edited bv: MICHAEL R. SLABAUGH HELENJAMES
chem I fupplement
Weber State College Ogden Utah 84408
Liquid Crystals-The Chameleon Chemicals Glenn H. Brown Liquid Crystal Institute, Kent State University, Kent, OH 44242
found in either liquids or solids. Some specific properties of thermotronic liauid crvstals include (1)formation of "monocrystals;' (moiecu1a;order in one dimension) with application of ordinary magnetic and/or electric fields; (2) optical activity of cholesteric liquid crystals of a magnitude without parallel in other states of matter; (3) sensitivity of the cholesteric structure to a temperature change which results in color changes.
Liquid crystals have been known for about one hundred years. Friedrich Reintzer is given credit for their discovery. However, papers written in the mid-1850s describe systems that match the properties we now ascribe to liquid crystals. Studies of the properties and structures of liquid crystals began in the 1950's after the publication of the review by G. H. Brown and W. G. Shaw in Chem. Rev., 57,1049 (1957). Conventionallv. scientists think of matter existine in one of three well-de&ed states of aggregation, namel; solids, liquids, and gases. In the gaseous state the particles are free to roam through the entire volume of their container with almost no constraints. The liauid state does not exhibit lone
Molecular ShaDe and Liauid Crystallinity As an aid in selecting compounds that may exhibit liquid crvstallinitv. some eeneral statements can he made about the geometry hf m o l e h e s that form liquid crystals. Some ruidelines for selection of organic thermotropic liquid crystalline compounds may be summarized as f b ~ ~ o w(1) i : The majority of liquid crystalline compounds have aromatic nuclei which are polarizable, planar, and rigid; (2) the central group (x) in the molecule usually contains a multiple bond along the long axis of the molecule or a system of conjugated double bonds, or involves a dimerization of carhoxyl groups; (3) the central group (x) connecting two benzene rings should constitute a lath or rod-like core of the molecule; (4) the length of the molecule should be greater than its diameter (assuming a cylindrical geometry); (5) a strong polar group near the center of the molecule and along the molecular axis generally enhances liquid crystallinity; and, (6) weak polar groups a t the extremities of the molecule enhance liquid crystallinity. The most common molecular shape is the cylindrical one. A typical molecular structure may be illustrated as follows:
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In the solid state, the intermolecular distances are about the size of the molecules. The dominant feature of the crystalline state is the strong bonding between molecules giving this state of matter an orderly arrangement. Liquid crystals are another state of matter, structurally falling between the solid and the liquid states. Reintzer found the compound cholesteryl benzoate (thermotropic liquid crystal) changed phases on heating. At room temperature the benzoate is a white crystalline solid. On heating to 145' the crystal structure collapses to form a turhid liquid (liquid crystal). On further heating the turhid structure collapses a t 179' to a liquid. On cooling the process reverses i t.. w.l.f.. .
Reintzer's second and most interesting observation was that the turhid liquid changed color as the temperature changed, going from red to blue with an increase in temperature and reversing the color pattern as the system was cooled. Not all liauid crvstalline compounds show color changes - with change intemperature. Presently liquid crystals can be divided into two categories, thermotropic and lyotropic, according to the principal means of destroying the order in the parent solid state. Probably the most widely studied category is the thermotropic. Thermotropic liquid crystals are prepared by heating a substance or substances. Lyotropic liquid crystals c a n be formed when certain compounds are treated with a controlled amount of water or other polar solvent. The properties of this birefringent fluid resemble those of the hirefringent fluid formed by heating certain sulids. Liquid crystals exhibit properties that are not
Central group
A and Ei are terminal units such as methyl (CHa), ethyl (C2Hi), methoxy (OCHa), ethoxy (OCaHS),halogens etc. The central group or linkage can be, for example, 0
I l : ~ l dis~lavs attractive, . " oarticularlv . As more scientists, engineers, and medical researchers enter this exciting research field, new discoveries and applications will he made including those dealing with living systems. The future is full of exciting possibilities. Literature Cited
General I l l Brown,G.H.,nndCrooker. P. P.,Chern.l.'nl..Nazus, 61.28 119RBl. 121 Chandrssekhar. S., "l.iquId Crystals," Cnmixidpe ilitiuersity Press. Camhridge. 1977.
1:Il deOennei, P.C.."ThaPhgsiaof l.iquidCrv~tals,"Oxford UniversitvPrers, I.ondon. 1974. I41 Woiken. J .I., and Rmwn. (;. H.. "Llquid C r j r i a l a a n d Bioloe~calSystems." Academic Pres. New York, 1980. 151 "Advancar in LiquidCrystnlr: \'olumes 1.2.B.4.and 5.Hrciwn.G. H..lEdi~orl.Acn~ demicPrerr. New Ysrk. 1975. 1976, 1978.1979,and 1982. 16) Shaa,W. G..and Bmwn, 1;. H., C h r m Rru..57, 10.19 119571. (7) Chandrasekhnr, S.,Sadashivn. R. K.,Suresh. K. A . Madhunidnna, N. Y..Kumar. S.. Shashidhsr.R..andVanksirsh, G.. J I'hys 1Paiisl. 40, C3-120 119791. 18) See fur example Pershan, 1,. S., Phyrics Today, 35, 34 lMay I9821 and references therein. 191 Fur acurrcnt reviewsee Lilsur.J.D.,and Birgenenu,R..I.. Ihysrcs Todoy.35,26(May LSR2). 1101 Demur, D.,and Richter.L.,"Textures oiLiquid Crystals,"Verla~Chemie, Weinhelm, 1978. Ill) Guillon, I).. C1adis.P.E.. and Stamatnli:S..Phvr. Rm Ld1 .41.1598 (19281. 1121 Gray, G.. and Winsor. P., "Liquid Crystalsand Plaitic Cwsinlr."John Wiley end %na, Now Yurk. 1974. p. 1811. 1131 See for example "The Physics and Chemirlry d1.iquid Cryrtal Devices? Spmkel, G. J.. IEdilorI. Plenum Presr, New Ymk, 1980.
Polymers 1111 Krisbsum. W. R.. Lader, Harry J.. and Cifferri, Albert". M~rrornolerulc~. 13, 554 11980). 115) Morgan, P. W..Matromoincuiur. 10, 1381 11977). 116) Black, W. H., and Preston. 11.. "High Modulus Ammatic Polymers." Marcel Dekker. lnc.,New York. 1979. (171 Skoullor.A.in "Advancesin l.iquidCryrulr."Volumc 1.Hrnwn.G.H., (Edilnr1.A~. ademic Presr, N e w York. 1975.
Applications 1181 Lim, H. S.. Msrgerum.J. U.,and Gr8ube.A.J. Eiectrirchrm S u i , 124, 1889 119771. 1191 Shanks,Ian A , Conirmp. 1'h.v..23.65 l19H21. 1201 "ThePhysicrandChemisrryofLiquidCryrlslnericer."Spr~~kel,GeraldJ.,1Ed:ditorl. Plenem I'resr. New York, 1980.
Volume 60
Number 10 October 1983
905
