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Nov. 5, 1968 J. L. FERGASON 3,409,404 ANALYTICAL METHODS and DEVICES EMPLOYING CHOLESTERIC LIQUID CRYSTALLINE MATERIALS Filed Nov

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Nov. 5, 1968 J. L. FERGASON 3,409,404 ANALYTICAL METHODS and DEVICES EMPLOYING CHOLESTERIC LIQUID CRYSTALLINE MATERIALS Filed Nov Nov. 5, 1968 J. L. FERGASON 3,409,404 ANALYTICAL METHODS AND DEVICES EMPLOYING CHOLESTERIC LIQUID CRYSTALLINE MATERIALS Filed Nov. 13, 1963 2 Sheets-Sheet 1 4/70 M j 20 2/ 26 22 INVENTOR. JAMES L . FEQGASO/V A 77'ORNEY. 1 3,409,404 United States Patent 0 1C@ Patented Nov. 5, 1968 1 2 It is optically negative, while smectic and nematic struc ' ' 3,409,404 ' ANALYTICAL METHODS AND DEVICES EM tures are optically positive. (2) The structure is opti PLOYING 'CHOLESTERIC LIQUID CRYSTAL cally active. It shows strong optical rotatory power. (3) LINE MATERIALS When illuminated with white light, the most striking James , L. Fergason, Verona, Pa., assignor to Westing property of the cholesteric structure is that it scatters house Electric Corporation, Pittsburgh, Pa., a corpora light selectively to ‘give vivid colors. A cholesteric mate tion of Pennsylvania ‘ rial exhibits a scattering peak having a bandwidth of Filed Nov. 13, 1963, Ser. No. 323,341 about 200 angstroms that occurs in or between the infra 19 ‘Claims. (Cl. 23-230) red and ultraviolet portions of the spectrum. (4) In the 10 cholesteric structure, one circular polar component of the incident beam is completely unaffected. For the dextro ABSTRACT OF THE DISCLOSURE cholesteric structure, it is only the circular. polarized The optical properties of a chloresteric liquid crystal beam with counter-clockwise rotating electric vector line material are changed when the cholesteric material which is re?ected. (The sign of ‘rotation refers to an ob is contacted with another material. A variety of mate 15 server who looks in the direction of the incident light.) rials, particularly vapors, are identi?ed by observing their Levo cholesteric structures have the reverse effect. (5) effect on cholesteric liquid crystalline materials. The most When circular polarized light is scattered from these ma convenient observable effect is a change in the color of terials, the sense of polarization is unchanged. In ordi the cholesteric material and, if necessary, comparing the nary materials, the sense of circular polarization is re change to the change effected by a known standard ma— versed. (6) The mean ‘wave length of the re?ection band terial. An analytical device may comprise one or more depends upon the angle of incidence of the beam. The distinct elements of cholesteric liquid crystalline mate relationship can be roughly approximated by the Bragg rial. Suitable cholesteric liquid crystalline materials in diffraction equation for a birefringent material. These enu clude a wide variety of compounds, and mixtures thereof, merated properties effectively de?ne cholesteric liquid derived from cholesterol. 25 crystals. A review of existing knowledge of liquid crystal line materials is found in an article by G. H. Brown and W. G. Shaw entitled “The Mesomorphic State—Liquid - This invention relates to the detection and analysis of Crystals,” Chemical Reviews, v. 57, No. ‘6, December matter, for example, gases and in particular concerns 1957, beginning on p. 1049. methods, articles of manufacture and apparatus particu 30 It has now been discovered, and it is on this discovery larly adapted to analysis on a qualitative and quantitative that the present invention is in large part predicated, that basis. gases, liquids and solids can affect the structure of cho It is a primary object of the present invention to pro lesteric liquid crystals so that one or more optical prop vide a novel method of analysis of both a quantitative erties thereof is at least temporarily changed. It has and qualitative nature, that is easily practiced, and is ‘further been discovered that, upon providing a compa highly sensitive to small concentrations of the materials rable basis, the resultant change is speci?c for the un to be detected. known involved which is thereby determined. By utiliz 7 Another object of the invention is to provide a novel ing these general principles an utterly new mode of analy method of identifying an unknown material. sis is provided. It is a further object of the invention to provide a The optical property most readily utilized in the prac method in which an unknown is brought into contact tice of this invention is that of selective scattering since with a new detector material and the character and it requires no polarizers or analyzers for observation. As amount of the unknown are indicated upon observing beforementioned, each cholesteric liquid crystal, at 1a an optical change in the system contacted. , given temperature and composition, exhibits, 'when ex An additional object of the invention is to provide a posed to white light, a scattering peak. In accordance novel detector, responsive by changes in an optical prop with this invention, the shift in the scattering peak may erty brought about by contact of the detector with an ‘be utilized for the analysis of unknown materials since unknown material. _ the direction of shift is a qualitative indication of the Other and further objects will be apparent from the unknown and the extent of shift is an indication of the following detailed description and discussion of the in quantity of the unknown. However, it is also possible to vention. utilize changes in other optical properties of the liquid Liquid crystalline materials have properties that are crystals. intermediate those of a true liquid and a true crystal since For example, it has also been found that the circular -_they have an ordered structure while also having ?uidity. dichroism and optical rotation of cholesteric materials Liquid crystalline materials are also referred to as mate are similarly affected by foreign matter. The component ' rials in the mesomorphic state. Liquid crystalline mate of circularly polarized light that is affected by the choles rials are known and are characterized or identi?ed by teric material ‘has a waveband of minimum transmission. one of three phases or structures. One is the smectic This waveband shifts in the same direction and to the structure, which is characterized by its molecules being same extent as the scattering peak. Similarly, the wave arranged in layers vwith the long axis approximately nor 60 band of peak optical rotation exhibits such a shift. Since mal to the plane of the layers. The second is the nematic cholesteric liquid crystalline materials have negligible structure, which is characterized by thread like molecules optical absorption, the transmitted radiation may be uti that tend to be and remain in nearly parallel orientation. lized for the purposes of this invention as well as the The third is known as the cholesteric phase the molecu scattered radiation. 65 lar con?guration of which has not yet been determined. _ In general, a material that is at least partially inter The present invention is concerned with materials ex soluble with the cholesteric liquid crystalline material hibiting a cholesteric liquid crystalline phase. will affect the optical properties of the liquid crystal in The cholesteric phase has certain characteristics which a reversible manner. Also it is the case that a material are markedly different from either the smectic or the 70 that chemically reacts with the cholesteric liquid crystal nematic phase. The characteristic properties of the ch0 line material will affect its optical properties in an ir lesteric structure may be summarized as follows: (1) reversible manner. In instances in which the effect is re a... versible, the liquid crystal provides an optical indication speed with which these changes occur also differ sub of the nature and quantity of the foreign material present stantially in many of the crystals. In instances, the gas at that instant. Because of the reversible nature of the permeating the crystal causes a vivid color change in but effect, the liquid crystal may‘ be continually reused. In seconds, or fractions of seconds. With the reversible sys instances in which they effect‘ is irreversible, thev liquid tems, the effect usually dissipates almost as rapidly, ap crystal provides ranloptica‘l indication of‘the same type parently as the volatile gas diffuses‘ back'»toiand'through that is cumulativeQHenée, each type of effect has ‘ad the atmosphererOther cholesteric liquid ‘crystals appear var'rt'ageous applications - and the j present‘ invention 'is con to have a “memory” and "the"resulttbropght'abotit' by,'\f'or cerhed‘with both reversible'and" irreversible effects. example, solubilizatiorfof gasz‘therei'n may remairr‘af'least ‘The present‘ invention permits identi?cation of a variety 10 partially for periods of hours apparently dueto ‘the; ‘forma of materials by reason of-their affect on cholesteric’liq'uid tion of relatively weak bond'slin theliquid crystal. crystalline materials-For example, reversible effects on Where the unknown to be determined is in the liquid the liquidcrystar optical properties‘have been observed or solid state, contact with thecholesteric liquid crystal withcommon'prganic solvents, amines, simple alcohols material generally diifersyfrom ‘that, employed with gaseous and’organic acids while irreversible ‘effects have been ob-' unknowns.- In these instances, it is‘ useful to‘ incorporate served'with-halogens, oxidizing agents,‘ alcohols,‘ amines, the unknown liquid or solidv in the liquid crystal material acids,‘ bases and'r‘educin'g agents.‘ ‘ while-‘the latter- is dissolved in a‘suitable solvent-There; ' By way of ‘example'and more 'speci?cally'in accordance upon, the-resulting mixture can be poured to-the" sub-: with this invention, vaporsor'gases'are detected and de strate. When the solvent evaporates, thereremains ‘the termined by‘contact thereof with a cholesteric liquid 20 liquid crystal having incorporated therein the added~un—> crystal.‘ In instances,‘ this contact ‘results ‘in a chemical known, either by reactionwith the crystal or by being reaction thatiis irreversible. Consequently, there is brought dissolved therein. Since the resulting cholesteric liquid about a change in the ‘contacted liquid crystal 'so that the crystal is intrinsically different from that which would resultant material'(vapor plus crystal) has diiferent'optical ' result-from the liquid crystal material solution alone, i.e.
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