Porous Antibacterial Membranes Derived from Polyethylene (PE)/Polyethylene Oxide (PEO) Blends and Engineered Nanoparticles

Abstract

The steep rise in the contamination of natural water sources, has led to an increasing demand for alternate solutions to cater safe drinking water to mankind. Water treatment by separation technology utilizes semipermeable membranes to filter the contaminants commonly present in potable water. In this context, the current work focuses on the development of membranes that are affordable, exhibit chemical resistance and can be developed at industrial scale. By blending two immiscible polymers like polyethylene (PE) and polyethylene oxide (PEO), different morphologies can be generated and porous structures can be developed by selectively etching the water soluble phase (PEO). Microorganisms in the feed stream often tend to foul the membrane by forming biofilms on the surface that tends to increase the resistance offered by the membrane. Therefore, preventing this biofilm is a key challenge in this field and can be overcome by use of functional group or materials that prevent the attachment or growth of microorganisms on the surface, while maintaining a good permeation rate of water. This thesis entitled “Porous Antibacterial Membranes Derived from Polyethylene (PE)/Polyethylene oxide (PEO) Blends and Engineered Nanoparticles” systematically studies the various morphologies generated by melt blending polyethylene (PE)/polyethylene oxide (PEO) in presence and absence of a compatibilizer (maleated PE). Porous structures are developed by selectively etching PEO from the blends and the nature of the pores, which is dependent on the blend composition, is assessed by tomography. The potential of these membranes are discussed for water purification application. Further, various modifications either on the surface or in the bulk have been systematically studied. For instance, incorporation of biocidal agents like graphene oxide (GO) and modified GO in the matrix and coating/grafting of membrane surface with biocidal agents like silver (Ag), GO for preventing the biofouling and to meet the specific requirements for safe drinking water. The thesis consists of ten chapters. Chapter 1 is a review on polymer blends for membrane applications. This chapter covers the fundamentals of polymer blends in transport processes and compares the merits and demerits of the conventional methods. This chapter mainly covers the melting blending technique and the optimizing parameters for obtaining a desired morphology. Further, the various methodologies for stabilization of the morphology against post processing operation have been discussed. The various methodologies for designing membranes (for water purification) that suppress or inhibit the bacterial activity on the membrane surfaces have been discussed elaborately. Chapter 2 outlines the materials, experimental set-up and procedures employed. Chapter 3 focuses on the morphologies that are developed during the blending of PE/PEO with varying weight ratios. The morphologies developed are supported by SEM analysis. The factors governing the localization of particles in PE/PEO blends are discussed in detail. The gradient in morphology obtained during post processing operations is highlighted. Based on the type of morphologies obtained, the thesis is divided into two parts as (I) membranes designed using matrix droplet type of morphology and (II) membranes designed using co-continuous morphology. Part I consists of four chapters that involves the development of membranes utilizing matrix droplet morphology. Chapter 4 focuses on the development of morphology, the length scales of which are smaller than a bacterial cell. This ensures sieving of the contaminants that are commonly present in the drinking water though the surface of the membranes may not be antibiofouling. Thus a passive strategy of antibiofouling has been employed by blending biocidal agents like GO and amine modified GO during melt mixing. The antibacterial mechanism and its effect on bacterial activity have been thoroughly studied. Chapter 5 focuses on modification of membrane by incorporating silver decorated GO in the bulk. The effect of incorporation of these particles and their effect on bacterial activity have been discussed systematically. Chapter 6 emphasizes on the surface coating of membrane with chitosan to enhance the antibacterial activity and antibiofouling. Chapter 7 focuses on the development of membrane with pore sizes that are larger than a bacterial cell. These membranes are grafted with antibacterial polymers like polyethylene imine (PEI) and Ag to achieve antibacterial and antibiofouling surface. The possible mechanism of bacterial inactivity is described and the leaching of Ag from the membranes has been discussed. Part II of the thesis focuses on the development of co-continuous morphology in PE/PEO blends and has been assessed using 3D tomography. Chapter 8 describes the development of co-continuous morphology in PE/PEO blend. 2D and 3D micrographs have been corroborated for understanding the morphology evolution during post processing operation like remelting or hot-pressing. The blend has been strategically compatibilized to arrest the morphology and retain the co-continuity in the blends. GO was anchored onto the surface of the membrane by rendering suitable surface active groups. The antibiofouling and bacterial inhibition was studied in detail. The effect of anchoring GO on the membrane surface has been discussed with respect to their membrane performance and its antibacterial activity. Chapter 9 discusses the development of membranes using PE based Ionomer (Surlyn) and PEO. The Ionomer provided active sites for reducing silver nitrate directly onto the surface of PE to render antibacterial surface which otherwise requires a two-step protocol in the case of inert PE. The effect of coating Ag on the membrane performance and its antibacterial activity is elaborated. Chapter 10 sums up the major conclusions from each chapter and highlights the outcome of the work.