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The water vapour transmission properties of selected hydrophilic, lipophilic, and mixed hydrophilic‐lipophilic polymer systems, cast as free films, have been examined as a function of time, film thickness, plasticiser concentration, and film formulation. An inverse linear relationship was established between the logarithm of the water vapour transmission rate (Rwvt) and the logarithm of film thickness in the presence of the various plasticiser concentrations for the three systems studied. The lipophilic n‐butyl methacrylate films were found to be less permeable to moisture than either the hydrophilic hydroxypropyl cellulose or mixed methyl hydroxypropyl cellulose‐ethyl cellulose films. The butyl methacrylate films closely follow Fickian diffusion, whereas the permeability constants of the other two films increased linearly with film thickness over the range of thicknesses studied. These phenomena are discussed with respect to the relative attractions these films have for water vapour.

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