Preparation hollow fiber membranes via thermally induced phase separation by using green solvents

Abstract

In this study a new, environmental friendly diluent (ATBC) was introduced to prepare flat sheet and hollow fiber poly(vinylidene fluoride) (PVDF) membranes via thermally induced phase separation (TIPS). Phase diagrams of PVDF/diluent are presented and the effect of different parameters such as polymer concentration, quenching temperature, air gap and bore fluid temperature on the morphologies, properties and water permeability of the PVDF membranes were investigated. In the second step, the effect of preparation conditions and polymer molecular weight on membrane morphology and performance were investigated. By increasing the quenching bath temperature, the overall porosity increased and the tensile strength of the hollow fiber membranes decreased, whereas the washing temperature did not affect these properties significantly. High molecular weight PVDF enhanced the mechanical properties and reduced porosity of the resulting hollow fiber membranes. Hollow fiber membranes were prepared with a very sharp pore size distribution and used for direct contact membrane distillation to demonstrate one of the potential applications of this membrane. In the third step, a new water soluble green solvent was introduced as a low toxic diluent to fabricate PVDF membranes via TIPS method. A specific fibrillar structure which yields the PVDF membranes with high elongation and permeability was formed. Morphologies, structures, properties and permeation performances of the PVDF membranes showed that they presented α phase, narrow and unimodal pore size distribution, bicontinuous and fibrillar structure, good mechanical strength and competitive permeability. In the last step, a water- soluble biodegradable green solvent was used to prepare PVDF hollow fiber membranes. Due to the solubility of solvent in water, a combination of thermally induced phase separation and non-solvent induced phase separation was observed during the membrane formation. The effects of additives such as poly(vinyl pyrrolidone) (PVP) with different molecular weights (10k, 55k, 360k and 1300k) and glycerol (5 wt% and 15 wt%) were studied at different quenching bath temperatures (5, 15 and 25°C). The properties of the hollow fiber membranes, such as their morphology, roughness, mechanical properties, overall porosity, water permeability, polymorphism in the skin layer and polymorphism in the bulk of the membrane structure were investigated as well. A phase diagram for the PVDF/solvent system showed the PVDF crystallization temperature around 53 oC. Solvent along with additives induces porous structure and the formation of β-polymorphs in the membranes, resulting in high water permeability.