Microheterogeneous model

Our simplest continuous microheterogeneous model assumes that the luminophore exists in a distribution of spectroscopically different environmental sites. For a tractable, yet plausible, model each site is assumed to be quenched by normal Stem-Volmer quenching kinetics. For luminescence decays each individual component is assumed to give a single exponential decay with the following impulse response ... 

The second approach or microheterogeneous model [1, 19-22] is based upon the principle, that the kinetics of the reaction in its initial stage are not that of a homophase polymerisation in a liquid monomer-polymeric solution, but a heterophase one. The reaction proceeding at the boundary liquid monomer - solid polymer microgranules surface under gel conditions. 

From this point of view, the difference between microheterogeneous model and DCR approach consists only in the way that they consider or do not consider the microheterogeneity of polymerising system as an essential and important factor determining the main features of polymerisation process to the high degrees of conversions. 

From this, as well as from the other physical studies, Foster et al. concluded that while the pH-dependent IV - F transformation is rapidly and completely reversible, at intermediate pH values, bovine plasma albumin consists of stable noninterconvertible N and F isomers. To explain this apparent contradiction, they proposed the microheterogeneity model for plasma albumin. This model postulates the... 

The first conception (or microheterogeneous model) is based on the assumption that the main contribution to the kinetics of the process in the initial state does not lead to homophaseous polymerization in the liquid monomer/polymeric solution, but the heterophaseous one, proceeding on the solid polymer-liquid monomer boundary under the gel-effect conditions. 

A microheterogeneous model of the radical polymerization process of polyunsaturated compounds at the present time is based on the investigations in which the kinetics and chemical mechanism of acrylic oligomer polymerization, structure and properties of the forming polymers are complexly studied. This model draws a general... 

The main principles of the microheterogeneous model can be formulated as follows. The densely-cross-linked polymer formation process is characterized by a parallel proceeding of inter-connected chemical and physical phenomena. Chemical transformations (or interaction of the reactive-able groups) are accompanied by physical phenomena, namely chain aggregation, phase division (or syneresis) with the elimination of the starting monomer from the sufficiently dense polymeric networks, local glass transition of the densely-cross-linked microvolumes, re-distribution of the inter-molecular interactions in connection with phase surface division, etc. 

The so-called microheterogeneous model of 3-D radical polymerization, based on an image of the microheterogeneous character of 3-D polymerization, has been proposed for the first time by Korolev in the 1970s. 

The microheterogeneous model foresees a sudden leap-like change of the properties of the cross-linked polymeric material in the monolytization state in sufficiently narrow depths of the transformation. This model also assumes the existence of two topologically... 

According to the kinetic interpretation of the microheterogeneous model, the autodeceleration is caused by reduction of the total surface area of the grains in the monolytization state via joining of grains. In this case, the polymerization rate is not described by equation (3.34) due to the loss of a reactive area part at the grains Joint. 

However, a comparison of the kinetic equations (3.34) and (3.36) with the experimental data shows, that they cannot quantitatively describe the process both in the autoacceleration state and in the autodeceleration states and do not agree in the section of maximal polymerization rate, since in the variant of the microheterogeneous model [2] the regularities of the polymerization at the interface layer are considered to be the same as the regularities in the volume of liquid monomer. Analysis of the experimental data shows that this assumption is not true. 

Two basic concepts are used for the interpretation of the postpolymerization process (1) diffusion-controlled reactions (DCR) and (2) the microheterogeneous model. With regard to DCR, the basis I or the analysis of experimental data is the famous kinetic equation of the initial state of the stationary process. The parameters of this equation are the function of the mobility of macroradicals. 

The second concept, the microheterogeneous model, is based on the assumption about the microheterogeneity of the system. 

As we can see, equation (5.1) is in good agreement with the main peculiarities of kinetic curves of 3-D polymerization [8-10]. At this Stage, the form of equation (5.1) means that the microheterogeneous model can be used as a base for derivation of this equation. The main positions of the above-mentioned model have been formulated by us as follows [8] ... 

The proposed microheterogeneous model (see equation 5.1) for thermal polymerization needs modification, It is necessary to take into account the presence of a light gradient in the layer of the photopolymerization composition, gradients in the concentrations of the photoinitiator of the oligomer and the depth of the photopolymerization. That is why in this variant of the photoinitiated polymerization equation (5.1) should be written as a partial derivative [8] ... 

Table 5.1. Kinetic parameters of the photoinitiated 3-D polymerization of di(meth)acrylates in the microheterogeneous model...

In conclusion, reviewing the literature and experimental material obtained from stationary photoinitiated radical polymerization of bifunctional (meth)acrylates in bulk allowed us to obtain the kinetic equation describing the process at all depths of conversion. A mechanism of 3-D polymerization combining the kinetic schemes of homophaseous and heterophaseous processes has been proposed on the basis of the microheterogeneous model and also effective rate constants of these processes have been estimated. 

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