Dielectric relaxation studies on two systems exhibiting the induced smectic A phase

Recently we reported some physical studies (layer spacings using X-ray scattering, low frequency dielectric constants) on two systems exhibiting a maximum (indicating an induced smectic A) as well as a minimum in the A-N transition boundary. In this paper we report measurements of the dielectric relaxation of ε|| (up to 13 MHz) for a few compositions of the two systems. The activation energy in the A phase of the mixtures corresponding to the maximum in the A-N boundary is higher than that of the pure polar component. The results are discussed in relation to the molecular interactions giving rise to the induced smectic A phase

Experimental determination of the curvature-induced reduction in the smectic A-nematic transition poin

On the basis of an analogy between smectic A-nematic and superconductor-normal metal transitions, de Gennes has predicted that a twist or bend distortion should reduce TAN with respect to that of a curvature-free sample. We report measurements of the reduction of TAN on samples prepared in two different geometries. In the first, a magnetically induced twist distortion has been used to obtain preliminary data on 8 OCB and CBOOA samples. In the second, a wedge-shaped twisted nematic cell has been used to get quantitative data on the reduction of TAN of CBOOA as a function of twist distortion

Effect of skewed cybotactic structure on the dielectric constants and conductivities of some binary mixtures exhibiting the nematic phase

2-cyano-4-heptylphenyl-4'-pentyl-4-biphenyl carboxylate (7P(2CN)5BC), a nematogen with a cyano group making a large angle (~60°) with the long axis of the molecule, has a moderately strong negative dielectric anisotropy (Δε). A mixture of 85 mole per cent of 7P(2CN)5BC with 15% of 4-n-heptyl-4'-cyanobiphenyl (7CB) exhibits a reversal of the dielectric anisotropy Δε as the temperature is varied, becoming negative at higher temperatures. Δε of mixtures with higher concentrations of 7CB is positive throughout the nematic range. The conductivity anisotropy ΔS' of these systems has been investigated for a range of frequencies from 300 Hz to 30 kHz. In 7P(2CN)5BC, ΔS' changes sign becoming negative at higher temperatures for all frequencies investigated. The results are discussed in terms of the temperature variation of the tilt angle of the skewed cybotactic structure of 7P(2CN)5BC

Studies on binary mixtures of systems which exhibit a maximum as well as a minimum in the A-N transition boundary

We have studied the binary phase diagrams of (I) p'-nitrophenyl-p-n-octyloxybenzoate with (2-hydroxy)-p-ethoxybenzylidene-p'-butylaniline, and (II) p'-n-octyloxy-p-cyanobiphenyl with p-butoxyphenyl-(p'-pentyloxy)benzoate. Both of them show a strong maximum in the A-N boundary, indicative of an induced A phase, and further, a well defined minimum for a composition rich in the highly polar component. The interactions responsible for the induced phase are much stronger for the system II than for the system I. We have investigated the temperature variations of the layer spacings for various compositions of both the systems. In system I, there is a considerable thermal expansion of the spacing, the expansion coefficient attaining a maximum value for the composition corresponding to the minimum in the A-N boundary. We have given a physical interpretation of this observation. The dielectric anisotropies of various compositions of system I have also been investigated and their temperature dependences interpreted in terms of the physical model mentioned above. The conductivity anisotropy changes sign for all compositions exhibiting the A phase. The temperature of crossover in relation to TAN is the highest for the mixture which exhibits the minimum value of TAN, since the A-N transition is almost of a second order character for this composition

Paramagnetic nematic liquid crystals

Five nematogenic complexes, bis[1-(p-n-decyl-biphenyl) 3-(p-substituted phenyl) propane-1, 3-dionato]-copper (II), have been prepared. The mesophase, which occurs monotropically in all of them, is of the usual nematic type, but has paramagnetic properties. Magnetic, dielectric and electron paramagnetic resonance measurements are presented. A preliminary analysis of the data indicates the existence of antiparallel correlations in the nematic phase

Comparative X-ray and dielectric studies on some structurally related smectogenic compounds

In continuation of our earlier work on smectogenic compounds exhibiting large bilayer spacings, we report in this paper a comparative study of several compounds with lateral methyl or methoxy substituent. Some of the compounds do not have any strongly polar end groups while others have a cyano end group. The dielectric (both static and relaxation studies up to 13 MHz) and X-ray investigations on these compounds enable us to elucidate the physical mechanisms responsible for the observed differences in the properties of the compounds. We also discuss the possible origin of the often observed reduction of the activation energy corresponding to the ε|| -relaxation in the smectic A phase compared to that in the nematic phase

Studies on some smectogenic compounds with large bilayer spacings

In mesomorphic compounds with strongly polar end groups, near neighbor molecules have antiparallel correlations. This results in a bilayer structure which in turn leads to several interesting phenomena like reentrance and smectic polymorphism. After a brief review of the development of this subject, we report some unusual properties in several compounds with relatively large bilayer spacings (≥ 1.6 times the molecular length). For example, 4-n-decylphenyl-3'-methyl-4'-(4"-nitrobenzoyloxy)benzoate (10 PMNBB) shows a remarkably large expansion (~20%) of the bilayer spacing on cooling in the SA phase, the rate of expansion increasing with decrease of temperature. Further, the dielectric anisotropy reverses sign, becoming negative at lower temperatures. When the nitro group is replaced by a cyano group (10 PMCBB), (a) the rate of expansion of the bilayer spacing decreases with decrease of temperature, and (b) an SA - SA transition takes place with a jump of ~0.4 Å in the layer spacing at TAN - T ≈ 15°. We present results on several homologues of both the compounds as also on compounds with a lateral methoxy substituent

Insulin autoantibodies as determined by competitive radiobinding assay are positively correlated with impaired beta-cell function — The Ulm-Frankfurt population study

Out of a random population of 4208 non-diabetic pupils without a family history of Type I diabetes 44 (1.05%) individuals had islet cell antibody (ICA) levels greater or equal to 5 Juvenile Diabetes Foundation (JDF) units. 39 of these ICA-positives could be repeatedly tested for circulating insulin autoantibodies (CIAA) using a competitive radiobinding assay. The results were compared with the insulin responses in the intravenous glucose tolerance tests (IVGTT) and with HLA types. Six pupils were positive for CIAA. All of them had complement-fixing ICA, and 5 of them were HLA-DR4 positive. Three of the 6 showed a first-phase insulin response below the first percentile of normal controls. Our data indicate that in population-based studies CIAA can be considered as a high risk marker for impaired beta-cell function in non-diabetic ICA-positive individuals

Antibodies to the Mr 64,000 (64K) protein in islet cell antibody positive non-diabetic individuals indicate high risk for impaired Beta-cell function

A prospective study of a normal childhood population identified 44 islet cell antibody positive individuals. These subjects were typed for HLA DR and DQ alleles and investigated for the presence of antibodies to the Mr 64,000 (64K) islet cell antigen, complement-fixing islet cell antibodies and radiobinding insulin autoantibodies to determine their potency in detecting subjects with impaired Beta-cell function. At initial testing 64K antibodies were found in six of 44 islet cell antibody positive subjects (13.6%). The same sera were also positive for complement-fixing islet cell antibodies and five of them had insulin autoantibodies. During the follow-up at 18 months, islet cell antibodies remained detectable in 50% of the subjects studied. In all six cases who were originally positive, 64K antibodies were persistently detectable, whereas complement-fixing islet cell antibodies became negative in two of six and insulin autoantibodies in one of five individuals. HLA DR4 (p < 0.005) and absence of asparic acid (Asp) at position 57 of the HLA DQ chain (p < 0.05) were significantly increased in subjects with 64K antibodies compared with control subjects. Of 40 individuals tested in the intravenous glucose tolerance test, three had a first phase insulin response below the first percentile of normal control subjects. Two children developed Type 1 (insulin-dependent) diabetes mellitus after 18 and 26 months, respectively. Each of these subjects was non-Asp homozygous and had persistent islet cell and 64K antibodies. We conclude that 64K antibodies, complement-fixing islet cell antibodies and insulin autoantibodies represent sensitive serological markers in assessing high risk for a progression to Type 1 diabetes in islet cell antibody positive non-diabetic individuals