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Comparative analysis of phase transition temperatures and rheological characteristics of p-n-alkyl- and p-n-alkyloxy-benzoic acids
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Biochemical sensors have important applications in biology, chemistry, and medicine. Nevertheless, many biochemical sensors are hampered by intricate techniques, cumbersome procedures, and the need for labeling. In the past two decades, it has been discovered that liquid crystals can be used to achieve the optical amplification of biological interactions. By modifying recognition molecules, a variety of label-free biochemical sensors can be created. Consequently, biochemical sensors based on the amplification of liquid crystals have become one of the most promising sensors. This paper describes in detail the optical sensing principle of liquid crystals, sensing devices, and optical detection technologies. Meanwhile, the latest research findings are elucidated. Finally, the challenges and future research directions are discussed.
We study the dynamics of a driven spherical colloidal particle moving in a fluid with a broken rotational symmetry. Using a nematic liquid crystal as a model, we demonstrate that when the applied force is not aligned along or perpendicular to the orientational order, the colloidal velocity does not align with the force, but forms an angle with respect to the pulling direction. This leads to blue an anisotropic hydrodynamic drag tensor which depends on the material parameters. In the case of nematic liquid crystal, we give an analytical expression and discuss the resulting implications for active microrheology experiments on fluids with broken rotational symmetry.
Thermotropic mesogens typically exist as liquid crystals (LCs) in a narrow region of high temperatures, making lowering their melting point with the temperature expansion of the mesophase state an urgent task. Para-substituted benzoic acids can form LCs through noncovalent dimerization into homodimers via hydrogen bonds, whose strength and, consequently, the temperature region of the mesophase state can be potentially altered by creating asymmetric heterodimers from different acids. This work investigates equimolar blends of p-n-alkylbenzoic (kBA, where k is the number of carbon atoms in the alkyl radical) and p-n-alkyloxybenzoic (kOBA) acids by calorimetry and viscometry to establish their phase transitions and regions of mesophase existence. Non-symmetric dimerization of acids leads to the extension of the nematic state region towards low temperatures and the appearance of new monotropic and enantiotropic phase transitions in several cases. Moreover, the crystal–nematic and nematic–isotropic phase changes have a two-step character for some acid blends, suggesting the formation of symmetric and asymmetric associates from heterodimers. The mixing of 6BA and 8OBA most strongly extends the region of the nematic state towards low temperatures (from 95–114 °C and 108–147 °C for initial homodimers, respectively, to 57–133 °C for the resulting heterodimer), whereas the combination of 4OBA and 5OBA gives the most extended high-temperature nematic phase (up to 156 °C) and that of 6BA and 9OBA (or 12OBA) provides the existence of a smectic phase at the lowest temperatures (down to 51 °C).
We have carried out phase polymorphism studies of 4- n -hexadecyloxybenzoic acid (16OB) in temperature range: from 10 to 150 °C with the use of Differrential Scanning Calorimeter, Thermal Microscopic Polarization and Transmitted Light Intensity. 16OB is a liquid crystal which has smectic C phase (SmC) and rich polymorphism in solid phase. Particularly interesting is one of the transitions in solid state possessing characteristic clear exothermic effect (during heating cycle) which is associated with the transition between the metastable and stable phases. Fourier transform infrared spectroscopy as a function of temperature was used to analyze changes in the nature of hydrogen bonds. The exact analysis of effects shows an existence of the metastable phases. The stable and metastable phases differ from each other in the structure made of hydrogen bonds.
The aim of this paper was to analyze the structure, vibrations and to do global analyses of molecules of p-n-alkyl benzoic acid (nBAC). The energy, IR, and Homo-Lumo optimised parameters were calculated using a density functional method. The global reactivity descriptors of molecules, including electro-negativity, electron affinity, ionisation potential, global softness, chemical potential, and energy gaps is further shown by band gap value drops. It offers important details on the stability of nBAC molecules (n=4,5,6,7,8,9). It has been demonstrated that the molecular series displays the energy of an isolated molecule as the length of the alkyl chain rises. The molecular series is useful for insulating applications since it also has a high band gap.
In this paper, Legendre wavelet transform–based texture analysis is proposed to study the phase transitional characteristics of hydrogen-bonded liquid crystals. Optical textures are the essential features of liquid crystals and analysis of these textural features is useful to investigate the material characteristics. Hydrogen-bonded liquid crystals: alkyl benzoic acid: p-(p′-Octyloxybenzylidene)-p-cyano aniline (nBA:OBCA, n = 6, 7, 8) are considered for investigation. Intermolecular interactions between the mesogen p-n- alkyl benzoic acid (nBA) and mesogenic compound p-(p′-Octyloxybenzylidene)-p-cyano aniline (OBCA) yield the hydrogen-bonded liquid crystals nBA:OBCA. Here, Legendre moments are computed from the wavelet-transformed optical texture using MATLAB computational tool. As a function of temperature, the phase transformations of liquid crystal from crystalline phase to isotropic liquid phase and vice versa can be observed as series of liquid crystal textures using polarizing optical microscope. All these textures show some variations in their features with respect to temperature and most of these variations can be observed as sudden or abnormal changes in transmitted intensity values of textures. They influence the moment values which are computed from the gray level transmitted intensities. Legendre wavelet transform captures the texture information in both frequency and spatial domains. And analysis of this information is useful to study phase behavior of liquid crystals. The results of the proposed methodology are in good agreement with thermal techniques available in the literature.
Graphene is the strongest known material. However, the challenge of translating that strength from the microscale to the more useful macroscale remains unmet. Preparing solid structures from self-assembled graphene oxide liquid crystals has allowed the creation of paper and fibers with excellent mechanical properties. Conventionally, vacuum filtration, wet spinning, and freeze-drying are used to prepare such structures from graphene oxide liquid crystals. Here, we introduce photocuring as an additional option to create solid structures of self-assembled graphene oxide liquid crystals that allows for thicker samples and other shapes to be realized. The photocured graphene oxide paper prepared here exhibited mechanical properties comparable to those of benchmark samples prepared by vacuum filtration.
New 1:2 liquid crystalline supramolecular H-bonded complexes (SMHBCs) were synthesized through double H-bond interactions between 4-(nicotinoyloxy) phenyl nicotinate as the base component and two molecules of 4-n-alkoxybenzoic acid (An). The base component was expected to be in two conformers according to the orientation of the N atom and the carboxylate group, syn conformer (I) and anti-conformer (II). DFT calculations revealed that only one of the two possible conformers of I exists, and the addition of the two molecules of the alkoxy acids (An) did not affect its conformation. The mesomorphic properties of all of the prepared complexes (I/An), bearing different terminal flexible alkoxy chains were investigated, and the formation of the H-bonds were confirmed by differential scanning calorimetry (DSC), and the phases were identified by polarized optical microscopy (POM), and FT-IR spectroscopy. Highly thermally stable mesophases possessing broad temperature ranges were observed for all investigated complexes compared to their individual components. Depending on the length of the terminal flexible alkoxy chain, the prepared SMHBCs were shown to exhibit di- or tri-morphic enantiotropic mesophases. The effect of replacing one of the –COO– connecting units by an azo group (-N=N-) in the basic molecule (I), on the mesomorphic properties has been investigated experimentally (via DSC) and theoretically (via DFT). The DFT calculations revealed that the polarizability, the dipole moment, and the aspect ratio of the investigated SMHBCs are lower than those of their corresponding ester/azo analogs. All these factors rationalize the enhanced smectic mesophase ranges of the complexes compared with those of the ester/azo analogs. The high aspect ratios and dipole moments of the SMHBCs of the azo derivative enforces the lateral intermolecular attraction that permits the formation of the more ordered smectic C mesophase with respect to the enhanced polymorphic mesophases of the diester derivative.
New geometrical architectures of chair- and V-shaped supramolecular liquid crystalline complexes were molded through 1:1 intermolecular hydrogen bonding interactions between 4-(4-(hexyloxy)phenylazo)methyl)phenyl nicotinate and 4-alkoxybenzoic acids. The length of terminal alkoxy acid chains varied, n = 6 to 16 carbons. The mesomorphic behaviour of these complexes was examined through differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fourier-transform infrared spectroscopy (FT-IR) was carried out to confirm the presence of Fermi bands that appeared for the hydrogen bonding formation. Enantiotropic nematic phases were observed and covered all lengths of alkoxy chains. The geometrical structures of the prepared supramolecular complexes geometries were estimated by Density functional theory (DFT) calculations. The supramolecular complexes I/An are projected to exhibit a nonlinear geometry with V-shaped and chair-shaped geometry. The chair-shaped conformers of I/An were found to be more stable than V-shaped isomeric complexes. Moreover, the effect of the change of the mesogenic core on the mesophase thermal stability (TC) has been investigated by a comparative study of the present azo supramolecular H-bonding LCs (SMHBCs) I/An and our previously reported their Schiff base analogue complexes, II/An. The findings of the DFT illustrated the high impact of CH=N as a mesogenic core on the mesomorphic behavior in terms of the competitive lateral and terminal intermolecular interactions as well as the molecular electrostatic potential (MEP).
The review summarizes the literature data and the authors’ own research results on the application of liquid crystals in tribology. It has been shown that both thermotropic (calamitic, discotic, cholesteric) and lyotropic (surfactants, chromonics) mesogens as tribological additives are able to optimize the properties of lubricating compositions when introduced even at low concentrations to oils and greases. A wide possibility of varying the chemical structure of mesogens and studying the relationship between their structure and tribological properties can be used for the desired (programmed) change of the quality of tribotechnical processes. The synergism of the combined use of mesogenic esters of cholesterol and carbon nanostructures as additives in improving tribological properties has been established. The use of synthetic lubricants in biological systems still requires further research as the experimental results obtained on models of joint prostheses in vitro conditions are significantly worse than the results obtained in vivo. Considering the annual loss of billions of US dollars worldwide due to the low efficiency of friction processes in the industry and the resulting wear, liquid crystals and the systems based on them can be the most effective way to optimize these processes. The present review will be useful for researchers and industrialists.
Bistable smectic-A (SmA) liquid crystal display (LCD) is one of the most promising devices for smart glass applications due to long-term bistability, low haze at clear state, low transmittance at scatter state and low power consumption. The need of a good simulation model for an LCD becomes apparent during the design of driving system. Liquid crystal (LC) capacitance is critical in the simulation of LCD pixels and is voltage dependent due to the crystal characteristics. Bistable smectic-A LC capacitance model is introduced in the paper. The study describes the most relevant conclusions obtained from the measurements of electric properties of bistable SmA LCD samples and electric equivalent circuit characterization.
Nowadays liquid crystal display (LCD) is an integral part of humans’ everyday life. High demand for new and innovative LCD products force LCD industry to develop and implement new types of LCDs. Bistable smectic-A (SmA) LCD is one of the most promising devices for smart glass applications due to long-term bistability, low haze at clear state, low transmittance at scatter state and low power consumption.
The study describes the most relevant conclusions obtained from frequency response testing and electric current measurements of bistable SmA LCD samples. Bistable LCDs have two states: opaque (light scattering state) and focal conic (transparent state). Switching between clear and scatter states and vice versa is a frequency dependant process. The conducted research on bistable SmA LC frequency response provides important knowledge about operation principles of the smart glass devices.
A suitable running-in process is advantageous for reducing friction. The aim of the present work was to study the influence of the running-in with acetylacetone on tribological performance of 4-Cyano-4’-pentylbiphenyl (5CB) liquid crystal. Friction tests were performed between steel surfaces in a ball-on-disk sliding system. After a running-in period of 240 s, the COF of 5CB was measured to be 0.013, which is about a quarter of the value (0.055) without running-in. The reduced contact pressure, caused in running-in process, does not directly lead to a drop in COF. The generation of tris(acetylacetonato) iron(III) induced by the tribochemical reactions between acetylacetone and steel surfaces, and the unique physical properties of liquid crystal are assumed to be reasons for the ultralow COF. Surface analysis was performed to correlate COF with the topography of wear surfaces. An evenly distributed specific grooved structure observed on wear area of the ball may have a beneficial effect on COF as well. We believe our findings can provide an effective and simple solution to reduce COF of liquid crystal between steel surfaces. A better understanding of the tribological behavior is needed for the development of this tribological system and for the possible future applications.
The lubricity of 4-acryloyloxyphenyl ester of 4'-n-octyloxybenzoic acid and the system composed of p-npropyloxybenzoic acid and p-n-propyloxy-p'-cyanobiphenyl in the conditions of boundary friction was investigated. The studies were carried out both for pure substances and their solutions in the liquid paraffin base. It was established that the studied additives possess good tribological properties. The tribological activity is defined by chemical structure and concentration of the additive in solution. The presence of the triboactive additives in lubricant stabilizes tribological properties of the lubricant on heating.
Two novel intermolecular hydrogen bonding mesogenic series comprising two distinct structural iso mers nABA:R:nABA and «ABA:H:«ABA (where R = resorcinol; H = hydroquinone; nABA = p-n-al-kyloxybenzoic acid with the alkyl carbon number varying from propyl-to decyl-and dodecyl-) have been synthesized using resorcinol and hydroquinone as central bent and linear cores, respectively. The thermal and phase behaviour of these complexes is studied by thermal microscopy and differential scan ning calorimetry. The IR spectral data confirm the formation of intermolecular hydrogen bonding between the OH group of the non-mesogen and -COOH group of the nABA moiety. Thermal studies reveal the existence of new phases: smectic-A and smectic-G. Comparative phase transition studies suggest that these isomers exhibit a different trend of mesomorphism.
Nematic liquid crystals (NLC) that were traditionally applied for detection of surface inhomogeneities in non-biological objects are proposed as an instrument for the objective microscopic diagnosis of malignancies in oncological practice. The specific alignment of NLCs on different types of tissue is exploited, a previously unknown phenomenon. Malignant tissue decorated with NLC due induces a homeotropic alignment while on benign tissues nematic materials are oriented close to planar mode. The relationship between this phenomenon and the di erent surface tension of malignant and nonmalignant tissues is revealed and the physical nature of these new phenomena is discussed.
Phase transition temperatures of p,n-alkyloxy benzoic acids (nOBA, n = 3 to 10 and 12) are investigated basing on the textural image analysis of liquid crystal. The analysis is carried out by the computation of Legendre moments. Textures of the homeotropically aligned compounds are recorded as a function of temperature using POM in arthroscopic mode attached to the hot stage and high resolution camera. A recurrence formula is used to compute the liquid crystal textures based on Legendre polynomial. The discontinuities and fluctuations in the values of Legendre moments as a function of temperature are related to the phase transition temperatures of the sample. This method is successful in conforming or detecting the phase transition temperatures and the present findings are comparable with literature.
The magnetite (Fe3O4) nanoparticles (0.5 wt%) of size less than 20 nm doped in p-n-alkyl benzoic acids where n varies from heptyl (7) to nonyl (9) are prepared and the presence of Fe+3 is confirmed through UV-Visible spectrophotometer. Textural and phase transition temperature studies are carried out using polarizing optical microscopy on pure and nano doped p-n-alkyl benzoic acids. These results are further confirmed by DSC at a scan rate of 5ºC/min and dielectric studies. Dielectric studies are carried out, in which the variation of dielectric constant, loss and the conductivity are analyzed with respect to temperatures and frequencies. Increment of relaxation times for nano doped heptyl and nonyl benzoic acids are observed which implies that the dielectric nature is strengthened for the nano doped mesogens. The preference of nano doped p-n-alkyl benzoic acids is discussed.
This paper presents a method which combines the statistical analysis with texture structural analysis called Local Binary Gray Level Cooccurrence Matrix (LBGLCM) to investigate the phase transition temperatures of thermotropic p , n -alkyloxy benzoic acid ( n OBA, n = 4,6 , 8,10 and 12) liquid crystals. Textures of the homeotropically aligned liquid crystal compounds are recorded as a function of temperature using polarizing optical microscope attached to the hot stage and high resolution camera. In this method, second-order statistical parameters (contrast, energy, homogeneity, and correlation) are extracted from the LBGLCM of the textures. The changes associated with the values of extracted parameters as a function of temperature are a helpful process to identify the phases and phase transition temperatures of the samples. Results obtained from this method have validity and are in good agreement with the literature.
The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids indicates the homologue number). The homologues with n ranging from 7 to 13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originated from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.
The retention behavior of various organic probes on the 4-(Nonyloxy) benzoic acid liquid crystal, which is used as a stationary phase, was investigated using the inverse gas chromatography method at infinite dilution. The thermodynamic parameters including the Flory-Huggins parameter, equation-of-state interaction parameter, the mole fraction activity coefficient, the effective exchange energy parameter, and residual thermodynamic parameters were determined in the temperature range of 423.15-433.15 K by using the retention behavior of the probes on the liquid crystal. It was determined from the thermodynamic parameters that all probes were poor solvents for the liquid crystal. Besides, the results of the 4-(Nonyloxy) benzoic acid liquid crystal was compared with a liquid crystal in the literature, and the effect of the number of alkyl groups on the liquid crystals on the Flory-Huggins interaction parameter and isomer separation was evaluated.
Electrical, thermal, and spectral characterizations of ferroelectric liquid crystal obtained from the precursors namely camphoric acid and heptyloxy benzoic acid abbreviated as CA + 7BAO. This mesogen exhibits two phases smectic C* and smectic G* in its exothermic run. DSC thermograms reveals the phase transition temperatures and enthalpy values of those phases. The spectral information recorded through Fourier transform infrared spectroscope reveals the possession of hydrogen bond. An interesting feature of this work lies in realizing the constant current device with variation to both temperature as well as potential. The same observation shall be utilized for the sensitive biomedical instruments where the current rating above few µA increment do have significant effect. Furthermore, the research work also reveals the information about the linearity of the thermo-electric graph with respect to phase transition temperatures. Thermoelectric Plot.
Two series of hydrogen-bonded liquid crystals (HBLC) were investigated: 4-alkoxy-2,3-difluorobenzoic acids (nOBAFF) with alkoxy chain lengths of n = 7–12 carbon atoms and mixtures of nOBAFF and 4-octyloxybenzoic acid (8OBA). Their phase behavior has been established using differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. At variance with non-fluorinated and monofluorinated homologues, the difluorinated nOBAFF compounds only form SmA and SmC liquid crystalline phases. An additional nematic phase is obtained upon mixing nOBAFF and 8OBA, showing the sequence: Isotropic liquid–Nematic–SmA–SmC–crystalline solid upon cooling from the isotropic phase. All the transition temperatures decrease with increasing alkoxy chain length of nOBAFF, which reveals a decrease of order parameter for longer chains. The dielectric and electro-optic behaviors of the nematic phase of nOBAFF/8OBA mixtures reveal a decrease of the threshold voltage and response time upon increasing the alkyl chain length of nOBAFF.
The SAR of 4-hydroxybenzaldehyde inhibitors of the trypanosomal alternative oxidase (TAO), a critical enzyme for the respiration of bloodstream forms trypanosomes, was investigated. Replacing the aldehyde group with a methyl ester resulted in a 10-fold increase in TAO inhibition and activity against T. brucei. Remarkably, two analogues containing the 2-hydroxy-6-methyl scaffold (9e and 16e) displayed single digit nanomolar TAO inhibition, which represents the most potent 4-alkoxybenzoic acid derivatives described to date. 9e was 50-times more potent against TAO and 10-times more active against T. brucei compared to its benzaldehyde analogue 23. The farnesyl derivative 16e was as potent a TAO inhibitor as ascofuranone with IC50 = 3.1 nM. Similar to ascofuranone derivatives, the 2-hydroxy and 6-methyl groups seemed essential for low nanomolar TAO inhibition of acid derivatives, suggesting analogous binding interactions with the TAO active site.
A series of thermotropic hydrogen-bonded liquid crystalline structures based on 4,4′-bipyridyl and aliphatic carboxylic acids was prepared by a mechanosynthesis technique. This series was characterised by polarising optical microscope, differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction and 1H NMR relaxometry experimental techniques. In these complexes, the bipyridyl component, a non-mesogenic substance by itself, acts as a double H-bond acceptor, whereas the alkylbenzoic acid acts as a H-bond donor, in a 1:2 proportion. The so-formed complexes exhibit mesophases that are not observed by the single components. A characteristic phase (smectic A) is identified and shown to be affected by the alkyl chain length. The isotropisation temperature is increased by the supramolecular aggregation through the H-bonds.
Lodge's molecular network theories are quite successful in describing the linear viscoelastic behavior of polymer solutions and melts, but cannot account for the rate‐of‐strain dependence of various material functions. By allowing the junction‐creation rate and the probability of loss of junctions to depend on the second invariant of the rate‐of‐strain tensor, more realistic constitutive equations were obtained. Two rheological models are proposed by assuming two different mechanisms for the effect of the rate of strain on the kinetics of the network. The experimental data on three fluids (representative of eight viscoelastic fluids) are used to test the models in various flow situations. For steady simple shearing and small‐amplitude, sinusoidal simple shearing, both model A and model B are capable of fitting the four functions η, −(τ 11 −τ 22 ), η′, and G′ rather well over many decades of shear rate or frequency. For suddenly changing flow experiments model A is inadequate. Model B however appears to be the only rheological equation which can fit simultaneously the steady shear, complex viscosity, stress growth, and stress relaxation functions. For stress growth, the agreement with the experimental data is remarkable, especially after the other models were shown to fail drastically. Finally, an interpretation of the stress growth and relaxation phenomena is given in the light of the modified theory.
The viscosities of three benzoic acid derivatives (p-n-heptyloxy-, p-n-decyloxy-, and p-n-dodecyloxy-) were measured on a unique viscometer of the class of CS-rheometer-viscometers with controlled shear stress over
the whole temperature range of the liquid crystalline state. Shear rates were calculated and flow and viscosity curves constructed
from the experimental shear stress values taking into account the Rabinovich-Moony correction. The smectic and nematic phases
were characterized by non-Newton and Newton viscosities, respectively, in all the samples studied. The activation parameters
of viscous flow were calculated for Newton viscosity. The results are discussed in terms of intermolecular interactions and
structural peculiarities of liquid crystalline phases.
The alkoxy benzoic acids, above n-propyl, exhibit liquid crystal properties. The homologous series of acids from n-propyl to n-octadecyl has been synthesized, the transition temperatures of each member of the series measured by means of a heated-stage, polarizing microscope, and the corresponding transition energies determined by differential scanning calorimetry. Some of the acids showed solid state transitions not previously reported.
Alkoxy benzoic acids are well-known liquid-crystalline compounds with nematic phase of long thermal ranges. The refractive indices measurements are carried out using a wedge-shaped cell with the help of a modified spectrometer. The birefringence (δn = ne − no) is obtained directly by using the method developed by Kuczynski et al. [1010.
Kuczynski , W. , Zywucki , B. , & Malecki , J. ( 2002 ). Mol. Cryst. Liq. Cryst. , 381 , 1 . [Taylor & Francis Online], [Web of Science ®]View all references] in all the available compounds (alkoxy chain number n = 3 to 12 and 16). The higher homologues exhibit the smectic-C phase along the nematic phase. The molecular polarizabilities are calculated from the refractive indices and the density using the well-known internal field models. The molecular anisotropy is estimated from the Lippincott δ-function model and molecular vibration method. The order parameter, S, is estimated (a) using molecular polarizabilities and the molecular anisotropy for both the models, (b) from Haller's extrapolation using molecular polarizabilities, and (c) directly from the birefringence, a method developed by Kuczynski et al. [1010.
Kuczynski , W. , Zywucki , B. , & Malecki , J. ( 2002 ). Mol. Cryst. Liq. Cryst. , 381 , 1 . [Taylor & Francis Online], [Web of Science ®]View all references] along with the nematic phase. Further, the order parameter from δn, without the consideration of any internal field model, to the nematic molecule is compared with S obtained from density due to Maier et al. [2323.
Maier , W. , & Saupe , Z. ( 1960 ). Z. Naturforsch , 15a , 287 . View all references]. The results are compared with one another and the advantages and disadvantages are discussed.
The crystal structure of 4-hexylbenzoic acid C6H13-C6H4-COOH, which forms a nematic mesophase upon melting, is determined. The crystal contains three crystallographically independent
molecules. Their molecular skeletons are made up of two almost planar fragments: a benzene ring, π-conjugated with the carboxyl
group and a planar zigzag aliphatic fragment. One of the independent molecules forms centrosymmetric dimers via pairs of hydrogen
bonds between carboxyl groups, whereas the two others are linked via hydrogen bonds. The dimers in the crystal are packed
into pseudostacks with a pronounced nonparallel arrangement of conjugated fragments. There is no good mutual projecting of
benzene rings in the stacks, which corresponds to efficient π-stacking interaction. The graph describing the mesophase of
this compound contains only one structure-forming element (a hydrogen bond) and corresponds to the nematic mesophase.
Thermal and extensive rheological characterization of a nematic liquid crystal gelated with a novel monodisperse dipeptide, also a liquid crystal, has been carried out. For certain concentrations, the calorimetric scans display a two-peak profile across the chiral nematic-isotropic (N*-I) transition, a feature reminiscent of the random-dilution to random-field crossover observed in liquid crystal gels formed with aerosil particles. All samples show shear thinning behavior without a Newtonian plateau region at lower shear rates. Small deformation oscillatory data at lower frequencies exhibit a frequency dependence of the storage (G') and loss (G'') moduli that can be described by a weak power-law, characteristic of soft glassy rheological systems. At higher frequencies, while lower concentration composites have a strong frequency dependence with a trend for possible crossover from viscoelastic solid to viscoelastic liquid behavior, the higher-concentration gels show frequency-independent rheograms of entirely elastic nature G' > G''. The plateau modulus of G' is described by a power-law with an exponent again common to soft materials, such as foams, slurries, etc. Other features which are a hallmark of such materials observed in the present study are: (i) above a critical strain, a strain softening of the moduli with a peak in the loss modulus, (ii) power-law variation of the storage modulus in the nonlinear viscoelastic regime, and (iii) absence of Cox-Merz superposition for the complex viscosity. An attractive feature of these gels is the fast recovery upon removal of large strain and qualitatively different temporal behavior of the recovery between the low and high concentration composites, with the latter indicating the presence of two characteristic time scales.
We propose a new theoretical method to study hydrogen-bonded supramolecular liquid crystals. It is based on our recent theory of associating polymer solutions combined with McMillan's molecular theory of liquid crystallization. We derive phase diagrams of supramolecular liquid crystals consisting of two different species of molecules, such as dimers and trimers. In such phase diagrams, liquid crystallization and two-phase separation coexist. There are two different types of phase separation: one is caused by the first-order transitions of liquid crystals, and the other is demixing by repulsive interaction between the different species of molecules. As a result, multicritical phenomena such as tricritical. point, Flory's chimney type two-phase region, etc., appear. Comparison of phase diagrams of dimerized supramolecular liquid crystals with those of corresponding flexible polymers suggests that a new microphase separation transition may occur inside the layers of the smectic A phase.
Rheological studies in p-n-alkoxy benzoic acid liquid crystals
- Sreehari Sastry
- S Bindu Madhavi
- A Vishwam
- T Ha Sie Tiong
Sreehari Sastry S., Bindu Madhavi A., Vishwam T., Ha
Sie Tiong. Rheological studies in p-n-alkoxy benzoic
acid liquid crystals. IJERT. 2017, 6, 353-568.
https://api.semanticscholar.org/CorpusID:104339068.
Visco-elastic properties of low molar mass p-nalkoxybenzoic acid mesogens
- Sreehari Sastry
- S Bindu Madhavi
- A Ha Sie Tiong
Sreehari Sastry S., Bindu Madhavi A., Ha Sie Tiong.
Visco-elastic properties of low molar mass p-nalkoxybenzoic acid mesogens. IJIRSER. 2013, 3,11417-11424.