The characterisation of ultrafiltration membranes used in water purification

Abstract

The increase of urbanisation has caused water scarcity concerns in developed nations. Natural sources of freshwater (ground water, river water, rain fall) are becoming insufficient, requiring man-made technologies for the purification of water. Ultrafiltration membranes are currently becoming more relied upon for water purification due to their selectivity and applicability in different environments. Fabrication of some ultrafiltration membranes require the mixing of two (binary blend) or three (ternary blend) synthetic polymers to form an amphiphilic polymer blend. Overall properties of the blend are dependent on the chemical nature of the precursors; however, chemical incompatibilities between polymers causes incomplete mixing, thereby forming a partially miscible system. The presence of different domains of varying miscibility creates a complex matrix where minimal changes in local chemical composition can drastically change the membranes properties. The material becomes increasingly complex with a ternary blend. Hence, the understanding of function and composition relationship is important to the development of design and functionality. This project aims to characterise a number of properties of ultrafiltration membranes at various stages, from fabrication to production. In this work, industrial membranes (both binary and ternary) and their respective precursors were studied. Difference in end groups within the system affects solubility of the precursor leading. This can lead to undissolved poly(N-vinyl pyrrolidone) (PVP) affecting the properties of the membrane produced. Free solution Capillary Electrophoresis (CE) was employed for the separation of PVP via end groups. Representative electrophoretic mobility distributions of different PVP samples were obtained showing the presence of different populations. The membranes were characterised through solid-state Nuclear Magnetic Resonance (NMR) spectroscopy, providing a means to determine the molecular structure and molecular mobility within the membranes. NMR measurements identified that polymer A is extracted out of the membrane during production and the ternary membrane is a miscible system. Deductions made contributed to interpretations on functionality Relationships between the surface composition and the functionality of the membrane were established. Functional properties were determined through tensile strength tests and tensiometry tests. The surface composition was determined through Scanning electron microscopy / energy dispersive x-ray spectroscopy and 1H NMR spectroscopy. Surface localisation of polymer A and polymer C affected the hydrophilicity of the membranes. It was also found the formation of macrovoids affect the tensile strength of the membrane. In conclusion, methods were developed to determine the chemical structures of the membrane, at various stages of production, and relating it to functionality. Analysis of structure-function relationships allowed for the improvement of design to optimise membrane properties. The AB membrane was determined to have better functional properties than the ABC membrane; however, the chemical stability of the ABC membrane makes it a promising system to be used for future design. Future design proposals can incorporate different polymerisations yielding PVP with end groups that are soluble. These improved designs will not only increase performance but also allow for cost-efficient measures of membrane production.

Date of Award	2018
Original language	English