Ping-Yun Zhang

Investigation of Polyvinylidene Fluoride Membranes Prepared by Using Surfactant OP-10 Alone or with a Second Component, as Additives, via the Non-Solvent-Induced Phase Separation (NIPS) Process

Polyvinylidene fluoride (PVDF) flat-sheet membranes were prepared via a non-solvent-induced phase separation (NIPS) method at 60°C using a hydrophilic surfactant OP-10 (octylphenol polyoxyethylene ether) solely (Blank) or with a second additive [H2O or lithium chloride (LiCl)] as pore-forming agents. The influence of OP-10 concentration on the surface tension, viscosity, and precipitation rate of PVDF/(H2O, LiCl, or Blank) systems were investigated, and the ultrafiltration and mechanical properties of the resultant membranes were measured. It was found that an increased demixing rate during the coagulation process was the reason for the change in membrane morphology and properties. An obviously improved flux and slightly decreased mechanical properties and rejection were found in membranes prepared using a high concentration of OP-10 and the second component as additives. SEM pictures revealed an increased porous structure on the resultant membrane surface. A hypothesis was proposed to explain these phenomena; the reoriented surfactant molecules at the interface facilitated the water diffusion channels, which finally became the porous structure on the membrane surface. The weakened mechanical properties were due to the macrovoid structure in its membrane cross-section, which developed from the micelle structure in the casting solution. This hypothesis was further confirmed in a PVDF/OP-10/polyethylene glycol (PEG) system. A consistent conclusion was obtained.