Measurement of diffusion coefficients (NT POLY 188)

Abstract

  This Nordtest method specifies a procedure for measuring diffusion coefficients in polymer and coatings films. Weight gain or loss is followed from one equilibrium condition to another one. The time required for one-half of the total weight change, the half-time, t½, is measured. This is then used with the appropriate solution to the diffusion equation for a constant diffusion coefficient to calculate the diffusion coefficient, D0. This assumes that the surface concentration at the exposed surfaces immediately changes from the old equilibrium value to the new equilibrium value. The procedure requires documentation that both the initial, conditioned value and new equilibrium values are achieved, and that the climate during the absorption or desorption steps is maintained constant. The application of the results of this method may be related to practical application through experience. The diffusion coefficient allows estimations of diffusion phenomena for a different geometry, a different film thickness, and for either longer or shorter exposure times than those involved in the experimental procedure. Concentration gradients can also be calculated.