Tailored Polymer Surfaces
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MRS BULLETIN/JANUARY 1997
affect the bulk mechanical properties of the polymer matrix. Hence methods are sought by which surface segregation of macromolecular additives can be used to produce more robust surfaces of welldefined chemistry, without compromising bulk properties of the material. With notable advances in the modeling of polymer surfaces and interfaces over the past decade, we can now begin to design multicomponent polymer materials from the ground up. Statistical mechanical models such as self-consistent-mean-field (SCF) treatments and computer simulation (both Monte Carlo and molecular dynamics) can offer valuable insights into the role of both enthalpic and entropic driving forces in surface-segregation processes. This knowledge can be applied to formulate candidate blend systems likely to exhibit large surface enrichment of a functionalized macromolecular component. SCF Models of Surface Segregation In SCF models, the thermodynamics of interfacial polymer systems is calculated by assuming that the chains experience a self-consistent potential that derives from the local density. Analytical SCF methods employ a continuum description for the polymer chain, which facilitates calculations through the use of path integrals.1 This approach has the advantage that closed form expressions for segment density profiles can be obtained. At the opposite extreme, numerical SCF methods such as those pioneered by Scheutjens and Fleer2 and by Szleifer3 employ discrete models of polymer chains that can be accommodated within the spatial constraints placed by a quasicrystalline lattice. Since the analytical SCF theories can be derived from the lattice formalism in the limit where the lattice size goes to zero,2 the information content of both theories is virtually identical. Self-consistent-mean-field theory has been used to model polymer conforma-
tions near a surface,3 surface segregation in bimodal polymer blends,4 segregation of end-adsorbed polymer layers from a polymer matrix,5 distribution profiles in ordered block-copolymer films,6 and segregation at the interface between immiscible polymer phases.7 In cases where detailed experimental comparisons have been made with SCF predictions, good agreement between measured and calculated profiles has been demonstrated.6" Ultimately though the goal is to move beyond the use of such models as a means to reproduce experimental results and to apply them instead as tools in the design of new materials. An interesting and technologically relevant example of this is the recent application of SCF methods to design polymer blends that selforganize to expose a stable hydrophilic air surface on a hydrophobic matrix. Designing Hydrophilic Surfaces The vast majority of commercial polymers have hydrophobic surfaces that exhibit high electrical resistance. These surfaces are easily charged, causing such common problems as static cling in textiles, attraction of dust and dirt to plastics, and damaging electrical discharge from electronic-components packaging. Hydrophilic surfaces ar
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