Transport Process
Moisture migration through a polymer matrix happens at a rate that is not proportional to the concentration gradient alone but is controlled by the relaxation rate of the polymer chains. This behavior of non-fickian diffusion is common in synthetic fibers when they are dyed or washed at temperatures near their glass transition point. Research labs model this behavior to optimize the dye cycle times for complex synthetic yarns.
The boundary of this transport model is reached when the temperature rises well above the glass transition, where normal diffusion behavior returns.
Polymer Relaxation
Mechanical stress and structural rearrangement within the fiber occur as the solvent penetrates the polymer network. When the rate of polymer chain relaxation is slower than the penetrant speed, non-fickian diffusion becomes the dominant transport mechanism. This leads to a sharp front of solvent moving into the fiber rather than a smooth gradient.
Dyeing Cycle
Industrial dyeing processes for polyester and nylon must account for this non-uniform transport rate. The transition to non-fickian diffusion during pressurized dyeing explains why certain dyestuffs require extended hold times at high temperatures. This understanding allows mills to adjust their temperature ramp rates to prevent uneven dye distribution and blotchy fabrics.
Fiber Swelling
Significant swelling of the polymer structure occurs during this penetration process. This change alters the physical dimensions of the yarn and can create temporary stress cracks in the fiber surface.
