Bakir Timimi - Academia.edu (original) (raw)
Papers by Bakir Timimi
Liquid Crystals, 2018
On orientational order in nematic and twist-bend nematic phases: A 2 H-NMR study of binary mixtur... more On orientational order in nematic and twist-bend nematic phases: A 2 H-NMR study of binary mixtures of the odd dimer,1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane, (CB9CB), and the monomer, 4-pentyl-4′-cyanobiphenyl, (5CB-d2)
Chemischer Informationsdienst, Jan 25, 1972
Materials Today: Proceedings, 2018
The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipid... more The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids may be elucidated from the use of deuterium NMR (2 H-NMR) spectroscopy. The 2 H-NMR spectrum of a deuterated material (either selectively or fully) is rather simpler to interpret compared with the corresponding proton NMR (1 H-NMR). Here, we apply this technique to understand the effect of chain branching on orientational order of the glycolipid self-assembly by using deuterated probes. The 2 H-NMR resonance of n-octanol-d 17 in n-octyl-β-D-glucopyranoside (β-GlcOC 8) and the branched-chain glycolipid 2-hexyldecyl-β-Dglucopyranoside (β-GlcOC 10 C 6) were measured at different temperatures. We also measured the 2 H-NMR spectrum of 2-ethyl-nhexanol-α,α-d 2 , a branched-chain alcohol which is selectively deuterated at α-position in β-GlcOC 8. All systems gave quadrupolar splittings in their respective 2 H-NMR spectra, which are attributed to the formation of anisotropic phase. The noctanol-d 17 dissolved in β-GlcOC 8 give us spectra in the lamellar phase and when used with a branched-chain lipid, i.e. β-GlcOC 10 C 6 , gave us new spectra in the inverse hexagonal phase. The magnitude of the quadrupolar splittings of the n-octanol-d 17 is indicative of the degree of orientational ordering in the given system. The results show that the quadrupolar splitting is larger in β-GlcOC 8 lipid than in β-GlcOC 10 C 6 .
Materials Today: Proceedings, 2018
The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipid... more The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids may be elucidated from the use of deuterium NMR (2 H-NMR) spectroscopy. The 2 H-NMR spectrum of a deuterated material (either selectively or fully) is rather simpler to interpret compared with the corresponding proton NMR (1 H-NMR). Here, we apply this technique to understand the effect of chain branching on orientational order of the glycolipid self-assembly by using deuterated probes. The 2 H-NMR resonance of n-octanol-d 17 in n-octyl-β-D-glucopyranoside (β-GlcOC 8) and the branched-chain glycolipid 2-hexyldecyl-β-Dglucopyranoside (β-GlcOC 10 C 6) were measured at different temperatures. We also measured the 2 H-NMR spectrum of 2-ethyl-nhexanol-α,α-d 2 , a branched-chain alcohol which is selectively deuterated at α-position in β-GlcOC 8. All systems gave quadrupolar splittings in their respective 2 H-NMR spectra, which are attributed to the formation of anisotropic phase. The noctanol-d 17 dissolved in β-GlcOC 8 give us spectra in the lamellar phase and when used with a branched-chain lipid, i.e. β-GlcOC 10 C 6 , gave us new spectra in the inverse hexagonal phase. The magnitude of the quadrupolar splittings of the n-octanol-d 17 is indicative of the degree of orientational ordering in the given system. The results show that the quadrupolar splitting is larger in β-GlcOC 8 lipid than in β-GlcOC 10 C 6 .
Preprints of symposium on liquid crystals, 2000
Preprints of symposium on liquid crystals, 2009
Liquid Crystals, 2017
One of the current challenges in liquid crystal science is to understand the molecular factors le... more One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (NTB) and determine its properties. During our earlier hunt for the NTB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the NTB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the 2H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase. Graphical Abstract
Liquid Crystals, 2016
Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N TB)... more Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N TB phase, has been also invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1",7"-bis-4-(4'-cyanobiphenyl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase.
日本液晶学会討論会講演予稿集, Oct 14, 2003
Journal of the Chemical Society B: Physical Organic, 1968
ABSTRACT
Physical Review E, 2011
The liquid-crystal dimer 1 ,7-bis(4-cyanobiphenyl-4-yl)heptane (CB7CB) exhibits two liquid-crysta... more The liquid-crystal dimer 1 ,7-bis(4-cyanobiphenyl-4-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and 2 H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.
Molecular Crystals and Liquid Crystals, 2003
Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phas... more Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phase of perdeuteriated 4-octyl-4'-cyanobiphenyl-d 25 (8CB-d 25 ) When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2000
Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric... more Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-α,α-d2-octyl-4′-cyanobiphenyl (8CB-d2), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment
Journal of the American Chemical Society, 2001
... 13) Hegmann, T.; Kain, J.; Diele, S.; Pelze, G.; Tschierske, C ... (16) Emsley, JW; Hamilton,... more ... 13) Hegmann, T.; Kain, J.; Diele, S.; Pelze, G.; Tschierske, C ... (16) Emsley, JW; Hamilton, K.; Luckhurst, GR; Sundholm, F.; Timimi, BA; Turner, DL ... Kwang-Un Jeong, Alexander J. Jing, Bart Mansdorf, Matthew J. Graham, Deng-Ke Yang, Frank W. Harris, and Stephen ZD Cheng. ...
The Journal of Chemical Physics, 1994
We have studied the molecular orientational order and director kinetics of the nematic phase form... more We have studied the molecular orientational order and director kinetics of the nematic phase formed by dissolving poly-p-phenylene-terephthalamide (PPTA) in sulphuric acid using deuterium NMR spectroscopy. Measurements were made on standard polymer samples using deuterated sulphuric acid and with deuterated PPTA in normal H2SO4. The influence of temperature and polymer concentration on the quadrupolar splittings which reflect the orientational order
Electrochimica Acta, 1974
The use of the temperature variation of the dissociation constant for evaluating changes in AC', ... more The use of the temperature variation of the dissociation constant for evaluating changes in AC', is reviewed with a view to establishing the degree of reliability of such values. AC", values obtained from dissociation constant measurements are compared, where possible, with the calorimetrically determined values. Significant differences are apparent, suggesting serious shortcomings in the indirect methods of evaluation used so far. The related question of the appropriate form of the pK(T) equation is also discussed. When the 'I most appropriate " form of pK(T), viz a five-constants equation is used with the accurate data of Ives for the dissociation of cyan-acetic acid, according to the method of Clarke and Glew, the results still retain large uncertainties on AC",. Model exact pK data can be calculated from known assumed AG", AH", AC",, first and second temperature derivatives of ACO, at a chosen reference temperature. By introducing variations of various magnitudes and signs on these model values, very accurate "experimental-like" pK values can be generated. When these accurate pK values are analysed on the computer, using the method of Clarke and Glew, and the results are compared, it becomes apparent that over the normal experimental range (0"~50°C or 60°C) even very small errors (ca 04lOO25 pK units) can lead to very large uncertainties on AC", (especially at the two extremes of the temperature range) exceeding in some cases 20 cal. The identification of the true AC",(T) becomes impossible. It is concluded that the dissociation constant method is not capable as yet of furnishing reliable AC', values; the extension of the temperature range to the neighborhood of 100°C and an increase in the degrees of freedom (es by measurements at 5" intervals) could combine to furnish fairly reliable AC', values. * AC, values are in cal/deg/mole. ' The same data of Harned and coworkers[l+] are analysed in each case. c This work. d The data were analysed using a procedure somewhat similar to that of Ives and Marsden[24]. 5 A cubic polynomial was used, with a standard deviation of &0.0004 pK units. 'A quartic polynomial had to be employed and even then the fitting of the pK data was not very satisfactory. A difference of 0*0025 pK unit between the calculated and the experimental value was retained at one temperature. s The two sets of AC", values under equations (2) and (4) refer each to an independent set of pK values of references 23 and 27 respectively. The difference between the two sets of pK values is within f0.002 pK units. ' The pK data of Bates et al.[8,91 and of Datta et aZ.[lO, 1 II were analysed according to equation (4). The difference between the two sets of data for each acid is within &O-O03 pK units except at one temperature where, for each acid, the difference is about 10-0055 pK units.
Current Applied Physics, 2006
ABSTRACT It is important, both for basic science and device applications, to investigate the stat... more ABSTRACT It is important, both for basic science and device applications, to investigate the static and dynamic director distribution in a thin nematic liquid crystal (NLC) slab confined between two transparent electrodes. Optical and electrical measurement methods are generally used to determine the director distribution as a function of an electric field. It is to be expected that deuterium NMR spectroscopy combined with simultaneous in situ observation of the light transmittance should give us a good understanding of the director distribution for a thin NLC slab because the former is the sum of the spectra for each position in the slab and the latter by the optical anisotropy averaged over the slab. This combined method is used to investigate the director distribution in a thin NLC slab of 4-pentyl-40-cyanobiphenyl subject to competing constraints.
Liquid Crystals, 2016
The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers havi... more The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers having odd spacers. Typically the phase is preceded by a nematic phase (N) via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)-N-I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9, and 11 methylene groups are found to possess the conventional phase sequence NTB-N-I; surprisingly for the propane spacer the NTB phase is formed directly from the isotropic phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2015
The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has... more The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has been investigated. The intermediate states during the relaxation process were found, with the aid of time-resolved deuterium NMR spectroscopy, to be markedly nonuniform. The macroscopic order was perturbed, although the initial and final states of the director appear to be essentially uniform. However, the initial state does have a profound influence on the uniformity of the director in the intermediate states. We have developed a fundamental model based on the effect of spontaneous director fluctuations to explain these unusual NMR observations.
Liquid Crystals, 2018
On orientational order in nematic and twist-bend nematic phases: A 2 H-NMR study of binary mixtur... more On orientational order in nematic and twist-bend nematic phases: A 2 H-NMR study of binary mixtures of the odd dimer,1″,9″-bis(4-cyanobiphenyl-4′-yl) nonane, (CB9CB), and the monomer, 4-pentyl-4′-cyanobiphenyl, (5CB-d2)
Chemischer Informationsdienst, Jan 25, 1972
Materials Today: Proceedings, 2018
The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipid... more The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids may be elucidated from the use of deuterium NMR (2 H-NMR) spectroscopy. The 2 H-NMR spectrum of a deuterated material (either selectively or fully) is rather simpler to interpret compared with the corresponding proton NMR (1 H-NMR). Here, we apply this technique to understand the effect of chain branching on orientational order of the glycolipid self-assembly by using deuterated probes. The 2 H-NMR resonance of n-octanol-d 17 in n-octyl-β-D-glucopyranoside (β-GlcOC 8) and the branched-chain glycolipid 2-hexyldecyl-β-Dglucopyranoside (β-GlcOC 10 C 6) were measured at different temperatures. We also measured the 2 H-NMR spectrum of 2-ethyl-nhexanol-α,α-d 2 , a branched-chain alcohol which is selectively deuterated at α-position in β-GlcOC 8. All systems gave quadrupolar splittings in their respective 2 H-NMR spectra, which are attributed to the formation of anisotropic phase. The noctanol-d 17 dissolved in β-GlcOC 8 give us spectra in the lamellar phase and when used with a branched-chain lipid, i.e. β-GlcOC 10 C 6 , gave us new spectra in the inverse hexagonal phase. The magnitude of the quadrupolar splittings of the n-octanol-d 17 is indicative of the degree of orientational ordering in the given system. The results show that the quadrupolar splitting is larger in β-GlcOC 8 lipid than in β-GlcOC 10 C 6 .
Materials Today: Proceedings, 2018
The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipid... more The structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids may be elucidated from the use of deuterium NMR (2 H-NMR) spectroscopy. The 2 H-NMR spectrum of a deuterated material (either selectively or fully) is rather simpler to interpret compared with the corresponding proton NMR (1 H-NMR). Here, we apply this technique to understand the effect of chain branching on orientational order of the glycolipid self-assembly by using deuterated probes. The 2 H-NMR resonance of n-octanol-d 17 in n-octyl-β-D-glucopyranoside (β-GlcOC 8) and the branched-chain glycolipid 2-hexyldecyl-β-Dglucopyranoside (β-GlcOC 10 C 6) were measured at different temperatures. We also measured the 2 H-NMR spectrum of 2-ethyl-nhexanol-α,α-d 2 , a branched-chain alcohol which is selectively deuterated at α-position in β-GlcOC 8. All systems gave quadrupolar splittings in their respective 2 H-NMR spectra, which are attributed to the formation of anisotropic phase. The noctanol-d 17 dissolved in β-GlcOC 8 give us spectra in the lamellar phase and when used with a branched-chain lipid, i.e. β-GlcOC 10 C 6 , gave us new spectra in the inverse hexagonal phase. The magnitude of the quadrupolar splittings of the n-octanol-d 17 is indicative of the degree of orientational ordering in the given system. The results show that the quadrupolar splitting is larger in β-GlcOC 8 lipid than in β-GlcOC 10 C 6 .
Preprints of symposium on liquid crystals, 2000
Preprints of symposium on liquid crystals, 2009
Liquid Crystals, 2017
One of the current challenges in liquid crystal science is to understand the molecular factors le... more One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (NTB) and determine its properties. During our earlier hunt for the NTB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the NTB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the 2H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase. Graphical Abstract
Liquid Crystals, 2016
Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N TB)... more Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N TB phase, has been also invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1",7"-bis-4-(4'-cyanobiphenyl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase.
日本液晶学会討論会講演予稿集, Oct 14, 2003
Journal of the Chemical Society B: Physical Organic, 1968
ABSTRACT
Physical Review E, 2011
The liquid-crystal dimer 1 ,7-bis(4-cyanobiphenyl-4-yl)heptane (CB7CB) exhibits two liquid-crysta... more The liquid-crystal dimer 1 ,7-bis(4-cyanobiphenyl-4-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and 2 H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.
Molecular Crystals and Liquid Crystals, 2003
Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phas... more Deuterium NMR spectroscopy has been used to investigate the director dynamics in the nematic phase of perdeuteriated 4-octyl-4'-cyanobiphenyl-d 25 (8CB-d 25 ) When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 2000
Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric... more Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the electric field-induced alignment of the director of the smectic A phase of the liquid crystal, 4-α,α-d2-octyl-4′-cyanobiphenyl (8CB-d2), at 303.3K. The electric field is arranged to be orthogonal to the magnetic field. The alignment process has been investigated at different electric field strengths and the rate of director alignment
Journal of the American Chemical Society, 2001
... 13) Hegmann, T.; Kain, J.; Diele, S.; Pelze, G.; Tschierske, C ... (16) Emsley, JW; Hamilton,... more ... 13) Hegmann, T.; Kain, J.; Diele, S.; Pelze, G.; Tschierske, C ... (16) Emsley, JW; Hamilton, K.; Luckhurst, GR; Sundholm, F.; Timimi, BA; Turner, DL ... Kwang-Un Jeong, Alexander J. Jing, Bart Mansdorf, Matthew J. Graham, Deng-Ke Yang, Frank W. Harris, and Stephen ZD Cheng. ...
The Journal of Chemical Physics, 1994
We have studied the molecular orientational order and director kinetics of the nematic phase form... more We have studied the molecular orientational order and director kinetics of the nematic phase formed by dissolving poly-p-phenylene-terephthalamide (PPTA) in sulphuric acid using deuterium NMR spectroscopy. Measurements were made on standard polymer samples using deuterated sulphuric acid and with deuterated PPTA in normal H2SO4. The influence of temperature and polymer concentration on the quadrupolar splittings which reflect the orientational order
Electrochimica Acta, 1974
The use of the temperature variation of the dissociation constant for evaluating changes in AC', ... more The use of the temperature variation of the dissociation constant for evaluating changes in AC', is reviewed with a view to establishing the degree of reliability of such values. AC", values obtained from dissociation constant measurements are compared, where possible, with the calorimetrically determined values. Significant differences are apparent, suggesting serious shortcomings in the indirect methods of evaluation used so far. The related question of the appropriate form of the pK(T) equation is also discussed. When the 'I most appropriate " form of pK(T), viz a five-constants equation is used with the accurate data of Ives for the dissociation of cyan-acetic acid, according to the method of Clarke and Glew, the results still retain large uncertainties on AC",. Model exact pK data can be calculated from known assumed AG", AH", AC",, first and second temperature derivatives of ACO, at a chosen reference temperature. By introducing variations of various magnitudes and signs on these model values, very accurate "experimental-like" pK values can be generated. When these accurate pK values are analysed on the computer, using the method of Clarke and Glew, and the results are compared, it becomes apparent that over the normal experimental range (0"~50°C or 60°C) even very small errors (ca 04lOO25 pK units) can lead to very large uncertainties on AC", (especially at the two extremes of the temperature range) exceeding in some cases 20 cal. The identification of the true AC",(T) becomes impossible. It is concluded that the dissociation constant method is not capable as yet of furnishing reliable AC', values; the extension of the temperature range to the neighborhood of 100°C and an increase in the degrees of freedom (es by measurements at 5" intervals) could combine to furnish fairly reliable AC', values. * AC, values are in cal/deg/mole. ' The same data of Harned and coworkers[l+] are analysed in each case. c This work. d The data were analysed using a procedure somewhat similar to that of Ives and Marsden[24]. 5 A cubic polynomial was used, with a standard deviation of &0.0004 pK units. 'A quartic polynomial had to be employed and even then the fitting of the pK data was not very satisfactory. A difference of 0*0025 pK unit between the calculated and the experimental value was retained at one temperature. s The two sets of AC", values under equations (2) and (4) refer each to an independent set of pK values of references 23 and 27 respectively. The difference between the two sets of pK values is within f0.002 pK units. ' The pK data of Bates et al.[8,91 and of Datta et aZ.[lO, 1 II were analysed according to equation (4). The difference between the two sets of data for each acid is within &O-O03 pK units except at one temperature where, for each acid, the difference is about 10-0055 pK units.
Current Applied Physics, 2006
ABSTRACT It is important, both for basic science and device applications, to investigate the stat... more ABSTRACT It is important, both for basic science and device applications, to investigate the static and dynamic director distribution in a thin nematic liquid crystal (NLC) slab confined between two transparent electrodes. Optical and electrical measurement methods are generally used to determine the director distribution as a function of an electric field. It is to be expected that deuterium NMR spectroscopy combined with simultaneous in situ observation of the light transmittance should give us a good understanding of the director distribution for a thin NLC slab because the former is the sum of the spectra for each position in the slab and the latter by the optical anisotropy averaged over the slab. This combined method is used to investigate the director distribution in a thin NLC slab of 4-pentyl-40-cyanobiphenyl subject to competing constraints.
Liquid Crystals, 2016
The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers havi... more The intriguing twist-bend nematic phase (NTB) is formed, primarily, by liquid crystal dimers having odd spacers. Typically the phase is preceded by a nematic phase (N) via a weak first-order transition. Our aim is to obtain dimers where the NTB phase is formed directly from the isotropic phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)-N-I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9, and 11 methylene groups are found to possess the conventional phase sequence NTB-N-I; surprisingly for the propane spacer the NTB phase is formed directly from the isotropic phase. The properties of these dimers have been studied with care to ensure that the identification of the NTB phase is reliable.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2015
The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has... more The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has been investigated. The intermediate states during the relaxation process were found, with the aid of time-resolved deuterium NMR spectroscopy, to be markedly nonuniform. The macroscopic order was perturbed, although the initial and final states of the director appear to be essentially uniform. However, the initial state does have a profound influence on the uniformity of the director in the intermediate states. We have developed a fundamental model based on the effect of spontaneous director fluctuations to explain these unusual NMR observations.