Network bonds formation

In paper [66] the degree of network connectivity was estimated with the aid of the probability of network bonds formation r. This parameter was defined as follows ... 

Gelatin stmctures have been studied with the aid of an electron microscope (23). The stmcture of the gel is a combination of fine and coarse interchain networks the ratio depends on the temperature during the polymer-polymer and polymer-solvent interaction lea ding to bond formation. The rigidity of the gel is approximately proportional to the square of the gelatin concentration. Crystallites, indicated by x-ray diffraction pattern, are beUeved to be at the junctions of the polypeptide chains (24). 

Reaction kinetics at phase houndaiies. Rates of adsorption and desorption in porous adsorbents are generally controlled by mass transfer within the pore network rather than by the kinetics of sorption at the surface. Exceptions are the cases of chemisorption and affinity-adsorption systems used for biological separations, where the kinetics of bond formation can be exceedingly slow. 

The essential property of a cementitious material is that it is cohesive. Cohesion is characteristic of a continuous structure, which in the case of a cement implies an isotropic three-dimensional network. Moreover, the network bonds must be attributed to attractions on the molecular level. Increasingly, recent research tends to show that cements are not bonded by interlocking crystallites and that the formation of crystallites is incidental (Steinke et al., 1988 Crisp et al., 1978). The reason is that it is difficult to form rapidly a mass which is both cohesive and highly ordered. 

In the foregoing considerations, formation of elastically inactive cycles and their effect have not been considered. For epoxy networks, the formation of EIC was very low due to the stiffness of units and could not been detected experimentally the gel point conversion did not depend on dilution in the range 0-60% solvent therefore, the wastage of bonds in EIC was neglected. For polyurethanes, the extent of cyclization was determined from the dependence on dilution of the critical molar ratio [OH] /[NCO] necessary for gelation (25) and this value was used for the statistical calculation of the fraction of EIC and its effect on Ve as described in (16). The calculation has shown that the fraction of bonds wasted in EIC was 2-2.5% and 1.5-2% for network from LHT-240 and LG-56 triols, respectively. 

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. 

To form a polymer network, at least one of the starting components must have functionality,/, (equal to the number of functional groups per molecule) larger than two if > 2). This is a necessary but often not a sufficient condition. The precursors of networks differ in two ways (1) they are of low or of high functionality, (2) they bear functional groups that are engaged in bond formation either by stepwise or chain mechanisms. 

Lattice percolation models were the first spatial simulation models applied to the network build-up. Classic lattice or off-lattice percolation modeling is based on random introduction of bonds between components placed randomly on the lattice or in space [56-58]. They suffer from the rigidity of the system and disregard of conformational changes accompanying the structure growth. These assumptions implicitly mean that the bond formation is much faster than conformation changes. Such assumption is somewhat closer to reality for fast bond-... 

Note Bond formation or attraction between particles or network chains within a gel induces the contraction and thereby the exudation of liquid from the network. 

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