Polymer collapse in supercritical solvents

We show analytically that in dilute solutions of high molecular weight polymers, a collapse transition of the chain can be induced by proximity to the critical point of the solvent. The transition is driven by the fluctuations in the medium, which lead to an effective attractive interaction of long range between different parts of the polymer. At the critical point itself, however, the chain adopts the same average conformations that characterize its size in the off-critical limit. In other words, on approach to the critical point, the polymer is found first to contract and collapse, and then subsequently to return to its original dimensions. This behavior has recently been observed in simulations of polymer-solvent mixtures near the lower critical solution temperature of the system, and it is also known to be characteristic of solutions of polymers in bicomponent solvent mixtures near the critical consolute point of the two solvents. (C) 1999 American Institute of Physics. [S0021-9606(99)50431-5].

Solvation dynamics in nonassociated polar solvents

Ultrafast solvation dynamics in three nonassociated polar solvents, namely, acetonitrile, dimethyl sulfoxide, and acetone, have been studied by using the molecular hydrodynamic theory. For solvation in acetonitrile, the solvent memory function required for this study has been obtained from recent dielectric relaxation measurements of Venabales and Schuttenmaer; earlier theoretical studies used only the Kerr relaxation data. As the latter provides only an indirect information regarding the polar dynamical response of the dipolar liquid, it fails to provide a fully quantitative description of the solvation time correlation function, S(t). The present study with full dielectric data, on the other hand, gives excellent agreement with the experimental results. The theory shows that the ultrafast part of the solvation dynamics originates almost entirely from the high-frequency component of dielectric relaxation (with time constant 0.177 ps), although the latter represents only a small part of the latter. For DMSO and acetone, however, the present theory predicts a decay slower than the experimental observation. It is proposed that for these two solvents specific chromophore-solvent interactions might be responsible for the-large discrepancy. On the basis of the theory, two experimental studies have also been proposed.

Two regiospecific alternative strategies in the synthesis of (-)-valeranone

Two regiospecific modifications have been developed in the synthesis of valeranone. The first one is based on the regiospecific protection of a diol and the second is based on the Wittig reaction of a hemiacetal.

Variable-speed wind power generation using doubly fed wound rotor induction machine - A comparison with alternative schemes

In this paper, a wind energy conversion system (WECS) using grid-connected wound rotor induction machine controlled from the rotor side is compared with both fixed speed and variable speed systems using cage rotor induction machine. The comparison is done on the basis of (I) major hardware components required, (II) operating region, and (III) energy output due to a defined wind function using the characteristics of a practical wind turbine. Although a fixed speed system is more simple and reliable, it severely limits the energy output of a wind turbine. In case of variable speed systems, comparison shows that using a wound rotor induction machine of similar rating can significantly enhance energy capture. This comes about due to the ability to operate with rated torque even at supersynchronous speeds; power is then generated out of the rotor as well as the stator. Moreover, with rotor side control, the voltage rating of the power devices and dc bus capacitor bank is reduced. The size of the line side inductor also decreasesd. Results are presented to show the substantial advantages of the doubly fed system.

Fluorescence quenching of biologically active carboxamide by aniline and carbon tetrachloride in different solvents using Stern-Volmer plots

Fluorescence quenching of biologically active carboxamide namely (E)-2-(4-chlorobenzylideneamino)-N-(2-chlorophenyl)-4,5,6,7-tetrahydrobe nzo[b]thiophene-3-carboxamide [ECNCTTC] by aniline and carbon tetrachloride (CCl(4)) quenchers in different solvents using steady state method and time resolved method using only one solvent has been carried out at room temperature to understand the role of quenching mechanisms. The Stern-Volmer plot has been found to be linear for all the solvents studied. The probability of quenching per encounter p (p') was determined in all the solvents and was found to be less than unity. Further, from the studies of rate parameters and life time measurements in n-heptane and cyclohexane with aniline and carbon tetrachloride as quenchers have been shown that, the phenomenon of quenching is generally governed by the well-known Stern-Volmer (S-V) plot. The activation energy E(a) (or E(a)') of quenching was determined using the literature values of activation energy of diffusion E(d) and the experimentally determined values of p (or p'). It has been found that, the activation energy E(a) (E(a)') is greater than the activation energy for diffusion E(d) in all solvents. Hence, from the magnitudes of E(a) (or E(a)') as well as p (or p') infer that, the quenching mechanism is not solely due to the material diffusion, but there is also contribution from the activation energy. (C) 2011 Elsevier B.V. All rights reserved.

Functionalized and Postfunctionalizable Porous Polymeric Films through Evaporation-Induced Phase Separation Using Mixed Solvents

Condensation of water droplets during rapid evaporation of a polymer solution, under humid conditions, has been known to generate uniformly porous polymer films. Similar porous films are also formed when a solution of the polymer in THF containing small amounts of water, is allowed to evaporate rapidly under air flow; this suggests that water droplets may be formed during the final stages of film formation. In the presence of added surfactants, the interface of water droplets could become lined with the surfactants and consequently the internal walls of the pores generated, upon removal of the water, could become decorated with the hydrophilic head groups of the surfactant molecules. In a series of carefully designed experiments, we have examined the effect of added surfactants, both anionic and cationic, on the formation of porous PMMA films; the films were prepared by evaporating a solution of the polymer in THF containing controlled amounts of aqueous surfactant solutions. We observed that the average size of the pores decreases with increasing surfactant concentration, while it increases with increasing amounts of added water. The size of the pores and their distribution were examined using AFM and IR imaging methods. Although IR imaging possessed inadequate resolution to confirm the presence of surfactants at the pore surface, exchange of the inorganic counterion, such as the sodium-ion of SDS, with suitable ionic organic dyes permitted the unequivocal demonstration of the presence of the surfactants at the interface by the use of confocal fluorescence microscopy.

Zircon Sand--a Viable Alternative Moulding System for Titanium Castings

The experimental observations of casting titanium in sodium silicate bonded zircon sand mould are presented in this paper. Metal-mould reactions, in general, involved dissolution of oxides in liquid titanium resulting in contamination of the casting. Minimal metal-mould reactions occurred when titanium was cast in zircon sand mould containing about 7.5 wt% of ZrO2. It has been further shown that the metal-mould reaction is considerably reduced if moulds were fired at high temperatures (> 1273K). This ensured elimination of moisture from the mould and also resulted in some beneficial changes in the mould chemistry. The reduction in metal-mould reaction is reflected in the decrease in oxygen and hydrogen contamination and decrease in hardness. Thus microhardness profile and oxygen analysis seems to provide a good index for evaluation of severity of metal-mould reaction. The method has been demonstrated to be satisfactory for casting titanium components.

Hydrogen generation fromammonia borane using nanocatalysts

The rapidly depleting petroleum feed stocks and increasing green house gas emissions around the world has necessitated a search for alternative renewable energy sources. Hydrogen with molecular weight of 2.016 g/mol and high chemical energy per mass equal to 142 MJ/kg has clearly emerged as an alternative to hydrocarbon fuels. Means for safe and cost effective storage are needed for widespread usage of hydrogen as a fuel.Chemical storage is the one of the safer ways to store hydrogen compared to compressed and liquefied hydrogen. It involves storing hydrogen in chemical bonds in molecules and materials where an on-board reaction is used to release hydrogen. Ammonia–borane, (AB,H3N·BH3) with a potential capacity of 19.6 wt% is considered a very promising solid state hydrogen storage material. It is thermally stable at ambient temperatures. There are two major routes for the generation of H2 from AB: catalytic hydrolysis/alcoholysis and catalytic thermal decomposition. There has been a flurry of research activity on the generation of H2 from AB recently. The present review deals with an overview of our efforts in developing cost-effective nanocatalysts for hydrogen generation from ammonia borane in protic solvents.

Solvent effect on the fluorescence quenching of biologically active carboxamide by aniline and carbon tetrachloride in different solvents using S-V plots

The fluorescence quenching studies of carboxamide namely (E)-N-(3-Chlorophenyl)-2-(3,4,5-trimethoxybenzylideneamino)-4,5,6,7 tetrahydrobenzob]thiophene-3-carboxamide ENCTTTC] by aniline and carbon tetrachloride in six different solvents namely toluene, cyclohexane, n-hexane, n-heptane, n-decane and n-pentane have been carried out at room temperature with a view to understand the quenching mechanisms. The Stern-Volmer (S-V) plots have been found to be nonlinear with a positive deviation for all the solvents studied. In order to interpret these results we have invoked the ground state complex formation and sphere of action static quenching models. Using these models various quenching rate parameters have been determined. The magnitudes of these parameters suggest that sphere of action static quenching model agrees well with the experimental results. Hence the positive deviation is attributed to the static and dynamic quenching. Further, with the use of Finite Sink approximation model, it was possible to check these bimolecular reactions as diffusion-limited and to estimate independently distance parameter R' and mutual diffusion coefficient D. Finally an effort has been made to correlate the values of R' and D with the values of the encounter distance R and the mutual coefficient D determined using the Edward's empirical relation and Stokes Einstein relation. (C) 2011 Elsevier B.V. All rights reserved.

INDUCTION OF FOLDED STRUCTURES IN DESIGNED PEPTIDES USING CONFORMATIONALLY CONSTRAINED RESIDUES

Folding into compact globular structures, with well-defined modules of secondary structure, appears to be a characteristic of long polypeptide chains, with a specific patterning of coded amino acid residues along the length of sequence. Cooperative hydrogen bond driven secondary structure formation and solvent forces, which contribute favorably to the entropy of folding, by promoting compaction of the polymeric chain, have long been discussed as major determinants of the folding process. First principles design approaches, which use non-coded amino acids, employ an alternative structure directing strategy, by using amino acid residues which exhibit a strong conformational bias for specific regions of the Ramachandran map. This overview of ongoing studies in the authors' laboratory, attempts to explore the use of conformationally restricted amino acid residues in the design of peptides with well-defined secondary structures. Short peptides composed of 20 genetically coded amino acids usually exist in solution as an ensemble of equilibrating conformations. Apolar peptide sequences, which are readily soluble in organic solvents like chloroform and methanol, facilitate formation of structures which are predominately driven by intramolecular hydrogen bond formation. The choice of sequences containing residues with a limited range of conformational choices strongly favors formation of local turn structures, stabilized by short range intramolecular hydrogen bonds. Two residue beta-turns can nucleate either helical or hairpin folding, depending on the precise conformation of the turn segment Restriction of the conformational space available to amino acid residues is easily achieved by introduction of an additional alkyl group at the C alpha carbon atom or by side chain backbone cyclization, as in proline. Studies of synthetic sequences incorporating two prototype residues alpha-aminoisobutyric acid (Aib) and D-proline (DPro) illustrate the utility of the strategy in construction of helices and hairpins. Extensions to the design of conformationally switchable sequences and structurally defined hybrid peptides containing backbone homologated residues are also surveyed.