Transport Classification
Solvent penetration into a glassy polymer represents a highly non-Fickian transport mechanism governed by mechanical relaxation rather than simple concentration gradients. Textile wet processing frequently encounters case II diffusion during the solvent dyeing of synthetic yarns where the rate of penetrant front movement remains constant over time. This linear rate of penetration occurs because the swelling process introduces a sharp boundary between the unpenetrated glassy core and the swollen rubbery shell.
Kinetic Rate
Relaxation times of the polymer chains control the entry of molecules during this state of transport. Traditional diffusion models rely on concentration-dependent rates, but case II diffusion displays a zero-order relationship where the solvent front moves at a uniform velocity. This difference determines the processing time needed for uniform dye distribution across the filament cross-section.
Polymer Softening
High internal mechanical stress arises at the phase boundary where the glass transition temperature of the substrate drops below the ambient operating temperature. When the penetrant acts as a solvent, the localized swelling generates tension that forces the adjacent glassy region to yield. As a result, the polymer transitions to a rubbery state, permitting immediate migration of dye molecules through the swollen region.
This relaxation-controlled behavior dictates the speed of the finishing cycle, ensuring that dye penetration occurs uniformly across every filament without risking structural breakdown of the synthetic yarn.
Inspection Standard
Verification of this transport behavior requires precise tracking of the boundary propagation through optical microscopy of dyed fiber cross-sections. Process technicians must monitor bath temperatures closely to ensure that the yarn undergoes the necessary relaxation without causing macro-structural failure. Excessively rapid solvent penetration can lead to fiber deformation or loss of tensile strength.
