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In this study, the effect of a random copolymer of polyoxyethylene (POE, 38 mol) / polyoxypropylene (POP, 10 mol) pentaerythrytol tetramethyl ether [PEPTME (38/10)], which is unable to form a self-organizing structure on account of its bulkiness, on the microemulsion phase was examined. The phase diagram of the liquid paraffin -nonionic surfactant PEPTME (38/10) -water system was compared with that of polyoxyethylene (POE) / polyoxypropylene (POP) dimethyl ether (EPDME) system which has a straight-chain structure. In the previous study, the authors reported that a special droplet type microemulsion (highly concentrated microemulsion) was formed, when EPDME was distributed in the hydrophilic moiety of a surfactant in a lamellar liquid crystalline phase. In contrast, it was found that the use of PEPTME (38/10) resulted in disappearance of lamellar as well as hexagonal and cubic liquid crystals, and the formation of an optically isotropic and fluid phase owing to the relationship between the original microemulsion phase and the highly concentrated microemulsion phase. Thus, PEPTME (38/10) was considered to be most suited for preparing highly concentrated microemulsions.

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