Polymer Adsorption
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ent monomers. Over longer length scales, long polymers are flexible objects that can adopt many conformations.5 The statistical distribution of these conformations depends upon the architecture of the chain, the number of monomer types and their natures, and the external fields to which the chains are subjected (e.g., those imposed by a solid surface). The conformations adopted by polymer chains in a given situation directly determine many macroscopic properties. Thus, if we are to understand the relationships between the properties of adsorbed polymer layers and the nature of the polymer, substrate, and other conditions, we must elucidate the molecular conformations of the polymer chains in a given situation. Appropriate choice of the molecular features of polymeric agents offer the opportunity to control macroscopic surface and interfacial properties. In this article, we briefly review what has been learned in the last few decades about confirmational statistics of adsorbed polymers in a variety of situations. Adsorbed Homopolymers The adsorption of homopolymers onto solid surfaces has been studied from a molecular viewpoint for over three decades.6"9 The confirmational statistics of polymers in dilute solution is determined by the nature of the polymer and the quality of the solvent. The solvent quality is often measured by the excluded volume parameter v, an effective volume of shells around each segment (created by solvation) that cannot be penetrated by another solvated segment. The parameter v is temperature dependent and becomes zero at a specific temperature 6. For T > 9, v is positive, and v is negative for T < 9. The size of a polymer chain is measured either by the squared and averaged end-to-end distance or by the radius of gyration (Rg), the average squared distance between segmental positions and the center of mass of the chain. Both quantities exhibit the same dependence upon chain length (molecular weight). The conformations adopted by polymer chains in good solvents
(v > 0) correspond to the statistical behavior of self-avoiding walks. The size of polymer chains in good solvents therefore exhibits the following scaling1" with chain length (JV): (Rl) * N"
(1)
The temperature 9 at which v equals zero is called the theta point. At the theta point, the self-avoiding walk that characterizes good solvent conditions becomes (approximately) a random walk. The statistics of a random walk imply1" that chain size in theta solvents scales with molecular weight in the following fashion: N.
(2)
The chain size is obviously smaller in theta solvents than in good solvents. As the quality of the solvent becomes poorer than that of a theta solvent, the chain size shrinks even further and the coil segment density becomes very high, ultimately leading to a collapsed coil. For a collapsed coil, the mean-squared radius of gyration grows with the 2/3 power of N. We may now ask what happens to the conformational statistics of homopolymer chains in the vicinity of surfaces. Consider situations in which the segment
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