Network rigidity

The origin of the diffusional response may arise from either 1) an intrinsically slow rate of electron transfer between redox sites (self-exchange) or 2) fast electron transfer limited by the existence of a structural barrier due to incorporation of the sites into a polymeric film. The latter effect could be brought about in one of two ways a) effective isolation of redox sites because of polymer network rigidity, or b) inability of the film to incorporate or expel a sufficient quantity of charge-compensating counterions during the redox process. 

The required network rigidity results from the three-dimensional cross-linking of the [SiC>4]4 tetrahedrons. Bulk modification by introduction of organic units through =Si-C- bonds leads to a change of network connectivity and should also affect porosity or surface area. On the other hand, in this case the modification becomes an intrinsic property that should not be affected by surface corrosion (Scheme II), because after removal of the surface layer by hydrolytic processes, the following layer exhibits the corresponding structure and properties. The properties to be developed determine whether a surface or a bulk modification is more appropriate. 

The single-phase uniformly crosslinked open network can be stressed in two opposite ways, on shrinkage and extension. As was discussed earlier, stresses of shrinkage appear in the dry hypercrosslinked material as a result of confrontation between the network rigidity and the attraction forces... 

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