Desorption from polymers

The rate constant k3 for the population with the fastest decay rate was shown to be equal to the rate constant, k, for desorption from polymer in the rubbery state [148], and the rate constants ki for the set of n populations trapped in the glassy state (attained when a, becomes equal to oeg) are related one to another by ... 

The kinetic behavior of emulsion polymerization is greatly affected by radical desorption from polymer particles. This has been shown by Dgelstad et al. (1969)> Litt et al. (1970), Harada et nl. (1971), Friis and Nyhagen (1973), and Nomura et al (1971). It is believed that the deviation of the kinetic behavior of the emulsion polymerization of water-soluble monomers such as vinyl acetate and vinyl chloride from the Smith and Ewart (1949) Case 2 kinetic theory is mainly due to dominant desorption of... 

The exchange of proteins (albumin, globulin, and fibrinogen) and the time course of adsorption-desorption from polymers have been studied by... 

The chain transfer to VC monomer is found to have a strong influence on the polymerization process through the formation of less reactive radicals and their desorption from polymer particles to the aqueous phase. The probability of such events is enhanced because the number of entangled or occluded radicals is high. The high efficiency of chain-transfer to monomer results from the low molecular weights of polymers formed in the emulsion systems and the value of polydis-persity index close to 2.0. 

Usually, reactions 1 and 2 take place in the aqueous jiiase, yttiile all the other kinetic events can occur both in the aqueous and in the polymer phases. Note that Pj,n indicates the concentration of active polymer chains with nTronaner units and tenninal unit of type j (i. e. of monomer j) Hi is the concentration of monomer i and T is the concentration of the chain transfer agent. Reactions 4 and 5 are responsible for chain desorption from the polymer pjarticles reactions 6 and 7 describe bimolecular temination by conJoination and disproportionation, respiectively. All the kinetic constants are depsendent upon the last monomer unit in the chain, i. e. terminal model is assumed. 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

Wittmer, J., Johner, A., Joarmy J. F. and Binder, K. (1994) Chain desorption from a semidilute polymer brush - a Monte-Carlo simulation./. Chem. Phys., 101, 4379-4390. 

Desorption chemical ionisation (DCI) mass spectrometry has been used for detecting additives extracted from polymers [51,52] by a solvent as volatile as possible. To use the DCI probe, 1 -2 iL of the sample, in solution, are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. The sample is then subject to fast temperature ramping. DCI does not seem to be the most suitable mass-spectrometric method for analysis of dissolved polymer/additive matrices, because ... 

Most analytical studies using FT-ICR mass spectrometry, where ions have been produced inside (or just outside) the analyzer cell, have used lasers as ionization sources. Other than some very limited Cs secondary ion mass spectrometry (SIMS) studies [77], most research utilized direct laser desorption to form various organic [78] and inorganic [79] ions, including metal [80] and semiconductor [81] (including carbon) clusters. More recently matrix assisted laser desorption ionization (MALDI) has been used to form ions of high molecular weight from polymers [82] and many classes of biomolecules [83]. 

Fig. 21. A typical example of many time-studies of desorption from liquid saturated poly(Sty-co-DVB) particles enmeshed in PTFE microfibers. The Inset records the number, at> of residual sorbed molecules [in this case CHC13 in (Sty)98(DVB)2] per phenyl group of polymer over the first 10 min of desorption at 23 °C. The main plot records the logarithm of at over the first 70 min of the time-study. The breakpoints in the kinetics of desorption at aj, otg and ag mark sequentially the compositions at incipient elimination of molecules immobilized by adsorption, incipient transition from the rubbery state of the system to the glassy state, and finally completion of this transition as discussed in the text...

A much simpler model for the radical capture (absorption) efficiency F can be derived by introducing the concept of radical desorption from a polymer particle, developed in Section 3.2.1. The probability F for a radical to be captured inside a particle containing n radicals by any chemical reaction (propagation or termination) is given by... 

The desorption (exit) of free radicals from polymer particles into the aqueous phase is an important kinetic process in emulsion polymerization. Smith and Ewart [4] included the desorption rate terms into the balance equation for N particles, defining the rate of radical desorption from the polymer particles containing n free radicals in Eq. 3 as kftiN . However, they did not give any... 

In the presence of catalysts, heterogeneous catalytic cracking occms on the surface interface of the melted polymer and solid catalysts. The main steps of reactions are as follows diffusion on the surface of catalyst, adsorption on the catalyst, chemical reaction, desorption from the catalyst, diffusion to the liquid phase. The reaction rate of catalytic reactions is always determined by the slowest elementary reaction. The dominant rate controller elementary reactions are the linking of the polymer to the active site of catalyst. But the selectivity of catalysts on raw materials and products might be important. The selectivity is affected by molecular size and shape of raw materials, intermediates and products [36]. 

As shown in Fig. 1, the value of n becomes independent of the value of kf in the range of n 0.5. This means that tbe rate coefficient for radical desorption from the particles is important in Ibe range (n < 0.5) where the polymer particle contains at most one radical. For this reason, we consider an emulsion polymerization system where (i) the particles contain at most one radical and (ii) instantaneous termination takes place when another radical enters the particle that already contains a radical. 

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