Stuur 4.50 A polymeric material is being considered as a coating film to protect an optical storage disk from water vapor damage. Suppose we need to protect a disk surface from an ambient me- dium that will be at 40% relative humidity at 21C. How thick must the film be if the concen- tration at the disk/protective film interface is to be below 0.01 wt% water in the film, for a period a of 5 years? The film density is 1.224 g/cm. At 100% relative humidity (at 21C) the equilibrium uptake of water in the film is 5.4 wt %. The absorption isotherm for water in this polymer is linear. The diffusion coefficient of water in the film is 8.2 x 10-10 cm-/s