TABLE OF CONTENTS The matter transfers taking place between polymer packaging and a liquid are controlled either by convection at the liquid-solid interface or by diffusion through the polymer.
Diffusion is the process by which matter is transported from one part of a polymer to another, as a result of random molecular motions. Diffusion is observed essentially in solid materials such as polymers, under the influence of a concentration gradient of diffusing substance, without appreciable displacement of the macromolecules of polymer.
This phenomenon takes place in liquids it involves the transfer of matter by mixing one part of fluid with another. The motion of the fluid may be entirely the result of differences of density or of concentration as in natural convection, or the motion may be produced by mechanical means, as in forced convection.
Heat conduction occurs when heat passes through the solid under the influence of a gradient of temperature, and there is an analogy between the processes of heat conduction and the matter transfer controlled by diffusion. This was recognised by Fick in 1855, who put diffusion on a quantitative basis by adopting the mathematical equation of heat conduction established some years earlier by Fourier in 1822.
The mathematical theory of diffusion in isotropic materials is thus based on the hypothesis that the rate of transfer of diffusing substance through unit area of a section of the material is proportional...
