Construction of High Efficiency Thin Film Composite Pervaporation Membrane with the Synergistic Effect of Graphene Oxide and Nanofibrous Support

Author:Cheng Cheng

Supervisor:wang xue fen


Degree Year:2019





Pervaporation,as a highly efficient membrane separation technique,has a bright and broad application prospect in environment protection,chemical,oil,food and pharmaceutical industries.The intensification of pervaporation process and enhancement of membrane separation efficiency are critically dependent on developing high-performance membranes.Graphene oxide(GO),an oxidized derivative of graphene,is a two dimensional(2D)sheet with modifiable and polar oxygen groups.It has been demonstrated that water molecule moves quite fast in non-oxidized region of GO sheets and oxidized region has a good water affinity,which endows the greatest development potential for fabricating high performance preferred water permeable pervaporation membranes.Electrospun nanofibrous support with high porosity and interconnected porous structure used as supporting layer could markedly improve the permeate flux of pervaporation membrane.However,it’s still a challenge to prepare a uniform and integrated skin layer on porous and undulating nanofibrous support,thereby restricting its application in pervaporation.Herein,in order to fabricate novel pervaporation membranes with high performance,various approaches by combining the synthetic effect of graphene oxide sheets and nanofibrous support are proposed based on their intrinsic properties and structures.The mechanism of membrane formation,interfacial interaction between barrier layer and substrate,as well as microstructure of skin layer are systematically investigated.Meanwhile,the mass transfer mechanism and separation properties of nanofibrous composite membranes are probed.The as-prepared nanofibrous composite membranes have appropriate microstructures and excellent performances,which offers a new strategy for the membrane fabrication and process intensification of pervaporation.For the purpose of taking full advantages of GO membrane in pervaporation desalination,a new type of thin film nanofibrous composite(TFNC)membrane comprising of a robust ultra-thin GO skin layer and an electrospun polyacrylonitrile(PAN)nanofibrous substrate was sturdily constructed by a simple vacuum assisted filtration method for pervaporation desalination application,in which GO nanosheets as the building blocks were cross-linked by glutaraldehyde with the assistance of flexible spacing mortar polyvinyl alcohol(PVA).The hydrophilic flexible PVA chains not only offered the GO skin layer higher flexibility and permeabilty,but also stabilized the GO nanoplatelets and enhanced the integrity of the composite membrane with sufficient bonding.When the GO/PVA mass ratio was 1:1 during preparation,the resultant optimized robust GO/PAN TFNC membranes possessed an excellent permeate flux of 69.1 L/m~2h and a stable high rejection(99.9%)from aqueous salt solution with NaCl concentrations of 35 g/L at 70°C.To expand the application of nanofibrous composite membrane in organic solvent dehydration,a novel polyamide(PA)TFNC membrane with an ethanediamine(EDA)modified graphene oxide(eGO)interlayer was fabricated by interfacial polymerization(IP)of EDA monomer and trimesoyl chloride(TMC)monomer on PAN nanofibrous substrate.Ultrathin eGO layer could be uniformly assembled by vacuum assisted method with well-controlled thickness on nanofibrous substrate,which provided smooth and hydrophilic surface in favor of forming defect-free PA top layer via interfacial polymerization.Importantly,these immobilized amino groups on the surface can also react with TMC,participating in the interfacial polymerization.The imported covalent interaction at the interface could induce the formation of compacter PA selective layer and also increase the interfacial interaction between the support layer and selective layer,which has been demonstrated by various characterizations.The optimized TFNC membranes exhibited an excellent separation performance for dehydrating 90 wt%aqueous isopropanol solution with remarkably enhanced separation factor(4150)and high permeate flux(1866 g/m~2h)at 70°C.Inspired by eGO interlayer in controlling the interfacial polymerization process,GO sheets were directly introduced into amine aqueous phase to form an integrated and compact PA layer on nanofibrous substrate via IP.Under the synergistic effect of high permeability of the nanofibrous scaffold and flexibility of GO lamellae,tiled GO lamellae closely onto the porous nanofibrous substrate acting as a skeleton got involved in the initial reaction for the construction of more selective PA layer.The hybrid PA membrane structures were systematically characterized,and the corresponding membrane performances were evaluated.We found that trace amount of tiled GO sheets were beneficial for preparing a more uniform and denser PA layer,which could enhance the molecular sieving function.A permeate flux of 6593 g/m~2h with a separation factor of 1491 was obtained using the hybrid membrane with just 0.001 wt%of GO loading for pervaporation dehydrating 90 wt%isopropanol aqueous solution at 70°C.In addition,the hybrid PA composite membrane exhibited good separation stability at different operation conditions.To develop nanofibrous composite membranes with higher performance,zwitterionic polymer modified graphene oxide(mGO)was synthesized by free radical polymerization and incorporated into EDA aqueous phase to fabricate novel hybrid PA barrier layer by interfacial polymerization.High-density zwitterionic groups on GO can not only mitigate the serious aggregation among GO sheets,but also confer electrostatic interaction sites with water molecules,leading to high water affinity and ethanol repellency.The effect of mGO loading in aqueous phase on membrane structure and separation performance were systematically investigated.Using dehydration of 90 wt%isopropanol aqueous solution as the model system,the membrane exhibits the simultaneous enhancement of permeation flux and separation factor with a permeation flux of8835 g/m~2h,and a separation factor of 2036.To sum up,series of thin film nanofibrous composite membranes with proper chemical structures and topographies were systematically designed by making full use(such as building blocks,interlayer ans assisted filler)of functional GO during preparation.And these integrated and uniform barrier layer were successfully fabricated by facile and mild methods on nanofibrous sypport,which have quite high separation efficiency in pervaporation.This study may devote a novel route to the design and fabrication of high-performance composite pervaporation membranes to meet the diverse energy and environment-related application requirements.