Fine structure of regenerated cellulose films as revealed by dye accessibility

Abstract

AbstractWe have demonstrated the utility of dyeing techniques, using Solophenyl Fast Blue Green BL, to characterize a variety of regenerated cellulosic films. Measurement of the optical density ratio and the rate and magnitude of dye uptake show that differences in structure exist in regenerated cellulose films processed under different conditions. These differences are attributed to differences in lateral order and orientation of cellulose chains and are believed similar to the so‐called skin and core observed in rayon fibers. These differences have been found related reproducibly to differences in regenerating and processing conditions. Diffusion of dye into a film is very rapid initially, slowing later to an almost constant rate, depending on the dye concentration in solution and on the type of film. A modified diffusion equation of the form In C/C0 – (1 – C/C0) = Dγ2‐tC0/Cf has been found adequate to express the non‐steady state of the observed absorption process, where C0 is the initial dye concentration of the test solution, C is the instantaneous dye concentration at time t, Cf is the dye concentration in the film, γ2 is the area of film per unit volume of dye solution, and D is the diffusion coefficient. Thus, a mathematical basis is provided for structural differentiation in cellulosic films processed in various ways. Plate‐cast viscose films appear to have a dense surface (skin) exposed to the regenerating bath and a porous surface (core) on the side touching the plate, whereas machine‐cast viscose films have a dense surface on both sides.