The formation mechanism of an “asymmetric” porous structure for the film cast from ethyl acrylate‐methyl methacrylate copolymer emulsion prepared by emulsifire‐free emulsion polymerization was investigated. The formation of this structure was affected by the emulsion stability which was varied by the postaddition of sodium sulfate or sodium dodecyl benzene sulfonate. It is concluded that it is derived from the production of porous skin film at the emulsion‐air interface and the precipitation of flocculated particles at the bottom of emulsion during drying process.

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SynopsisI t was found that the film cast on a glass plate from emulsifier-free ethyl acrylate-methyl methacrylate copolymer emulsion when separated from there shrank and curled immediately. This film had an "asymmetric" porous structure. It was discussed the relationship between the porous structure and the shrinkage due to the residual internal stress produced during film formation.