Preparation Of Novel PCL-Based Superhydrophobic Membranes And Their Application On Oil-Water Separation

With the rapid development of industrialization in various countries,the problem of water pollution has become more serious.The emission of oily wastewater into rivers,oceans and others threats to the survival of aquatic organisms and destroys the ecological balance.Humans ingesting fish from contaminated areas can cause a series of illnesses which are harmful to our health.Therefore,how to properly deal with oily wastewater has become an urgent problem to be solved.At present,the oily wastewater mainly has the following treatment methods:gravity separation method,centrifugal separation method,flotation method,coagulation method,combustion method,photocatalytic oxidation method,adsorption method and membrane separation method.However,most of these methods are complicated in operation,cumbersome in process,discharging toxic substances which result in secondary pollution to the environment.Superhydrophobic/superlipophilic membranes have become a suitable candicate to separate oily wastewater in recent years.At present,the materials commonly used for preparing oil-water separation membranes are mainly polymer materials such as polyvinylidene fluoride（PVDF）,polystyrene（PS）and polyurethane（PU）.However,these materials are not only diffcult to degrade,but also toxic such as PVDF and PS.They are easy to cause secondary pollution to the environment when used as oil-water separation membranes.Polycaprolactone（PCL）is an environmental-friendly and biodegradable polymer what has been widely used in biomedical and tissue engineering fields.However,due to the limitations of PCL chemical composition,its hydrophobicity is limited compared to PVDF.In order to avoid the above shortcomings,we used PCL as substract to prepare superhydrophobic oil-water separation membranes by the method of electrospraying/electrospinning to change the physical microstructure and adding a small amount of low surface energy materials.The hierarchical structural mat obtained by changing the polymer morphology not only improved the hydrophobicity of the film,but also increased the adsorption space of the oil and promoted the process of the oil-water seperation.In this paper,PCL-based composite fiber membranes with different hierarchical structures were prepared by two-step and one-step method with environmental-friendly PCL as matrix.The structure and morphology of hierarchical PCL-based composite fiber membranes were analyzed,and their oil-water separation performance was systematically studied.The main research contents include the following three parts:（1）With PCL as the matrix,different morphology of single PCL was studied by electrospinning/electrospraying method.When the concentration of PCL was 20%（w/v）and chloroform（CF）and N,N-dimethylformamide（DMF）were used as mixed solvent,the electrospun fibers with smooth surfaces and the average diameters of 500 nm could be obtained.By changing the flow rate of the spinning solution,different diameters of the electrospun fibers were obtained.The standard deviation of the diameter of the nanofibers obtained at flow rate of 2.5 mL h^-1 was the smallest,and the obtained fibers were most uniform.PCL electrospray microspheres were prepared at relatively low concentration（4%,5%and6%;w/v）using CF as single solvent.The effects of different polymer solution concentrations and flow rates on the morphology of electrospraying microsphere were investigated.The results showed that when CF was used as the electrospraying solvent,PCL microspheres with smooth surface and diameter of several micrometers to ten micrometers were obtained.The effect of PCL concentration on the diameter of the microspheres was most significant.As the concentration of PCL increased,the diameter of the microspheres also increased.（2）Using the principle of bionics,the surface of the lotus leaf is superhydrophobic because it include surface waxy layer and micron-sized convex structure.To prepare such a biomimetic structure,superhydrophobic PCL film of hierarchical structure was prepared by alternate electrospinning/electrospraying technique.The PCL/MSO-PCL_MS hierarchical mat was prepared by using PCL/methyl silicone oil（PCL/MSO）nanofibers as the substrate and electrosprayed PCL microspheres(PCL_MS)as surface layer of the hierarchical membrane.The FE-SEM image showed that the hierarchical structure PCL/MSO-PCL_MS film was successfully prepared.The water contact angle（WCA）tests showed that the hydrophobicity of the MSO modified PCL film was significantly improved compared with the pure PCL nanfiber mat.In addition,the WCA of the PCL microspheres membrane could reach 146±2.8°without the addition of low surface energy materials.On this basis,the PCL/MSO-PCL_MSS hierarchical film had a WCA of 150±0.6°.The maximum n-hexane adsorption capacity of PCL/MSO-PCL_MS membrane was 32.53 g g^-1,and the oil-water separation efficiency was as high as 99.93%.After 10 cycles oil-water separation experiments,the hydrophobicity and oil-water separation efficieny of the superhydrophobic membranes were still stable.（3）In view of the above research,the preparation process of the oil-water separation membrane obtained by the two-step method was complicated,and the multi-layer structure had stability problems such as delamination due to the preparation conditions,we used one-step method to prepare PCL/MSO/silica（PCL/MSO/SiO₂）nanofiber-microsphere superhydrophobic mat.Low surface energy MSO and hydrophobic SiO₂ nanoparticles were added to the 15%PCL solution to increase the hydrophobicity of the membrane.The results of FE-SEM,FTIR and XPS showed the chemical structure and micro-structure of PCL/MSO/SiO₂ microsphere-fiber composite membrane.The WCA of the PCL/MSO and PCL/SiO₂ films were improved compared to pure PCL membrane.It is worth noting that the WCA of the PCL/MSO/SiO₂ composite mat reached 151.6±1.7°.The microsphere-fiber interconnect structure and the superhydrophobicity of membrane contributed to the excellent performance of oil adsorption and oil-water separation.The maximum n-hexane adsorption capacity of the PCL/MSO/SiO₂ membrane was 41.37 g g^-1,and the separation efficiency of the oil-water mixture was as high as 99.92%.The oil flux during the oil-water separation process reached 3300 L m^-2 h^-1.In addition,the composite membrane also had good recyclability,the membrane structure integrity and stable superhydrophobicity could be maintained after 10 cycles of oil-water separation tests.