Applications de la sonde moléculaire aux composites a teneur elevee en solides—I. Determination des temperatures de transition vitreuse et etude des phenomenes de surface

The molecular probe technique has been shown to be useful for measuring the glass transition temperature of a highly-filled composite, or of a polybutadiene prepolymer covering surfaces of varying types. The measured Tg of 193°K is independent of the curing stage as well as of the nature of functional groups. Transitions between 349 and 364°K were detected in a Fe₂O₃-containing composite, showing the general applicability of the technique. Studies of the slope of the curve relating the logarithm of the specific retention volume to the reciprocal of the absolute temperature can lead to useful information concerning the orientation of the polybutadiene prepolymer at the polymer-solid interface.     

Effect of temperature on carbon-black agglomeration in hydrocarbon liquid with adsorbed dispersant

Suspensions of carbon black in oil, stabilized with adsorbed polyisobutylene succinimide (PIBSI) dispersant, are commonly used as model systems for investigating the soot-handling characteristics of motor oils. The structure of the carbon-black agglomerates changes dramatically with temperature; this results in a concomitant change in the suspension rheology. Linear and nonlinear rheological experiments indicate a large increase of the interparticle attractions as the temperature is raised. To elucidate the origin of this behavior, we investigate the effect of temperature on the stabilizing effect of the dispersant. Measurements of adsorption isotherms of the dispersant on carbon black indicate that there is little variation of the binding energy with temperature. Intrinsic viscosity measurements of PIBSI dispersants in solution clearly exhibit an inverse dependence of the dispersant chain dimension with temperature. These results suggest that the temperature-dependent changes in the chain conformation of the PIBSI dispersant are primarily responsible for the changes in the dispersion rheology, and we propose a simple model to account for these data.     

RCM of tripeptide dienes containing a chiral vinylcyclopropane moiety: impact of different Ru-based catalysts on the stereochemical integrity of the macrocyclic products

[structures: see text] Tripeptide dienes containing an (1R,2S)-vinyl aminocyclopropylcarboxylate residue were cyclized to beta-strand scaffolds under ring-closing metathesis (RCM). Conformational factors, ligand effects, and reaction conditions were evaluated. A protocol was developed for the efficient synthesis of 15-membered ring peptides in high diastereomeric purity. These peptides are key synthetic precursors to antiviral agents that target the hepatitis C virus and represent the first class of clinically validated pharmaceutical agents that are synthesized in large scale using RCM.     

Electron transfer driven by proton fluctuations in a hydrogen-bonded donor-acceptor assembly

The temperature-isotope dependence of proton-coupled electron transfer (PCET) for a noncovalent molecular dyad is reported. The system consists of an excited-state Zn(II) porphyrin that transfers an electron to a naphthalene diimide acceptor through an amidinium-carboxylate interface. Two different isotope effects are observed for variant temperature regimes. A reverse isotope effect (i.e., kH/kD < 1) is observed as T approaches 120 K (kH/kD = 0.9, 120 K), whereas a normal isotope effect (i.e., kH/kD > 1) is recovered as the temperature is increased (kH/kD = 1.2, 300 K). The transition between these limits is smooth, with a crossover temperature of T approximately 160 K. These observations are in accordance with charge-transfer dynamics that are susceptible to bath-induced fluctuations in the proton coordinate.     

Anomalous ionic conductivity increase in Li2S + GeS2 + GeO2 glasses

Numerous studies of the ionic conductivities in oxide-doped chalcogenaide glasses have shown the anomalous result that the ionic conductivity actually increases significantly (by more than a factor of 10 in some cases) by the initial addition of an oxide phase to a pure sulfide glass. After this initial sharp increase, the conductivity then monotonically decreases with further oxide addition. While this behavior is important to the application of these glasses for Li batteries, no definitive understanding of this behavior has been elucidated. To examine this effect further and more completely, the ionic conductivities of 0.5Li(2)S + 0.5[(1 - x)GeS(2) + xGeO(2)] glasses have been measured on disc-type bulk glasses. The ionic conductivity of the 0.5Li(2)S + 0.5GeS(2) (x = 0) glass was observed to increase from 4.3 x 10(-5) (Omega cm)(-1) to 1.5 x 10(-4) (Omega cm)(-1) while the activation energy decreased to 0.358 eV from 0.385 eV by the addition of 5 mol % of GeO(2). Further addition of GeO(2) monotonically decreased the conductivity and increased the activation energy. On the basis of our previous studies of the structure of this glass system, the Anderson and Stuart model was applied to explain the decrease in the activation energy and increase in the conductivity. It is suggested that the "doorway" radius between adjacent cation sites increases slightly (from approximately 0.29(+/-0.05) A to approximately 0.40(+/-0.05) A) with the addition of oxygen to the glass and is proposed to be the major cause in decreasing the activation energy and thereby increasing the conductivity. Further addition of oxides appears to contract the glass structure (and the doorway radius) leading to an increase in the conductivity activation energy and a decrease in the conductivity.