Planar Molecular and Macromolecular Gradients: Preparation and Properties
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Planar Molecular and Macromolecular Gradients: Preparation and Properties J. GENZER, T. WU, K. EFIMENKO Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905
ABSTRACT We present a method for fabricating polymer brushes with a gradual variation of grafting density on solid substrates. The technique for generating such structures consists of: i) deposition of a molecular gradient of polymerization initiator on the solid substrate, and ii) polymerization from the substrate bound initiator centers (“grafting from”). In this publication we describe the preparation of gradient polymer brushes of poly(acryl amide) on silica-covered substrates. We show that the polymer density within the gradient polymer brush can be varied by controlling the polymerization time. INTRODUCTION Recent advances in the field of self-assembly have led to the development of a plethora of new technologies based on soft lithography [1], that enable alternative ways of fabricating two and three dimensional patterns on material surfaces. Most of the soft lithography techniques are based on controlled deposition of self-assembled monolayers (SAMs) [2]. Various structural patterns with dimensions ranging from hundreds of nanometers to several micrometers are created on the material surface using a “pattern-transfer element” or stamp that has a threedimensional structure moulded onto its surface. Because of the molecular nature of the SAMs, the surface patterns generated via “soft lithography” are rather thin (several Angstroms to several nanometers). Some applications, particularly those involving subsequent microfabrication steps, such as etching, require that thicker layers of the surface coating be formed. Hence techniques, involving the patterning of thicker polymer layers grafted to the substrate have been developed [3]. The latter group of technologies is based on selectively decorating the material surfaces with polymerization initiators and then performing the polymerization directly on the surface (“grafting from”). Using this technology, the thickness of the overcoat film can be adjusted by simply varying the polymerization conditions (time, monomer concentration, temperature). While useful for creating substrates with well-defined dimensional chemical patterns of various shapes and dimensions, the soft-lithography technologies always produce sharp boundaries between the distinct chemical regions on the substrate. However, for some applications, it is desirable that the physico-chemical characteristics, such as wetting of the substrate change gradually. This can be accomplished by producing surfaces with a position dependent and gradually varying chemistry. In these so-called “gradient surfaces”, the gradient in surface energy is responsible for a position-bound variation in physical properties, most notably the wettability [4]. Recent studies have reported on the preparation of molecular gradients on length scales ranging from nanometers to centimeters [5,6]. All gradient techniques presented to date le
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