Diffusion control in the kinetics of cross-linking

Diffusion control of a cross-linking reaction manifests itself by a decrease or increase in the reaction rate. The increase is characteristic of diffusion-controlled termination in free-radical polymerizations. Segmental diffusion control is typical of reactions occurring in the glass transition region and is characterized by a sharp fall of the rate constants. Diffusion control is either specific, when the bond formation rare depends on the mobilities of individual reacting molecules or substructures, or non-specific, when the reaction rate depends only on the average properties of the system. Within the specific diffusion control, the diffusivity of molecules and substructures also affects the structure evolution on the rime and conversion scales. The effect of diffusion is discussed on the basis of the transition complex model. For slow (relative to diffusion) reactions, the specific diffusion control is not operative unless the critical region is reached. The observed deviations from the ideal branching process are also determined by static effects, such as volume exclusion, which make the rate constants dependent on the size and structure of the reacting molecules. The existing network formation theories have been analysed with respect to their ability to describe the effects of diffusion and static obstructions. An adequate network formation model working in the whole conversion range including the critical state should respect the changing mobility of the molecules or substructures which bear the reacting functional groups. Copyright (C) 1996 Elsevier Science Ltd.