AbstractThe influence of certain aspects of film surface on the diffusion of urea in aqueous solutions through films cast from two acrylic–methacrylic ester polymers has been investigated. Greater rate of permeation by urea occurred in films of both polymers when the lower surface (that in contact with the substrate during casting) was the entry surface for urea than when the upper surface (that in contact with the atmosphere during casting) was the entry surface. Furthermore, this difference between the upper and lower surfaces was more marked in film cast from one of the polymers; and scanning electron micrographs of films of the two polymers revealed differences in gross surface structure. Changes occurred in both upper and lower film surfaces during exposure to urea solution, the lower surface developing larger pores on exposure while smaller pores were observed on the upper side after exposure. The upper‐lower surface differences occurred in films of both polymers. The less hydrophilic film (Eudragit RS100) was associated with lower overall permeation rate for urea and smaller postpermeation mean pore size on both upper and lower surfaces than the more hydrophilic film (Eudragit RL100). The relationship between permeability and changing microscopic appearance of the surfaces is discussed and pictorial evidence of porous structure presen
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