We report the effects of polyethylene glycol (PEG) as a polymeric additive on the phase behavior of polyacrylonitrile (PAN)/dimethyl sulfone (DMSO2) mixtures and the structure formation of PAN membranes via thermally induced phase separation (TIPS). DMSO2 was chosen as a crystallizable diluent, while PEG was considered in terms of molecular weight (M-w) and dosage. Their effects on the phase behavior and the droplet growth kinetics of PAN/DMSO2/PEG ternary system were systematically derived from optical microscopy and differential scanning calorimetry as well as the solubility parameter theory. We found that the addition of PEG brings about the ternary system thermodynamically less stable, facilitates liquid-liquid (L-L) phase separation and suppresses the crystallization of DMSO2. The L-L phase separation can be promoted by raising the M-w or the dosage of PEG, and is accompanied by an increase in the growth rate of droplets. These results match with the pore size of the obtained PAN membranes. SEM images indicate that needle-like pores are obvious in the membrane when 10 wt% PEG-200 is added, whereas lacy pores gradually appear as the M-w of PEG increases from 200 to 1000. In addition, cellular pores can be created by raising the dosage of PEG-200 or reducing the cooling rate. Correspondingly, the membrane presents relatively high water flux (1528 L/m(2) h) and porosity (83.6%) when the ternary system contains 10 wt% PEG-1000 and cools in air bath at 30 degrees C. Besides, the presence of PEG in membrane matrix was examined by FTIR spectroscopy. PEG with different M-w almost completely leaches out and the pore structure keeps well after immersing the membrane in water for 144 h. (c) 2013 Elsevier B.V. All rights reserved.