Fine tuning the pore size and permeation performances of thermally induced phase separation (TIPS) -prepared PVDF membranes with saline water as quenching bath

Abstract

The PVDF membranes were prepared via thermally induced phase separation (TIPS) method with saline water as quenching bath and applied to the separation of oil/water emulsions. Surface pore size, permeation performance and other properties of the PVDF membranes were finely tuned by adjusting salt concentration in quenching bath. The results show that with the addition of NaCl in quenching bath, the thermal transfer resistance increases in the TIPS process due to the obvious increase of bath viscosity and the slight decline of thermal conductivity of saline solution. All the PVDF membranes present bi-continuously fibrillar structure in top surfaces. The fibrils are fused together and get thicker, and leads to rougher top surfaces due to the decline of cooling rate with the addition of NaCl in quenching bath. At the same time, the PVDF membranes exhibit expanded pore size and higher permeation flux for both pure water and oil/emulsions. The PVDF membrane of S20 (prepared with 20 wt% NaCl aqueous solution as quenching bath) exhibits the largest mean pore size and the highest pure water permeation flux of 2481.1 L m−2 h−1 bar−1. However, overhigh NaCl content leads to the inverse trend. For example, the PVDF membrane of S30 (quenching bath containing 30 wt% NaCl) exhibits a lightly decreased pore size and pure water permeation flux compared to S20. All of the PVDF membranes exhibit good oil rejections above 90.0% for surfactant-stabilized oil-in-water emulsions and above 95.0% for surfactant-free oil-in-water emulsions.