Case II diffusion of water in Na2O–3SiO2 glass: Constant tensile stress gradient at the diffusion interface

Some polymers and oxide glasses exhibit unusual diffusion of liquid or gas, with the depth of diffusion exhibiting a linear increase with time, instead of normal square root of time dependence. There have been many models, but very few experimental data that can help clarify the cause of the phenomenon's existence in glass. Residual stress in sodium trisilicate glass (Na₂O–3SiO₂) samples was characterized following Case II water diffusion at 80°C in a saturated water vapor environment. The surface-swelled layer of the glass was removed by dissolving it in water, and birefringence of the newly revealed surface layer was measured. The presence of a constant negative tensile stress gradient was revealed by indicating that Case II diffusion in sodium trisilicate glass originates from this stress gradient, which overwhelms the more typical Fick's law concentration-dependent flux.