Insight into the role of amphiphilic pluronic block copolymer as pore-forming additive in PVDF membrane formation

The use of additives in the polymer dope solutions is one of the most powerful strategies to control the pore structure of poly(vinylidene fluoride) (PVDF) membranes for various applications. In present work, the effects of amphiphilic pluronic block copolymer as an additive on PVDF membrane formation has been studied and a membrane formation mechanism involving pluronic has been proposed. The surface tension-concentration curve reveals that no micelles are formed in pluronic/N-methyl-2-pyrrolidone (NMP) solutions while the thermodynamics and kinetics are found to be similar for the systems containing pluronic and polyethylene glycol (PEG), respectively. The membranes prepared with pluronic as the additive exhibit larger surface pore, porosity, and macrovoids than that with PEG, indicating that pluronic has a stronger pore-forming ability. Instead of the commonly used concept of instantaneous or delayed demixing, it is believed that the stronger pore-forming ability of pluronic is caused by the hydrophobic-hydrophobic interaction between pluronic and PVDF that delays the solidification process. Since pluronic is not completely stable in PVDF membrane matrix, the membranes are soaked in 2-propanol (IPA) to purposely wash out pluronic. The loss of pluronic after IPA-treatment causes an increase in water permeation and a decrease in rejection, indicating that the presence of pluronic on the membrane pore surface can narrow down the pore size. By adding lithium chloride as a second additive, the growth of macrovoids is suppressed. Moreover, the resultant membranes exhibit both higher permeability and selectivity.