Novel Liquid Crystalline Derivatives of Cubanes and Hydrogenolyzed Cubanes

The N-I transition temperatures for a range of 4-propyl 1-monoesters and several 1,4-diesters of cubane, bicyclo(2.2.2)octane, cydohexane and benzene have been determined. The differences between the values for the monoesters and diesters of each system and between those for the various ring systems for the mono- and di-ester series are discussed. Cubane has a very poor ability to generate nematic phases of high thermal stability, and the relative nematic stability of the benzene derivatives are significantly different in the two series, the diesters with a central benzene ring having the highest N-I transition temperatures. The values for diesters and monoesters of dihydrocubane are compared with those for cubanes and the effect of deviations from colinearity of the substituent bonds and of changes in flexibility are discussed.     

Synthesis and Liquid Crystal Behavior of a Novel Ester Homologous Series

A novel ester homologous series of 4-[4’-n-alkoxy cinnamoyloxy] benzyl benzoates has been synthesized. The series consists of 12 homologues. Liquid crystal properties commence from the sixth member to the last member of the series with the exhibition of an enantiotropic nematic phase without the exhibition of any smectogenic mesophase. The remaining homologues do not exhibit liquid crystal behavior. The texture of the nematic phase is of the threaded or Schlieren type. The solid-nematic or isotropic transition curve adopts a zigzag path and the nematic-isotropic transition curve steeply rises and then falls in the phase diagram and behaves in normal manner. An odd–even effect is absent in the nematic-isotropic transition curve with an alteration of transition temperatures. The average thermal stability for the nematic mesophase is 153.1°C, and the nematogenic temperature ranges vary from 12°C to 50°C. Analytical data confirm the molecular structures of the homologues. The Liquid Crystal properties and transition temperatures were observed through an optical polarizing microscope, equipped with a heating stage. The mesomorphic characteristics of the novel ester series are compared with other, known and structurally similar series. The novel series is predominantly nematogenic with the absence of any smectogenic character, with relatively short mesophase temperature ranges and of the middle ordered melting type.     

Phase transition studies in mesogenic dimers

Schiff base liquid crystal dimers, both symmetric and non‐symmetric, in which two anisotropic groups are linked by a flexible spacer, exhibit a rich variety of smectic mesomorphism. The interest in this class of mesogens stems not only from their ability to act as model compounds for semi‐flexible mainchain liquid crystal polymers but also from their quite different properties compared to conventional low molar mass liquid crystals (monomers). We report here the phase transition studies on two examples of these schiff base liquid crystal dimers using the Differential Scanning Calorimetry and density measurements as a function of temperature. The symmetric liquid crystal dimer, α,ω‐bis (4‐n‐dodecylaniline benzylidene‐4′‐oxy)decane (10.O12O.10) exhibits a very rare Isotropic to G transition. Where as, the non‐symmetric dimer, α‐(4‐cyano biphenyl‐4′‐yloxy)‐ω‐(4‐n‐decylanilinebenzylidene‐4′‐oxy)decane (CB.O10O.10) exhibits a rare nematic to intercalated smectic A phase transition. The transitions studied, isotropic to nematic and isotropic to G exhibit a large density jump at the transition confirming their first order nature. Nematic to intercalated smectic A transition is found to be a second order transition whose behavior is similar to nematic to smectic A transition. Calculated values of pressure dependence of transition temperature and thermal expansion coefficient are also reported.     

Study of a new homologous series of liquid crystals: Isopropyl-p-(p/-n-alkoxy cinnamoyloxy) cinnamates

A new homologous series: isopropyl-p-[p^/-n-alkoxy cinnamoyloxy] cinnamates was synthesized and studied with a view to understanding and establishing the relation between liquid crystal property and molecular structure. Twelve homologues were synthesized. Methyl to butyl homologues are not liquid crystals, while pentyl, hexyl, heptyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl derivatives are enantiotropic liquid crystal in nature with nematogenic character. Smectogenic character is totally absent. A phase diagram is obtained by plotting a graph of transition temperatures versus number of carbon atoms is n-alkyl chain of left n-alkoxy terminal end group. Solid-isotropic or solid-nematic transition curve rises steeply from methyl to propyl derivatives and falls to pentyl homologue through butyl homologue, and follows a zigzag path of rising and falling values as the series is ascended. Nematic–isotropic transition curve shows descending tendency as series is ascended in a normal manner with exhibition of odd-even effect. Smectic mesophase does not appear even in the monotropic condition. Phase transition temperatures are determined by hot stage polarizing microscope. Analytical data support the structure of molecules. Texture of nematic mesophase is of threaded type. Mesomorphic properties are compared with structurally similar homologous series.     

Properties of the Liquid Crystals Formed by the N-Oxides of Certain 4-(4′-pyridyl)phenyl 4″-n-Alkoxybenzoates: Comparison of the N-Oxide and Cyano-groups as Mesogenic Terminal Substituents

The liquid liquid crystal transition temperatures of the N-oxides of certain 4-(4′-pytidyl)phenyl 4″ -n-alkoxybenzoates are reported. The effects on smectic and nematice thermal stability of the N-oxide function and the cyano-group in terminal positions in mesogens are compared and discussed.     

Density Studies in Polymorphic Liquid Crystal N- (p-n-Heptyloxybenzylidene) p-n-Pentylaniline

Density studies in polymorphic liquid crystal, N-(p-n-heptyloxybenzylidene) p-n-pentyl-aniline is carried out in isotropic liquid, nematic, smectic A, smectic C, smectic B and smectic G phases. The temperature variation of density in all the mesophases confirms that isotropic liquid to nematic, nematic to smectic A, smectic C to smectic B, smectic B to smectic G transitions are of first order while smectic A to smectic C transition is of second order. The density variation across smectic C to smectic B is greater than that of any other transition. Thermal expansion coefficients are also calculated.     

Mesomorphism in chloro substituted isomeric series including chalconyl central bridge

A novel homologous series of thermotropic liquid crystals α-4-(4′-n-alkoxy benzoyloxy phenyl) β-2″-chloro benzoyl ethylenes have been synthesized and studied with a view to understand and establish the effect of molecular structure on liquid crystal (LC) properties with reference to molecular flexibility in isomeric series with differing positional status of same functional group. Novel homologues series consist of 12 homologues C₁ to C_16. C₁, C₂, and C₃ homologues are nonliquid crystals (NLC) and rest of the homologues are liquid crystals. C₁₀ to C₁₆ homologues are enantiotropically smectogenic plus nematogenic and C₄ to C₈ homologues are enantiotropic nematic. The texture of nematogenic derivatives is threaded or schlieren and that of the smectic mesophase are focal conic of the type smectic A or C. Analytical, thermal and spectral data supported molecular structures of novel homologues. Transition temperatures as determine by a hot stage polarizing optical microscopy (POM) were plotted against number of carbon atom present in n-alkyl chain ‘R’ of left n-alkoxy (-OR) group and the phase transition curves Cr–I/M, Sm–N, N–I were obtained on linking like or related points. The odd–even effect is observed for the N–I transition curve and thus transition curves behaved in normal manner. The even-membered nematic transition curve occupied higher position than the odd-membered transition curve. Present series is predominantly nematogenic and partly smectogenic with middle-ordered melting type.     

Mesomorphism dependence on molecular rigidity

A novel ester homologous series of thermotropic liquid crystal (LC) has been studied with a view to understanding and establishing the relation between LC property and the molecular structure. The series consists of 11 members, the C₁–C₅ and C₁₆ members of the series are nonliquid crystals. LC properties commence from the C₆ homolog and continue up to the C₁₄ homolog as enantiotropic nematic and smectic in addition to nematic. Transition temperatures of the homologs were determined by an optical polarizing microscope equipped with a heating stage. Textures of nematic phase are threaded or Schlieren and that of smectic is focal conic of the type-A. Analytical and spectral data confirm the structures of homologs. Thermal stability for nematic is 93 °C and 136 °C, respectively. The N–I and Sm–N transition curves of phase diagram do not exhibit odd-even effect. The N–I transition curve partly behaves in an abnormal manner. The Cr–I and Sm–N transition curves behave in normal manner. The LC behavior of the present series is compared with structurally similar known homologous series.     

Synthesis and Mesomorphic Behavior of Calamitic Liquid Crystals with a Biphenyl Moiety

A new mesogenic homologous series having a biphenyl moiety has been synthesized by condensing 4-hydroxy-4′-nitrobiphenyl- and methoxy-substituted 4-n-alkoxy cinnamoyl chlorides, and their liquid crystalline properties have been studied. All the members of the series are enantiotropic liquid crystals. The methyl to n-pentyl derivatives exhibit both Smectic A (SmA) and Nematic (N) phases; the higher members, starting with the n-hexyl derivative show only a SmA phase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits zig-zag pattern for Sm-N and N-Isotropic (Iso) transition temperature curves. The average thermal stability is compared with other related homologous series. The introduction of polar nitro group increases significant intermolecular force of attraction which stabilizes the molecular orientation. This results into the increase in the thermal stability of the system.     

Synthesis and Evaluation of Liquid Crystal Behavior of a Novel Homologous Series: 4-(4′-n-Alkoxy benzoyloxy) Phenylazo-2″, 6″-dimethylbenzenes

A novel homologous series entitled 4-(4′-n-alkoxy benzoyloxy) phenyl azo 2^″, 6^″-dimethyl benzenes consisting of 12 members is reported. Liquid crystal behavior as a nematogenic mesophase commences from the pentyloxy homologue monotropically and then continues enantiotropically until the last hexadecyloxy homologue without exhibition of any smectogenic behavior. An odd-even effect is observed for nematic-isotropic transition curve of the phase diagram. The textures of the nematic mesophase are of the threaded or Schlieren type. Analytical and spectral data support the molecular structure of the homologues. Transition temperatures of the novel homologues were determined by an optical polarizing microscopy. Liquid crystal properties of a novel homologous series are compared with other known homologous series. The average thermal stability for the nematic phase is 77.1°C.     

Study of Formation,Phase Behavior,and Microdroplet Size of a Polyurethane-based Polymer-dispersed Liquid Crystal

Using calorimetry and scanning electron microscopy, we have investigated the formation process and phase behavior of a polyurethane-based polymer-dispersed liquid crystal system. We have measured the kinetics and energetics of the cure process during which liquid crystal microdroplets form by phase separation from the matrix as it cross-links. The greatest degree of cure occurs for samples cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linearly with increasing liquid crystal concentration due to a dilution effect. The time constant for the cure process decreases rapidly with increasing temperature but is much less sensitive to liquid crystal content.

Samples cured below 375 K are apparently not fully phase separated, but subsequent treatment at higher temperatures evidently increases the degree of cure. The highest nematic-isotropic transition temperatures were achieved for liquid crystal concentrations above 40 volume percent. The nematic isotropic transition enthalpy, ΔH_NI, is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of ΔH_NI with increasing liquid crystal concentration.

Optimum microdroplet formation occurs at 375 K, but only for liquid crystal concentrations below about 53 volume percent. At higher concentrations a reversed phase (“polymer ball”) morphology is seen. For the lower concentrations droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model.     

Linear Double Hydrogen-bonded Thermotropic Liquid Crystals Formed Between Oxaloacetic Acid and p-n- Alkyloxy Benzoic Acids

Double hydrogen-bonded thermotropic liquid crystal complexes (DHBLC) have been isolated from molar ratios of Oxaloacetic acid (OAC) and eight alkyloxy benzoic acids (nBAO) whose carbon number varied from pentyloxy to dodecyloxy. The complexes are referred as OAC+nBAO where n varied from 5 to 12. In each of the synthesized complexes, complementary hydrogen bonds formed are confirmed by FTIR study and the complexes are further investigated by Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) for the construction of phase diagram. Characteristic phases like nematic, smectic C, smectic F, and smectic G are characterized by various textures. From DSC studies, odd–even effect is evinced in transition temperatures across isotropic to nematic phase transition and across smectic F to smectic G phase transition in enthalpy values. The order of all transitions observed in eight complexes is calculated by thermal analysis. The magnitude of optical tilt angle in smectic C is fitted to a power law and the scale of critical exponent is found to concur with the Mean Field theory predicted value.     

Some Novel Nematic Liquid Crystals of Negative Dielectric Anisotropy

A number of ether and ester derivatives of 1-(trans-4-n-alkylcyclohexyl)-2-(4-hydroxyphenyl)ethane bearing lateral substituents on the phenyl ring are reported. Several of the novel compounds exhibit enantiotropic nematic phases at room temperature. All of the liquid crystals described are of negative dielectric anisotropy (-0.5 -3.5). The effect of lateral substituents on the liquid crystal transition temperatures of these ethanes has been studied.     

Mesomorphism dependence on lateral substitution and central bridge

A novel ester homologous series was synthesized and studied with a view to understanding and establishing the effects of molecular structure on liquid crystal properties with a common laterally substituted –OCH3 group and changing terminal groups. The novel series consists of twelve members. The C₁ to C₃ members are not liquid crystals and the rest of the members C₄ to C₁₆ are smectogenic without exhibition of nematic character. Textures of smectic phase are focal conic fan shaped of the type A or C. The transition temperatures and textures of smectic mesophases were observed through hot stage polarizing microscopy (POM). The transition curves (Cr-I or Cr-M and Sm-I) behaved in normal manner. Analytical and spectral data support the molecular structures. Average thermal stability for smectic is 77.25°C and smectogenic mesophase ranges from 15°C to 33°C. Liquid crystal properties of the present series are compared with structurally similar homologous series. Thus, the series is smectogenic only with three homologs nonmesogenic.     

X-Ray Diffraction Studies of Liquid Crystalline Polymers

Thermotropic liquid crystalline copolyesters display an ordered fluid phase at elevated temperatures which permits the development of unusually high orientation at ambient temperatures. The transition which occurs upon cooling the high temperature nematic liquid crystalline phase very rapidly (e.g. fiber spinning) results in a polymeric glass with nematic structural order. Annealing increases structural order from the nematic glass toward ideal three dimensional crystalline order. Precision X-ray diffractometry has been used to directly observe changes in structural correlations which occur with annealing. Structural transitions range in character in these copolyesters from no change upon annealing to a transition from fully two dimensional to fully three dimensional structural order. Increased three dimensional order also results in substantially increased first order character of the high temperature phase transition. The chemical structure of co-monomers determines, in part, the final degree of dimensionality.     

Phase Transition Studies in N(p-n-Heptyloxy Benzylidene)p-Toluidine, 70.1, Across I-N,N-SA and SA-SB Phases

The results of density and ultrasonic velocity variation with temperature in N(p-n-heptyloxy benzylidene)p-toluidine, HYBT, 70.1 exhibiting nematic, smectic-A and crystalline smectic-B phases, are presented. The isotropic-nematic, nematic-smectic-A phase transitions are found to be first order while the smectic-A crystal smectic-B transition is weak first order. The thermal expansion coefficient α computed from the molar volume data is presented. The molar sound velocity and molar compressibility of HYBT computed from the density and ultrasonic velocity data in isotropic phase are compared with the values of the other compounds in a homologous series. They are found to be in good agreement with theory.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Coefficient of Volume Expansion and Thermoacoustic Parameters of Alkyl-Cyano-Biphenyl Liquid Crystals

Dilatometric studies were performed on the nematic and isotropic phases of the homologous series of alkyl-cyano-biphenyl liquid crystals C_nH_2n+1C₆H₄C₆H₄. CN for n = 5 to 9. From these studies the coefficient of volume expansion (a) of the liquid crystal is estimated. Using this data a number of thermoacoustic and an harmonic parameters of the liquid crystal are evaluated. The temperature dependence of these parameters have also been investigated. All these thermoacoustic and an harmonic parameters evaluated in this case show characteristic change in the immediate vicinity of the phase transition temperature.     

Mesomorphic Behavior of Some New Fluorene Compounds

Several new fluorene derivatives have been synthesized and their liquid crystalline polymorphic behavior examined. Specifically, 2-fluorenylmethylidene-4′-n-alkylanilines with different alkyl groups are compared with corresponding compounds wherein the alkyl groups have been replaced either by the alkoxy or other different groups. In addition, some of the 4-alkyl and alkoxybenzylidene-2-aminofluorenes have also been prepared for comparison. Many of these compounds show not only the nematic phase but also two smectic phases. The presence of a lateral substituent in analogous compounds decreases the nematic-isotropic transition by about 60[ddot]C.     

Mesomorphism dependence on molecular rigidity with reference to ‒CH˭CH‒ unit of central bridge

A novel ester homologous series of rich mesomorphism and low temperatures with unexpected phase behaviors of eleven homologues was synthesized and studied with a view to understanding and establishing the relation between mesomorphic behaviour and the molecular structure of a series 4-(4′-n-alkoxybenzoyloxy)-4″-chlorobenzyl cinnamates. All the members of the novel series are enantiotropically smectogenic and the octyloxy (C₈) to hexadecyloxy (C₁₆) homologues are enantiotropically nematogenic, in addition to smectogenic. Odd–even effect is observed for Sm?I/Sm?N transition curve but it is absent for N?I transition curve. Textures of nematic phase are threaded or Schlieren and that of the smectic phase are fan shaped or batonates of smectic-A type phase or Smectic-C type for C₁₆ homologues as judged directly from a heating stage of an optical polarizing microscopy. Analytical and spectral data confirmed the molecular structures of novel homologues. Mesomorphic properties of present series are compared with the structurally similar other known series. The average smectic and nematic thermal stabilities are 92.78°C and 100.8°C, respectively. Mesophase length minimum to maximum for smectic and nematic are 21.0°C to 31.1°C and 8.4°C to 42.6°C respectively. Thus, the present novel series is partly nematogenic and fully smectogenic with considerable degree of mesomorphism and low melting type.