Nematic liquid-crystal compositions
United States Patent Chang et al.  
 [451 Apr. 29, 1975 1 1 NEMATIC LIQUID-CRYSTAL COMPOSITIONS [75] Inventors: Roger Chang, Thousand Oaks; John P. Dobbins, San Marino, both of Calif.  
 [73] Assignee: Rockwell International, El Sequndo,  
 Calif.  
 [22] Filed: Oct. 12, 1972 [21] Appl. No.: 297,172  
  Related US. Application Data [63] Continuation of Ser. No. 87,515. Nov. 6. 1970.  
 abandoned.  
 [52] US. Cl...... 252/299; 252/408 LC; 350/160 LC [51] Int. Cl. G091 1/28; G09f 1/36; G03b l/24 [58] Field of Search. 96/15; 252/501, 304, 408 LC, 252/299; 350/160 LC, 150; 161/410 [56] References Cited UNITED STATES PATENTS 3,499.702 3/1970 Goldmachcr et a]. 350/160 LC 3.690,745 9/1972 Jones 252/408 FOREIGN PATENTS OR APPLICATIONS 1.170.486 11/1969 United Kingdom 252/408 OTHER PUBLICATIONS Usoltseva et al., Russian Chemical Reviews, Sept., 1963. Vol. 32, No. 9, pp. 495-507.  
 Kelker et al. Angew. Chem. International Edit, Vol.  
 8, No. 11, 1969, pages 884-885.  
 Primary Examiner-Norman G. Torchin Assistant Examiner-J. P. Brammer Attorney, Agent, or Firm-H. Fredrick Hamann; G. Donald Weber, Jr.; Robert Ochis [57] ABSTRACT 4 Claims, 1 Drawing Figure MIXTURE 0F LIQUID CRYSTAL TRANSITION TEMPERATURE, (&#34;K I so so 100 3&#34; &#34;A&#34; methoxy-berzylidene-buIyI-nnilim &#34;B&#39; film -mm buIyl -uni|ine mimwmz i is MIXTURE OF LIQUID CRYSTAL 0 0 O 5 w m 9 7 w 3 3 3 2 2 2 &#34;A&#34; -me1hoxy-benzylidenebuiyl-qniline &#34;B&#34; -ethoxy-benzylidene-butyl -aniline INVENTORS CHANG ROGER BY JOHN P. ooasms ATTORNEY NEMATIC LIQUID-CRYSTAL COMPOSITIONS This is a continuation of application Ser. No. 87,5 filed Nov. 6, 1970, and now abandoned.  
 BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to nematic liquid crystals and more particularly to eutectic mixtures of nematic substances.  
 2. Description of Prior Art Nematic substances and nematic liquid-crystal light valves and display devices are described in British Pat. No. 441,274 and in US. Pat. No. 3,322,485. The use of pure nematic substances in these light valves has not been entirely satisfactory due to the relatively narrow temperature range in which they behave as liquid crystals and due to the fact that in most cases these nematic ranges are considerably above room temperature.  
  The nematic range of a liquid crystal is defined as the temperature range between the crystal/nematictransistion temperature and the nematic/isotropictransition temperature. For example, the nematic liquid crystal p-azoxyanisole has an operating range of 19C between its crystal/nematic-transition temperature of 1 16C and its nematic/isotropic-transition temperature of 135C. Light valves which use a nematic substance which has a high nematic range require a heating element in order to heat the nematic substance to its required mesophasic condition or range. The use of a heating element in a light valve not only increases the cost and complexity of the apparatus but also results in a higher power consumption and a slower startup time for all operations involving the light valve apparatus than is the case without the use ofa heating element.  
  Forming a nematic liquid crystal mixture containing 10-90 weight percent of p-n-ethoxybenzylidene-pamino-benzonitrile and 90-10 weight percent p-n butoxybenzylidene-p-amino-benzonitrile to obtain a lower crystal/nematictransition temperature is described in the US. Pat. No. 3,499,702. Crystal/nematic-transition temperatures as low as 41 and 50C were obtained.  
 SUMMARY OF THE INVENTION It is a primary object of this invention to provide an improved nematic liquid crystal composition.  
  It is another object of this invention to provide a nematic liquid crystal composition having a wide nematic range.  
  It is yet another object of this invention to provide a nematic liquid crystal composition suitable for nematic use at temperatures below and above room tempera ture,  
  The invention is accomplished by the use ofa eutectic mixture of two nematic substances. A preferred embodiment is a eutectic mixture of p-methoxybenzylidene-p-n-butylaniline (MBBA) and p-ethoxybenZylidene-p&#39;-n-butylaniline (EBBA).  
 BRIEF DESCRIPTION OF THE DRAWING The drawing shows a phase diagram for the eutectic mixture of p-methoxy-benzylidene-pn-butylaniline and p-ethoxy-benzylidene-p-n-butylaniline.  
 DETAILED DESCRIPTION OF THE INVENTION In general, a light valve containing a thin layer of nematic liquid crystal exhibiting negative dielectric anisotropy is relatively transparent to light in the absence of an electric field. When an electric field above a threshold value is applied to the liquid-crystal material, the light valve device when veiwed by transmitted light appears to darken in the region of the field. The operative region where this action occurs is shown in the phase diagram between the crystal/nematic-transitiontemperature curve 12 and the nematic/isotropictransition-temperature curve 14. The wider the difference between the two aforementioned temperature curves, the broader the mesophasic or operating range for the liquid-crystal mixture. The eutectic mixture 16 contains 60 weight-percent p&#39;methoxy-benzylidene-p&#39;- n-butylaniline and 40 weight-percent p-ethoxybenzylidene-p-n-butylaniline (hereinafter referred to as MBBA and EBBA, respectively.) The nematic operating range for this eutectic mixture is about 65C. This broad range is from 2 1C to a +42C, a desirable temperature range which covers temperatures both above and below room temperature (20-25C).  
  A preferred composition is generally the eutectic mixture of the two nematic liquid crystals. As shown by the phase diagram, a wide operating range is obtained when the composition varies i2 weight percent from the eutectic mixture. In the case of the 60 weightpercent MBBA and 40 weight-percent EBBA, a satisfactorily wide operating range is still obtained within 5 weight-percent of the eutectic mixture. Thus, a useful eutectic mixture is obtained with 55% MBBA and 45% EBBA, for example.  
  The lowest crystal/nematic-transition temperature is obtained with a eutectic mixture. In many mixtures, the widest operating range, that is the range between the crystal/nematic-transition temperature and the nematic/isotropic-transition temperature, is obtained with an eutectic mixture. There are some mixtures in which the shape of the nematic/isotropic-transitiontemperature curve differs from that shown in the drawing and, as a result, the widest operating range will vary somewhat from the eutectic mixture composition described herein.  
  This invention is applicable to mixtures, especially eutectic mixtures, of all nematic liquidcrystal compounds. The broad class of organic compounds commonly known as Schiff bases is of special interest. Eutectic mixtures of specific Schiff base nematic liquidcrystal compounds having the following structure are preferred embodiments of this invention:  
  where R CH; and C 11 R c H n is from 2 to 12 and R R have a total of at least four carbon atoms. Examples of these compounds are:  
  p-methoxy-benzylidene-p-n-butylaniline, p-methoxy-benzylidene-p-n-propylaniline, p-methoxy-benzylidenep-n-hexylaniline, p-ethoxy-benzylidene-p&#39;n-butylaniline, p-ethoxy-benzylidene-p-ethylaniline, p-ethoxy-benzylidene-p-n-propylaniline and p-ethoxy-benzylidene-p&#39;-n-hexylaniline. In this particular group of Schiff base compounds, an increase of the size of R will raise the crystal/nematictransition-temperature curve. As a result, R, is limited to methyl and ethyl to provide a mixture which is liquid at room temperature. With respect to the size of the R radical, it was found that increasing the size of the radical will decrease the transition temperature. It has been determined that, in order to produce suitable nematic liquid-crystal mixtures which are liquid at room temperature, the carbon atoms in R and R in the compound should total four or more. Compounds not having a total of at least four carbon atoms are not nematic liquid-crystal compounds.  
  Eutectic mixtures which are not liquid at room temperatures are presented as other examples of this invention and are set forth in the following table:  
  1. A composition having a relatively wide nematic range suitable for use in a nematic liquid-crystal light valve comprising a eutectic mixture of two or more nematic substances each having the structure:  
 where R, C,,H n is l or 2, R C I-1 n is from 2 to 12,  
 said first nematic substance is p-ethoxy-benzylidenepn-butylaniline;  
 the concentration of said first nematic substance is about 35 to 45 weight percent of said composition; and  
 said second nematic substance is p-methoxybenzylidene-p&#39;-n-butylanilene;  
 the concentration of said second nematic substance is about 55 to 65 weight percent of said composition.  
  2. A nematic liquid crystal composition suitable for use in a light valve, said composition consisting of a nematic liquid crystal eutectic mixture having a nematic range which extends from below 0C to above room temperature, said mixture consisting of about 35 to 45 weight percent p-ethoxy-benzylidene-p&#39;n-butylaniline and about 65 to 55 weight percent p-methoxy-benzylidene-pn-butylaniline.  
  3. A composition suitable for use in a light valve having an operating range extending from below 0C to above room temperature, said composition consisting of a eutectic mixture of about 35 to 45 weight percent p-ethoxy-benzylidene-p&#39;n-butylaniline and about 65 to 55 weight percent p-methoxy-benzylidene-p&#39;nbutylaniline.  
  4. A composition suitable for use in a light valve having an operating range extending from below 0C to above room temperature, said composition including a nematic liquid crystal consisting of a eutectic mixture of about 39 to 41 weight percent p-ethoxy-benzylidenepn-butylaniline and about 59 to 61 weight percent pmethoxy-benzylidene-p&#39;n-butylaniline.