Fickian kinetics

Hartauer et al. [58] reported that peroxide residues in povidone (binder) and crospovidone (disintegrant) were attributable to the formation of the A-oxide oxidation product of raloxifene. The authors correlated residual levels of peroxide in the excipients with A-oxide formation and thereby gained understanding of the degradation mechanism. A radical-initiated oxidation mechanism would be expected to show a typical S -shaped autocatalytic curve, whereas these curves showed fickian kinetics that is, rapid initial formation of the A-oxide followed by a plateauing of the rate, with consumption of the peroxides, leading to a slowing of the reaction rate. 

The diffusion of CO2 in polymers follows Fickian kinetics over the entire pressure range diffusivity increases with concentration, in line with other evidence of the plasticizing action of C02 ... 

While many resin systems exhibit Fickian diffusion, others show non-Fickian behaviour. The experimental reabsorption diffusion curve shown in Figure 12.2 represents a smooth process which is linear over the hrst 60% before reaching equilibrium. For the resin system used, Fickian diffusion could only be obtained after preconditioning because often the early stage of moisture absorption deviates from Fickian kinetics. This creates difficulties in the calculation of a precise diffusion constant. It is thought that the non-Fickian behaviour results from the reorganisation of the network as the material becomes plasticised [9—11]. 

If the Deborah number is large (large molecular relaxation time or small diffusion time), the diffusion process is described by Fickian kinetics and is denoted by an elastic diffusion process. The polymeric structure in this process is essentially unaffected and coefficients of mutual and self-diffusion become identical. Elastic diffusion is observed at low solvent concentrations below the glass transition temperature. ... 

Using Fickian kinetics and the time required to reduce the conductivity to one-half of its initial value, i.e. a/[Pg.473]

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