Fabrication And Performance Of Poly (L-lactic Acid) Antifouling Ultrafiltration Membrane

With the rising demand for fresh water resource and declining availability of fossil resources,it is crucial to develop advanced water treatment technology and make use of renewable biotic resources for the sustainable development of human society.Poly（L-lactic acid）（PLLA）attracts extensive attention as one kind of promising biopolymers because of its renewability and biodegradability.Herein,PLLA was firstly made into ultrafiltation membrane to replace some traditional polymer based on fossil resources.This study focuses on the regulation of PLLA membrane str ucture,optimization of PLLA membrane performance and hydrophilic modification of PLLA membrane.On the one hand,binary solvent and Tween 80 were severally employed to tailor the membrane structure and enhance the membrane permeabililty.On the other hand,titanium dioxide（TiO₂）and polyaniline（PANI）were respectively incorporated into the casting solution to fabricate PLLA composite membranes,eventually improving the antifouling performance of PLLA membrane.The details are listed as follows:Firstly,1,4-dioxane（DX）,N-methyl-2-pyrrolidone（NMP）,N,N-dimethylform-amide（DMF）,N,N-dimethyl-acetamide（DMAc）were used as solvents,and DX/NMP,DX/DMF and DX/DMAc were used as binary solvent mixtures.With a focus on the PLLA/DX/NMP/H2O system,the effect of solvent compostion on PLLA membrane was systemically investigated by altering the ratio of DX/NMP,and various membrane morphologies were obtained,which were further correlated with the solubility parameter and viscosity of casting solution.It was found that the membrane cast with DX/NMP（1/1）,exhibited ideal structure and superior performance compared with the membranes cast at different solvent ratios and the same concentration of PLLA.Secondly,PLLA ultrafiltration membrane was fabricated by immersion precipitation method using Tween 80 as a pore-forming agent.It was shown that the addition of Tween 80 into the casting solution significantly increased the permeability and molecular weight cut-off of membrane.Tensile test confirmed that PLLA membranes exhibited acceptable mechanical properties for ultrafiltration.Further,the role of Tween 80 in the process of membrane formation was analyzed and proposed.The addition of Tween 80 favored the formation of larger pores by decreasing the viscosity of casting solution and increasing the miscibility between solvent and coagulant.Thirdly,PLLA/TiO₂ composite membranes were fabricated by immersion precipitation method using TiO₂ as a hydrophilic modifier.The results of XRD,SEM and ATR-FTIR indicated that TiO₂ was successfully enriched and distributed onto the membrane surface,thus significantly improving the membrane hydrophilicity.DSC and TGA indicated that the thermal stability of membrane was also enhanced obviously.SEM images revealed that the pore size and porosity on the membrane surface varied with the TiO₂ loading.It was confirmed by filtration experinents that PLLA/TiO₂ composite membranes had excellent antifouling and recycling performance.Finally,PLLA/PANI composite membranes were fabricated by non-solvent induced phase separation method using PANI as a hydrophilic modifier.Compared with the pure PLLA membrane,the incorporation of PANI significantly enhanced the hydrophilicity of PLLA composite membrane,and eventually improved the antifouling performance of composite membrane.It was demonstrated that PLLA composite membrane cast with 1 wt%PANI possessed better separation and antifouling performance among the composite membranes.