Comparative study of the phase behavior of liquid-crystalline components in closely related blends and copolyesters

Phase behavior of blends of a liquid-crystalline (LC) polymer with a non-LC polymer and of a series of copolymers containing mesogenic and nonmesogenic units was studied by thermal, optical, and dynamic mechanical methods. The polymers composing the blends and the copolymers had the same constituent monomers. The blends exhibited phase separation over the whole range of compositions studied as observed by DSC and dynamic mechanical analysis. Two glass transition temperatures (T_g) corresponding to the two components and independence of melting (T_m) and isotropization temperatures (T_i) to changes in composition were observed for the blends. The copolymers did not show phase separation over most of the composition range studied. Only one T_g corresponding to that of the major component could be detected for the copolymers, and the T_g was found to increase with an increase in the amount of nonmesogenic monomer in the copolymers. The difference in phase behavior was explained on the basis of the chemical environment of the constituent units in the blends and in copolymers. Phase inversion in the blends was observed by microscopy when the blends contained 60 mol% or more of the non-LC polymer.     

Formation of highly caged compounds through Diels—Alder cycloaddition of 3-bromo-7-(bromomethyl)tetracyclo[5.3.1.02,6.04,8]undeca-10(12)-ene-9,11-dione with itself and with cyclopentadiene

Diels–Alder reaction of the cage compound2 with itself, leading to the highly caged compound3, as well as with cyclopentadiene, leading to compounds 4 and5, is described.     

Catalytic activity of anti-ground state antibodies,antibody subunits,and human autoantibodies

Catalytic antibodies may be produced over the natural course of antibody-affinity maturation by placement of chemically reactive residues in antibody-active sites by somatic hypermutation or V-D-J-gene rearrangement. This hypothesis has received support from recent observations on the chemical reactivity of antibodies to vasoactive intestinal peptide (VIP), DNA, and steroid-and dinitrophenyl-esters. Recent studies reveal that monoclonal antibodies raised against the ground state of VIP can accelerate the cleavage of peptide bonds. The light-chain (L-chain) subunit of human autoantibodies display increased hydrolytic rate and diminished VIP-binding affinity compared to the parent antibody, consistent with increased turnover owing to weaker binding of the substrate ground state. These observations reveal an essential limitation of catalytic antibodies, i.e., large turnover rates may be associated with diminished substrate specificity. The hydrolysis of VIP by IgG purified by affinity chromatography from asthma patients and nonasthmatic controls was compared. IgG from the majority of asthma patients displayed VIP-hydrolyzing activity. V_max values for IgG from asthmatics tended to be higher than those from the nonasthmatic group. In principle, catalysis by antibodies may be an important mediator of immunological defense, regulation, and autoimmune dysfunction. The verification of these possibilities will require studies that utilize efficient assays of antibody catalysis during experimental immunization and autoimmune disease, as well as mechanistic investigation of catalysis by antibodies and their subunits.     

Surface complexation modeling of zinc sorption onto ferrihydrite

A previous study involving lead(II) [Pb(II)] sorption onto ferrihydrite over a wide range of conditions highlighted the advantages of combining molecular- and macroscopic-scale investigations with surface complexation modeling to predict Pb(II) speciation and partitioning in aqueous systems. In this work, an extensive collection of new macroscopic and spectroscopic data was used to assess the ability of the modified triple-layer model (TLM) to predict single-solute zinc(II) [Zn(II)] sorption onto 2-line ferrihydrite in NaNO(3) solutions as a function of pH, ionic strength, and concentration. Regression of constant-pH isotherm data, together with potentiometric titration and pH edge data, was a much more rigorous test of the modified TLM than fitting pH edge data alone. When coupled with valuable input from spectroscopic analyses, good fits of the isotherm data were obtained with a one-species, one-Zn-sorption-site model using the bidentate-mononuclear surface complex, (triple bond FeO)(2)Zn; however, surprisingly, both the density of Zn(II) sorption sites and the value of the best-fit equilibrium "constant" for the bidentate-mononuclear complex had to be adjusted with pH to adequately fit the isotherm data. Although spectroscopy provided some evidence for multinuclear surface complex formation at surface loadings approaching site saturation at pH >/=6.5, the assumption of a bidentate-mononuclear surface complex provided acceptable fits of the sorption data over the entire range of conditions studied. Regressing edge data in the absence of isotherm and spectroscopic data resulted in a fair number of surface-species/site-type combinations that provided acceptable fits of the edge data, but unacceptable fits of the isotherm data. A linear relationship between logK((triple bond FeO)2Zn) and pH was found, given by logK((triple bond FeO)2Znat1g/l)=2.058 (pH)-6.131. In addition, a surface activity coefficient term was introduced to the model to reduce the ionic strength dependence of sorption. The results of this research and previous work with Pb(II) indicate that the existing thermodynamic framework for the modified TLM is able to reproduce the metal sorption data only over a limited range of conditions. For this reason, much work still needs to be done in fine-tuning the thermodynamic framework and databases for the TLM.     

Influence of the anion on polyaniline

Polyaniline exists in various forms distinguished by the number of protons bonded to nitrogen, which determines the degree of oxidation of the polymer. At a macroscopic scale, doping involves electron transfer as well as protonation of the nitrogen heteroatom, which stabilises localised bound states to impart electronic conductivity to the system. However, it has been observed that the processability of this intractable polymer is greatly influenced by the anion associated with the protonating agent. Here we briefly summarise the influence of the anion on the behaviour of polyaniline. Received: 27 May 1997 / Accepted: 15 August 1997     

Second Molecular Hyperpolarizability of Mixed Transition Metal, Non-Metal Clusters

The real and imaginary parts of third order nonlinear optical susceptibilities ( ⁽³⁾) of 10^–4M solutions of [{Fe₂(CO)₆}( ₃-Y₃P){CpCr(CO)₂}] (Y=S (1) or Se (2)) and the well known precursor clusters [Fe₃(CO)₉( ₃-Y)₂] (Y=S (3) or Se (4)) in toluene were measured using Z-scan and ARINS techniques respectively. Compounds 1 and 2 possess nearly three times the _R values of their corresponding precursor compounds, 3 and 4. The results suggest a rich potential of mixed-metal, mixed-nonmetal class of clusters as materials exhibiting large nonlinearity.     

4,4′-Bipyridyl complexes of iron: Magnetic,Mössbauer and other spectroscopic studies

Summary 4,4-Bipyridyl (4,4-bipy) complexes of ferrous salts of the Fe(4,4-bipy)_x(anion)_y type (where x or y=1 or 2) and of ferric salts of the Fe(4,4-bipy)_m(anion)_n type (where m=1 or 2 and n=3) have been synthesised. Elemental analyses, i.r. and electronic spectra, magnetic and Mössbauer studies have been performed to characterize the complexes. 4,4-Bipy and some anions are inferred to act as bridging ligands. The magnetic moments, electronic and Mössbauer spectra suggest that the complexes are of high spin type with distorted octahedral structures. The value of the isomer shift and quadrupole splitting are discussed in terms of bonding of the ligand and anions.     

Ion-exchange equilibrium distribution studies of some metal ions in nitrite media

Ion-exchange equilibrium distribution of some metal ions has been studied in nitrite media, with cation- as well as anion-exchangers. The distribution coefficients (K(D)) show the utility of a new nitrite medium which is a complexing medium for most transition metal ions giving anionic complexes in aqueous solution. Similar studies have been performed in aqueous ethanol. Increase in the concentration of ethanol led to increased complex formation and changed K(D) values.     

Synthesis of Pt, Pd, Pt/Ag and Pd/Ag nanoparticles by microwave-polyol method

Pt, Pd, Pt-Ag and Pd-Ag bimetallic nanoparticles were synthesized in ethylene glycol and glycerol using the microwave technique in the presence of a stabilizer poly(N-vinylpyrrolidone) (PVP). It has been observed that PVP is capable of complexing and stabilizing nanoparticles. Mixed clusters were formed by simultaneous reduction of the metal ions. The clusters were characterized using UV-Vis spectra, XRD and dynamic light scattering. To understand the mechanism of formation of mixed nanoparticles, several experimental parameters such asin situ irradiation of mixed metal salts and mixing of individual sols were attempted.     

Preparation and electrochemical characterization of cation- and anion-exchange/polyaniline composite membranes

Composite membranes were prepared by chemical polymerization of a thin layer of polyaniline (PANI) in the presence of a high oxidant concentration on a single face of a sulfonated cation-exchange membrane (CEM) and quaternary aminated anion-exchange membrane (AEM). IR and SEM studies for both types of membranes confirmed PANI loading on the ion-exchange membranes. PANI composite ion-exchange membranes were characterized as a function of the polymerization time by ion-exchange capacity, coating density, and membrane conductance measurements. Membrane potential measurements were performed in various electrolyte solutions in order to observe the selectivity of these membranes for different types of counterions. Membrane potential data in conjunction with membrane conductance data was interpreted on the basis of frictional considerations between membrane matrix and solute. Electrodialysis experiments, using PANI composite ion-exchange membranes with 4 h polymerization time, were performed in single and mixed electrolyte solutions for observing electromigration of solute across PANI composite ion-exchange membranes. Relative dialytic rates of Na(2)SO(4), CaCl(2), and CuCl(2) were estimated with reference to NaCl on the basis of electrodialysis experiments and it was concluded that it is possible to separate different electrolytes using PANI composite ion-exchange membranes.