Fabrication Of Superhydrophobic PVDF Nanofibrous Membranes And Investigation Of Waterproof And Breathable Performance

With the improvement of living standards,traditional textile materials could hardly meet our requirements for protection from the cold,and functional textile materials with various specific applications have attracted extensive attention.Waterproof and breathable（W&B）fabrics that can not only prevent the penetration of external liquids but also exchange moisture and air with the environment are typical examples.A variety of W&B fabrics,such as high-density,coated,and laminated fabrics,have been widely used in the market.However,high-density and coated fabrics are difficult to balance waterproofness and breathability.Laminated fabric obtains excellent waterproof and moisture permeability by combining the core functional layer W&B membrane with other materials.The W&B membranes are mainly divided into hydrophilic flat membranes and hydrophobic microporous membranes.The representative product of the hydrophilic flat membranes is polyurethane（PU）dense membranes.The membranes own compact nonporous structure,which gives them excellent waterproof performance,as well as,leading to poor moisture permeability.The representative product of microporous membranes is polytetrafluoroethylene（PTFE）microporous membranes,which involve the use of perfluorooctane sulfonates and other harmful substances in the preparation process.These harmful substances would not be degraded in the environment and possess high bioaccumulation,causing long-term harm to the human body and the environment.Besides,PTFE membranes also struggle with complex machining processes and stringent production standards,which brings big obstacles to mass application.Due to the advantages of creating high-performance W&B membranes with adjustable pore shapes,interconnecting channels,and an abundance of raw materials,electrospinning technology has recently received considerable interest.PU,polyvinylidene fluoride（PVDF）,polyacrylonitrile polyamide,and other polymers have been investigated for use in the development of W&B membranes.However,building superhydrophobic and mechanically robust W&B membranes remains a significant challenge.Therefore,we combined humidity-induced electrospinning thermal-pressing to prepare nanofibrous membranes with excellent superhydrophobic and mechanical properties.Firstly,PVDF and the hydrophobic agent fluorinated polyurethane（FPU）were dissolved in N,N-dimethylformamide（DMF）,and finally obtained a mixed solution of PVDF/FPU.Humidity-induced electrospinning was used to create PVDF/FPU nanofibrous membranes with a micro/nano hierarchical rough structure.The effect of PVDF concentration on the macroscopic morphology,microstructure,and overall properties of PVDF/FPU nanofibrous membranes was then discussed.As the PVDF content went up,the viscosity of the solution gradually increased,and the bead structure on the surface of the nanofibrous membranes gradually decreased.At the same time,the diameter of PVDF/FPU nanofibrous membranes also kept increasing,resulting in the maximum pore size,thickness,and porosity of the membranes continually increasing.For a comprehensive comparison,the nanofibrous membranes exhibited the optimal comprehensive performance at the PVDF concentration of 10 wt%.The unique micro/nano hierarchical rough structures and the introduction of low surface energy FPU endow the membranes with excellent superhydrophobic performance,and the water contact angle of membranes reaches 151.9°.In addition,the nanofibrous membranes also exhibited excellent waterproof and breathability performance with hydrostatic pressure of 67.6 k Pa and water vapor transmitting of 9111.4 g m^-2 d^-1.In order to endow PVDF/FPU nanofibrous membranes with excellent mechanical and waterproof properties,in this work,the thermal-pressing treatment technique was introduced to further treat the membranes to obtain dense PVDF/FPU nanofibrous membranes,and the effects of temperatures and pressures during thermal-pressing on the morphology and properties of the nanofibrous membranes were analyzed.The results showed that the thermal-pressing treatment brought the nanofibrous membranes uniform adhesive bonded structures.As the temperature and pressure rose during the thermal-pressing process,the number of bonding points on the surface of the membrane continually increased.In the meantime,the fluffy surface of the membranes became dense,which eventually resulted in high packing density,smaller max pore size,and lower porosity of the membranes.The nanofibrous membranes that exhibited the best comprehensive properties were obtained with the controlled thermal-pressing process under the temperature of 120℃ and the pressure of 5 MPa.The tensile strength of the membranes reached up to 8.1 MPa.Its water and oil contact angles were 150.8°and 130.7°,while hydrostatic and oil intrusion pressures were 81.8 k Pa and 36.3 k Pa,respectively.Also,the membranes owned excellent water vapor transmitting of 7948.0g m^-2 d^-1.The successful preparation of the dense PVDF/FPU nanofibrous membranes provides a new strategy for the research and development of superhydrophobic waterproof and breathable membranes and has potential application value in sports clothes,medical and health,building exteriors,electronic appliances,and other fields.