Surface Evolution of Copolymer thin Films as Revealed by Atomic Force Microscopy
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SURFACE EVOLUTION OF COPOLYMER THIN FILMS AS REVEALED BY ATOMIC FORCE MICROSCOPY
G. COULON, B. COLLIN. D.CHATENAY AND D.AUSSERRE LPI2M2C. Universit6 P.M.Curie and CNRS Institut Curie, Section de Physique et Chimle, ii rue Pierre et Marie Curie, 75231 Paris cedex 05, France ABSTRACT Atomic Force Microscopy has been used to study the early stage evolution of the free surface of annealed symmetric poly(styrene-b-n-butylmethacrylate) diblock copolymer thin films. As the lamellar ordering propagates through the film thickness, Islands or holes are formed on the free surface. It Is shown that, depending on the Initial film thickness, I.e. on the fraction of the film surface occupied by the islands (or holes) In the ordered state, the existence or non-existence of spatial correlations characterizes the ordering kinetics of both islands and holes. However, the limit between these two regimes is not the same in the two cases : in the case of holes, spatial correlations occur for a higher value of the surface coverage than In the case of islands. INTRODUCTION Block copolymers are finding increasing applications as thin-film adhesives and surfactants. It is, thus, of fundamental Importance to understand the behavior of copolymers at surfaces and Interfaces. In the past, there have been a number of studies focused on the ordering process near surfaces or interfaces. Most of them dealt primarily with symmetric diblock copolymers which exhibit one-dimensional lamellar morphology In the bulk. The lamellar period L corresponds to the thickness of a -ABBA- or -BAABbilayer. In thin films, an ordering of the lamellar microdomains parallel to the film surfaces has been observed after annealing above the glass transition temperature Tg. Minimization of Interfacial energies dictates which block is present at both interfaces (air/polymer and substrate/polymer). As a consequence, after ordering, the film thickness is quantized [1.2,3], i.e. : e d=(n+l)L if the same block is present at both interfaces =(n+I)L If not For thin films prepared by spin-coating a copolymer solution onto a substrate at room temperature (T
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