Behavior of Block Copolymers in Thin Films
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of Block Copolymers in Thin Films
T.P. Russell, V.R. Deline, V.S. Wakharkar, and G. Coulon Introduction The uses of polymeric materials in today's world are vast. Polymers are finding applications in the microelectronics industry as dielectric insulators and photoresists, in the aerospace and automobile industry as lightweight substitutes for metals, and in biotechnology as replacement components for bone, heart, and other organs. Thèse are just a few of the many end uses of polymers. Often, a polymer may hâve a particular, désirable property but processing of the polymer is difficult or the polymer's surface characteristics are undesirable. To circumvent such shortcomings there are several options. The first is to synthesize a new material, which is quite costly and time consuming. Alternatively, two polymers with complimentary properties can be mixed. Unfortunately, most polymer pairs are immiscible unless there are spécifie interactions (e.g., hydrogen bonding) between the two components. Consequently, coarse phase séparation is often observed, leading to an ill-defined material. Finally, two chemically distinct homopolymers can be joined together at one point, forming a block copolymer. While phase séparation may occur, the scale of the domains is restricted to the sizes of the individual homopolymers, which is typically on the tens of nanometers scale. The added advantage of this approach is that the size of the différent blocks can be altered, varying the concentration of the différent components. For copolymers that "microphase" separate, this variation in composition leads to a variation of the morphology of the microdomains ranging from spherical to cylindrical to lamellar.
MRS BULLETIN/OCTOBER1989
This article will focus on symmetric diblock copolymers — block copolymers comprised of two chemically distinct monomers, namely poly(styrene) (PS) and poly(methylmethacrylate) (PMMA) where the lengths of the block are equal and consequently a lamellar morphology is expected. The physical linking of two chemically distinct homopolymers has important ramifications in any application where surface behavior is important. Consider, for example, an emulsion of particles in a fluid or a composite comprised of filler particles in a host polymer matrix. The emulsion or the filler particles can be stabilized by selecting a block copolymer where one of the
blocks strongly adsorbs on the suspended particles in the emulsion or the composite and the other block interacts favorably with the surrounding médium. Such a copolymer would tend to stabilize the suspension and promote a uniform distribution of particles in the médium. In biotechnology, blood compatibility is always of concern. One can easily see that a copolymer comprised of one block that adsorbs onto an implant and another block which is biocompatible would circumvent this problem. Indeed such copolymers are in use today. Block copolymers find important applications as adhésion promoters for immiscible polymers where one block interacts favorably with one compo
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