Activated diffusion through polymers

When a polymer film is exposed to a gas or vapour at one side and to vacuum or low pressure at the other, the mechanism generally accepted for the penetrant transport is an activated solution-diffusion model. The gas dissolved in the film surface diffuses through the film by a series of activated steps and evaporates at the lower pressure side. It is clear that both solubility and diffusivity are involved and that the polymer molecular and morphological features will affect the penetrant transport behaviour. Some of the chemical and morphological modification that have been observed for some epoxy-water systems to induce changes of the solubility and diffusivity will be briefly reviewed. 

The release of the active ingredient from the capsule is governed by a number of steps. The active material will have a high concentration cq inside the capsule. It must first dissolve in the wall - as we shall see, the solubility K has to be low. It then diffuses through the polymer wall into the surroundings. 

When the active centre is surrounded by a layer of solid polymer, further propagation will be controlled by the rate of monomer diffusion through the polymer layer. Usually it will be retarded. With a porous polymer layer surrounding the active centres, monomer transport will be easier. These effects must be considered when highly crystalline polymers are formed, especially when the chains grow from a non-transferring monomer as, for example, with coordination polymerizations [56],... 

However, much uncertainty remains as to the role played by the monomer diffusion to the active center through the polymer layer which covers it or through the pores of the catalyst itself. 

Alternatively, a drug and matrix polymer may be coated onto the cores simultaneously. The active drug is released over an extended period either via diffusion through the polymer or through to the controlled erosion of the polymer coating. 

The two basic processes depicted within a given analysis are the primary diffusive transport of the active agent through the polymer delivery vehicle Into the biological receiving medium and the subsequent transport, distribution and elimination of the drug. 

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