Diffusion Anomaly
A molecular transport phenomenon describes liquid or vapor absorption in glassy polymers where the rate of diffusion does not follow classical Fick’s laws. In the finishing of synthetic textiles, non-Fickian sorption is observed when nylon or polyester fibers absorb moisture or dyes while undergoing structural relaxation. This behavior occurs because the polymer chains cannot adjust instantaneously to the presence of the penetrant molecules.
Polymer Mechanics
Swelling stress and polymer relaxation control the rate of chemical transport in these materials. During non-Fickian sorption, the absorption curve exhibits an anomalous profile with an initial delay followed by a rapid increase in weight gain as the polymer transitions from a glassy to a rubbery state. This transition is highly dependent on both the temperature of the dyeing bath and the concentration of the dye molecules.
The relaxation of the polymer network creates extra free volume, which in turn accelerates further solvent penetration.
Analytical Modeling
Quantifying this anomalous behavior requires advanced diffusion models that incorporate relaxation time constants alongside standard diffusion coefficients. These models help textile engineers predict the exact time required for dye molecules to penetrate the core of the fiber during high-temperature dyeing.
Industrial Impact
Controlling the rate of dye uptake is essential to avoid unlevel dyeing or shading variations in the finished fabric. If relaxation happens too slowly, the dye remains concentrated on the fiber surface, reducing its wash and rub fastness.
