The articles in this issue illustrate how various theoretical and computer models have been used to probe behavior of polymers at penetrable and impenetrable interfaces. Interest in polymer interfaces stems from the control interfaces commonly have over macroscopic properties—such as the strength or biocompatibility of the material. Consider the “alloying” or blending of existing polymers. This technique provides an inexpensive means of fabricating new materials that can display the desirable properties of the individual components. Most polymer pairs however are immiscible, and the mixture segregates into macroscopic domains separated by relatively weak interfaces. To enhance the structural integrity of the blend, copolymer “compatibilizers” are added to the mixture. These chains localize at the interface between the immiscible polymers, enhance the adhesion between the phase-separated regions, and thereby improve the mechanical properties of the blend.

On an impenetrable interface, polymer films or coatings are commonly used to modify the properties of the underlying substrate. For example the polymer layer can be utilized to tailor biocompatibility, wettability, or roughness of the surface. Polymers anchored to solid surfaces can also be used to control the interaction between these surfaces. Thus the tethered layer can promote the adhesion between dissimilar solids or prevent the aggregation of colloidal particles.

Polymer films that contain well-defined patterns can be used in a variety of novel applications. For example such films can serve as the scaffolding in fabricating organic/inorganic composites with controlled architectures. One means of forming patterned films is to anchor the ends of homopolymers onto a substrate (so that the ends are fixed and cannot move) and immerse the system in a poor solvent. The incompatibility between the polymer and solvent drives the system to phase-separate. Since the ends are immobilized however, the polymers can only escape the unfavorable solvent by clustering with neighboring chain s into distinct aggregates or “pinned micelles.” These micelles have a uniform size and spacing, and form a regular array on the surface. In this article, we use theoretical models to extend this concept and show that, by tethering copolymers—chains that contain more than one type of monomer—we can drive the system to form more complicated surface patterns. These copolymer patterns provide a handle for engineering the interaction between surfaces and thus facilitate the fabrication of novel optical devices. If the copolymer films are composed of both hydrophilic and hydrophobic domains, the surface can also be used as a template for growing biological cells with tailored shapes and sizes.

Polymers are routinely placed between solid walls to provide lubrication or adhesion. Their function in these roles depends critically on the degree of dissipation within the polymer film and at the film/wall interface as the film shears or ruptures. Good lubrication is achieved by minimizing frictional dissipation while dissipation increases the strength of adhesive bonds.

Gent and Schultz suggested a direct link between frictional losses and the adhesive performance of polymers. This correspondence has been supported by recent experiments and by some of the molecular-dynamics simulations to be described. However we find that the correspondence breaks down when the molecular motion producing dissipation occurs at different locations during shear and rupture. In the following sections, we discuss the types of rate-dependent dissipation observed in thin films and the different factors that control whether dissipation occurs within the polymer or at the wall/film interface. The results suggest an origin for interesting memory effects observed in surface-force-apparatus (SFA) experiments on thin films and expose the atomic-scale processes that produce dissipation during internal rupture of a thin film.

From a single chain in a dilute solution to an entangled polymer melt, from bulk systems to more complex interfacial problems, computer simulations have played a critical role not only in testing the basic assumptions of various theoretical models but also in interpreting experimental results. Early computer simulations of polymers were mostly carried out on a lattice using Monte Carlo methods. This approach has led to significant progress in recent years and will continue to do so in many areas. In some cases however, for example in the study of shear, lattice models have serious limitations. For this reason and also due to the availability of more powerful computers, continuum, off-lattice polymer models have recently become popular. In this article, we review some of the recent progress in studying polymers at surfaces and interfaces using continuum models.

Studying the properties of endanchored polymer layers has been a fashionable occupation for numerous physicists, chemists, and material scientists for more than 10 years. Theoreticians have realized that grafted macromolecules are nice statistical objects wriggling around under thermal motion, which give rise to nontrivial long-range entropic effects. These can be described by elegant scaling laws and analogies with quantum or classical mechanics. For experimenters the area turned out to be a marvelous playground in which both very simple and sophisticated techniques such as x-ray or neutron scattering and reflectivity, nuclear magnetic resonance (NMR), Rutherford backscattering, and optical and atomic force microscopy (AFM) have been used to discover interesting and subtle phenomena. All this effort was also motivated by the importance of grafted layers in applications such as paints, adhesives, lubricants, colloidal stabilizers, and composite materials. By anchoring a thin, soft polymer layer to a solid surface, one can tune the surface properties. In this short article, we will discuss how the wetting and spreading of liquids and polymer melts can be profoundly altered by the presence of such protective layers.

Most pairs of chemically different homopolymers of any useful molecular weight are thermodynamically immiscible. This is because any tiny repulsive interaction per monomer, multiplied by the large number of monomers in a typical polymer, overwhelms entropy of mixing (of the order of kBT per molecule where kB is the Boltzmann constant and T the temperature). Nonetheless blends of immiscible pairs of homopolymers are industrially ubiquitous. Two broad categories of such materials are rubber-toughened plastics and stiffened elastomers. Both are fine dispersions of one polymer in another, with the desirable properties of the resulting material (e.g., stiffness-toughness balance) depending crucially on its composite structure and the mechanical contrast between the two components (e.g., difference in moduli).

Typically a fine dispersion, consisting of submicron particles, is necessary for good final properties. Also necessary is a strong interfacial bond between the particles and the matrix so that the resulting material does not fail at these interfaces. A typical volume fraction of the suspended component is perhaps 20–30%. Such polymer blends are prepared by two time-honored techniques (ancient, really, if one considers their antecedents in cookery): stirring and the use of polymeric surfactants—copolymers. The copolymers can be synthesized beforehand and added to a mixer where the components are stirred, but more often they are produced from the homopolymers during stirring by a grafting reaction, which somehow joins the two dissimilar homopolymers to form some sort of block copolymer.

The control of surface chemistry and topography has great technological relevance for numerous applications of polymers in textiles, adhesives, coatings, packaging, membranes, and biomedical implants. Conventionally, chemical modification of polymer surfaces has been achieved through kinetically governed practices that allow little control over the final surface composition or morphology. These chemically generated surfaces are also prone to reconstruction. Hence the development of inexpensive, scaleable routes to impart stable and more complex chemical functionality to polymer surfaces continues to be an active area of research. Apart from surface chemistry, the topography of a polymer surface often plays a determinant role in the adhesive, optical, and wetting characteristics of the surface. Consequently methods to produce surfaces of controlled texture are also of interest. Toward these goals, new, statistical, mechanics-based theoretical approaches, coupled with increased computing power, can now facilitate the first-principles design of polymer surfaces that are chemically and structurally “tailored” for a given application. In this article, we review some of the recent, significant developments in this area.

A long deoxyribose-nucleic-acid (DNA) molecule can be seen directly under an optical microscope if it is suitably decorated by fluorescent dyes. This makes it possible to see how a long chain responds to external forces f. The conformation of a uniformly stretched chain has been discussed by Pincus. Under a force f, acting on the two free ends, the chain configuration is modified only above a scale length ξ = kT/f where T is temperature and k is Boltzmann's constant. The chain (Figure 1a), containing N monomers, can be pictured as a string of blobs of size ξ containing g monomers with ξ = gva, where a is the monomer size (v = 1/2 in a θ solvent: the chain is ideal, v = 3/5 in a good solvent). The end-to-end distance is L = (N/g)ξ, that is,

For DNA, which is a semirigid chain, ideal behavior has been observed for chain lengths up to 20 μm when f is small. At higher f ( ≅ 100 pN), Cluzel et al. found a structural phase transition, corresponding to a plateau in the curve f versus elongation. Going even further, one reaches the break point.

Another method to stretch DNA is based on optical tweezers. Here one end of the DNA is attached to a bead of glass of high polarizability, which is attracted by a region of large (optical) electric field. This was used by the Stanford group: In one series of experiments, they observed the conformations of the chain, pulled at one end by a constant force f. The friction on monomers are cumulative, and the tension along the chain is not uniform—it increases from the free end to the tethered end.