Preparation and characterization of graphene doped ultrafiltration membranes

Ultrafiltration, a low-pressure membrane process, has attracted more attention due to the increasing advances in membrane technology. Ultrafiltration is a widely used process especially in obtaining potable quality clean water, wastewater treatment and membrane separation processes. The hydrophilic and porous structure of the membranes plays an important role in membrane separation processes. Among all inorganic-organic materials, polyvinylidene fluoride (PVDF) is an excellent material capable of forming membranes. PVDF based membranes show superior oxidation, thermal stability as well as good mechanical and film-forming properties. Therefore, PVDF membranes are widely used in many ultrafiltration processes through various modifications. Graphene has received considerable scientific interest in recent years due to its superior mechanical, thermal and electrical properties as well as its large surface area. In composite preparation, graphene has been frequently used as a nano filler in recent years to improve the mechanical, electrical and thermal properties of polymers. In this thesis, nanofiber mats were prepared by electrospinning from PVDF solutions prepared using suitable solvents. For comparison, PVDF films were also prepared by solvent evaporation. Composite PVDF materials were prepared to change the hydrophilic-hydrophobic properties of the membranes. Graphene was chosen as reinforcement material. First, the graphene was modified with HNO3 to give hydrophilic character to PVDF, then the miscibility of composite formed by addition of HNO3 modified graphene was changed. PVDF / HNO3 modified graphene nanofiber mats were prepared by electrospinning method using PVDF as matrix with HNO3 modified graphene. Characterization studies were performed using FT-IR, XRD, SEM, TGA / DTG, DSC. In addition, contact angle measurements were performed to measure surface hydrophobicity. Then, water-gasoline mixtures were separated using PVDF / HNO3 modified graphene nanofiber mats and films and their separation performances were compared. In the electrospinning experiments, process and system parameters affecting fiber diameter and morphology; solvent, polymer solution concentration, molecular weight were investigated. It has been found that the most suitable solvent mixture for electrospinning and subsequent film preparation is acetone / DMF. In SEM images of nanofibers, it was observed that nanofiber diameter increased due to both solution concentration and molecular weight of PVDF. The composite structure obtained with HNO3 modified graphene added to provide hydrophilic structure to the PVDF was fiberized by electrospinning. The addition of HNO3 modified graphene to the structure has led to a reduction in nanofiber diameters, thereby reducing bead formation and obtaining more homogeneous fibers. It was confirmed from the FT-IR and XRD results that the acid modification gave the hydrophilic character to the graphene and thus PVDF with functional ends (such as –OH) formed on the edges of the graphene, as well as the observations during the solution preparation. As a result of TGA / DTG and DSC studies, it was observed that composite nanofibers and films obtained by adding HNO3 modified graphene to the PVDF matrix were thermally stable and mechanically more resistant than pure PVDF. Keywords: Electrospinning, nanofiber, polyvinylidene fluoride, graphene, ultrafiltration.