Polymeric interaction

The interaction energy between particles in a dispersion can also be manipulated by the addition of adsorbing or non-adsorbing non-ionic polymers . At low concentration, adsorbing polymers can induce bridging agglomeration. At higher concentration, where the surfaces are completely covered, there can be 

8 The effect of charged polymers (polyelectrolytes) is mainly electrostatic at low electrolyte concentration, although in some cases they show charge effects emd steric repulsion (see text). 

An approximate expression first derived by Fisher (see Ref. [32] p. 460) for the adsorbed polymer steric interaction energy AGpoi t etween two spheres is  

When particles approach each other a repulsion occurs (in a good solvent) which is osmotic in origin, rather similar to the repulsion due to ED overlap. 

In Fig. 6.15 the interaction free energy distance curves are shown for several layer thicknesses adsorbed polymer in a good solvent. The weak minimum due to the van der Waals force decreases with increasing layer thickness. In Fig. 6.16, a typical interaction free energy curve is shown in the presence of van der Waals attraction, electrostatic repulsion and steric repulsion due to adsorbed polymer. Note the absence of a primary minimum. 

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AGpoi, polymeric interaction This results in a total interaction Gibbs free energy... 

G due to electrostatic interaction [J] polymeric interaction [J] gravity acceleration Goucher number separation distance [m]... 

JOM(642)275). Sodium boratastilbene with [ZrCl4(THF)2] forms sandwich 40. Species 37 and 38 are efficient catalysts for ethylene polymerization. Interaction of the appropriate boratabenzene ligand with lithium di-wo-propylamide and then zirconi-um(IV) chloride gives 41 (030M203). Similarly, 42 can be obtained. 

Amoeboid movement (cell extension on one ATP Actin polymerization interaction between cytoskeletal filaments Amoebae (e.g., Dictyostelium discoideum), some ... 

A new research field in the chemistry of neptunium, plutonium, and other light actinides developed in the late 1970s in the context of environmental contaminations and nuclear waste management. The aim was to unravel their behavior in natural environments (see, e.g., Watters et al. 1983 Kim 1986). This is a difficult task because of the complex chemistry many oxidation states, polymerization, interaction with less defined natural partners such as humic acids, clays, and various rocks play a role. Included is also the development of techniques for the determination of actinide elements and their speciation in extremely low concentrations. 

To summarize at this stage, the inteactions between the adsorbed polymer chains may be separated into (1) a mixing effect that produces either a repulsion or an attraction and (2) an elastic effect that is always repulsive (Table 4.4). The mixing effect is also described as an osmotic effect whereas the elastic effect is described as an entropic effect or a volume restriction effect. The free energy of the polymeric interaction can be written... 

Ziegler catalysts Complex catalysts prepared by interaction between an organometallic derivative and a transition metal derivative. A typical catalyst is the product of the interaction of TiCU and AIBU3. These catalysts polymerize olefins, particularly ethylene, to polyolefins, the polymerization generally being in a siereoregular manner. 

Protein adsorption has been studied with a variety of techniques such as ellipsome-try [107,108], ESCA [109], surface forces measurements [102], total internal reflection fluorescence (TIRE) [103,110], electron microscopy [111], and electrokinetic measurement of latex particles [112,113] and capillaries [114], The TIRE technique has recently been adapted to observe surface diffusion [106] and orientation [IIS] in adsorbed layers. These experiments point toward the significant influence of the protein-surface interaction on the adsorption characteristics [105,108,110]. A very important interaction is due to the hydrophobic interaction between parts of the protein and polymeric surfaces [18], although often electrostatic interactions are also influential [ 116]. Protein desorption can be affected by altering the pH [117] or by the introduction of a complexing agent [118]. 

The interaction between ions of the same sign is assumed to be a pure hard sphere repulsion for r < a. It follows from simple steric considerations that an exact solution will predict dimerization only if i < a/2, but polymerization may occur for o/2 < L = o. However, an approximate solution may not reveal the fiill extent of polymerization that occurs in a more accurate or exact theory. Cummings and Stell [ ] used the model to study chemical association of uncharged atoms. It is closely related to the model for adliesive hard spheres studied by Baxter [70]. 

As is evident from the fomi of the square gradient temi in the free energy fiinctional, equation (A3.3.52). k is like the square of the effective range of interaction. Thus, the dimensionless crossover time depends only weakly on the range of interaction as In (k). For polymer chains of length A, k A. Thus for practical purposes, the dimensionless crossover time is not very different for polymeric systems as compared to the small molecule case. On the other hand, the scaling of to is tln-ough a characteristic time which itself increases linearly with k, and one has... 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

In homopolymers all tire constituents (monomers) are identical, and hence tire interactions between tire monomers and between tire monomers and tire solvent have the same functional fonn. To describe tire shapes of a homopolymer (in the limit of large molecular weight) it is sufficient to model tire chain as a sequence of connected beads. Such a model can be used to describe tire shapes tliat a chain can adopt in various solvent conditions. A measure of shape is tire dimension of tire chain as a function of the degree of polymerization, N. If N is large tlien tire precise chemical details do not affect tire way tire size scales witli N [10]. In such a description a homopolymer is characterized in tenns of a single parameter tliat essentially characterizes tire effective interaction between tire beads, which is obtained by integrating over tire solvent coordinates. 

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

This isomerization, which must proceed through a 1,2,3-trienylanine, is not "contra-thermodynamic", since with a catalytic amount of potassium tert.-butoxide the same result is obtained. Enyne ethers, H2C=CH-CsC-0R, undergo a similar conversion into HCeC-CH=CH-OR upon interaction with alkali metal amides in liquid NH3, followed by hydrolysis . Enyne sulphides, H2C=CH-CsC-SR, and the hydrocarbons H2C=CH-CsC-R (R = or phenyl) give only tars or polymeric products under... 

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