L. H. Sperling

Departments of Chemical Engineering and Materials Science and Engineering, Materials Research Center, Center for Polymer Science and Engineering, and Polymer Interfaces Center,

Lehigh University, 5 E. Packer Ave., Bethlehem, PA 18015-3194

This communication will examine some of the nomenclature recommended for use in rubber-toughened plastics, an interdisciplinary field of interest to polymers, materials, chemistry, and chemical engineering. Recent literature in the field includes works by Folks and Hope(1), Utracki(2), Arends(3), and Sperling(4). Two classics, summarizing the mechanics side of the field, were written written by Williams(5) and by Kinloch and Young(6). Michler(7) wrote a review with excellent diagrams to illustrate the terminology.

One of the most important rubber-toughened plastics is high ampact polystyrene, HIPS. This material is prepared by dissolving 5-10% of polybutadiene in styrene monomer and polymerizing with stirring. The stirring induces a phase inversion, i.e., the earlier continuous rubber-rich phase becomes discontinuous, and vice versa. This material is sometimes called a graft copolymer, or a solution-graft copolymer. Indeed, grafting constitutes an important improvement over the earlier mechanical blends, primarily because the grafts lower the interfacial tension and help bond the two components together. Also, grafting, actually AB-crosslinked copolymer formation (two polymers forming one network) causes the rubber domains to become crosslinked. However, the extent of grafting is limited to only a few percent of the polystyrene chains. The final product, used in commercial materials, flows as an ordinary thermoplastic. It would be useful to call these materials solution graft copolymers to be understood as producing the kind of morphology with limited actual grafting characteristic of HIPS and related materials.

At one time, the term interpolymer was used to mean a chemically reacted mixture of two or more polymers, such as block or graft copolymers. That term should be discouraged as not being specific enough. Similarly, the term modifier is used to express "the rubber modification of a plastic." While it is surely modified, the authors usually mean toughened or improved. The term modified might equally mean worsened, almost never implied by the authors using these terms.

Almost always, rubber-toughened plastics are phase-separated, often with a phase-within-a-phase-within-a-phase morphology, see Figure 1. It is just becoming recognized that the complex morphology results primarily from spinodal decomposition, although nucleation and growth kinetics are sometimes important. Nucleation and growth results in spheroidal domains, while spinodal decomposition often results in interconnected cylinders, not always obvious in thin section electron microscopy.

Figure 1 illustrates a continuous plastic phase and a discontinuous rubber phase, called a rubber domain, or sometimes a rubber cellular domain. Within the rubber domains are occluded cellular domains, usually composed of substantially the same material as the continuous phase. The rubber cellular domains together with the occluded cellular domains are sometimes called salami structures after their appearance. The toughness obtained in such materials is often related to the rubber phase volume, which is the rubber domain volume plus the occluded cellular domain volume.

A crack is an open fissure in the material. However, a craze constitutes a relatively stable structure, with usually highly oriented fibrillar or microfibrillar structures running from the roof to the floor of the craze, see Figure 2. The rubber domains may cavitate, or form voids inside; this relieves traxial stresses inside the plastic. Another mechanism, shear band formation, involves the orientation of the polymer chains parallel to the applied stress. The shear bands themselves usually run at 45^o to the stress direction, and may form a crisscross pattern. Rubber particles may also be said to bridge a crack or a craze, providing toughening via rubber elasticity forces.

For composite materials, those containing a polymer and a non-polymer component such as fibers, platelets, or particulates, several different mechanisms may arise: crack pinning by several closely spaced particles may retard crack passage through the plastic, microcracking via bifurcation and crack path deflection, where the particle behaves as a mirror to the growing crack, all contribute to toughness.

Thus, each part of a multicomponent polymer material should have a precise name, and each action that the material takes part in, such as fracture processes, should also be named.

1. M. J. Folks and P. S. Hope, Polymer Blends and Alloys, Blackie Academic & Professional, London, 1993.
2. L. A. Utracki, Polymer Alloys and Blends, Hanser, Munich, 1990.
3. C. B. Arends, Ed., Polymer Toughening, Dekker, New York, 1996.
4. L. H. Sperling, Polymeric Multicomponent Materials: An Introduction, Wiley, New York, 1997.
5. J. G. Williams, Fracture Mechanics of Polymers, Ellis Horwood, Chichester, 1984.
6. A. J. Kinloch and R. J. Young, Fracture Behavior of Polymers. Applied Science, London. 1983.
7. G. M. Michler, Trends Polym. Sci. (TRIP), 3(4), 124 (1995).

Figure 1. Morphological features of a typical HIPS material. (PS = polystyrene, BP = polybutadiene, SBR = styrene-butadiene rubber)

Figure 2. Some features associated with failure in rubber-toughened plastics.

First published: ACS Division of Polymeric Materials: Science and Engineering (PMSE), 78, 1998.