Network structure active fraction

Polymer networks are formed from functional precursors by covalent bond formation [1], As a result, molecular weights and polydispersity increase and the system passes through a critical point, the gel point. At this point, an infinite structure (molecule) is formed for the first time. Beyond the gel point, the fraction of the infinite structure (the gel) increases at the expense of finite (soluble) molecules (the sol). The sol molecules become gradually bound to the gel and eventually all precursor molecules can become a part of the gel - the network. This is not always the case for different reasons sometimes sol is still present after all functional groups have reacted. In passing from the gel point to the final network not only the gel fraction increases, but also the network becomes denser containing increasing amounts of crosslinks and strands between them called elastically active network chains. 

The structure and properties of a network polymer are determined by the relation between the inter- and intramolecular reactions of functional groups. The latter gives rise to ineffective cycles, and has been studied fairly well3-5 7°.71-8i-85.i°8 u2 Their occurrence in the system results in the gelation point being shifted towards higher conversions, higher sol fraction, fewer elastically active chains, and smaller equilibrium modulus of the network. 

The percolation model suggests that it may not be necessary to have a rigid geometry and definite pathway for conduction, as implied by the proton-wire model of membrane transport (Nagle and Mille, 1981). For proton pumps the fluctuating random percolation networks would serve for diffusion of the ion across the water-poor protein surface, to where the active site would apply a vectorial kick. In this view the special nonrandom structure of the active site would be limited in size to a dimension commensurate with that found for active sites of proteins such as enzymes. Control is possible conduction could be switched on or off by the addition or subtraction of a few elements, shifting the fractional occupancy up or down across the percolation threshold. Statistical assemblies of conducting elements need only partially fill a surface or volume to obtain conduction. For a surface the percolation threshold is at half-saturation of the sites. For a three-dimensional pore only one-sixth of the sites need be filled. 

Along with T, further structural parameters of networks, e.g. the sol fraction, w, and the concentration of elastically active network chains in the gel, s v /(l — w, ... 

The hypothetical active site lattice (HASL) method (Doweyko 1988) identifies the points of the network associated to the atoms of the molecule of interest. Then it gives a fraction of the value of the biochemical property to these points. This fraction is characteristic of the analyzed molecule. By repeating the procedure for all the molecules in a given set, some points of the network acquire the summing of the assigned values that are different from null values. These points describe a structure as a map of the active site of the receptor macromolecule that interacts with the effector molecules. 

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