Functional Modification Based On Poly(Vinylidene Fluoride) Membrane

Poly(vinylidene fluoride)material is a good performance polymer with excellent properties such as chemical resistance and high mechanical strength due to the presence of strong C-F bonds,which has been widely used in industrial production and life.Thus,PVDF materials have been extensively applied in ultrafiltration and microfiltration for general separation purposes in the study of membrane science.Modification of PVDF membrane for applying in separation field has been the research hotspot in membrane technology.In this paper,PVDF composite membranes with different functions were prepared by blending and in-situ synthesis respectively.The structure and properties of the membranes were examined by various characterization and test experiment.This study is divided into two parts with the main results summarized as follows:1.PVDF-PAA block polymers were used as a novel blending materials for the first time to make the environmental pH response PVDF membranes.pH-responsive membranes have found wide and important applications such as controlled release of drugs,sensors,flow regulation.The precursor of PVDF-PAA is PVDF-PtBA.Membranes containing PVDF-PtBA block copolymer were prepared via a non-solvent induced phase separation(NIPS)method.The obtained membranes were treated by trifluoroacetic acid(TFA)to deprotect the carboxylic groups to generate PAA chains.By observing the SEM images of the surface and the cross section of the membrane,one can see that the pores in the membrane surface first increase and then decrease with the increase of the blending amount of the PVDF-PAA block copolymers.From the FT-IR measurements data,one can see a significant increase of the signal at the wavenumber of 1730 cm-1,which corresponds to the characteristic stretching of the carbonyl groups(0-C=0).The hydrophilicity of the membranes improved after blending PVDF-PAA block polymers from the water contact tests.From the pH value of 3 to 10,the zeta potential of the composite membranes was decrease from ca.-4 mV to-30 mV.These results suggest the blending PVDF-PAA block polymers in PVDF membrane.We tested the water flux of membrane at different pH solution.The results showed that the flux and the adsorption capacity towards Cu2+ of PVDF-PAA composite membrane increased with the increase of pH.This phenomenon is due to the shrinkage of the PAA chains at low pH and the swelling of the PAA chains at high pH.The study shows that the PVDF composite membrane prepared by the above method has the pH-responsive properties.2.In order to control the eutrophication of water body caused by excess phosphorus,we report the first example of in situ formation of La(OH)3 nanorods in poly(vinylidene fluoride)(PVDF)membrane by adding LaCl3.7H20 precursor in the membrane casting solution,after a non-solvent induced phase separation process in alkaline solution.The SEM images show that La(OH)3-PVDF membranes possess holes on the surface of the membranes and finger-like pores on the cross-section of the membranes.The TEM images suggested that La(OH)3-PVDF membranes have the well-dispersed La(OH)3 nanorods with lengths of ca.70 nm and widths of ca.10 nm.The morphology of nanorods of La(OH)3 was transformed into nanospheres of LaPO4 after the phosphate adsorption.The water contact angel measurements showed that the hydrophilicity of the La(OH)3-PVDF membrane was enhanced after the adsorption of phosphorus.The results of XRD,FT-IR and XPS showed that the La(OH)3 crystal was successfully synthesized in the La(OH)3-PVDF membrane and nanorods of La(OH)3 was transformed into LaPO4 after the adsorption of phosphorus.The adsorption isotherms and adsorption kinetics of phosphorus on La(OH)3,PVDF membrane showed that the maximum amount of La adsorbed on the La(OH)3-PVDF membrane could reach 256.6 mg P/g(La),which was much higher than that of commercial La(OH)3 nanoparticles,and had faster adsorption kinetics for P.After membrane filtration test,the La(OH)3-PVDF membrane has good adsorption and filtration performance on phosphorus.After the regeneration in alkali solution,the La(OH)3-PVDF membrane can still reach 98%of the initial value,indicating that the composite membrane has a good reusability.To sum up,we studied two kinds of modifications of PVDF membrane,mainly focused on the two aspects.(1)PVDF-PAA materials were firstly introduced into PVDF membrane to prepare pH-responsive PVDF membrane.(2)We reported a facile one-step preparation of La(OH)3 nanorods in PVDF membrane to combine filtration and phosphate removal.These studies have extended the application range of PVDF membrane,which shows that the material has a wide application value in intelligent response,water treatment and so on.