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Reaction Environment (reaction + environment)
Selected AbstractsMechanisms Controlling Crystal Habits of Gold and Silver ColloidsADVANCED FUNCTIONAL MATERIALS, Issue 7 2005C. Lofton Abstract Examples of gold and silver anisotropic colloids, such as prisms and rods, have appeared in the literature for many years. In most cases, the morphologies of these thermodynamically unfavorable particles have been explained by the particular reaction environment in which they were synthesized. The mechanisms used to explain the growth generally fall into two categories, one in which chemically adsorbed molecules regulate the growth of specific crystal faces kinetically, and the other where micelle-forming surfactants physically direct the shape of the particle. This paper raises questions about the growth of anisotropic metal colloids that the current mechanisms cannot adequately address, specifically, the formation of multiple shapes in a single homogeneous reaction and the appearance of similar structures in very different synthesis schemes. These observations suggest that any growth mechanism should primarily take into consideration nucleation and kinetics, and not only thermodynamics or physical constrictions. The authors suggest an alternative mechanism where the presence and orientation of twin planes in these face-centered cubic (fcc) metals direct the shape of the growing particles. This explanation follows that used for silver halide crystals, and has the advantage of explaining particle growth in many synthesis methods. In this mechanism, twin planes generate reentrant grooves, favorable sites for the attachment of adatoms. Shape and structural data are presented for gold and silver particles synthesized using several different techniques to support this new model. Triangular prisms are suggested to contain a single twin plane which directs that growth of the initial seed in two dimensions, but limits the final size of the prism. Hexagonal platelets are suggested to contain two parallel twin planes that allow the fast growing edges to regenerate one another, allowing large sizes and aspect ratios to form. Rods and wires were found to have a fivefold symmetry, which may only allow growth in one dimension. It is expected that a superior mechanistic understanding will permit shape-selective synthesis schemes to be developed. [source] A DFT study on the (2,+,3) cycloaddition reactions of 2-nitropropene-1 with Z- C, N -diarylnitrones,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2009Radomir Jasi Abstract B3LYP/6-31G* calculations for competing (2,+,3)-cycloaddition pathways for 2-nitropropene-1 (1) to Z- C, N -diarylnitrones (2a,e) suggest a concerted reaction mechanism. However, the results point to the strongly asymmetric nature of transition complexes. Increasing polarity of the reaction environment and presence of electron-donating substituents in the nitrone phenyl rings contribute to the higher asymmetry of these structures. Copyright © 2009 John Wiley & Sons, Ltd. [source] Prediction of Polymer Properties in LDPE ReactorsMACROMOLECULAR MATERIALS & ENGINEERING, Issue 4 2005Gary J. Wells Abstract Summary: A new analysis tool is presented that uses the governing kinetic scheme to predict properties of low-density polyethylene (LDPE) such as the detailed shape of the molecular weight distribution (MWD). A model that captures mixing details of autoclave reactor operation is used to provide a new criterion for the onset of MWD shouldering. Kinetic effects are shown to govern the existence of MWD shoulders in LDPE reactors, even when operation is far from perfectly-mixed. MWD shoulders occur when the mean reaction environment has a relatively high radical concentration and has a high polymer content, and is at a low temperature. Such conditions maximize long chain formation by polymer transfer and combination-termination, while limiting chain scission. For imperfectly-mixed reactors, the blending of polymer-distributions produced in different spatial locations has a small effect on the composite MWD. However, for adiabatic LDPE autoclaves, imperfect mixing broadens the stable range of mean reactor conditions, and thereby increases the possibility for MWD shouldering. Polymer MWD produced in an LDPE autoclave reactor by various kinetic mechanisms. [source] Matrix Representation of Polymer Chain Size Distributions, 2,MACROMOLECULAR THEORY AND SIMULATIONS, Issue 2 2007Fundamental Analysis of Linear Polymerization Mechanisms at Transient Conditions Abstract Analysis of the mass balance equations that describe a reaction system may be useful to provide information about its dynamics, such as the restricted set of compositions that can be achieved from a given set of initial compositions and the effect of feeding reactants to the reaction environment along the reaction course. Since these results may be important for the formulation of reaction policies, this work presents the properties of a matrix polymerization model previously developed and extended to describe transient conditions. This model is based on the definitions of two matrices: the consumption matrix (A,,,Kt), which contains information about chemical transformations among the many active polymer species in the system, and the propagation matrix Kp, which contains information about chain growth. It is shown that the set of mass balance equations that describes the dynamics of active chemical species in polymerization reactions has a stable and unique solution, which is bounded if feed rates are also bounded. It is also shown that the set of compositions that may be reached through manipulation of the feed rates is restricted and may not include all possible chemical compositions. Finally, it is shown that the obtained molecular weight distributions are special multiple time convolutions of the initiation rates. [source] Synthesis, characterization and application of poly[(1-vinyl-2-pyrrolidone)- co -(2-hydroxyethyl methacrylate)] as controlled-release polymeric system for 2,4-dichlorophenoxyacetic chloride using an ultrafiltration techniquePOLYMER INTERNATIONAL, Issue 7 2008Guadalupe del C Pizarro Abstract BACKGROUND: Polymers supporting chemicals used in agriculture have recently been developed to overcome the serious environmental problems of conventional agrochemicals. The success of these formulations is based on a suitable choice of polymer support. Degradable polymeric hydrogels are of particular interest. The gradual release of the bioactive agent can be achieved by hydrolytic or enzymatic cleavage of the linking bond. RESULTS: In this context, poly[(1-vinyl-2-pyrrolidone)- co -(2-hydroxyethyl methacrylate)] [poly(NVP- co -HEMA)] has been used as a bioactive carrier reagent. Herein, we report a controlled-release system with the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) using an ultrafiltration system. Hydrolysis was studied by testing the release at various pH values. A high release with poly(NVP- co -HEMA),2,4-D was observed at pH = 7 and 10 after two days (Z = 2). The release percentage of copolymer,herbicide increased at pH = 10. It showed release values between 79.0 and 94.5%. Poly(NVP- co -HEMA),herbicide can release a bioactive compound in aqueous solution at pH = 3, 7 and 10. CONCLUSION: Based on the results of homogeneous hydrolysis, it is argued that the herbicide release rate depends on the pH of the reaction environment. This functional polymer could be employed as a biodegradable material for applications in agrichemical release. Copyright © 2008 Society of Chemical Industry [source] | ||||||||||