Network formation general features

As distinct from almost all polymer materials, the kinetics of IPN formation is a governing factor in development of the system morphology. For traditional polymeric materials their structure and morphology depend on the ways of processing, heat treatment, and other physical, not chemical, factors, while for IPNs all depends on the kinetic conditions. One may say that thermodynamics gives the general rules of the equiUbrium state and determines the path to equilibrium, whereas the kinetics allows the reahzation of the path and predetermines the real structure far from equilibrium. This specific feature of the IPN formation is connected with the fact that in the reaction system two processes proceed simultaneously the chemical process of network formation and the physical process of phase separation. As will be shown below, these processes are interconnected. 

Only recently has a rather general and rigorous approach been developed relating these observations to the characteristic features of the reactions and of the catalyst. These developments are reviewed in the following sections, without entering into the mathematical details. Situations with increasing complexity will be dealt with coke formation on sites only, and on sites located in pores or in networks of pores, with or without diffusion limitations. When only sites are considered, the deactivation functions will be written for a number of sites in a pore and for a particle <1). Experimental values, deduced from measurements on particles or beds of particles, will be represented by O. 

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

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