Diblock Copolymers at Surfaces
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DIBLOCK COPOLYMERS AT SURFACES*
Peter F. Green Thomas M. Christensen# Sanda National Laboratories Albuquerque N.M. 87185-5800
Thomas P. Russell
Spiros H. Anastasiadis IBM Research Division Almaden Research Center 650 Harry Road San Jose, California 95120-6099
Abstract The surface properties of symmetric microphase separated diblock copolymers of polystyrene (PS) and polymethylmethacrylate (PMMA) were investigated using X-ray photoelectron spectroscopy (XPS), the specular reflectivity of neutrons and secondary ion mass spectrometry (SIMS). PS, the lower surface energy component, exhibited a preferential affinity for the free surface. For copolymers that are far from the bulk microphase separation transition (MST), the surface consists of a layer of pure PS. When the system is close to the MST the surface is a mixture of PS and PMMA. The PS surface excess can be described by a N-1/2 dependence, where N is the number of segments that comprise the copolymer chain. It is shown that the surface undergoes an ordering transition at a temperature T, that is above that of the bulk MST. The ordering of the bulk lamellar morphology is induced by an ordering at the surface. This is analogous to the ferromagnetic order observed in systems such as Gd at temperatures above the bulk Curie temperature. The results here are discussed in light of previous work on copolymer surfaces and in light of mean field theory. # Current Address: Dept. of Physics, Univ. of Colorado, Colorado Springs, Colorado 80933
Mat. Res. Soc. Symp. Proc. Vol. 171. @1990 Materials Research Society
318
Introduction Diblock copolymers are technologically an important class of materials. They behave like surfactants, analogous to the way in which soap molecules behave at oil water interfaces. Because of this they are used to promote adhesion between the phases of immiscible polymers and, hence to improve the mechanical properties of these systems. Due to their unique surface properties block copolymers are used in a variety of biomedical and microelectronic applications. The inherent incompatibility between the components of a block copolymer chain necessarily induces microphase separation at temperatures below the bulk microphase separation transition (MST). Unlike A-B polymer-polymer mixtures the phases are unable to grow very large because of the connectivity of the components. The resulting spatially periodic phases that form exhibit varying symmetries (cubic, cylindrical, double diamond and lamellar) that lower the free energy of the system [1-3]. The symmetry of a given copolymer system is dictated by the relative length of each constituent.The total number of segments, N, that comprise each chain, determine the size of the phases. The bulk properties of diblock copolymers are well understood. However, the surface properties of these materials are less well understood. It is well documented in multicomponent systems that the lowest surface energy component preferentially segregates to a free surface in order to minimize the total free energy of the
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