Modification、Preparation And Performances Of Pervaporation Membranes Based On Polyelectrolyte Complexes

Polyelectrolyte complexes (PECs), which is also known as polyion complexes, show excellent hydrophilicity. It is a kind of new membrane materials. However, the applications of solid PECs have been badly limited as they are generally insoluble, infusible and not processable. Recently, a method of "acid protection-alkali deprotection" of preparing solution processable PECs and their separation membranes was proposed and successfully applied in pervaporation(PV) dehydration of organics by our research group. But this kind of new material has some deficiencies in the mechanical properties, anti-swelling property, film forming property and the selective performance. Blending, cross-linking and organic/inorganic hybrid methods are used to make up the deficiencies of PECs. The feasibility and the characteristics of the modification to PECs, and the PV performances of their membranes were systematically studied. Simultaneously, in this work, we explored a new method to prepare the solution processable PECs. The main results are as follows:(1) PEC blending membrane. The study of the PEC(PDDA/PAANa0.3) modified by blending with neutral PVA of30wt%found that the morphologies, structures and properties of PVA/PEC blends and their membranes are dependent on the preparation methods. When the blend manner is changed from the PVA solid blending to the PVA solution blending, the properties of PVA/PEC blend liquids are transferred from the solution to the suspension. The PVA/PEC blend membrane prepared by PVA solid blend method show better PV performances and good stability in dehydrating10wt%water-isopropanol. For example, the flux (J) and the separation factor (α) of PVA/PEC3070blend membrane remain almost unchanged after continuous operation for10h at50℃, which are1550g/m2h and1500respectively. The tensile strength, modulus, and elongation at break for the PVA solid blend membrane are1.5,1.4and3.6times of the pristine PEC(PDMC/CMCNa0.46) respectively, and it can be made into membrane without polysulfone substrate and show high PV performance in dehydrating10wt%water-isopropanol at70℃, of which the water in permeate and the flux are roughly100%and987g/m2h, respectively. The study of the PEC (PDDA/CMCNa0.19) modified by blending with negative charged CMCNa of20wt%found that the tensile strength of blend membrane is1.8times of the pristine PEC, and it can be used in dehydrating10wt%water-ethanol, resulting in the water in permeate and the flux at70℃are99.30%and1059g/m2h, respectively.(2) PEC hybrid membrane. A preparation method of MMT/PEC (PDMC/CMCNa0.46) nano-hybrid is proposed, that is first adding the solution of anionic polyelectrolyte CMCNa into the suspension of negative charged MMT, and then complexed with cation polyelectrolyte PDMC to form MMT/PEC nano-hybrids., Compared to adding the suspension of negative charged MMT into the solution of anionic polyelectrolyte CMCNa, the former method results in a high exfoliated degree of MMT layer in PEC matrix. The mechanical properties of MMT/PEC nano-hybrid membranes are largely improved compared with the pristine PEC. For example, the tensile strength, elongation at break and modulus of MMT/PEC nano-hybrid with3wt%MMT are1.53,1.81and1.31times of pristine PEC, and when it is applied into dehydrating10wt%water-isopropanol at70℃, the water in permeate is99.83%, which is higher than that of PEC membrane (99.6%), and the permeate flux can reach2600g/m2h. The separation factors of hybrid membranes were almost unchanged after continueous operation for20h at70℃.(3) PEC cross-linking membrane, γ-glycidyoxypropyltrimethoxysilane (GPTMS) is choosed to be the cross-linking agent, which can be carried out sol-gel reaction in the solution medium with high pH value (pH=6), resulting in GPTMS/PEC(PDDA/CMCNa0.19) cross-linking membrane. The tensile strength and elongation at break of GPTMS/PEC membrane with2wt%GPTMS are1.12and1.35times of pristine PEC, and the PV performances of its membrane is the best. For dehydrating10wt%water-isopropanol at70℃the water in permeate is almost100%and the permeate flux is not decreased, it can reach2670g/m2h, which is higher than the flux of PEC membrane (2480g/m2h) without cross-linking. (4) A new in-situ preparation method of solution processable PECs (NPECs) by dropping cation polyelectrolyte solution to excessive anionic polyelectrolyte solution without adding acid or alkali is proposed, based on the concept of polymeric solvent. When the content of CMCNa is not less than80wt%, the prepared NPEC(PDDA/CMCNa)-80by the new method has an IPN structure affording good toughness with an elongation at break of8%, which is3.03times of the PEC prepared by "acid protection-alkali deprotection" method (the acid-alkali method). The PV performance of NPEC membranes are better than the PEC membranes prepared by the acid-alkali method in isopropanol dehydration. For example, the flux of NPEC-80membrane can reach2821g/m h, being1.14times of the PEC membrane prepared by the acid-alkali method2480g/m2h in dehydrating10wt%water-isopropanol at70℃. The NPEC membranes can further be used in dehydrating10wt%water-ethanol at40℃and show excellent PV performances, the flux and the water in permeate of NPEC-90membrane are811g/m2h and1570(PSI=1.27×106), which are better than522g/m2h and1998(PSI=1.02×106) of CMCNa/(PDDA/CMCNa0.19)7030blend membrane. According to the new preparation method other two kinds of NPECs of PDMC/CMCNa and CMC+/CMCNa with good PV performance are made, indicating that the new preparation method is universal.