13C NMR investigations of molecular order of rod-like,bent-core,and thiophene mesogens

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy are described. Besides ¹³C chemical shifts, the ¹³C─¹H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the ¹³C─¹H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B₁ and B₂ mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state ¹³C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.     

Graphitic nanofillers in PMMA nanocomposites—An investigation of particle size and dispersion and their influence on nanocomposite properties

Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ～10 nm. Solution‐based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007     

Rod–coil block copolymers: An iterative synthetic approach via living free-radical procedures

A modular approach has been developed for the synthesis of rod–coil block copolymers involving the initial preparation of a macroinitiator based on the rod block followed by the growth of the coil segment with living free-radical procedures. The key feature of this strategy is the utilization of an alkoxyamine group from the beginning of the synthesis, which serves as a solubilizing group and ensures that each rod block contains a single initiating fragment. Using this approach permits block copolymers based on insoluble biphenyl ester oligomers to be conveniently prepared with coil segments that range from styrenes to acrylates to 1,3-dienes. The tendency of the rod segments to crystallize is strongly dependent on the weight fraction of the rod segment and the chemical nature of the coil segment. Rod–coil molecules containing at least 25–35 wt % polystyrene or poly(n-butyl acrylate) coil segments show a two-dimensional hexagonal arrangement of rod aggregates, as characterized by transmission electron microscopy and small-angle X-ray scattering. Polyisoprene block copolymers exhibit a lamellar microstructure with short rigid domains in which the rod units lie in an interdigitated smectic C arrangement. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3640–3656, 2003     

TDPAC studies of Hf doped YBCO

Time differential perturbed angular correlation measurements of the 133–482 keVγ-γ cascade of¹⁸¹Ta in Hf-doped YBa₂Cu₃O_7−x are presented. The¹⁸¹Hf precurser nuclei are incorporated into the sample by thermal neutron irradiation. Two quadrupole interaction frequencies are observed in the as-irradiated sample:v _Q1=161±10 MHz with intensityf ₁=75%, asymmetry parameterη ₁=0.32 and damping parameter Λ₁=0.42, andv _Q2=1108±40 MHz withf ₂=25%,η ₂=0.62, and Λ₂=0.60. On annealing the sample in air at various temperaturesT _a and quenching to room temperature,f ₁ remained nearly constant forT _a<600°C andv _Q1 for all annealing temperatures indicating that these are insensitive to oxygen stoichiometry. This frequency is interpreted to be due to¹⁸¹Hf substitutingY sites. BeyondT _a=600°C,f ₁ increased and reached a constant value of 90% forT _a=800°C. The value ofv _Q2 showed a slight variation between 1086 and 1160 MHz, whilef ₂ remained nearly constant at 25% forT _a<600°C. This component is identified to be due to¹⁸¹Hf substituting Cu 1 sites in the Cu-O chains of YBCO. Above 600°Cv _Q2 decreased and reached a value of 808 MHz beyond 750°C.     

Coulometric determination of NAD+ and NADH in normal and cancer cells using LDH, RVC and a polymer mediator

An electrochemical method for the measurement of NAD(+) and NADH in normal and cancer tissues using flow injection analysis (FIA) is reported. Reticulated vitreous carbon (RVC) electrodes with entrapped l-lactate dehydrogenase (LDH) and a new redox polymer containing covalently bound toluidine blue O (TBO) were employed for this purpose. Both NAD(+) and NADH were estimated coulometrically based on their reaction with LDH. The latter was immobilized on controlled pore glass (CPG) by cross-linking with glutaraldehyde and packed within the RVC. The concentrations of NAD(+) and NADH in the tissues, estimated using different electron mediators such as ferricyanide (FCN), meldola blue (MB) and TBO have also been compared. The effects of flow rate, pH, applied potential (versus Ag/AgCl reference) and adsorption of the mediators have also been investigated. Based on the measurements of NAD(+) and NADH in normal and cancer tissues it has been concluded that the NADH concentration is lower, while the NAD(+) concentration is higher in cancer tissues. Amongst the electron mediators TBO was found to be a more stable mediator for such measurements.     

Preparation and properties of novel polyhydrazides

A new class of polyhydrazides having aromatic as well as aliphatic units has been synthesized by low-temperature solution polycondensation from equimolar amounts of diacid chloride and dihydrazides in HMPT as solvent. These polyhydrazides have poor solubilities and their intrisnsic viscosities ranged from 0.2 to 0.6 dl/g. X-ray diffractograms of these polyhydrazides show that the polymers having terephthalic acid dihydrazide unit were more crystalline and all the polymers have densities in the range of 1.2-1.8 g/cm³. Thermogravimetric analysis of these polyhydrazides showed initial weight losses commencing at 220–300°C in both air and nitrogen.     

Kinetics of polymerization of acrylonitrile initiated by the Mn3+-thioacetamide redox system

Kinetics of the polymerization of acrylonitrile has been investigated in aqueous sulfuric acid in a temperature range of 25–45°C. The rate of polymerization (R_p) and the rate of managanic ion disappearance, etc., have been measured. The effect of the various additives, such as water-miscible organic solvents, neutral electrolytes, complexing agents, and surfactants, on the rate has been thoroughly studied. A mechanism that involves the initial complex formation between the thiol form of the thioamide and Mn³⁺, whose decomposition yields the initiating free radical with the polymer chain terminated by mutual combination of growing radicals, has been suggested.