Network formation, control

In the case of network formation controlled by (irreversible) kinetics programmed polymerization regime (starved feed conditions, etc.). 

The article is an overview on network formation theories, their experimental verification and application to more complex systems of Industrial Importance. Network formation controlled by specific or overall diffusion Is also discussed. 

K. Dusekand J.P. Pascault, in The Wiley Polymer Networks Group Review, Vol. 1, Chemical and Physical Networks Formation Control of Properties, K. te Nijenhuis and WJ. Mijs, Eds, John Wiley Sons, Chichester (1998) p. 277. 

The low quantum yield of the photografting process (0 = 2 X 10 ) provides a good opportunity to control the network formation (curing time control), and accordingly, the desirable properties of the crosslinked or grafted copolymer might be obtained. 

With appropriate choice of reaction conditions, hyperbranched polymers can be formed by sclf-condcnsing vinyl polymerization of monomers that additionally contain the appropriate initiator (NMP, ATRP), when the compounds are called inimers, or RAFT agent functionality. Monomers used in this process include 340,716 341717 and 34204 (for NMP), 108714,714 and 344 and related monomers720 723 (for ATRP) and 343408 (for RAFT). Careful control of reaction conditions is required to avoid network formation. 

Generalization of Flory s Theory for Vinyl/Divinyl Copolvmerization Using the Crosslinkinq Density Distribution. Flory s theory of network formation (1,11) consists of the consideration of the most probable combination of the chains, namely, it assumes an equilibrium system. For kinetically controlled systems such as free radical polymerization, modifications to Flory s theory are necessary in order for it to apply to a real system. Using the crosslinking density distribution as a function of the birth conversion of the primary molecule, it is possible to generalize Flory s theory for free radical polymerization. 

Alternatively, Leung and Eichinger [51] proposed a computer simulation approach which does not assume any lattice as the classical and percolation theory. Their simulations are more realistic than lattice percolation, since spatially closer groups form bonds first and more distant groups at later stages of network formation. However, the implicitly introduced diffusion control is somewhat obscure. The effects of intramolecular reactions were more realistically quantified, and the results agree quite well with experimental observations [52,53],... 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

Bokobza, L. Clement, F. Monnerie, L. Lapersonne, P. In Chemical and Physical Networks, Formation and Control Properties Nijenhuis, K., Mijs, W. J., Eds. The Wiley Polymer Networks Group Review Series, Wiley New York, 1998 Vol. 1, 321. 

It is important to note that this statistical calculation is only vaHd as long as the kinetics of network formation is totally controlled by the reactivity between the precursor monomers. With the formation of an infinite network at gelation and corresponding increase in viscosity, the reaction is slowed down considerably. Consequently, Eq. (15) is only valid prior to gelation. 

The network formation theories are based mainly on the assumption of the validity of the mass action law and Arrhenius dependence of the rate constants. However, diffusion control can be taken into account by some theories in which whole molecules appear as species developing in time. 

Therefore, concentrations of the polyvinyl monomer, the branching and network formation proceeds as shown in Figure 12. Mlcrogel-llke species are formed first which are highly internally crossllnked. The pendant double bonds in the interior are very Immobile so that their reactivity is strongly diffusion controlled and they almost cannot react at all, even with the monomers. Only the more mobile pendant double bonds in the periphery of these species can enter into reactions with macroradicals and participate in Interbinding of the species together. 

The branching theories In their present state can treat a number of complex branching reactions of Industrial importance. It is to be stressed, however, that there does not exist any universal approach to all systems. The understanding of the reaction mechanism and kinetics is a necessary prerequisite for adaptation of the proper theory to give relations for structural parameters. Further progress in the network formation theory seems highly desirable particularly in the field of cycllzatlon and diffusion control and in understanding the network structure-properties relations. 

Model networks of known structure can be prepared by end-linking functionally-terminated polymer chains [3, 4, 157]. Because of the nature of this network formation process, the molecular weight of the starting chains becomes the critically important molecular weight between cross-links, Mc. Control of Mc and its... 

In this case of three-monomer polyurethane synthesis, there is no thermodynamic driving force for phase separation. The formation of clusters is fully controlled by the initial composition of the system, the reactivity of functional groups, and the network formation history (one or two stages, macrodiol or triol reacted with diisocyanate first, etc.). 

Epoxy networks may be expected to differ from typical elastomer networks as a consequence of their much higher crosslink density. However, the same microstructural features which influence the properties of elastomers also exist in epoxy networks. These include the number average molecular weight and distribution of network chains, the extent of chain branching, the concentration of trapped entanglements, and the soluble fraction (i.e., molecular species not attached to the network). These parameters are typically difficult to isolate and control in epoxy systems. Recently, however, the development of accurate network formation theories, and the use of unique systems, have resulted in the synthesis of epoxies with specifically controlled microstructures Structure-property studies on these materials are just starting to provide meaningful quantitative information, and some of these will be discussed in this chapter. 

Mixed metal alkoxide systems are also of interest as a means of creating additional hybrid systems. However, recognition of the large differences in their hydrolysis and condensation rates is crucial. For example, if titanium isopropoxide is made to react under the same conditions as might be used for TEOS, hydrolysis and condensation rapidly occur and lead to particulate rather than network formation of Ti02- Cocondensation with TEOS under these conditions does not occur because of the fast precipitation of the titanium dioxide species. Indeed, of the general metal alkoxides, those based on silicon tend to be more easily controlled because of their slower hydrolysis... 

As an extension of our research program concerned with the elucidation of the mechanism of three-dimensional network formation in the radical polymerization of multivinyl compounds, we attempted to control network formation with the intention of collecting the basic data for the molecular design of three-dimensional vinyl-type polymers with high performance and high limctionality. 

As is evident fium the above mechanistic discussion, the problems involved in the network formation of fi-ee-radical multivinyl polymerization are quite complicated and many factors are relevant to the network formation processes and their contributions depend on the polymerization conditions. Thus, finding an appropriate model for a comprehensive gelation theory and, moreover, controlling the network structure are very important but they are quite difficult subjects. In the polymerization of multivinyl compounds having different es of vinyl... 

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