Preparation of polyethylene microporous membranes with high water permeability from thermally induced multiple phase transitions

- We prepare microporous membranes based on the HDPE/DIDP/DBS matrix.
- The addition of HDPE-g-MAH changes the distribution of DBS.
- Multiple phase separations exist in the cooling process.
- Different cooling rate affects the morphology of membranes.
- High permeability and good rejection rate membranes are obtained.

Thermally induced phase separation (TIPS) has been widely used to prepare microporous membrane. The interaction and dynamic asymmetry between polymer and solvent sometime make the transition happen through viscoelastic phase separation (VPS) mechanism. Under such condition, cell like structure is obtained with low water permeability. In this paper, a new method is proposed to obtain high permeability membranes by TIPS. Dibenzylidene sorbitol (DBS) and HDPE-g-MAH are introduced into the polyethylene/di-iso-decyl phthalate (HDPE/DIDP) system, which exhibits an upper critical solution temperature (UCST) behavior and undergoes VPS mechanism during cooling. DBS can self-assemble and form fibril network both in HDPE and DIDP. The addition of HDPE-g-MAH makes DBS mainly distributed in HDPE-rich domains, while it mostly locates in the DIDP-rich domain in the ternary system due to their similar polarity. The self-assembly of DBS in the HDPE-rich domains can induce the secondary liquid-liquid phase separation, which significantly change the structure in walls between cells. High permeability of pure water and high retention of permeability in silica suspension can be obtained under proper control of the extent of these phase transitions.