Research Of PVC Composite Nanofiltration Membrane Based On Dendritic Macromolecules

Nanofiltration(NF)is a pressure-driven membrane separation process between reverse osmosis(RO)and ultrafiltration(UF).It is generally believed that NF membrane has a pore size of about 1 nm and can be used for effective separation of nanoscale components.The composite NF membrane prepared by interfacial polymerization method is the mainstream product of the current market.However,there are still some practical problems such as high preparation cost,permeate flux and separation performance of the membrane remain to be improved,that restrict the development and application of NF membrane technology.In this paper,nearly spherical hydroxyl-terminated dendritic macromolecules with intramolecular voids were successfully introduced into the polyamide separation layers by interfacial polymerization on low-cost PVC hollow fiber substrate membranes to prepare a series of PVC composite NF membranes with both high permeability and selectivity.Three composite NF membranes were prepared by interfacial polymerization on the inner surface of different PVC hollow fiber membranes obtained by wet spinning.The performance test results showed that the physical and chemical properties of each substrate membrane,especially the pore size and porosity could significantly influence the permeation and separation performance of the composite NF membrane.The UF-PVC10 hollow fiber membrane was selected as the substrate membrane used for the subsequent NF membrane preparation.For the NF-PVC10 membrane prepared on the UF-PVC10 substrate,the MgSO4 rejection and permeate flux reached 98.0%and 28 L/m2 h respectively at 0.4 MPa.A series of HPE/PIP composite NF membranes were successfully prepared by introducing the fourth generation of hydroxyl-terminated HPE into the polyamide matrix by interfacial polymerization.The prepared HPE/PIP membrane surfaces were rough and packed with small nodules,which is the typical morphology of poly(piperazine-amide)layer interfacially polymerized by PIP and TMC,and the membrane surface was negatively charged,but the surface roughness exhibited obvious differences.The performance test results showed that the permeate flux and MgSO4 rejection of the prepared NF membranes were both enhanced when the content of HPE in the aqueous phase was less than 30 wt%.With the increase of HPE content,the permeate flux increased steadily while the salt rejection ability began to decline due to the changes in the cross-linked structure of polyamide layer.For the prepared NF6 membrane with the HPE/PIP ratio of 3:2,the MgSO4 rejection and permeate flux reached 98.1%and 45.6 L/m2 h respectively at 0.4 MPa.Also,the NF6 membrane maintained a high monovalent/divalent ions selectivity in the mixed salt solution.Based on the previous work,three HPE/PIP composite NF membranes were prepared by introducing different generations of hydroxyl-terminated HPE into the corresponding polyamide matrix by interfacial polymerization.The chemical composition analysis showed that HPE had a gradient distribution on the surface of the separation layer and could be embedded across the thickness of the separation layer.Also,the chemical compositions of HPE/PIP separation layers formed from different generation HPEs with the same concentration in PIP aqueous solution indeed existed with some differences.Combined with the performance test results,the hypothetical molecular structure of the formed HPE/PIP polyamide separation layer was proposed:HPE macromolecules would constrict themselves in three-dimensional space influenced by the packing of surrounding polymer chains because their molecular structures are not rigid,resulting in the effective incorporation of HPEs into the aggregate pores,or even the network pores of the cross-linked polyamide matrix.Both the enlargement of some aggregate pores and the increase in the hydrophilicity of the polyamide matrix with the incorporation of H40 HPE molecules into the aggregate pores led to the permeate flux of the TFC membrane increasing significantly.Meanwhile the nearly spherical structure and intramolecular cavities of H40 HPE could also impose extra steric hindrance on salt ions but have little affect on the transport of water molecules,resulting in an increase in the salt rejection rate.The prepared H40/PIP NF membrane exhibited Na2SO4 rejection of 99.1%and permeate flux of 62.2 L/m2 h respectively at 0.4 MPa,and showed good permeation and separation performance for the separation process of a series of dye/salt mixed solutions.Three PAMAM-OH/PIP composite NF membranes were prepared by introducing different generations of hydroxyl-terminated PAMAM into the corresponding polyamide matrix by interfacial polymerization according to the same blending ratio with HPE/PIP.The common aspects of the performance test results suggested:the G3-OH embedded into the polyamide matrix with the similarity of nearly spherical structure and intramolecular cavities to that of H40 HPE,could also effectively improve the permeation and separation performance of the composite NF membrane.This result strongly validated the rationality and universality of the former proposed hypothetical molecular structure and related mechanisms.Finally,a series of PAMAM-NH2/PIP composite NF membranes were prepared by interfacial polymerization using amino-terminated PAMAM as the blending reaction monomer of PIP.G3-NH2 with a large number of terminal primary amine groups could be involved in the formation of the polyamide separation layer together with the PIP monomer.Also,the G3-NH2 concentration in the mixed monomer composition had significant influences on the network cross-linking degree and surface charge of the separation layer.Compared with the NF6 membrane,the permeate flux of PAM-1.2 membrane prepared with the same blending ratio was very low,which indicated that the dendritic macromolecule with higher reactivity of terminal groups could not maintain its nearly spherical structure during the interfacial polymerization process,resulting in unable to be effectively embedded in aggregate pores of the polyamide matrix,and then play a role in improving the permeation and separation performance of the composite NF membrane.It was also proved that the necessity of hydroxyl-terminated dendritic macromolecule in the polyamide separation layer to maintain the integrity of its structure and the effectiveness of permeation and separation performance improvement for the composite NF membrane.