Constant Front Speed in Weakly Diffusive Non-Fickian Systems

In certain polymer penetrant systems the e ects of Fickian di usion are dominated by nonlinear viscoelastic behavior Consequently such systems often exhibit concentration fronts unlike those seen in classical Fickian systems These fronts not only are sharper than in standard systems but also propagate at constant speed The mathematical model presented is a moving boundary value problem where the boundary separates the polymer into two distinct states glassy and rubbery where di erent physical processes dominate The moving boundary condition that results is not solvable by similarity solutions but can be solved by integral equation techniques In the case under consideration namely one where the standard Fickian di usion coe cient is small asymptotic solutions where a comparatively sharp front moves with constant speed are obtained