Meaning
Molecular transport of a substance through a polymer or other medium occurs down a concentration gradient under steady conditions. In many industrial packaging scenarios, Fickian diffusion describes this rate of mass transfer where the flux is proportional to the concentration gradient. This mathematical framework applies when the polymer chain relaxation rate is much faster than the penetrant diffusion rate.
Transport Kinetics
Linear relationship governs the movement of a penetrant through the polymer matrix. When an engineer models the barrier properties of a plastic film, Fickian diffusion acts as the baseline for predicting shelf life. The diffusion coefficient remains constant at a given temperature, allowing straightforward calculations of gas transmission.
If the polymer undergoes structural transitions during the process, the linear model ceases to hold. Polymer scientists rely on these measurements to qualify raw materials before production scale up.
Concentration Dependency
Plasticization effects sometimes alter the rate of penetrant movement through the material. Under high humidity or solvent exposure, the transport behaviour of many polymers deviates from the classical Fickian diffusion model due to polymer swelling. Absorbed water molecules open up the polymer structure and increase free volume.
This increase accelerates the passage of subsequent penetrants through the barrier.
Boundary Limit
Non-Fickian behaviour typically emerges when the temperature approaches the glass transition point of the polymer. Under these specific conditions, viscoelastic relaxation times become comparable to diffusion times.
