Block Copolymer Thin Films Above and Below the Order-Disorder Transition Temperature
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Block Copolymer Thin Films Above and Below the Order-Disorder Transition Temperature
Ratchana Limary and Peter F. Green Dept. of Chemical Engineering and the Graduate Program in Materials Science The University of Texas at Austin, Austin TX
ABSTRACT Symmetric diblock copolymers undergo a disorder to order transition below a microphase separation transition temperature. In this temperature range the structure is characterized by alternating lamellae of thickness L. In thin film geometries, the lamellae are oriented normal to the substrate if there is a preferential interaction between either of the block constituents and the substrate/copolymer or copolymer/vacuum interfaces. Depending on the relation between the film thickness and L, the topography of the film might comprise of holes, islands or spinodal-like structures. We show that in a polystyrene-b-poly(methyl methacrylate) diblock copolymer of molecular weight 20, 000 g/mol, above the microphase separation transition temperature, the topography of the film depends on the thickness. A heirarchy of bicontinuous patterns and holes is observed with increasing film thickness for films thinner than 35 nm.
INTRODUCTION Applications that require thin polymer films include coatings for protection and/or lubrication, membranes, microelectronic processing and organic thin film device technologies. Polymer films with thicknesses h approximately 100 nm are subject to destabilizing intermolecular forces, and under certain conditions, can spontaneously rupture, creating patterns that reflect variations in the local film thickness. These range from bicontinuous patterns to discrete cylindrical holes that appear randomly throughout the surface of the film [1-8]. The reasons for the destabilization of the film are reasonably well understood, at least in simple liquids. Consider the situation involving a thin liquid film on a substrate. The free energy of interaction per unit area between the polymer/substrate interface and the polymer/air interface can be written as [9] ∆G = -A/(12πh2) This assumes that the interactions are purely non-polar and long-range. In this equation, A is the Hamaker constant, a measure of the strength of the interactions and is determined by the polarizability of the molecules in the media. If A > 0, then the interaction between the polymer/substrate and polymer/air interfaces is attractive. This implies that small amplitude oscillations at the free surface can become amplified by long-range forces. The Laplace pressure, ratio of the surface tension to the local radius of curvature, acts to stabilize the film. The competition between the Laplace pressure and the disjoining pressure, due to the van der Waals forces, determines a critical wave length instability beyond which the instability will grow [8]. If the film is sufficiently thin, it will rupture, creating a range of patterns. FF1.5.1
It was recently shown by Sharma and Khanna that the form of the short- and long-range interatomic potentials determines the early and intermediate stage morpholo
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