L. H. Sperling

Department of Chemical Engineering, Department of Materials Science and Engineering, Materials Research Center, Center for Polymer Science and Engineering, Polymer Interfaces Center, Lehigh University, Bethlehem, PA 18015

Polymer surfaces and interfaces are far different than the interior or the bulk of polymers, and thus differ in properties. Accordingly, a whole new nomenclature has sprung up regarding polymers at surfaces and interfaces. It is the objective of this preprint to delineate the current state of affairs.

There are several kinds of polymer surfaces and interfaces. First, there is the free surface, where a polymer is ideally exposed to a vacuum, but in reality may be exposed to air. In the bulk state, a polymer may be in contact with another polymer, called a polymer blend interface, or in contact with a non-polymer such as a filler. This is termed a polymer composite interface. There is yet one other case of extreme importance, that of a polymer solution in contact with a solid, wherein the polymer is either attracted to or repelled from the solid surface.

Consider a polymer in solution, the solution being in contact with a solid. First, the polymer may be attracted to the surface, see Figure 1. This model was first described by Jenkel and Rumbach(1), and developed by Fleer, et al.(2) and many others. In this model, the mers in contact with the surface are called trains. These are adsorbed on the surface, usually by physical forces. The loops have no contact with the surface, but connect two trains, and the tails are non-adsorbed chain ends. People refer to the bound fraction as that fraction of chains forming trains.

When a surface exerts no net attraction on the polymer, the case of negative adsorption arises. The cause is the reduced entropy of polymer chains near the surface, in turn caused by the reduced number of conformations possible in chains near a surface, or in any confined circumstance. Hence, the fluid immediately adjacent to the interface will have a lower mer concentration than in the solution. This phenomenon is termed depletion(3). The depletion phenomenon will always exist when a polymer solution is near a surface. However, if the attractive forces are strong enough, the enthalpy gained overcomes the entropy lost, leading to the model in Figure 1.

Figure 1. A polymer chain partly adsorbed on a surface.

At very low solution concentrations, the chains in contact with the surface tend to spread out, having a relatively large bound fraction. This conformation is termed a pancake, see Figure 2. At higher solution concentrations, the chains must compete for space at the surface. A frequent result is that the trains are shorter, and long tails and loops stick out into the solution. This is termed a brush. A special type of brush involves terminally attached chains, and block copolymers where only one block (usually short) is attracted to the surface.

Block copolymers in solution may have a long block that is soluble, such as poly(ethylene oxide) in water, and shorter blocks on either end that are water-insoluble. These may be aliphatic hydrophobes, for example. Besides forming brushes on the surface of particles, the insoluble blocks may associate, termed associative polymers(4). These associative blocks are usually held together by weak forces, easily disturbed by shear. Thus, at low shear, the polymer solution/dispersion has a high viscosity, and at high shear a low viscosity. Incidentally, this is the basis for many non-drip paints.

A Langmuir-Blodgett film may be formed by dipping a solid substrate into a solution containing a monolayer film on its surface. The substrate is then then withdrawn, usually under conditions where the monolayer surface pressure is kept constant(5). The monolayer is transferred from the solution to the substrate. For the case of polymers, frequently it is the monomer that is transferred, followed by polymerization on the surface of the substrate(6).

A Langmuir-Blodgett film can be transferred onto a substrate by three possible deposition types: X, Y, and Z. Each type is formed by immersing (or emersing) the substrate repeatedly. In the X-type, the more polar end of the molecule or mer is always pointed away from the substrate. In the Z-type, the more polar end is always pointed toward the substrate. In the Y-type, the polar ends alternate in direction.

Figure 2. Two extreme cases of polymer adsorption on a substrate. Left, pancakes; right, brushes.

Polymer blends are usually immiscible. This is a direct result of the low entropy of mixing of two polymers, and the often positive heat of mixing. The modern story begins with the works of Helfand and Tagami(7), who developed a mean field theory of polymer interfaces, or interphases as they are commonly referred to now. An interphase is a region of space occupied by both molecular species, often of the order of 6-12 mer lengths. Clearly a gradation of composition exists between the two phases on either side.

The densities of polymers may go up or down at surfaces and interfaces, depending on the interactions. Positive heats of mixing as in immiscible blends and/or entropic depletion at free surfaces and composite interfaces causes a decrease in density.

Polymer blends that have unacceptable properties are called incompatible. (This must be distinguished from the scientific term immiscible, which means that the free energy of mixing is positive.) Thus, industrially, polymer blends are called incompatible or compatible, depending on the usefulness of the material. Lacking at this time seems to be a range of terms expressing quantities between immiscible and miscible, and incompatible and compatible. Compatibility, however, can be improved by a variety of techniques.

Compounds that are used to bond a polymer to a surface are called coupling agents. The most common coupling agents are the silanes(8). Another interaction of importance is called the acid-base interaction(9). Hydrogen bonds form the most important case of the acid-base interactions, where the heats of hydrogen bonding are a function of the acid strength of the hydrogen donor and the base strength of the hydrogen acceptor.

Often, block or graft copolymers are used at the interface, to promote adhesion. Such materials are known as compatibilizers. Other terms are modifiers, coupling agents (above), adhesion promoters, and interfacial agents. Utracki(10) defines a polymer alloy as an immiscible polymer blend having a modified interface and/or morphology. The general term for the bonding of a polymer and a substrate is called adhesion, which in general involves interfacial surface tension, London dispersion forces, hydrogen bonds, covalent bonds, etc.

The above briefly summarizes the current state of affairs. While the use of such terminology is becoming widespread, these terms and many others are not always well defined. Significant effort in this direction would be desirable.

1. E. Jenkel and B. Rumbach, Z. Elektrochem. 55, 612 (1951).
2. G. J. Fleer, M. A. Cohen Stuart, J. M. H. M. Scheutjens, T. Cosgrove, and B. Vincent, "Polymers at Interfaces", Chapman and Hall, London, 1993.
3. J. F. Joanny, L. Leibler, and P. G. de Gennes, J. Polym. Sci, Polym. Phys. Ed., 17, 1073 (1979).
4. Z. Gao and H. D. Ou-Yang, in Complex Fluids, E. B. Sirota, D. Weitz, T. Witten, and J. Israelachvili, Eds., MRS Symposium Proceedings, 248, 425; MRS, Pittsburgh, 1992.
5. G. Roberts, Langmuir-Blodgett Films, Plenum Press, New York, 1990.
6. S. H. Ou, V. Percec, J. A. Mann, J. B. Lando, L. Zhou, and K. D. Singer, Macromolecules, 26, 7263 (1993).
7. E. Helfand and Y. Tagami, J. Chem. Phys., 56, 3592 (1972).
8. E. P. Plueddemann, in Encylopedia of Polymer Science and Engineering, 2nd Ed., J. I. Kroschwitz, Ed., Vol. 4, Wiley, New York, 1986.
9. F. M. Fowkes, in Encylopedia of Polymer Science and Engineering, 2nd Ed., Supplement, p. 1, J. I. Kroschwitz, Ed., Wiley, New York, 1989.
10. L. A. Utracki, Polymer Alloys and Blends, Hanser Publishers, Munich, 1990.

First published: ACS Division of Polymeric Materials: Science and Engineering (PMSE), (1995).