Since their commercialization in the late 1940s, applications for synthetic polymers have grown at an extraordinary rate, to the point where polymers are now ubiquitous in our society. The demands placed upon polymer performance have paralleled the growth in applications and has driven the development of sophisticated multiconstituent polymer formulations with many outstanding physical and mechanical properties. Many applications require that a polymeric material be attached to or in contact with another material. In some cases, such as the classic nonstick fry pan, as well as lubrication and release paper, it is desirable to create surfaces that do not interact with the material in contact. On the other hand, in applications such as rubber toughening of blends, filled or fiber-reinforced polymers, and coatings, dissimilar materials must adhere to each other if high performance is to be obtained. While the majority of the research and development efforts to date have centered on optimizing bulk properties, the focus is now shifting toward the development of polymer systems with controlled surface and interfacial properties.

The recent flurry of activity in polymer interfacial science can be traced to the simultaneous emergence of three factors: a strong commercial need to control the surface properties of advanced multicomponent polymer systems, the availability of sophisticated theoretical methods for studying polymer surface and interphase problems, and the devel opment of new characterization techniques capable of investigating molecular level structure at polymer interphases. The seven articles that comprise this issue provide an overview of basic polymer interface science by discussing some of the current advances in polymer interface theory, by presenting many of the new techniques available for polymer interphase characterization, and by illustrating some of the interesting and challenging problems associated with their applications.

The properties of polymeric surfaces and interfaces are ubiquitous in their myriad commercial applications: paints and coatings, adhesives, lubrication, biocompatible materials, flocculation and steric stabilization of colloids, membranes and separation media, immiscible polymer blends, and filled composites. Some of these applications require low-energy surfaces that are chemically inert and are not easily wet with other materials. Other applications require high adhesion and strong interactions between the polymer and substrate. This article discusses fundamental principles governing the behavior of polymer surfaces and interfaces, then illustrates various means available for polymer-interface modification.

The adhesion of a polymer to itself, to a different polymer, or to an inorganic material can often be one of its most important properties. Adhesives are normally polymers, but then so are nonstick coatings. Clearly, polymer adhesion can vary from very strong to extremely weak. Adhesion is a sufficiently complex and broad area that there are a range of approaches to its understanding. My approach here is one that attempts to bridge between the mechanics of crack propagation and the atomic-scale processes that occur at a crack tip. I will not be concerned with the details of the interface chemistry and its modification. The interface chemical approach is very useful in many practical situations but is often very specific to particular systems. Adhesion (or perhaps its inverse) can be considered a failure property. We can only measure the adhesion between two items by breaking the joint between them. I will be mainly concerned with failure by crack propagation using a fracture-mechanics approach.

I will first consider the general question of the effects of bulk mechanical properties on adhesion and then describe some recent work on fracture mechanics of biomaterial interfaces as it relates to polymer glasses. I will then change the focus to the scale of the polymer chain to consider local processes involved in adhesion.

Long polymer chains adsorbed onto solid surfaces are important in a wide range of applications and are relevant to many issues in biology and medicine. Adsorbed polymer layers used widely in the stabilization of colloidal suspensions, for example, are essential to the formulation of paints, coatings, printing inks, drilling needs, and ceramics processing. Adsorbed polymer layers also play a crucial role in many tribological applications such as boundary lubricants.

The interfacial behavior of biological macromolecules (such as proteins) plays an important role in biomedical applications as well as in the function of living organisms. The development of biocompatible solid materials that can be used safely and efficiently in vascular and joint prostheses, catheters, artificial-heart valves and whole hearts, cardiac-arrest devices, and hemodialysis cartridges is critically important. In some instances, the adsorption of biological macromolecules onto the artificial material can lead to deleterious effects. For example, many vascular grafts and catheters fail because of thrombotic occlusion initiated by protein adsorption. Protein adsorption is also the initial subprocess that leads to plaque formation in teeth and the fouling of contact lenses. Given the central role of protein adsorption in many physiological systems, and the great benefits that could be derived by designing materials that do not adsorb biological macromolecules, understanding the interfacial behavior of biological polymers is important.

The ultimate goal of research in polymer adsorption is to facilitate the manipulation of the properties of adsorbed polymer layers (or polymer-solid interfaces) so that materials with required properties can be fabricated. To take steps toward this goal, understanding how the nature of the polymer, the substrate, and other prevailing conditions (such as the type of solvent) affect the macroscopic properties of the interface is crucial.

A major surge of activity is underway to understand the dynamics of polymer chains at interfaces. This stands in contrast to the situation a generation ago when much of polymer-materials research revolved around understanding dynamics in the bulk (isotropic) state. Building in part on earlier studies that had been somewhat neglected, striking new findings have been obtained. The new methods and equipment include surface-specific spectroscopies; advanced, in situ time-resolved methods to determine surface structure and composition; and the surface-forces apparatus for measuring adhesion and interfacial rheology. Also, older methods (such as contact angle) have been revitalizated and applied to new problems. Theoretical calculations and molecular-dynamics simulations are also emerging.

Appreciation is growing that scientific understanding is possible of these systems that are so complex and, often, so far from equilibrium. Polymer surfaces are becoming recognized as an area with many opportunities to do exciting and useful surface science, particularly regarding kinetics, diffusion, surface chemistry, and other rate-dependent processes.

The engineering significance is that while polymers and plastics-based applications are rooted in our economic life, too often the technologies and formulations are empirically derived. One tends to take plastics and their communication with adjoining materials for granted. A molecular understanding is needed so that better design can emerge by rational extension.

During the course of these new activities, the community of polymer science has rubbed shoulders with and has thereby become increasingly integrated with other disciplines-colloid science, surface science, biomedical science, and microelectronics, to cite a few examples. When dealing with interfaces, one's parochial materials interests quickly become generalized.

Ion-beam analysis of chemical composition as a function of depth is by now well-established for inorganic materials and is an important method of investigating growth of thin films. It has been applied to polymers much more recently, perhaps because fairly obvious problems with radiation damage discouraged workers in this field initially. Ion-beam analysis has developed, however, into a analytical tool that complements other methods, such as x-ray photoelectron spectroscopy and neutron reflection, very well. The purpose of this short article is to give the reader an introduction to its current uses in polymers.

The ion beams of ion-beam analysis are typically highly energetic (1–5 MeV) beams of 4He++. While other beams are used, for example, 3He and 15N, alpha particle beams are used in the vast majority of experiments reported in the literature. Two major categories of experiments are carried out with such beams. Rutherford backscattering (RBS) spectrometry to detect heavy elements in the polymer and forward recoil spectrometry (FRES) (also known as elastic recoil detection) to detect the isotopes hydrogen and deuterium. The basic principles for each method are similar.

The surface science of polymer materials has recently grown to a dynamic field, largely because of application in such areas as composite materials, wetting, coatings, adhesion, friction, and biocompatibility. The synthesis of new polymer materials, resulting in desired polymer-surface structures and composition, has become more sophisticated and is driving the development of new spectroscopic probes and continuing evolution of more established methods.

A good example of how instrumentation development has led to better applications in polymer-surface science is to follow the growth of studies using x-ray photoelectron spectroscopy (XPS)—also called electron spectroscopy for chemical analysis (ESCA). ESCA is now routinely used to obtain surface composition of polymers, and to follow processing steps and degradation chemistry. Advances in instrumentation have driven many of these more sophisticated applications.

Nevertheless, to improve the understanding of polymer-surface chemistry, more information is needed about surface structure with further sophistication, at a higher level of precision. For example, the knowledge of orientation and subsequent reactive availability of functional groups and of monomer ar rangement along a chain in copolymers and intrachain interactions is important. Macromolecular chain arrangement, termination, branching, and micromorphological information (i.e., domain size and distribution), molecular-weight distributions at or near the surface (in comparison with the average, bulk distribution), and higher degrees of spatial resolution in all three dimensions are all important in determining sophisticated surface structure-property relationships. To approach this level of structural and reactivity information at surfaces and interfaces, evolution of established methods and development of new methods must both be accomplished.

The presence of a surface or interface can markedly alter the configuration and spatial distribution of polymer molecules. In the bulk, polymer molecules—comprised of numerous monomers covalently linked together—pervade 10s of nanometers spatially. However, packing such chains at an interface—under the constraint that a solid polymer is essentially incompressible—necessitates perturbations to the chain configurations near the interface. This may result in a collapse of coils at the surface or in a preferential orientation of the monomers with respect to the interface plane. For amorphous homopolymers, however, simulations indicate that, on a segmental level, surface effects are generally damped within several segment diameters from the surface. Interactions between the surface and the polymer chains place additional constraints on the molecular configurations. As the architecture of the polymer chain becomes more complex, as with diblock copolymers in which two chemically distinct polymer chains covalently bond together at one end, the packing of chains at the interface must take into account the relative interactions of the two portions of the chains with the interface. Due to the connectivity of the blocks, preferential interactions of the blocks with the interface can influence the spatial distribution of the chains far from the interface. As the number of components increases, as with a simple binary-polymer mixture, not only must packing constraints be satisfied, but also the interactions of the two chains with the surface and with each other must be taken into account. In the case of homogeneous mixtures, the preferential interaction of one chain with an interface can lead to a substantial excess of that chain at the interface which, depending upon the proximity to the demixing point, can lead to surface effects that propagate many molecular diameters into the sample. The key, however, is the connectivity of the monomers in the polymer chain, which can enhance surface effects, as in the case of block copolymers or polymer mixtures, or suppress surface effects, as in the case of homopolymers.