IL34035A - Emulsions containing cholesteric liquid crystals and their use - Google Patents

Description

o'^tu o»®*aa o'^san o'a' nn Emulsions containing cholestsric liquid crystals and their us© SPARATODICA A.&.

Cs 32351 RAN 4090/15 This invention relates to a stable emulsion containing a hydrophobic oil phase and an aqueous phase.

Liquid crystalline materials have properties that are intermediate those of a true liquid and a true crystal since they have an ordered structure while also having fluidity.

These materials are known and are characterized or identified by one of three phases or structures known as the smectic phase, the nematic phase and the cholesteric phase which is a special form of the nematic phase. The present invention is concerned with materials exhibiting a cholesteric liquid crystalline phase.

Compounds with the cholesteric liquid crystalline structure exhibit certain characteristics which are markedly different from those having the smectic or the nematic structures. The characteristic properties of compounds with the cholesteric liquid crystalline structure may be summarized as follows: (1) they are optically negative, in contrast to the smectic and nematic structures which are optically positive; (2) the cholesteric liquid crystalline structure is optically active and shows strong optical rotatory power; (jj) when illuminated with white light, the most striking property of the compounds with the cholesteric liquid crystalline structure is that they scatter light selectively to give vivid colors. The color and intensity of the scattered light depends upon the ternpera- cidence of illumination. A cholesteric material exhibits a scattering peak having a band width of about 200 angstroms ' that occurs in or between the infrared and ultraviolet portions of the spectrum; (½) in the cholesteric structure, one circular polar component of the incident beam is completely unaffected.

For the dextro cholesteric structure, it is only the circular polarized beam with counterclockwise rotating electric vector which is reflected. (The sign of rotation refers to an observer who looks in the direction of the incident light). Levo cholesteric structures have the reverse effect; (5) when circular polarized light is scattered from these materials, the sense of polarization is unchanged. In ordinary materials, the sense of circular polarization is reversed;. (6) the mean wave length of the reflected band depends upon the angle of incidence of the beam.

The relationship can be roughly approximated by the Bragg di- fraction equation for birefringent materials. These enumerated properties effectively define cholesteric liquid crystals.

Thin films of cholesteric liquid crystals exhibit a property upon interaction with light, which may be termed "selective scattering". The term "scattering" is used rather than "reflection" in order to distinguish from the effect occurring on mirror surfaces wherein light is reflected at an angle equal to the angle of incident light. A scattered light ray may leave the scattering material at an angle unrelated to the angle of the incident light. A selectively scattering film, when observed with light impinging the film on the same the film.

The terms "light" and "color" as used herein have a broad connotation of referring to electromagnetic radiation generally, rather than to solely visible radiation.

The phenomenon of selective scattering as exhibited by cholesteric liquid crystalline films is independent of whether the light illuminating the film is polarized or not. The color and. intensity of the scattered light depends upon the temperature of the scattering material and upon the angle of incidence of illumination.

Because of the thermochromic properties of cholesteric liquid crystals, films containing them are useful for detecting temperature patterns on various objects, i.e., thermography and/or thermometry. Tnis temperature pattern is manifested by an irridescent color pattern exhibited by compounds in their cholesteric liquid crystalline phase.

Compounds capable of axisting in the cholesteric liquid crystalline phase exhibit thermochromic properties at temperature ranges which are unique for that compound. Therefore, the particular cholesteric liquid crystalline compound or mixtures of compounds utilized to detect a temperature pattern can be varied to result in color sensitivity at the particular temperature range being measured. At the lower temperature within the range, which can be varied from color exhibited is in the red end of . the spectrum and at the highe temperature within the range, the color is in the violet end of the spectrum. Intermediate temperatures result in intermediate colors, e.g., green. Thus, for example, if it is desired to measure and detect the temperature pattern of a particular portion of the anatomy of a person suspected of having a blood circulatory disorder or a tumor, a composition which shows a color change at the appropriate temperature can be formulated. Furthermore, cholesteric liquid crystals have been utilized to determine faults of metal parts of machines and airplanes by non-destructive testing techniques.

Previously, it has been found that in order to more easily visualize the colors exhibited by cholesteric liquid crystals, it is advantageous to utilize a black background.

However, the use of a black background gives rise to problems which make the use of cholesteric liquid crystals for detecting temperature patterns difficult and uneconomical. One problem is that the black background must be painted on in the form of a paint or a spray and then the liquid crystals must be applied to the black background so the colors can be readily observed. Because of the problems involved, the adaptability of these systems is limited. Further, these methods are disadvantageous since the oily cholesteric liquid crystals must be applied to the black background as a solution in a volatile solvent, thus causing obvious dangers. Furthermore, the removal of the background and particularly the liquid cr stals themselves is difficult articularl where lar e since it is very difficult if not 'impossible to get a uniformly even coating of the liquid crystals upon the background, thus rendering the pattern unreliable. Furthermore, by the known methods, the re-use of the liquid crystals is, for practical purposes, impossible.

In instances wherein the black . background is painted or sprayed on a plastic film prior to the application of liquid crystals, as well as wherein no plastic film is utilized, problems arise since the liquid crystals are unstable when exposed to the atmosphere which causes partial decomposition of the compounds and loss of color intensity as well as a shift in the color-temperature response. Even in the case wherein the liquid crystals are protected, e.g., minute transparent walled capsules, problems arise. This is true since films formed containing these materials tend to be rough and the protected liquid crystalline material can be rubbed off, thus causing losses of color intensity and thermographic reliability. Furthermore, the thin walls of the capsules can shatter under pressure, thus exposing unprotected liquid crystals to the atmosphere.

It is more advantageous to use a film of the colesteric Λ liquid crystals, preferably on a flexible substrate blackened prior to the application of liquid crystals, Since the utilization of a black paint or spray to serve as a background is particularly troublesome when dealing with human patients to remove.

It is, therefore, the purpose of the present invention to provide a stable cholesteric liquid crystalline composition that is amenable to re-use, exhibits good color properties at desired temperature ranges, can be formed into or onto a film and is easy to apply as a uniformly thick film, is easy to remove from the thermography subject and permits the use of an easily handled black or dark background.

This is achieved according to the invention by the fact that the hydrophobic oil phase of the emulsion comprises cholesteric liquid crystalline materials and the aqueous phase comprises film-forming colloids.

The particular cholesteric liquid crystalline material or mixtures thereof as well as their relative proportions which comprise the first phase is determined by the particular temperature response required for the intended thermographic use. Thus, if it is desired to have a cholesteric liquid crystalline temperature response, e.g., color change between 55°C and 37°C, a homogeneous mixture, on a dry weight basis, of 3.34 parts cholesteryl nonanoate, 2.42 parts cholesterol oleyl carbonate and 0.24 parts cholesteryl benzoate can be used.

The liquid crystal containing phase is formed by heating the cholesteric liquid crystalline materials until a uni temperature at which this occurs depends upon the particular cholesteric liquid crystals or mixtures thereof which are used. The temperatures which are suitable for forming the melt are generally between about 0°C and about 80°C.

It should be understood this invention comprehends the use of any cholesteric liquid crystalline material with thermo-chromic properties for. forming the emulsions. Examples of typical cholesteric liquid crystalline materials which are suitable for use in this invention are mixed esters of cholesterol and inorganic acids such as cholesteryl chloride , cholesteryl bromide, cholesteryl nitrate, etc.; organic esters of cholesterol such as cholesteryl benzoate, cholesteryl crotonate, cholesteryl nonanoate, cholesteryl formate, cholesteryl acetate, cholesteryl propionate, cholesteryl valerate, cholesteryl hexanoate, cholesteryl octonoate, cholesteryl do-cosonoate, cholesteryl vaccenate, cholesteryl chloroformate, cholesteryl linolate, cholesteryl linolenate, cholesteryl oleate, cholesteryl erucate, cholesteryl' butyrate, cholesteryl caprate, cholesteryl laurate, cholesteryl myristrate, cholesteryl clupanodonate, cholesteryl pxienyl propionate, cholesteryl 2, -dichlorobenzoate, etc.; ethers of cholesterol such as cholesteryl decyl ether, cholesteryl lauryl ether, cholesteryl dodecyl ether, etc.; carbonates and carbamates of cholesterol such as cholesteryl decyl carbonate, cholesteryl methyl carbonate, cholesteryl ethyl carbonate, cholesteryl butyl carbonate, cholesteryl docosonyl carbonate, cholesteryl cetyl heptyl carbamates, etc.; alkyl amides and aliphatic secondary amines derived from j5-P-amino--4-5-cholestene, the corresponding esters noted above of cholestanol and the like.

The second phase, i.e., the aqueous phase, is comprised of film-formers. In the preferred embodiments, the aqueous phase additionall contains a plasticizer. An added plasticizer however, is not necessary in all cases since the cholesteric liquid crystalline materials in the first hydrophobic phase can act as plasticizers, e.g., cholesteryl oleyl carbonate. The film-formers preferably are those which exhibit surface active properties. Additional preferred optional ingredients which can be used in the aqueous phase include bacteriastatic agents which are particularly needed if the emulsion is to be stored for a lengthy period.

Suitable types of film-formers which can be used are organic, water-soluble, film-forming polymers of plant or animal origin, typical of which are the protein-type of film-formers such as zein, gelatin and hydrolyzed collagen; cellulose derivatives such as ethylcellulose, methylcellulose, hydroxy propylcellulose, hydroxy ethylcellulose, sodium carboxymethylcellulose; natural, products such as acacia and starches; modified starches and polymers such as polyvinyl alcohol and polyvinyl pyrrolidone.

The preferred film-formers for use in this invention are the roteins, of which gelatin is .most suitable. Of the Since the desired use of the films formed from a particular emulsion determines the required characteristics of both the emulsion and the film eventually formed from it, the film-forming components of the aqueous phase can be varied to produce the desired results. Thus, mixtures of the aforesaid film-formers can be used in required proportions, which proportions can be ascertained by simple laboratory experiments.

The amount and identity of film-former utilized in the emulsions is variable but generally the total is from about 25 percent to about 75 percent by weight based on the dry weight of the finished film with about 30 percent to about 5 percent by weight preferred for most uses of the finished film.

If more than 75 percent of film-former by weight based on the weight of the dry film is used, then an insufficient amount of cholesteric liquid crystalline material will be in the film, resulting in poor color intensity. If less than 2 percent of film-former by' weight is used, then there is so much cholesteric liquid crystalline material present that the resulting film becomes undesirably greasy to the feel.

The identity, amount and type of plasticizer, when used in the aqueous phase, is dependent upon the particular film properties desired. Thus, in order to obtain a flexible film, on a dry weight basis about one-tenth the weight of the film-former. However, in the event a cholesteric liquid crystalline material which is suitable as a plasticizer is used, no added plasticizer is needed. Generally, if a harder film is desired, no plasticizer is necessary. The amount and identity of the plasticizer will determine the. flexibility of the films and this is in turn determined by the requirements of the intended use of the materials. Generally from about 0 percent to 7·5 percent by weight based on the dry film weight is suitable with about 4 percent preferred.

Suitable plastieizers for use in this invention are carbohydrates and polyhydric alcohols. Suitable typical carbohydrates are sugars such as sucrose, dextrose, levulose and invert sugars, e.g., sorbitol. Suitable typical polyhydric alcohols are glycols, glycerine and the like. The preferred plasticizer, when one is usi-d in the compositions of this invention, is glycerine.

Preferably, the relative proportions by weight of the cholesteric liquid crystal containing phase to' the aqueous film-forming phase in the emulsion composition is about 55 percent to about 70 percent by weight of cholesteric liquid crystals to about >0 percent to about percent of film-formers based on the dry weight of the finished film. These proportions can vary so long as they are within ranges which form stable emulsions and films. Generally, the most preferred relative proportions of the film-formers and the liquid crystals in the emulsion are calculated on a dry weight basis and these proportions depend upon the end use of the composition and the methods of applying them to substrates. The relative proportions are calculated on a dry weight basis since dilution of the two phases can be made to suit convenience. The emulsion is formed by mixing to homogeniety, the aqueous solution with a homogeneous melt of the cholesteric liquid crystalline materials. Temperatures of about 4o°C to about 80°C have been found suitable and convenient for mixing to homogeniety the components of the emulsion.

The particle sizes of the cholesteric liquid crystalline materials dispersed in the emulsion can vary, but in order to achieve a satisfactory film, particle sizes in the range of about 2 to about 10 microns are preferred because in this range the most intense colors are produced. If the particle size of the dispersed cholesteric liquid crystals is too small, the intensity of the color response is diminished and if the particle size of the cholesteric liquid crystals is too large, the emulsion is physically unstable and is not formable into a suitable film.

The emulsion can be applied with uniform thickness to the surface of the thermography subject and allowed to dry into a continuous film which is easily removable. Alternatively, the film can be formed on a non-adhering surface, e.g., Teflon, coating by suitable means on a supportive matrix to which it will adhere such as paper, cellulose acetate or other plastic film base. After the emulsion dried as a film on the supportive matrix material, it can be cut into desired shapes or sizes prior to use.

As previously indicated, the properties of the film or supportive matrix are modified by variations in the formulations in order to give them the properties required for a specific use. In any event the film formed when the emulsion dries contains the cholesterlc liquid crystalline material permanently imbedded therein, dispersed uniformly throughout and thus protected from the atmosphere and physical abrasion. In many cases, the black background which is needed to better visualize the colors developed by the cholesterlc liquid crystals in response to temperature is painted either on the supportive matrix or the thermographic -subject before the emulsion is applied, preferably the supportive matrix serves as the black background. It is also suitable to coat a transparent supportive matrix on one side with the black background and the other side with the emulsion. Another ilternative is to apply the emulsion to a transparent supportive matrix film and, after ■ the emulsion is thoroughly dry, to paint the emulsion black and to view the color response through the transparent film. It is further advantageous to utilize a supportive matrix having a black pigment or dye dispersed therein. is advantageous to coat the back of the film or supportive matrix with a pressure sensitive adhesive. Generally, the liquid crystal containing film formed from the emulsion is somewhat thicker than a monomolecular layer, but is uniformly thick throughout and is continuous. Films of from about 0.05 to about 0.15 mm thick are suitable when on a supportive matrix. Films of the emulsion .wherein no supportive matrix is used are suitable from about 0.1 to about 1.0 mm thick, with about 0.2 mm generally preferred. Unsupported films less than 0.10 mm thick can be made but they are too fragile for practical purposes.

As indicated previously, the films, either supported on matrix or not, can be rigid, brittle, flexible and/or elastic, depending on the intended uses.

In some cases, it is necessary to prepare the supportive matrix to recieve the emulsion and insure its adherence. For example, if Saran (a vinylidene polymer plastic) is used, it must be first coated with a primary layer, generally a self-reacting vinyl acrylic polymer. Preferred are the types known as X-Link marketed by National Starch Company and described as self-reactive vinyl acrylic terpolymer latexes. Other similar materials which are also suitable are vinylidene chloride copolymer latex (Vynaclor 362 ) , vinyl acrylic copolymer latex (Resyn 78-33^-6) and vinyl acetate copolymer latex (Resyn 1103), all marketed by National Starch Company.

The following examples illustrate the invention and Example 1 4.4 Parts by weight of cholesteryl nonanoate, 1.1 parts by weight of cholesteryl oleyl carbonate and 0.6 parts by weight of cholesteryl chloride are heated until a uniformly clear liquid is formed. A solution containing 4 parts by weight gelatin, 0.45 parts by weight glycerine and 25·55 parts by weight water is heated to 50°-6o° and added to the molten cholesterol esters and homogenized with an Eppenbach Homo Rod.

The resulting emulsion is then cast into a film and allowed to dry. It displays a temperature-color response having a mid-point (green) at 34°.

Example 2 .3 Grams of cholesteryl nonanoate, 2.42 g of cholesteryl oleyl carbonate and 0.24 g of cholesteryl benzoate are heated until a homogeneous uniformly clear liquid melt is formed. 4.0 Grams of a solution heated to about 55° comprised of 12.5 g of gelatin (Swift 00 bloom type S), 0.1 g sorbic acid and distilled water q.s. 100 g is added to the liquid crystal melt. ■ The mixture is then emulsified with a Gif ord Wood Mini-Mi11 at a rotor/stator clearance of 0.030" for about four minutes.

The resulting emulsion is applied to a Saran film primed with X-Link No. 28¾· The priming coat is applied with a o. 1 eier rod. The emulsion is a lied after the rime is allowed to air dry at room temperature.

The film displays a color response range between 35° and 37°, with a mid-point (green color) at 6.5-37°.

Examples -^-1 Following the procedures of Examples 1 and 2, emulsions containing the following liquid crystals and an appropriate film-former are cast into films. These thermographic films have various color/temperature responses as noted in the chart.

Liquid Example No.

Crystal 3 ^ 6 7 8 9 10 11 Material Parts by Weight Cholesteryl nonanoate 4.0 4.4 4.0 4.2 4.45 4.8 4.4 4.4 4.4 Cholesteryl oleyl carbonate 1.1 1.1 0.9 1. 1.2 0.7 1.1 1.1 1.1 Cholesteryl benzoate 1.0 0.6 1.0 - 0.2 0.7 0.45 Cholesteryl chloride - - 0.6 0.43 0.6 0.7 0.2 0.45 Mid-point Temperature Response ca.

(Green color) 32° 50° 46° 31° 35° 40° 25° 42° 4° Example 12 Following the procedures of Examples 1 and 2 an emulsion of the following formulation was cast into a film: Ingredients Parts by Weight Cholesteryl Nonanoate 29.0 Cholesteryl Oleyl Carbonate 29.0 Cholesteryl Benzoate 2.0 Polyvinyl Pyrrolidone 40.0 Water q.s.

The film displays a color response with a mid-point (green color) at 3 °.

Examples 13-20 Films containing the following ingredients were formed from emulsions prepared following the procedures of Examples 1 and 2: Example No. 13 14 15 16 17 18 19 20 Ingredients Parts by Weight Cholesteryl Nonanoate 4.4 4.4 4.6 4.6 2.2 2.3 4.4 4.4 Cholesteryl Oleyl Carbonate 1.1 1.1 0.9 0.9 0.55 0.45 1.1 0.9 Cholesteryl Percent Gelatin 33 45 . 33 45 57 57 39.6 39.6 Liquid crystal/ Gelatin Ratio 6.1/3 6.1/5 6.I/3 6.1/5 3.05/4 3.05/4 6.1/4 6.1/4 Color Intensity Good Good Good Good Poor Poor Good Good

Claims (16)

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. A stable emulsion containing a hydrophobic oil phase and an aqueous phase wherein the hydrophobic oil phase comprises cholesteric liquid crystalline materials and the aqueous phase comprises ilm-forming colloids.

2. The emulsion of Claim 1 wherein the hydrophobic oil phase and the aqueous phase are present in a weight ratio on a dry weight basis of about 25 percent to about 75 percent oil phase to about 75 percent to about 25 percent aqueous phase.

3. · The emulsion of Claim 2 containing on a dry weight basis up to about 7.5 percent of a plasticizer.

4. The emulsion of Claim 5 comprising on a dry weight basis about 55 percent to about 70 percent of cholesteric liquid crystalline material as the oil phase and the remainder the aqueous phase.

5. · The emulsion of Claim 4 containing on a dry weight basis about JO percent to about percent gelatin and about percent glycerine.

6. The emulsion of Claim 5 wherein the aqueous phase contains a bacteriastatic agent.

7. The emulsion of Claim wherein the particle size of the cholesterlc liquid crystalline materials dispersed therein is from about 2 to about 10 microns.

8. A method of stabilizing cholesteric liquid crystalline materials against the effects of the atmosphere which comprises forming an emulsion therewith comprising the composition of Claim 1.

9. A method of forming a film of cholesteric liquid crystals suitable for thermography which comprises applying i the emulsion of Claim 1 to a supportive matrix.

10. A method of forming a film of cholesteric liquid crystals suitable for thermography which comprises applying the emulsion of Claim 1 to a thermographic subject.

11. A device suitable for forming thermographic patterns comprising a film comprising cholesteric liquid crystalline materials imbedded in a gelatin matrix.

12. The device of Claim- 11 wherein the amount, on a dry weight basis, of cholesteric liquid crystalline material present in the film is about 25 percent to about 75 percent.

13. 1^· The device of Claim 11 wherein the particle size of the cholesteric liquid crystalline material dispersed in the film is between about 2 and 10 microns.

14. The device of Claim 11 wherein the film is coated on a supportive matrix.

15. The device of Claim 14 wherein the supportive matrix is colored black.

16. The method of obtaining a thermographic pattern which comprises applying to the subject, the device of Claim 14.