PROCESS FOR CONTROLLING LAYER SPACING IN MIXTURES OF SMECTIC LIQUID CRYSTAL COMPOUNDS

Abstract: A process for controlling layer spacing in mixtures of smectic liquid crystal compounds comprises the step of combining a liquid crystal composition having a net negative thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase and a liquid crystal composition having a net positive thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase, each of the compositions comprising at least one smectic or latent smectic liquid crystal compound. The process enables control of layer spacing in the tilted smectic mesophase(s) and thereby control or suppression of chevron layer geometry.

Description: PROCESS FOR CONTROLLING LAYER SPACING IN MIXTURES OF SMECTIC LIQUID CRYSTAL COMPOUNDS FjLeJ. Pf tftP InvβPtipP This invention relates to a process for preparing mixtures of smectic or latent smectic liquid crystal compounds having a controllable layer expansion or contraction behavior (with respect to temperature) in at least one tilted smectic mesophase. In other aspects, this invention relates to mixtures prepared by the process and to electrooptical display devices containing the mixtures. Background of the Invention Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available. However, slow response can impose limitations for many potential applications. Speed of response becomes especially important in proportion to the number of elements which have to be addressed in a device, and this limits the potential use of some types of liquid crystals. The modes of liquid crystal displays that are most extensively employed at the present time are twisted ne atic (TN) , εupertwisted birefringence effect (SBE) , and dynamic scattering (DS) , all employing nematic or chiral nematic (cholesteric) liquid crystals. These devices are based upon the dielectric alignment effects (Freedericksz effect) of the nematic and/or chiral nematic liquid crystal (or mixtures of nematic or chiral nematic liquid crystals) upon application of an electric field. The average molecular long axis of the liquid crystal material takes up a preferred orientation in the applied electric field, the orientation of which is dependent on the sign of the dielectric anisotropy of the material or mixture, and this orientation relaxes upon removal of the applied electric field. This reorientation and relaxation is slow, on the order of a few milliseconds. Although nematic and chiral nematic liquid crystals are the most extensively employed, there are liquid crystal devices that employ more highly ordered smectic liquid crystals. These devices are also based on the dielectric reorientation of the liquid crystals, and response times are on the order of milliseconds. A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic liquid crystals, which are also termed ferroelectric liquid crystals, in devices which give microsecond switching. Ferroelectric liquid crystals were discovered by R. B. Meyer et al. (J. Physique 3_6, 1-69 (1975)), and fluorine-containing ferroelectric liquid crystal materials have recently been developed (see, e.g., U.S. Pat. Nos. 4,886,619 (Janulis), 5,082,587 (Janulis), and 5,262,082 (Janulis et al.)). Ferroelectric liquid crystals possess a macroscopic electric dipole density which is perpendicular to the molecular tilt direction and parallel to the smectic layer planes. This provides a much stronger coupling of the molecular orientation to an applied electic field than is available via the dielectric anisotropy. Furthermore, the coupling is polar, so that reversal of an applied electric field can be effectively used to control molecular orientation. A high speed optical switching phenomenon was discovered for ferroelectric liquid crystals by N. A. Clark et al. (see Appl. Phys. Lett. 26, 899 (1980) and U.S. Pat. No. 4,367,924). Clark developed a surface- stabilized ferroelectric liquid crystal display (hereinafter, SSFLCD) which enabled bistable operation not possible in any of the device applications described above. The SSFLCD has been recognized as having high potential in regard to information content, viewing angle, contrast ratio, and switching time, but development of the SSFLCD has been hindered by problems with defects in the liquid crystal layer structure. These defects arise due to layer shrinkage upon cooling (through the temperature ranges associated with the tilted smectic mesophases) and the resulting formation of a "chevron" layer structure (see, e.g., the discussion by T. P. Rieker et al. in Phys. Rev. Lett. 59, 2658 (1987) and Ferroelectrics HI, 245 (1991), as well as the discussion by Y. Ouchi et al. in Jpn. J. Appl. Phys. 22, L1993 (1988)). Since the defects result in, e.g., a poor contrast ratio and unstable bistability, researchers have earnestly sought means for avoiding the formation of a chevron structure. European Pat. Publication No. 405,868 (Fujitsu Limited et al.) discloses a liquid crystal composition which exhibits a bookshelf structure. The composition comprises a mixture of ferroelectric (i.e., tilted chiral smectic) liquid crystal compounds, the mixture containing a predetermined amount of a chiral ferroelectric liquid crystal compound having a specific naphthalene ring structure. Takanishi et al. describe (in Jpn. J. Appl. Phys. 29. L984 (1990) and Mol. Cryst. Liq. Cryst. 199. Ill (1991)) the spontaneous formation of a quasi-bookshelf layer structure in new ferroelectric liquid crystals derived from a naphthalene ring. Mochizuki et al. obtain both bookshelf and quasi- bookshelf layer structures by using a "naphthalene base liquid crystal mixture with a rubbed polymer orientation films panel" (see Ferroelectrics 122. 37 (1991)). Research Disclosure 34573 (1993) discloses ferroelectric liquid crystal mixtures having a high percentage of perfluoroether-containing liquid crystal compounds. The mixtures exhibit a spontaneous bookshelf layer structure and have been formulated with negative layer expansion materials to reduce temperature dependence of the layer thickness. A bookshelf structure obtained by application of an electric field has been reported by M. Johno et al. in Jpn. J. Appl. Phys. £8_, L119 (1989) and is also described in U.S. Pat. No. 5,206,751 (Escher et al.). A bookshelf structure obtained by oblique vapor deposition techniques has been reported by A. Yasuda et al. in Liquid Crystals 14., 1725 (1993). Bmmn»τγ o the Invention It has been discovered that some liquid crystal compounds surprisingly exhibit layer expansion, rather than contraction, upon cooling through the temperature range(s) associated with at least one tilted smectic mesophase. It has also been discovered that such compounds, when used in admixture with certain other liquid crystal compounds, provide mixtures which have a controllable layer expansion or contraction behavior (with respect to temperature) . This can be used to effect an essentially temperature-independent layer spacing in the tilted smectic mesophase(ε) , as well as a reduced temperature dependence of the layer spacing in the temperature range associated with the transition between the non-tilted and the tilted mesophases. Thus, in one aspect, this invention provides a process for controlling layer spacing in mixtures of smectic liquid crystal compounds. The process comprises the step of combining a liquid crystal composition having a net negative thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase and a liquid crystal composition having a net positive thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase, each of the compositions comprising at least one smectic or latent smectic liquid crystal compound. (Latent smectic liquid crystal compounds are those which by themselves may not exhibit certain smectic mesophase(s) , e.g., tilted smectic mesophase(s) , but which, when in admixture with compounds having smectic mesophases or with other compounds having latent smectic mesophases, develop or exhibit smectic mesophases under appropriate conditions.) Preferably, the composition having a net negative thermal expansion behavior (hereinafter termed "negative composition") is utilized in an amount such that the resulting combination has a net thermal layer expansion within the range of from about -0.05 to about The process of the invention enables control of layer spacing in the tilted smectic mesophase(s) and thereby control or suppression of chevron layer geometry. Such suppression reduces or eliminates the formation of zig-zag alignment defects upon cooling through the tilted smectic mesophase(s) , resulting in improved electooptical switching performance, increased memory to driven cone-tilt angle ratios, and improved contrast ratios. The process also enables the reduction or elimination of the stress-induced defects and non-bookshelf layer geometry which result from heating and cooling cycles. Furthermore, the process of the invention can be carried out using either chiral or achiral liquid crystal compounds and is not limited to compounds having a particular type of core structure. Since low viscosity materials can be utilized, fast switching speeds can be achieved. In other aspects, this invention also provides mixtures prepared by the process of the invention and electrooptical display devices containing the mixtures. Brief Description of the Drawings Figures 1-5 and 7-9 are plots of layer spacing (obtained by variable-temperature, small-angle X-ray scattering) versus temperature for the liquid crystal samples (compounds or mixtures) described in the examples, infra. The portions of the plots designated by symbols of reduced size are those portions selected for linear fit. The slopes of the equations shown in the figures correspond to the thermal layer expansion values of the samples. Figure 6 is a plot of thermal layer expansion value versus composition for the liquid crystal compound mixtures described in Example 5, infra. Detailed Description of the Invention Liquid crystal compositions suitable for use as negative compositions according to the process of the invention are those compositions which have a net negative thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase and which comprise at least one smectic or latent smectic liquid crystal compound. Smectic (or latent smectic) liquid crystal compounds suitable for inclusion in the negative compositions include, e.g., those which exhibit a mechanism which offsets the layer shrinkage associated with the tilting of the molecules in at least one tilted (or latent tilted) smectic mesophase and which thereby exhibit negative thermal expansion behavior in the mesophase(s) . Such compounds can be used in admixture with liquid crystal compounds which do not exhibit such a mechanism (and which therefore exhibit positive thermal layer expansion behavior) , provided that the resulting composition has a net negative thermal layer expansion. Preferred smectic or latent smectic liquid crystal compounds which can be utilized in the negative compositions are chiral or achiral liquid crystal compounds which have at least one fluorinated terminal portion and which exhibit negative thermal layer expansion behavior in at least one tilted (or latent tilted) smectic mesophase. A preferred class of such compounds is that class of chiral or achiral liquid crystal compounds which have at least one fluoroether terminal portion containing at least two catenary, i.e., in-chain, ether oxygen atoms and which exhibit negative thermal layer expansion behavior. Such compounds can comprise, e.g., (a) an aliphatic fluorocarbon terminal portion containing at least two catenary ether oxygen atoms; (b) an aliphatic hydrocarbon terminal portion; and (c) a central core connecting the terminal portions. The aliphatic hydrocarbon terminal portion of the compounds can be either chiral or achiral. (Such achiral compounds are described in U.S. Pat. No. 5,262,082 (Janulis et al.).) The aliphatic fluorocarbon terminal portion can be represented by the formula -D(C J .F 2x O) I C y F 2y+ ,, where x is independently an integer of 1 to about 12 for each C x F 2x O group, y is an integer of 1 to about 12, z is an integer of 2 to about 10, and D is selected from the group consisting of a covalent bond, 0 -C II-0-C,H 2r -, -0-C.H 2r -, -OfC,H 2l O)- t C r .H 2r .-, -C^-, C^O^H .-, - -C«C-, -CH=CH-, -C II-, -O-CII-C-H,,-, -, and combinations thereof, where r and r' are independently integers of 0 to about 20, s is independently an integer of 1 to about 10 for each (C,H 2 ,0) , t is an integer of 1 to about 6, and p is an integer of 0 to about 4. The group of the fluorocarbon terminal portion can contain small amounts of residual carbon-bonded hydrogen atoms but is preferably completely fluorinated. Preferably, the fluorocarbon terminal portion is a linear group represented by the formula ~-~^ {C 1 ^ Jx ) z y F iyJt . l , where D is -0CH 2 -, x is independently an integer of 1 to about 8 for each C x F 2j .O group, y is an integer of 1 to about 8, and z is an integer of 2 to about 6. Such liquid crystal compounds can be represented by the general formula I: RfM T -.AfN T - b BfP -D-R, (I) where M, N, and P are each independently selected from the group consisting of 25 30 35 a, b, and c are each independently zero or an integer of from 1 to 3, with the proviso that the sum of a + b + c be at least 1; each A and B are non-directionally and independently selected from the group consisting of a covalent bond, 0 0 0 -C II-0-, -CII-S-, -CII-Se-, 0 -C II-Te-, -(CH 2 CH 2 ) k - where k is 1 to 4, 0 -CH=CH-, -CEC-, -CH=N-, -CH 2 -0-, - IIC-, and -O- ; each X, Y, and Z are independently selected from the group consisting of -H, -Cl, -F, -Br, -I, -OH, -0CH 3 , -CH 3 , -CF 3 , -0CF 3 -CN, and -N0 2 ; each 1, m, and n are independently zero or an integer of 1 to 4; D is selected from the group consisting of a covalent bond, 0 0 O II II II -C-0-C,H 2r -, -0-C,H 2r -, -O-C-C-H j ,-, -C=C-, -CH=CH-, -C-, -0-(C.H 2i O)- t C r .H 2r .-, -C.H 2 ,-, C i H 2l O t C H 2r .-, -0-, -S-, -0S0 2 -, -S0 2 -, -SO^C^-, -C,H 2r -N-S0 2 -, -N(C^, +1 )- f CpH∑p+i O II -C,H 2r -N-C-, -CH=N-, and combinations thereof, where r I CpH∑p+i and r' are independently integers of 0 to about 20, ε is independently an integer of 1 to about 10 for each (C,H 2 ,0) , t is an integer of 1 to about 6, and p is an integer of 0 to about 4; R is selected from the group consisting of -O-( (C,.^,,.-(R') v .)-0) - -H^^-(R') v , -( (C q -H*^.-(R') v .)-0) w -C q H 2q+1 . v -(R') v , and -CR'H-(D) g .-CR'H-, where each R' is independently selected from the group conεisting of -Cl, -F, -CF 3 , -N0 2 , -CN, -H, -CJij q+ i t -0- where q' is independently an integer of 1 to about 20 for each (C q .H-^.-O) , q is an integer of 1 to about 20, w is an integer of 0 to about 10, v is an integer of 0 to about 6, each v' is independently an integer of 0 to about 6, g is an integer of 1 to about 3, g' is an integer of 1 to about 3, each D is independently selected from the group set forth above, and R can be linear or branched and chiral or achiral; and Rj iε -(C x F 2 -.0) } .C y F 2y+1 , where x iε independently an integer of 1 to about 12 for each (C^O) , y iε an integer of 1 to about 12, and z iε an integer of 2 to about 10. Preferably, D iε -0CH 2 -, R iε linear, x iε independently an integer of 1 to about 8 for each (C x F 2x 0) , y iε an integer of 1 to about 8, and z iε an integer of 2 to about 6. A preferred εubclaεε of the compounds can be repreεented by the following formula: where x is independently an integer of 1 to about 8 for each C x F 2x 0; y iε an integer of 1 to about 8; z is an integer of 2 to about 4; j is an integer of 0 or 1; j' is an integer of 0 or 1; and R" is selected from the group consiεting of (R , ) v ~ c q H 2 q + ι-v and , where each q iε independently an integer of 2 to about 10; each R' iε independently selected from the group consiεting of hydrogen, fluorine, chlorine, methyl, and perfluoromethyl; v iε an integer of 0 to about 4; and can be linear or branched. Another preferred class of liquid crystal compounds which have at least one fluorinated terminal portion and which can be utilized in the negative compositions are those smectic or latent smectic, chiral or achiral liquid crystal compounds which have at leaεt one fluoroaliphatic terminal portion (e.g., the compoundε deεcribed in U.S. Pat. Noε. 4,886,619 (Januliε) and 5,082,587 (Januliε) ) and which exhibit negative thermal layer expanεion behavior in at leaεt one tilted (or latent tilted) smectic mesophase. For example, 5-hexyl-2-(4'-l,l- dihydroperfluorooctyloxy)phenylpyrimidine haε been found to exhibit such behavior and to be suitable for such use. Other liquid crystal compoundε which posεess a mechanism which offsets the layer shrinkage associated with the tilting of the molecules in at least one tilted (or latent tilted) smectic mesophaεe and which thereby exhibit negative thermal expanεion behavior in the mesophaεe(s) can alεo be utilized in the negative compositions. Liquid crystal compoεitionε εuitable for uεe in admixture with the above-deεcribed negative compoεitionε according to the process of the invention are thoεe compositions which have a net positive thermal layer expansion in at least one tilted (or latent tilted) smectic mesophase and which comprise at least one smectic or latent smectic liquid crystal compound. Smectic (or latent smectic) liquid crystal compounds suitable for inclusion in the liquid cryεtal compoεitionε having net poεitive thermal expanεion behavior (hereinafter termed "poεitive compoεitionε") include chiral and achiral liquid cryεtal compoundε which have aliphatic terminal portions and which exhibit positive thermal layer expansion behavior in at least one tilted (or latent tilted) smectic mesophase, e.g., compounds such aε the alkyl, alkoxy phenylpyrimidineε and the alkoxy, alkoxy phenylbenzoates described by D. Demus et al. in Flussiqe Kristalle in Tabellen. VEB Deutscher Verlag fur Grundεtoffinduεtrie, pages 65-76 and 260-63, Leipzig (1974) and by S. Kumar in Phyε. Rev. A 23., 3207 (1984) ; chiral and achiral liquid cryεtal compounds which have at least one fluoroaliphatic terminal portion (such compounds are described, e.g., in U.S. Pat. Nos. 4,886,619 (Janulis) and 5,082,587 (Janulis)) and which exhibit poεitive thermal layer expanεion behavior in at leaεt one tilted (or latent tilted) smectic mesophaεe, e.g., compoundε εuch as Compound B in Table 1, infra; and chiral and achiral liquid cryεtal compoundε which have at leaεt one fluoroether terminal portion containing only one catenary ether oxygen atom and which exhibit poεitive thermal layer expanεion behavior in at leaεt one tilted (or latent tilted) εmectic meεophaεe. The latter compoundε can be, e.g., compoundε which compriεe (a) an aliphatic fluorocarbon terminal portion containing one catenary ether oxygen atom; (b) an aliphatic hydrocarbon terminal portion; and (c) a central core connecting the terminal portionε. The aliphatic fluorocarbon terminal portion can be repreεented by the formula -D(C x F 2x O) z C y F 2y+1 , where x iε an integer of 1 to about 12 (preferably, 1 to about 8) , y iε an integer of 1 to about 12 (preferably, 1 to about 8) , z iε an integer of 1, and D iε εelected from the group defined above. Such compoundε can be represented by the general formula I above, wherein R, M, N, P, A, B, D, X, Y, Z, a, b, c, 1, m, and n are as defined above, and wherein f is -(C x F 2x O) 3 -C y F 2y . ( . 1 , where x is an integer of 1 to about 12, y iε an integer of 1 to about 12, and z iε an integer of 1. Preferably, D iε -0CH 2 -, , is linear, x is an integer of 1 to about 8, and y is an integer of l to about 8. When compound(s) having a fluoroether terminal portion containing at least two catenary ether oxygen atoms are used as the major component(s) of the negative compositions, the compositionε can preferably be combined with poεitive compositions comprising (aε the major component(ε) ) at leaεt one chiral or achiral liquid cryεtal compound having a fluoroaliphatic terminal portion or, more preferably, a fluoroether terminal portion containing only one catenary ether oxygen atom. Such combinations are preferred from the standpoint of compatibility. The process of the invention can be carried out by combining a negative compoεition and a poεitive composition. The combining or mixing of the compositions can be effected by introducing the compositions to a vessel, generally with simultaneous and/or subsequent agitation or stirring, e.g., roller mixing. The vesεel can be either an open or a closed vessel of a size which iε sufficient to hold both compositionε while allowing room for mixing. The compoεitionε can be formed prior to combination with each other, or, alternatively, one or more of the componentε of either can be combined with one or more of the componentε of the other prior to addition of the remaining componentε. Any order and manner of combination of the componentε of the compoεitionε is acceptable. The resulting combination is preferably agitated or stirred sufficiently that a homogeneous mixture is achieved. Thiε iε preferably facilitated by applying εufficient heat to melt the combination or by diεεolving the combination in a εolvent, e.g. , a polar aprotic εolvent, with εubεequent εolvent removal, e.g., by rotary evaporation. The compoεitionε (and componentε thereof) to be utilized can be εelected baεed upon the εignε and values of their thermal layer expansion in at least one tilted (or latent tilted) smectic mesophaεe, e.g., the smectic C mesophase. Both the sign and the absolute value of such expansion can be determined using variable temperature small angle X-ray scattering (SAXS) . The SAXS technique is described in, e.g., Small-An le Scattering of X-Ravs by A. Guinier et al. (translation by C. B. Walker) , John Wiley & Sons, pages 1-4, New York (1955), and its application to liquid crystalε iε described, e.g., by S. Kumar in Phyε. Rev. A 22, 3207 (1981) . The sign and the absolute value of the expanεion can be determined by calculating the εign and the magnitude of the εlope of a line fitted to an approximately linear region (selected by inspection) of a plot of layer spacing versus temperature for the desired mesophase(s) . Generally, the temperature region εelected for fitting can be that which correεpondε to the uεeful temperature range for a particular device application, and data aεεociated with the tranεition between the tilted and non-tilted mesophaseε can generally be excluded from the εelected region due to nonlinearity. A component or composition for which the layer spacing increaεeε (decreaεeε) with increaεing temperature has a positive (negative) thermal layer expansion. Preferably, the negative composition is utilized in an amount such that the resulting combination has a net thermal layer expansion within the range of from about -0.05 to about +0.05 A/°C, more preferably, from about -0.01 to about +0.01 A/°C, oεt preferably, from about -0.005 to about +0.005 A/°C. However, in εo e cases an expansion value outside of these ranges may be desirable for a particular purpose and can be achieved by the mixing of positive and negative compositionε. Net expansions within these ranges can generally be achieved by choosing an amount of the negative compoεition εuch that that amount multiplied by the absolute value of the composition's net negative expansion is esεentially equal to the amount of the poεitive compoεition multiplied by the absolute value of itε net poεitive expanεion. (Thiε corresponds to the fitting of a linear equation to a plot of expanεion versus weight percent or mole fraction.) Refinement of the value of the net expansion (to obtain a value which iε cloεer to that desired) can be achieved through an iterative process (in which the measured net expansion value of the mixture is used to refine the equation describing the relationship between the expansion values and weight percent or mole fraction) . When latent εmectic liquid crystal compounds are utilized, their thermal layer expanεion valueε (which are unmeasurable due to the latency of the smectic mesophase) can be approximated by preparing mixtures (according to the procesε of the invention) having a range of different compoεitionε, measuring the expansion values of the mixtures, plotting expansion verεuε weight percent or mole fraction, fitting a linear equation to the data, and solving the equation for a composition correεponding to 100% latent εmectic compound. The process of the invention provides combinations or mixtures which have a controllable layer expansion or contraction behavior (with respect to temperature) which can be used to effect an esεentially temperature- independent layer εpacing in at leaεt one tilted εmectic meεophase (as well as a reduced temperature dependence of the layer spacing in the temperature range associated with the transition between the non- tilted and the tilted mesophaseε) . The proceεε enables control of layer shrinkage and thus εuppreεεion of chevron layer geometry. Thiε reduceε or eliminateε the formation of zig-zag alignment defectε upon cooling through the tilted εmectic meεophaεe(ε) , reεulting in improved electooptical εwitching performance, increaεed memory to driven cone-tilt angle ratioε, and improved contrast ratios. The procesε alεo enables the reduction or elimination of the εtreεs-induced defects and non-bookshelf layer geometry which reεult from heating and cooling cycleε. Mixtureε having an eεεentially temperature- independent layer εpacing in at least one tilted smectic eεophaεe can alεo be prepared by combining εmectic or latent εmectic liquid cryεtal compoundε (or compoεitionε) having only small negative (or only εmall poεitive) thermal layer expansions in the mesophaεe(ε) , the expanεionε preferably having values in the range of from about -0.05 A/°C to about +0.025 A/°C, more preferably, from about -0.01 A/°C to about +0.01 A/°C, most preferably, from about -0.005 A/°C to about +0.005 A/°C. Mixtures having a particular deεired non-zero temperature dependence of the layer εpacing in at leaεt one tilted εmectic meεophaεe can be prepared by combining εmectic or latent εmectic liquid cryεtal compound(ε) or compoεition(ε) having a εelected negative (poεitive) thermal layer expanεion in the meεophaεe(ε) with other εuch compound(ε) or composition(s) having a selected negative (poεitive) expanεion. The negative (poεitive) expanεionε can be εelected to bracket a deεired intermediate expanεion value (and the amountε of the compoundε or compoεitions then based on an approximately linear relationεhip between the expansions and the amounts, as deεcribed supra) or can be selected to approximate εuch a value. Alternatively, a particular deεired non-zero temperature dependence can be achieved by combining compound(ε) or compoεition(ε) having a εelected negative (poεitive) expanεion with compound(ε) or compoεition(ε) having an expanεion of approximately zero. Such non-zero temperature dependence of the layer spacing may be desired, e.g., to counterbalance the density-driven formation of chevrons described by T. P. Ricker and N. A. Clark in Phase Transitionε in Liguid Crvstalε (edited by S. Martellucci and A. N. Cheεter) , Plenum Preεε, page 310, New York (1992) . Objectε and advantageε of thiε invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit thiε invention. The fluorine-containing liquid cryεtal compoundε utilized in the exampleε can be prepared by application of the ethodε deεcribed in U.S. Pat. Noε. 4,886,619 (Janulis), 5,082,587 (Janulis), and 5,262,082 (Janulis et al.) . EXAMPLES In the following examples, layer spacing was measured by the SAXS technique in which small angle X- ray scattering is obεerved aε a function of temperature. Data waε collected using a Kratky camera, copper K-alpha radiation, and a linear position εenεitive detector. A liquid crystal sample (either a single compound or a mixture) waε placed in an open- ended quartz capillary tube and poεitioned on the εurface of a εtainleεε εteel εpecimen holder with the tube axis in the horizontal plane and perpendicular to the beam path. The sample waε heated to the iεotropic εtate and then cooled for the initial determination of the layer εpacing, with a ten minute waiting period prior to data collection. The εa ple was then cooled in two to three degree increments, and data waε acquired at each temperature following a ten minute waiting period. Layer spacing data was plotted with respect to temperature, and the region of the deεired meεophaεe (smectic C) which was approximately linear was selected by inspection. Generally the data region between the tilted and non-tilted mesophases waε excluded from the εelected data due to nonlinearity. The layer εpacing data for the selected region was plotted, and a linear equation was fitted to the data. The slope of the linear equation repreεented the thermal layer expanεion value of the sample in the εelected temperature range. Thuε, a sample with layer εpacing that increased with increasing temperature had a positive thermal expansion. Using this technique, thermal expansions (TEs) were determined for a variety of liquid cryεtal compoundε suitable for use in the procesε of the invention. The reεultε are εhown in Table 1. Table 1 * Extrapolated Value ** Estimated Value Control of Layer Spacing by Mixing Negative and Positive Compositions The layer spacing of a sample of 5-decyloxy-2-(4'- (1,l-dihydroperfluoro-2-(2-butoxyethoxy)ethoxy)- phenylpyrimidine (Compound 2 of Table 1) was measured as a function of temperature, plotted, and itε thermal expanεion calculated by the method deεcribed above. The reεulting plot of layer εpacing (in Angεtromε) verεus collection temperature is shown in Figure 1. The calculated expansion was -2.64 x 10" 2 A/°C in the smectic C mesophase. The fit of the line used was y = 33.591 - 2.6404e-2x, R2=0.985. A series of mixtures of Compound 2 and 5-octyl-2- (4'-(1,1-dihydroperfluoro-(3- butoxypropoxy)phenylpyrimidine (Compound A of Table 1) were then prepared, and the thermal expansions of the mixtures were determined esεentially as previously described. The resulting plots of layer spacing (in Angstroms) versuε collection temperature (in degreeε centigrade) are εhown in Figure 1. The calculated thermal expanεions of the mixtures were as follows: 100% Compound 2 -2.64 x 10 "2 A/°C 75% Compound 2/25% Compound A -1.78 x 10 2 A/°C 50% Compound 2/50% Compound A -1.19 x 10 2 A/°C 25% Compound 2/75% Compound A -0.37 x 10 "2 A/°C By plotting the calculated values of the TE for each mixture versus the weight percent of Compound 2, it was possible to extrapolate to a theoretical composition of 100 weight percent Compound A (0 weight percent Compound 2) for which a value of the TE for pure Compound A was calculated to be +0.36 x 10" 2 A/°C. The above data shows that Compound 2 (having a negative thermal expansion) can be mixed with liquid cryεtal materialε having a positive thermal expansion (indicating contraction upon cooling within the tilted mesophaεe) to offεet the layer contraction of the poεitive materialε and thereby enable control of layer expanεion behavior to achieve an essentially temperature-independent layer spacing in the smectic C mesophaεe (a TE approximately equal to zero) . The above-liεted thermal expanεions (TEs) of the ixtureε approximately fit the equation 100 x TE= 0.3590 - 2.26 x (weight ratio of Compound 2 to Compound A) . If a TE value even cloεer to zero (than the -0.37 x 10 "2 A/°C value obtained for the mixture of 25% Compound 2 and 75% Compound A) iε deεired, refinement of the equation can be achieved by an iterative proceεε. Example 2 Control of Layer Spacing by Mixing Negative and Positive Compositions The TE of Compound 10 of Table 1 was determined using esεentially the procedure of Example 1. The TEs of mixtures of Compound 10 (exhibiting strong layer expanεion upon cooling) and 5-decyl-2-(4'-(l,1- dihydroperfluorohexyloxy)phenylpyrimidine (Compound B, exhibiting εtrong layer contraction upon cooling) were determined uεing a modification of the above-deεcribed SAXS technique in which the capillary tube waε replaced by a thin homeotropic film of the liquid cryεtal mixture on a εubεtrate (a glaεε microεcope cover slip) treated with a silane adheεion promoter (commercially available from DuPont as VM651) . The resulting εample was placed in a horizontal X-ray beam in Kratky geometry with the sample layer normal in the vertical orientation, with adjustments to maximize scattering. The resulting plots of layer spacing (in Angεtromε) verεuε collection temperature are εhown in Figure 2. The thermal expanεionε of the mixtureε were determined to be as followε: 100% Compound 10 -4.01 x 10' 2 A/°C 75% Compound 10/25% Compound B -6.39 x 10" 3 A/°C 50% Compound 10/50% Compound B -5.00 x 10" 3 A/°C Thiε data shows that Compound 10 (having a large negative thermal expanεion) can be mixed with liquid cryεtal materialε having a poεitive thermal expanεion and exhibiting εtrong layer contraction to greatly counterbalance that contraction. Example 3 Control of Layer Spacing by Mixing Compositions Having Thermal Expansions of the Same (Negative) Sign The TEε of Compoundε 11 and 12 of Table 1 were determined essentially as described in Example 1, and the values are εhown in Table 1. Both compounds were found to have a negative TE. Mixtures of the two compounds were prepared, and the TEs of the mixtures were determined by esεentially the εame procedure. The reεulting plotε of layer εpacing (in Angεtroms) versus collection temperature are shown in Figure 3. The thermal expanεionε of the mixtureε were determined to be aε followε: Thiε data εhowε that Compoundε 11 and 12 (both having negative thermal expansions) can be mixed to enable control of layer expansion behavior to achieve desired intermediate thermal expansion values which may be uεeful to compenεate for the bulk thermal expanεion of the compoεition or for the poεitive layer expanεion of a second composition. Bvampla A Control of Layer Spacing by Mixing Negative and Positive Compositions The TEs of 5-octyl-2-(4'-(l,l-dihydroperfluoro-2- (2-butoxyethoxy)ethoxy)phenylpyrimidine (Compound 9 of Table 1) and 5-octyl-2-(4'-(l,1-dihydroperfluoro-(3- butoxypropoxy)phenylpyrimidine (Compound A of Table 1) were determined esεentially as described in Example 1 and found to be negative for Compound 9 and positive for Compound A. A mixture of 75 weight percent Compound 9 and 25 weight percent Compound A was prepared, and the TE of the mixture was determined by essentially the same procedure. The reεulting plotε of layer εpacing (in Angεtroms) verεus collection temperature are shown in Figure 4. From the plotε of the data for pure Compound 9 and the mixture, it waε predicted (by iterative fitting of a line to TE verεuε composition data, as described in Example 1) that a mixture of 41 weight percent Compound 9 and 59 weight percent Compound A should have a TE of about zero. Thiε mixture waε prepared, and itε TE waε determined eεεentially aε deεcribed above and was found to be -0.31 x 10 "2 A/°C. This data shows that Compound 9 (having a negative thermal expansion) can be mixed with liquid crystal materialε having a poεitive thermal expanεion to enable control of layer expansion behavior to predictably achieve an essentially temperature- independent layer spacing in at leaεt one tilted εmectic meεophaεe. (If a TE value even cloεer to zero were deεired, an iterative process could be utilized as described in Example 1.) Example 5 Determination of the Thermal Expansion (in a Tilted Smectic Mesophase) of a Latent Tilted Smectic Liquid Crystal Compound In this example, a smectic liquid crystal compound having a negative TE (Compound 2 of Table 1) was mixed in various proportions with a latent tilted smectic liquid crystal compound (Compound C of Table 1) . The TEs of the various mixtures were determined esεentially as in Example 1 in order to enable approximation of the TE of Compound C (which could not be directly determined due to the latency of the smectic C mesophaεe) in this particular mixture. The resulting plots of layer spacing versus temperature are shown in Figure 5. By plotting the values of the TE for each mixture versus the weight percent of Compound 2, it was posεible to extrapolate to a theoretical composition of 100 weight percent Compound C (0 weight percent Compound 2) for which a value of the TE for pure Compound C was calculated to be -0.44 x 10 "2 A/ C C, as shown in Figure 6. Example 6 Control of Layer Spacing by Mixing Negative and Positive Compositions In this example, a smectic liquid cryεtal compound having a negative TE (Compound 10 of Table 1) waε mixed with a non-fluorinated εmectic liquid cryεtal compound having a poεitive TE (5-decyl-2- hexyloxyphenylpyrimidine. Compound D of Table 1, which can be prepared by the methodε described by H. Zaschke in J. prakt. Chem. 117, 617 (1975)). The TEε of the pure compoundε and of a mixture of 90 weight percent Compound 10 and 10 weight percent Compound D were determined essentially as in Example l, and the resulting plots are shown in Figure 7 below. The TE of the mixture was intermediate in value to the TEs of the two pure compounds, demonstrating that negative and positive compositionε can be combined according to the process of the invention to enable control of layer expansion behavior. Example 7 Liquid Crystal Display Device Containing a Ferroelectric Liquid Crystal Mixture Prepared by the Process of the Invention Two mixtures of ferroelectric liquid crystal compounds (Mixtures 7A and 7B, having the compositions shown in Table 2 below) were prepared according to the procesε of the invention. The TE for the εmectic C meεophaεe of each mixture waε determined eεεentially aε in Example 1, and the reεulting plotε of layer εpacing verεus temperature are εhown in Figure 8. The figure εhows that latent smectic C liquid crystal compounds (the two compounds unique to Mixture 7B) can be used as additives to further reduce the TE value of a mixture and achieve even greater temperature independence in the ferroelectric phase. Table 2 A working εurface-εtabilized ferroelectric liquid cryεtal device (SSFLCD) waε constructed in the following manner: Onto an indium tin oxide (ITO) coated (300 A ITO coating), ultrasonically cleaned glasε εubεtrate (2.85 x 3.5 x 0.1 cm) was placed several drops of a 0.52 weight percent solution of nylon 6/6 (Aldrich Chemical Co.) in formic acid. The substrate was spun at 1200 rpm for 40 εecondε and cured at 75°C for 16 hourε to provide a nylon coating about 400 A thick. The coated substrate was rubbed (20 strokeε) in one direction with a 115 gram rubbing bar (a glass rod 2.5 cm in diameter, 10 cm long) about which a 65% cotton, 35% rayon velveteen fabric was tightly wrapped, to provide an oriented alignment layer on the substrate. Onto another ITO coated (300 A ITO thickness) glass subεtrate (2.85 x 3.5 x 0.1 cm) having a pattern of polyimide spacer postε 1.5 μm in height (and which had been ultraεonically cleaned) waε placed εeveral drops of a 1.5 weight percent solution of polydimethylsiloxane (5.6% GR-651L, available from Owen-Illinoiε, Inc.) in butyl alcohol. The εubεtrate waε εpun at 8000 rpm for 20 εecondε and cured at 75°C for 16 hours to provide an alignment coating about 200- 300 A thick. Two setε of the above-deεcribed εubεtrateε were aεsembled using a UV curable adhesive (Norland™ 61 Optical Adheεive, available from Norland Productε, Inc.) with the ITO-conεtituted electrodeε and the alignment layerε facing inward to form two cellε. Each cell waε then filled with one of the two mixtureε of liquid crystal compounds (Mixtures 7A and 7B, shown in Table 2) by heating uεing capillary action under vacuum to form εwitchable deviceε. The ITO conεtituted electrodes of the device were connected to an arbitrary waveform generator with variable output voltage. The device waε placed on the rotation εtage of a polarized light microεcope. The incandeεcent light εource of the microεcope waε filtered to confine itε wavelength to between 450 and 700 nm. The optical tranεmission waε meaεured with a photomultiplier tube and diεplayed on an oεcilloεcope. The latching time for the device, meaεured with a field of 20 V/μm and at a temperature of 24°C was 80 με. The device waε driven with a voltage waveform conεiεting of bipolar, εquare pulεeε of 20 V/μm εpaced at least 30 s apart by a train of square pulεes having the same width and 6.7 V/μm amplitude. The memory to tilt angle ratio of the device was measured to be 0.87. Comparative E ampl Two commercially available ferroelectric liquid crystal mixtures (CS-1015, available from Chisεo Corp., Japan, and ZLI 4654-100, available from Merck, Germany) were selected as being representative of commercially available ferroelectric liquid crystal mixtures sold for use in SSFLCDs. The TEs of the mixtures were determined essentially as in Example 1, and the resulting plots of layer spacing versuε temperature are εhown in Figure 9. Alεo εhown in Figure 9 iε a plot of layer εpacing verεuε temperature for Mixture 7B of Example 7, prepared according to the process of the invention. The plotε of the data for the two commercially available mixtureε are nearly εuperimpoεable, giving nearly identical poεitive TE valueε. In contrast with the two commercial mixtures, Mixture 7B exhibits the esεentially temperature- independent layer εpacing which haε been highly εought in the art. Variouε modificationε and alterationε of thiε invention will become apparent to thoεe εkilled in the art without departing from the εcope and εpirit of thiε invention.

Claims: Claims 1. A procesε for controlling layer εpacing compriεing the step of combining a liquid crystal composition having a net negative thermal layer expansion in at leaεt one tilted or latent tilted εmectic mesophase and a liquid crystal composition having a net poεitive thermal layer expansion in at least one tilted or latent tilted smectic mesophaεe, each of said compositions comprising at least one liquid crystal compound selected from the group consisting of εmectic liquid cryεtal compounds and latent smectic liquid crystal compounds. 2. A procesε for controlling layer εpacing compriεing the εtep of combining a liquid cryεtal compoεition having a net negative thermal layer expanεion of known magnitude in at leaεt one tilted or latent tilted εmectic meεophaεe and a liquid cryεtal compoεition having a net positive thermal layer expansion of known magnitude in at least one tilted or latent tilted smectic mesophaεe, εaid compoεitionε compriεing at leaεt one liquid cryεtal compound εelected from the group conεisting of εmectic liquid cryεtal compoundε and latent εmectic liquid crystal compounds, and said compositions being utilized in amounts based upon said magnitudes such that the reεulting combination haε a desired net thermal layer expansion in at least one tilted smectic mesophaεe. 3. A liquid cryεtal compound mixture prepared by the process of Claim 1. 4. A liquid crystal compound mixture comprising a liquid crystal composition having a net negative thermal layer expansion in at least one tilted or latent tilted smectic mesophase and a liquid crystal composition having a net positive thermal layer expansion in at least one tilted or latent tilted smectic mesophaεe, each of εaid compoεitionε compriεing at least one liquid crystal compound εelected from the group conεiεting of εmectic liquid cryεtal compoundε and latent εmectic liquid cryεtal compoundε, wherein εaid compoεition having a net negative thermal layer expanεion iε preεent in an amount such that said mixture haε a net thermal layer expansion in at least one tilted εmectic mesophase within the range of from about -0.05 to about +0.05 A/°C. 5. A liquid crystal display device containing the mixture of Claim 3. 6. A liquid crystal diεplay device containing the mixture of Claim 4. 7. A proceεε for controlling layer spacing comprising the step of combining a liquid crystal composition having a net negative thermal layer expansion in the smectic C meεophaεe, said composition comprising at least one chiral or achiral, smectic or latent smectic liquid crystal compound having a fluoroether terminal portion containing at least two catenary ether oxygen atoms; and a liquid crystal composition having a net positive thermal layer expansion in the smectic C mesophaεe, εaid composition comprising at least one chiral or achiral, smectic or latent smectic liquid crystal compound having a terminal portion selected from the group consiεting of fluoroaliphatic terminal portionε and fluoroether terminal portionε containing one catenary ether oxygen atom; wherein εaid compoεition having a net negative thermal layer expansion is utilized in an amount such that the resulting combination has a net thermal layer expansion in the smectic C mesophaεe within the range of from about -0.05 to about +0.05 A/°C. 8. A proceεε for controlling layer εpacing comprising the step of combining at leaεt two liquid cryεtal compoεitionε having net thermal layer expanεionε of the εame sign and known magnitudes in at least one tilted or latent tilted smectic mesophaεe, εaid compoεitionε compriεing at leaεt one liquid cryεtal compound εelected from the group conεiεting of εmectic liquid cryεtal compoundε and latent εmectic liquid cryεtal compoundε, and εaid compositions being utilized in amounts based upon said magnitudes such that the reεulting combination haε a deεired net thermal layer expanεion in at leaεt one tilted εmectic meεophaεe. 9. A liquid crystal compound mixture prepared by the procesε of Claim 8. 10. A liquid cryεtal compound mixture compriεing at leaεt two liquid cryεtal compoεitionε having net thermal layer expanεionε of the same sign in at least one tilted or latent tilted smectic mesophaεe, each of εaid compoεitionε comprising at least one liquid crystal compound εelected from the group conεiεting of smectic liquid crystal compoundε and latent εmectic liquid cryεtal compoundε, wherein each of εaid compoεitionε iε present in an amount εuch that εaid mixture haε a net thermal layer expanεion in at leaεt one tilted smectic mesophaεe within the range of from about -0.05 to about +0.025 A/°C. 11. A liquid cryεtal display device containing the mixture of Claim 9. 12. A liquid crystal display device containing the mixture of Claim 10. 13. A process for estimating the net thermal layer expanεion of a latent εmectic liquid cryεtal compoεition in at least one latent tilted smectic mesophaεe compriεing the steps of (a) preparing at least one mixture of a latent smectic liquid crystal composition and a εmectic liquid crystal composition, said latent smectic liquid crystal compoεition compriεing at leaεt one latent εmectic liquid cryεtal compound and εaid εmectic liquid cryεtal compoεition compriεing at leaεt one εmectic liquid cryεtal compound; (b) determining the net thermal layer expanεion of εaid mixture in at leaεt one εelected tilted εmectic meεophaεe; (c) determining the net thermal layer expanεion of εaid εmectic liquid cryεtal compoεition in at leaεt one correεponding tilted εmectic meεophaεe; and (d) extrapolating to a composition of 100 percent said latent εmectic liquid cryεtal compoεition to obtain an approximate value of the net thermal layer expanεion for εaid latent εmectic liquid crystal composition in at least one corresponding latent tilted smectic meεophase. 14. A liquid crystal display device containing a liquid cryεtal compound mixture compriεing a liquid cryεtal composition having a net negative thermal layer expansion in at least one tilted or latent tilted smectic mesophase and a liquid crystal composition having a net positive thermal layer expansion in at least one tilted or latent tilted smectic mesophase, each of said compositions comprising at least one liquid crystal compound selected from the group consisting of εmectic liquid cryεtal compoundε and latent εmectic liquid crystal compounds, wherein said composition having a net negative thermal layer expansion iε present in an amount εuch that εaid mixture haε an eεεentially temperature-independent layer εpacing in at leaεt one tilted εmectic meεophaεe.

Inventors: EPSTEIN KENNETH A, KEYES MICHAEL P, RADCLIFFE MARC D, SNUSTAD DANIEL C, EPSTEIN, KENNETH, A., KEYES, MICHAEL, P., RADCLIFFE, MARC, D., SNUSTAD, DANIEL, C.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: C09K19
IPC codes: C09K19, G02F1