Computer simulation of the rheology of concentrated star polymer suspensions

We use particle-based computer simulations to study the rheology of suspensions of high-functionality star polymers with long entangled arms. Such particles have properties which are intermediate between those of soft colloidal particles and entangled polymer chains. In the simulations, each star polymer is coarse-grained to a single particle. In order to faithfully reproduce dynamical properties, it is very important to not only include time-averaged interactions (potentials of mean force) but to also account for transient interactions induced by entanglements between the arms of different star polymers. Using a model which has all these features, it is found that, for sufficiently high shear rates, the start-up shear stress displays an overshoot. With increasing concentration, the core interactions increasingly dominate the initial stress response, leading to a maximum in the stress overshoot at relatively low strain values (0.1 to 0.5). Transient forces start to dominate after this initial stage. In a simulated experiment in which the shear rate is suddenly stepped-down from a high to a lower value, the stress shows a clear undershoot, with the minimum stress again at a relatively low strain value (based on the new shear rate). Finally, it is shown that a stress plateau develops in the flow curve. This plateau is absent when the transient forces between the polymer stars are not taken into account.     

Use of heat capacities for the estimation of cubic equation-of-state parameters—application to the prediction of very low vapor pressures of heavy hydrocarbons

Improvement in the prediction of very low vapor pressures is checked by introducing heat capacity data into the estimation of cubic equation-of-state (EOS) parameters. As the key parameter is the temperature-dependent parameter a, several expressions (mainly of exponential form) were investigated. All of them were chosen in order to show a consistent behavior for the two considered properties (vapor pressures and heat capacities). The cubic EOS used as an illustration is of the Peng–Robinson type applied to heavy hydrocarbons. No satisfactory refinement in the prediction of the very low vapor pressures was observed in comparison with the results obtained by extrapolating the EOS from medium to very low pressures. This work has, however, the following benefits: (1) to point out the changes that should be made to improve these predictions; (2) to inform on the accuracy that may be obtained if vapor pressures of heavy organic compounds are predicted from heat capacity data as the sole alternative for estimating the temperature-dependent parameter a of a cubic EOS; (3) to confirm the reliability of the cubic group-contribution (GC)-based EOS proposed by Coniglio et al. [Ind. Eng. Chem. Res. 39 (2000) 5037] when extrapolated for modeling crude oils or gas condensates encountered in the petroleum industry.     

Intramolecular homolytic displacements 7 — Free radical additions to unsaturated peroxy compounds : Synthesis of oxygenated heterocycles

Free radical additions of hydrogen donor solvents to tert-butyl perpent-4-enoate have given 4-substituted γ-butyrolactones, with yields of 50 % or greater. Such reactions applied to allyl-tert-butyl peroxide have produced 2,3-epoxypropanation of these solvents. Similar addition-elimination processes occurred with tert-butyl 4-methyl-perpent-4-enoate, tert-butyl 2,2-dimethyl-perpent-4-enoate and tert-butyl-methallyl peroxide, but they failed with tert-butyl 5-methyl-perhex-4-enoate and tert-butyl-3-methyl-but-2-enyl peroxide.     

Synthesis and configuration of 4-alkyl-2-methyl-3,4-diphenyl-1,2-thiazetidine 1,1-dioxide

4-Alkyl-2-methyl-3,4-diphenyl-1,2-thiazetidine 1,1-dioxide 5, 6 and 7 were obtained from 2-methyl-3,4-diphenyl-1,2-thiazetidine 1,1-dioxide 1 by reaction of its anion 4 with alkyl halides. cis- and trans-Configuration of the 4-alkylated products were determined by ¹H-nmr and NOE difference spectro-scopy studies.     

Supervised clustering of variables

In predictive modelling, highly correlated predictors lead to unstable models that are often difficult to interpret. The selection of features, or the use of latent components that reduce the complexity among correlated observed variables, are common strategies. Our objective with the new procedure that we advocate here is to achieve both purposes: to highlight the group structure among the variables and to identify the most relevant groups of variables for prediction. The proposed procedure is an iterative adaptation of a method developed for the clustering of variables around latent variables (CLV). Modification of the standard CLV algorithm leads to a supervised procedure, in the sense that the variable to be predicted plays an active role in the clustering. The latent variables associated with the groups of variables, selected for their “proximity” to the variable to be predicted and their “internal homogeneity”, are progressively added in a predictive model. The features of the methodology are illustrated based on a simulation study and a real-world application.     

Resolutions of the Coulomb operator

We discuss a generalization of the resolution of the identity by considering one-body resolutions of two-body operators, with particular emphasis on the Coulomb operator. We introduce a set of functions that are orthonormal with respect to 1r(12) and propose that the resulting "resolution of the Coulomb operator," r(12) (-1)=mid R:phi(i)phi(i)mid R:, may be useful for the treatment of large systems due to the separation of two-body interactions. We validate our approach by using it to compute the Coulomb energy of large systems of point charges.     

Effect of sodium to barium substitution on the space charge implementation in thermally poled glasses for nonlinear optical applications

Thermally poled niobium borophosphate glasses in the system 0.55(0.95−y) NaPO₃+y/2 Ba(PO₃)₂+0.05Na₂B₄O₇)+0.45Nb₂O₅ were investigated for second order optical nonlinear (SON) properties. Bulk glasses were studied by Raman spectroscopy, thermal analysis, optical and dielectric measurements. The sodium to barium substitution does not lead to significant changes in optical properties, crystallization of glasses and coordination environment of polarizable niobium atoms. However, the ionic conductivity decreases drastically with the increase of barium concentration. Secondary ion mass spectroscopy has been used to determine the element distribution in the anode layer of the thermally poled glasses. The second order optical susceptibilities gradually decrease from χ⁽²⁾=2.8 pm/V to zero with the increase of barium content. Using a simple electrical model, variations of nonlinear optical responses have been correlated with dielectric characteristics.