Surface Engineering of Polymer Membranes

Polymeric separation membranes develop rapidly and have been applied in many fields. Filtration with polymer membranes covers the separation of industrial chemicals, purification of laboratory products and treatment of drinking water. For all these processes, the performance limits are clearly determined by the membrane itself. Flux and rejection of a membrane process such as microfiltration or ultrafiltration are mainly influenced by size exclusion. Nevertheless, surface properties of the membrane also play a crucial role. It is because membrane fouling, induced by adsorption of matter onto the membrane surface or deposition into the pores, is mainly controlled by the surface properties. For example, a hydrophobic membrane surface is apt to cause adsorption of protein or other solutes due to hydrophobic interactions. As is well known, membrane fouling will decline the flux and deteriorate the selectivity. Meanwhile, the products must meet the ever stricter environmental or safety standards. A membrane separation system should consist of a membrane with stable and reliable performance. Therefore, the first objective of polymer membrane surface engineering is to enhance the surface properties of the membrane by modulating its surface chemistry and physics, and hence improve the performance of the membrane.

In recent years, the surface engineering of polymer membranes through surface modification and surface functionalization has received significant attention. Since the properties of a membrane surface are very important for practical applications, it is important to have the means to characterize and measure those structures and properties. In fact, surface characterization is not only important for understanding the relationship between the membrane structure and its properties but also for guiding surface modification. It is well known that various aspects of a membrane surface, which include chemical composition, morphology and topography, wettability, and biocompatibility, can affect the properties and applications remarkably. Many kinds of surface characterization techniques may be applied to study the surface properties of polymer membranes. In this chapter, techniques for the characterization of a polymer membrane surface are reviewed, which include attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SSIMS), energy dispersive X-ray spectroscopy (EDS), optical microscopy, laser confocal scanning microscopy (LCSM), scanning electron microscopy (SEM), environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM), contact angle measurement, and some evaluation methods for the biocompatibility of membrane surfaces.

Surface functionalization of materials is one of the efficient techniques that can endow these materials with novel properties and transform them into valuable finished products. It has been widely applied to polymeric membranes in many fields and has progressed rapidly in recent years. In this chapter, therefore, important approaches to the surface functionalization of polymeric membranes are briefly described.

Membrane technologies have now been widely exploited and utilized on a large scale due to the unique separation principles of membranes. For further progress, however, membranes with favorable properties and functionabilities must be designed and prepared. Based on existing membrane materials, modification of them is necessary. Among different technologies, graft polymerization is a universal modification method for preparing a “tailored” membrane surface with desired functions. The grafted polymer chains on the membrane surface play an important role in membrane applications. In this chapter methods of graft polymerization on the membrane surfaces and the relative applications are introduced.

Membrane based processes have attracted great attention in recent years. However, the innate disadvantages of common membrane materials, such as the hydrophobic surface and poor biocompatibility, cause many side effects in practice and hinder further applications. Grafting a new layer onto a membrane surface by macromolecule immobilization is a promising solution for these problems. In addition, macromolecule immobilization can endow the existing membranes with new functions. In this chapter some research is introduced in which surface modification was achieved by macromolecule immobilization on the membrane surface.

Since phospholipids, a major component of the outside surface of a biomembrane, were demonstrated to be non-thrombogenic, much attention has been devoted to the use of phospholipid analogous polymers for surface modification in order to improve the biocompatibility of biomaterials with biological systems. In this chapter a variety of approaches for the synthesis of different phospholipid analogous polymers and the surface modification of a polymeric membrane with these phospholipid-containing polymers are briefly described.

Sugar-containing polymers, including natural polysaccharides and synthetic glycopolymers, are highly hydrophilic and biocompatible materials. Sugars also play important roles in many biological processes. Using them as modifiers to modify the membrane surface, which is also called membrane surface glycosylation, can offer properties of both anti-non-specific adsorption and specific recognition. In this chapter we introduce some research in which membrane surface glycosylation was carried out to reduce non-specific adsorption, improve anti-coagulation properties, endow the recognition ability to lectins and even serve as an additional layer to alter the separation performance.

Molecular imprinting technology, which originates from the molecular recognition phenomenon in biological systems, has been receiving much attention and already rapid developments have been achieved in recent years. A molecularly imprinted membrane is characterized by selective recognition, high binding capacity and excellent permeability. It is helpful for separation in large-scale applications and especially for the recognition of natural biomacromolecules. In this chapter the basic concept and theory of molecular imprinting are simply described for an understanding of the principles underlying the technique. Then the preparation and application of molecularly imprinted membranes are well presented and summarized.

Because of the attractive features associated with enzymes and membranes, their integration has been attracting much attention for many years. This chapter intends to outline the fabrication of a biocatalytic membrane surface through enzyme immobilization. The effects of these kinds of membrane materials and immobilization approaches, as well as methods of surface modification on the properties of enzyme-immobilized membranes are discussed. Furthermore, the preliminary applications of biocatalytic membranes, including membrane bioreactors and biosensors, are also simply reviewed.

Nanofibers are able to form a highly porous membrane and their attractive features promote their applications in many fields. This chapter covers the studies of the functionalization of the nanofibrous membranes and corresponding applications, including catalysis, micro-detection and target capturing. This review suggests that nanofibrous membranes offer great promise in bioapplications.