Liquid Crystals V. Nematic Materials Derived fromp-Alkylcarbonato-p'-Alkoxyphenyl Benzoates
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Abstract In our continued effort to synthesize materials which exhibit nematic behavior at room temperature, homologous p-alkylcarbonato-p-'-alkoxyphenyl benzoates were prepared. Of the 48 compounds that were synthesized, 27 exhibited enantiotropic and 14 monotropic nematic properties while only 7 were not liquid crystalline. One binary mixture of enantiotropic derivatives had a nematic range of 27–73°C while a ternary mixture exhibited nematic behavior between 24 and 76°C. In addition, seven other mixtures had crystal nematic transition temperatures below 30°C.Keywords:
Benzoates
Homologous series
Biaxial nematic
Abstract Mesomorphic characteristics of homologous series n-propyl p-(p'-n-alkoxy cinnamoyloxy)benzoates (I)1 are quite in keeping with the general molecular structure and molecular forces accruing therefrom. Continuing the work further, the right hand terminal group of the series (I) has been replaced by a branched group with iso disposition of the alkylchain but having the same number of carbon atoms. Thus the new homologous series iso-propyl p-(p'-n-alkoxy cinnamoyloxy)benzoates (II) with all other molecular geometry the same, gets mesomorphic characteristics changed considerably. The first few members become non-mesogenic, nematic orientation is altogether eliminates and the smectic mesophase range is reduced to about one third of that of the series (I). Another new homologous series (III) with all the molecular geometry of the series (II). but with a shorter central bridge —COO— than —CH=CH—COO—receives a further jolt in its exhibition of mesomorphic characteristics. The first seven members and the last member of the series (III) become non-mesomorphic; however, some middle members exhibit monotropic smectic mesophase. The effect of the branching of the terminal substituent and shortening of the central bridge are thus quite evident and arouse good interest. The smectic texture as determined by optical microscopy and by the study of mixtures2 is of fan shaped focal conic smectic A variety.
Homologous series
Benzoates
Mesogen
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Abstract The homologous series of 4-isothiocyanatophenyl 4-(trans-4-alkylcyclohexyl)benzoates (with n-alkyl chain from ethyl to dodecyl) were synthesized and their phase transition temperatures and enthalpies, smectic layer spacing and dielectric permittivity were measured. It was observed that the smectic A, layer spacing is expanding from d/l ∼ 1 to d/l ∼ 1.1 when the alkyl chain increases from butyl to dodecyl. It was proposed to term the smectic A1 with d/l > 1 as the enhanced smectic A1 (Ale).
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Benzoates
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Abstract A homologous series of 4′-bromophenyl 4′'-[(4-n-alkoxy-2,3,5,6-tetrafluoro-phenyl)ethynyl]benzoates has been synthesized. The transition temperatures were studied by polarizing optical microscopy. The compounds exhibit nematic and smectic A phases.
Benzoates
Homologous series
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A novel monomeric homologous series of α,β-ethylene derivatives was synthesized and studied with a view to understanding and establishing the relation between molecular structure and liquid crystal properties. The series consists of 12 novel compounds. The methoxy to propyloxy derivatives are not liquid crystals, the butyloxy homologue is monotropic nematic, the pentyloxy homologue is monotropic smectic and enantiotropic nematic, and the rest of the homologues (C6–C16) are enantiotropically smectic in addition to nematic. Transition temperatures and texture were determined by an optical polarizing microscope equipped with a heating stage, and show an odd–even effect for nematic–isotropic and smectic–nematic transition curves. Analytical and spectral data confirm the molecular structures. The average smectic and nematic thermal stabilities are 84.9°C and 94.15°C, respectively. The liquid crystal properties of present series are compared with the structurally similar known series.
Homologous series
Mesogen
Biaxial nematic
Polarized light microscopy
Texture (cosmology)
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Abstract In our continued effort to synthesize materials which exhibit nematic behavior at room temperature, homologous p-alkylcarbonato-p-'-alkoxyphenyl benzoates were prepared. Of the 48 compounds that were synthesized, 27 exhibited enantiotropic and 14 monotropic nematic properties while only 7 were not liquid crystalline. One binary mixture of enantiotropic derivatives had a nematic range of 27–73°C while a ternary mixture exhibited nematic behavior between 24 and 76°C. In addition, seven other mixtures had crystal nematic transition temperatures below 30°C.
Benzoates
Homologous series
Biaxial nematic
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Abstract A homologous series of 4-(4-alkoxyphenoxycarbonyl)phenyl 4-(4-nitrobenzoyl) benzoates has been prepared. The octyloxy and nonyloxy homologs give rise to three kinds of smectic A phases (S1, S2, and S3) and a nematic one, and the decyloxy homolog gives rise to two kinds of the smectic A phases (S1 and S3). The S1 and S3 phases are isomorphous with a partially bilayered smectic A phase (SAd) of CBOOA and a bilayered smectic A phase (SA2) of 4-(4-pentylphenoxycarbonyl)phenyl 4-cyanobenzoate, respectively. The S2 phase may correspond to the antiphase (SÃ) of a smectic A phase. These phase transitions are observed only by the differential scanning calorimeter. Their microscope textures are quite similar to each other.
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Benzoates
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Molecular mobility of cyanophenyl alkylbenzoates (CPnBs) (n = 2, 3, 7 – number of carbon atoms in the alkyl chain) in the bulk and in composites with aerosil A380 is investigated by broadband dielectric spectroscopy, while thermal analysis and infrared spectroscopy were applied to characterise the molecular species. The work completes preliminary results obtained for the members with n = 4 … 6. An interaction by hydrogen bonding, between aerosil surface – OH groups and – CN or ester groups of the CPnB molecules takes place. It slows down the relaxation process as observed for related composites in comparison to the pure materials. The existence of two types of bonding might be the reason that Vogel temperature for the relaxation process in the surface layer does not show the odd-even effect. Temperature dependence of the relaxation rates for composites shows a crossover behaviour from a high to a low temperature regime. Moreover, the temperature dependence of the dielectric strength is unusual. As the loading degree is similar, comparison of the dielectric, spectroscopic and thermal data obtained here and with the results obtained for the composites with n = 4 … 6 can be directly done. Increasing the number of the members of the homologous series confirms and hardens the preliminary conclusions.
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Fumed silica
Benzoates
Carbon fibers
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Mesomorphic Properties of A Homologous Series of 4-Cyanophenyl 3,4- Di(4-alkoxybenzoyloxy) benzoates
Abstract A new homologous series of 4-cyanophenyl 3,4-di(4-alkoxybenzoyloxy)benzoates (from ethoxy to decyloxy homologs) has been prepared. The homologs show both nematic and smectic A phases with fine schlieren and fan textures, respectively, whereas these have extremely large molecular breadth. The series is assumed to form an associated dimer and behave as a partially bilayer type smectic A phase. The dielectric properties have also been examined and the results support the formation of the associated dimer. The results have been discussed in terms of the geometry of the molecules.
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Benzoates
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Abstract Some derivatives of the homologous series of 4-cyanophenyl-3-methyl-4-(4-n-alkoxycinnamoyloxy) benzoates and of 4-cyanophenyl-4-(4-n-alkoxy-α-methyl cinnamoyloxy) benzoates have been synthesises. The higher homologs of these series exhibit reentrant nematic phases at atmospheric pressure.
Benzoates
Homologous series
Reentrancy
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