Non-Fickian Mechanism

Criteria 1-3 are the cardinal characteristics of Fickian diffusion and disregard the functional form of D(ci). Violation of any of these is indicative of non-Fickian mechanisms. Criterion 4 can serve as a check if the D(ci) dependence is known. As mentioned, it is crucial that the sorption curve fully adhere to Fickian characteristics for a valid determination of D from the experimental data. At temperatures well above the glass transition temperature, 7 , Fickian behavior is normally observed. However, caution should be exercised when the experimental temperature is either below or slightly above 7 , where anomalous diffusion behavior often occurs. 

A drug release mechanism from compressed cellulose ether matrices using dimensionless analysis has been discussed (17). A non-Fickian mechanism could be dismissed when calculating the Deborah and the Swelling interface numbers from relaxation, penetrant diffusion, swelling and drug diffusivity data (17). 

Graft copolymer of GG with acrylamide by crosslinking with glutar aldehyde Verapamil and Nifedipine Hydrogel Microspheres Drug release followed non-Fickian mechanism. [47,66]... 

It was demonstrated that concentration profiles of different components can be obtained utilizing the partial least squares method. With concentration profiles readily measurable, it is possible to obtain other quantities such as diffusion type, that is, Fickian versus non-Fickian mechanism of diffusion. The application of in situ ATR FT-IR spectroscopic imaging has been employed to investigate polymer interdiffusion of PVP and polyfethylene glycol) (PEG) under high pressure CO2 [74]. The diffusion mechanism of the system was described based on the spectroscopic imaging data, and it was found that CO2 molecules dissolved in the polymeric system greatly enhanced the interdiffusion process. 

Drug release from soluble polymers is accompanied by the gradual erosion-type dissolution of the polymer. Therefore, polymer dissolution and drug diffusion may be the overall hybrid mechanism of release. Drug release from nonsoluble hydrogels generally follows Fickian or non-Fickian diffusion kinetics [51]. The mechanism of... 

Although Fickian diffusion has been used to describe the diffusion mechanism of liquid through the rubbery material because of its simplicity and mathematical tractability, most of the rubber-solvent systems do not fit such a simplified explanation. Then, non-Fickian diffusion was developed. The case of non-Fickian diffusion was investigated using the following equation ... 

Whereas the value of n equals 0.5, the mechanism of diffusion described by Case I of Fickian model, when n values range between 0.5 and 1 that exhibits anomalous transport model and when n value equals 1, Case II of non-Fickian model is used to determine. Fickian model describing the rate of diffusion of penetrant molecule is much less than the relaxation rate of polymer chains while Case II of non-Fickian diffusion representing rate of diffusion is rapid than relaxation process. For the anomalous transport model, both solvent diffusion rate and polymer relaxation rate are comparable. Figure 27.2 shows various types of non-Fickian model anomalous transport and Case II of non-Fickian are included in this group. ... 

For diffusion of liquid through rubbery polymer composites, Fickian and non-Fickian diffusion theories are frequently used to describe the mechanism of transport, but for gas or vapour, other models have been developed to fit experimental data of diffusion profiles. The models of gas transport include Maxwell s model," free volume increase mechanism," solubility increase mechanism," nanogap hypothesis," Nielsen model, " " Bharadwaj model, ° Cussler model " " and Gusev and Lusti model, " etc. 

Hydrophilicity-to-hydrophobicity ratio It affects dmg solubihsation and release rate and mechanism (the increase of hydrophihcity enhances sweUing, causing a shift from Fickian to non-Fickian diffusion) Ikeda et al. (1990) Hafeman et al. (2010) Wang et al. (2013) Mattu et al. (2013) Ferreira et al. (2014) Gentile et al. (2015a)... 

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