Specific Versus Overall Diffusion Control

As chemical, mechanical and electrical properties attain their ultimate values during the last stages of cure, the diffusion-controlled regime appears to be a very important part of the curing process. An accurate quantitative description of the impact of mobility restrictions on cure is therefore essential. Diffusion control can be specific or non-specific (overall). 

Specific diffusion control concerns translational diffusion of individual species (or reactive groups) and is characteristic for dilute solutions (e.g. free radical (co)polymerisation with the monomer as diluent). The diffusion rate of the active centres then depends on the molecular weight of the molecules to which they are attached, on their symmetry, on the composition and on the interactions. For example, in free radical polymerisations, a large difference between the size of polymer and monomer molecules (which remains the case along the entire cure path) is found and so a large difference in their diffusion coefficients is observed. 

Note that some ionic reactions involving smaller scale mobilities might remain unaffected below the main (glass) transition. An example is the production of low-temperature IPGs [41,42]. 

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