72.4 C ii V90(45)=1.16 iii M20 = 1.73 iv M'20=1.34 v 1/V(#V/#T) 0 # 40 = 0.43%/ CV90(45 ) 0.48%/ CV50(10 ) 0.54%/ CV10(0 )

Claims (9)

1. A nematic liquid crystal material suitable for use in a twisted nematic liquid crystal display comprises a mixture of: a. a first component comprising at least one nematogenic compound having a positive dielec tric anisotropy having a magnitude greater than 3; together with b. a second component comprising at least one nematogenic compound having a formula

where R1 is an alkyl group, R2 is an alkyl or alkoxy group and

is a bicyclo (2, 2, 2) octane ring.

2. A material as claimed in claim 1 and wherein R' is an n-alkyl group and R2 is an n-alkoxy or n-alkyl group, R1 and R2 having less than 10 carbon atoms.

3. Amaterial as claimed in claim 1 or claim 2 and wherein the first component comprises one or more compounds selected from compounds having the following formulae:

where R3 is an n-alkyl or n-alkoxy group, R4 is an n-alkyl group and R5 is an n-alkyl group. R3, R4 and R5 having less than 10 carbon atoms.

4. A material as claimed in any one of the preceding claims and wherein the ratio by weight of the first component to the second component is in the range between 10:90 and 90:10.

5. A material as claimed in any one of the preceding claims and wherein the material additionally incorporates a third component comprising one or more compounds selected from the following:

where R3 and R7 have the same definition as R3 above and R4, R5, R6 and R8 and R9 have the same definition as R4 above.

6. A material as claimed in any one of the preceding claims and which additionally incorporates a further component comprising an optically active additive to the extent of about 1 % by weight of the overall Mixture.

7. A material which is substantially the same as any one of Mixtures 1 to 7 hereinbefore.

8. A liquid crystal electro-optical display including two dielectric substrates at least one of which is transparent, a film of liquid crystal material sandwiched between the substrates, the molecular Arrangement in the material being such as to give the twisted nematic effect, and electrodes on the inner surfaces of the substrates for applying the electric fields across the liquid crystal material, characterized in that the material is the material claimed in any one of the preceding claims.

9. A device as claimed in claim 8 and wherein the electrodes are arranged so as to be addressed in a multiplexed fashion.