Determination of phase diagram of a ternary PVDF/γ-BL/DOP system in TIPS process and its application in preparing hollow fiber membranes for membrane distillation

A mixed diluent composed of γ-butyrolactone (γ-BL) and dioctyl phthalate (DOP) was employed for preparing PVDF microporous membrane via TIPS method. The relation equations expressing the quantitative relationship between interaction parameter and phase separation temperature were deduced based on Flory–Huggins equation and solubility parameter theory. The unknown constants in the relation equations were solved out by fitting the binodal curve with cloud points in phase diagram. The binodal curves of PVDF/mixed diluent systems with different diluent compositions were calculated according to the relation equations. The compositions possible to yield bicontinuous structure inside microporous membrane were predicted in phase diagram. Finally, the hollow fiber membrane with interconnected sponge-like structures was prepared successfully from the predicted system (PVDF 12.74%, DOP 58.44%). The DCMD performances of the fabricated hollow fiber membranes were investigated with 3.5 wt% NaCl solution. The permeate flux of the membrane achieved 51.5 kg/m2 h with the NaCl rejection of 99.99% at the feed temperature of 90 °C. As a result, this prediction method is effective in constructing the polymer/mixed diluent system which is used for preparing MD hollow fiber membrane in TIPS process.

► The quantitative relationship between the interaction parameter and the phase separation temperature is established.
► The compositions possible to yield bicontinuous structure inside microporous membrane are obtained.
► The PVDF hollow fiber membrane applied in membrane distillation is prepared via TIPS method.