Polyethylene (PE) membranes having various porosities are required for use as microfilters and separators in lithium ion batteries. Phthalates, whose mixtures with PE exhibit upper critical solution temperature (UCST)-type phase behavior, are used as diluents for the fabrication of PE membranes. However, the use of phthalates as diluents is limited because of their toxicity. Triethylolpropane tris(2-ethylhexanoate)(TEPTEH) was synthesized for use as a new nontoxic diluent for the fabrication of the PE membranes. The PE/TEPTEH blends showed UCST-type phase behavior and their phase separation temperatures decreased with increasing PE content. TEPTEH and paraffin oil (PO) mixtures were also examined as diluents to control the porosity of PE separators. PE//TEPTEH/PO) blends also exhibited UCST-type phase behavior when TEPTEH/PO diluent mixtures contained greater than or equal to 70 wt% of TEPTEH, and their phase separation temperatures decreased with increasing PO content in the diluent mixture with a fixed PE content. The average size of pores and the porosity of the membrane increased with increasing TEPTEH content in the diluent mixture. The porosity of the PE separator fabricated using a lab-scale twin extruder equipped with biaxial stretching machines also increased with increasing TEPTEH content in the diluent mixture. As a result, when the air permeability of the separator prepared from the PE/PO blend was compared with that prepared from PE/TEPTEH blend, the latter was about 2.5 times higher than the former. (C) 2013 Elsevier B.V. All rights reserved.