Organic Polymers

JOHN D.HOFFMAN and ROBERT L.MILLER

Compared to metallurgy and ceramics, the field of organic polymers is new. For example, the now widely used polymer polyethylene was discovered in Britain only a little more than 50 years ago. New polymers continue to be introduced every year at a significant rate, and new applications in both science and commerce inevitably follow. The number of new polymers is sufficient to justify books on the subject.1

This review considers some of the opportunities and challenges related to organic polymers. It not only covers what polymers are but also develops the nature of certain opportunities. Chemical properties and physical properties (which are becoming very important) are illustrated, both in experiment and in theory. Our goal in part is to remove some of the mystique that sometimes surrounds the subject of organic polymers and polymer science and to put these substances in proper perspective in the larger world of materials generally. The need for understanding the processing of polymers is one of the main themes of the review, as is the nature of certain exciting new theoretical developments.

Roles of the molecular theories and of fundamental research are broadly illustrated throughout. The very strong interaction of polymers with other materials and their significant contribution to application technology in other fields are shown. To balance this, instances are mentioned where polymer science and technology owe a debt to other areas of materials research and development, and examples are given where polymers indeed have their limitations. Although this review covers organic polymers, it does not intrude far into the field of biotechnology, which would require a separate review of its own. Emphasis is given to major new trends, such as the use of polymers



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Advancing Materials Research Organic Polymers JOHN D.HOFFMAN and ROBERT L.MILLER Compared to metallurgy and ceramics, the field of organic polymers is new. For example, the now widely used polymer polyethylene was discovered in Britain only a little more than 50 years ago. New polymers continue to be introduced every year at a significant rate, and new applications in both science and commerce inevitably follow. The number of new polymers is sufficient to justify books on the subject.1 This review considers some of the opportunities and challenges related to organic polymers. It not only covers what polymers are but also develops the nature of certain opportunities. Chemical properties and physical properties (which are becoming very important) are illustrated, both in experiment and in theory. Our goal in part is to remove some of the mystique that sometimes surrounds the subject of organic polymers and polymer science and to put these substances in proper perspective in the larger world of materials generally. The need for understanding the processing of polymers is one of the main themes of the review, as is the nature of certain exciting new theoretical developments. Roles of the molecular theories and of fundamental research are broadly illustrated throughout. The very strong interaction of polymers with other materials and their significant contribution to application technology in other fields are shown. To balance this, instances are mentioned where polymer science and technology owe a debt to other areas of materials research and development, and examples are given where polymers indeed have their limitations. Although this review covers organic polymers, it does not intrude far into the field of biotechnology, which would require a separate review of its own. Emphasis is given to major new trends, such as the use of polymers

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Advancing Materials Research FIGURE 1 Schematic representation of a polymer chain adsorbed on a surface. The black dots represent polymer-surface bonding. in composites, and the special properties that can be achieved from polymer blends. Figure 1 shows one aspect of the sometimes unique behavior of polymers. It depicts a typical long, concatenated polymer chain adsorbed on a surface. The chain consists of similar chain units (monomers) and is flexible. This illustration is important because it shows that, to desorb the entire polymer molecule, all of the little “feet” in the long molecule that are attached to points on the surface must be lifted from the surface simultaneously. From a probability point of view this is difficult even when the occasional attachments involve relatively weak van der Waals forces, and it is still more difficult if the attachments involve chemical bonds. The figure thus indicates one reason that polymers make such good surface coatings: Once adsorbed, they can be very difficult to detach. Polymer molecules will adsorb on a surface in seconds but may take weeks to detach fully, even in the presence of pure solvent; in poor solvents or in the absence of solvents, they virtually never detach. For short chains this is not the case; from a statistical point of view they come off rapidly in solvents because of the relatively low number of attachments. This simple illustration shows that the treatment of polymers deals with a property not ordinarily thought of as an important materials parameter, namely, molecular length, or, in more customary terms, molecular weight. MORPHOLOGY AND PROPERTIES Crystalline Polymers In the field of organic polymers, a wide range of chemical structures is readily available. As a beginning, consider the remarkable variety of properties and morphologies one can obtain with a specific single polymer chain, i.e., with “constant chemistry.” For this purpose we emphasize polyethylene, —(CH2—CH2)n—, which is a very simple chain. The examples will be single crystals, lamellar spherulitic structures, and high-strength fibers— all with the same molecule (common polyethylene), but with different processing. Consider the following experiment: in ordinary xylene at, say, 135 to 138°C, a small amount of linear polyethylene (0.001 to 0.01 percent) is dissolved, and the solution is cooled to around 70 to 80°C. Chains with a

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Advancing Materials Research FIGURE 2 Shadowed electron micrograph of a ridged, chain-folded, polyethylene single-crystal lozenge with a hollow pyramidal center formed from dilute solution. Crystal was sedimented on glycerine to prevent damage. Scale bar, 1 µm. From Bassett, Frank, and Keller.3 Reprinted with permission. molecular weight of 50,000 (corresponding to a chain length of about 455 nm) are suitable. Crystals, such as the one shown in Figure 2, will form and precipitate. This is a polyethylene single crystal! That such beautiful crystals could be formed from a polymer came as a surprise to most researchers. In now classical work, Keller2 elucidated the basic nature of these crystals. Figure 2 is an electron micrograph of such a crystal taken by Bassett,3 formerly one of Keller’s students. Although a full discussion of polymer single crystals is beyond the scope of this review, salient features are presented. A somewhat idealized structure of such a crystal4 is shown in Figure 3, in which each continuous, accordion-like line represents a single long polyethylene chain. It has been well established that the polymer molecules are chain-folded as shown. The diagonal striations seen in Figure 2 are the (310) slip planes indicated in Figure 3. The thickness l of a crystal is, say, 9 to 20 nm and is dependent on the crystallization temperature, as suggested by nucleation theory.4 (Nucleation theory, invented by metallurgists, is highly useful in explaining the formation and thickness of these polymer single crystals.) Such crystals began a revolution in polymer physics, namely, as a consequence of the chain-folding phenomenon. Modern techniques such as infrared spectroscopy5 and neutron scattering6,7 suggest that the fold perfection in such crystals is about 75 percent—nature does in fact make mistakes in putting together a crystal consisting of such long molecules. (The concept

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Advancing Materials Research FIGURE 3 Schematic diagram of chain-folded polyethylene single crystal. The orthorhombic subcell with dimensions a and b typical of many of the η-paraffins is shown in the lower diagram. From Hoffman, Davis, and Lauritzen.4 Reprinted with permission. of chain folding—even in single crystals—was adamantly resisted by some of the doyens in the field. In the end, the concept proved too useful and well supported to be ignored.) Polymer single crystals are not of great commercial importance, although strong mats resembling a sheet of paper can be prepared from them. Their importance is that they started a whole new line of thought concerning the morphology and basic character of crystalline polymers and, moreover, provided new insights into structure-property relationships for crystalline and semicrystalline polymeric materials. Many (but not all) of the polymers of commerce are potentially crystallizable, and in practice do frequently exhibit crystalline, or more often semicrystalline, properties. Depending on intended use, the crystallinity can be either useful or detrimental. More frequently it

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Advancing Materials Research is useful. For example, the crystalline regions are relatively effective barriers to diffusion of gases and small molecules. Hence the use of semicrystalline polymers—e.g., polyethylene—in applications such as food wrapping. Also, the crystalline regions may act as physical, rather than chemical, cross-links in forming a three-dimensional network that imparts mechanical stability. Yet, when melted again they may be processed or even reprocessed. Ordinary commercially crystallized polyethylene, such as food and freezer wrapping material, is crystallized from the melt. Under such conditions, objects such as those shown in Figure 4 are frequently seen under a polarizing optical microscope.8 These are called spherulites (by analogy with mineralogical spherulites) and appear superficially to be exceedingly different from a single crystal such as that shown in Figure 2. The four fields in Figure 4 represent three different molecular weights of polyethylene (18,000; 30,000; and 60,000). The first three were crystallized isothermally at the temperature indicated; the fourth was quench-crystallized. Molecular weights studied varied from 3,600 to 807,000, that is, chains varying from 32 nm to 7,300 nm in length. Commercial polyethylene crystallized in an unstrained manner contains typical spherulites as do other crystallizable polymers, such as nylon, which is a polyamide. What is the structure of a spherulite and what is the relation, if any, with single crystals such as that in Figure 2? Figure 5 shows some of the structural details of a polymer spherulite. It consists of lamellae or blades radiating from a central point, which is usually a piece of dirt (more elegantly, a “heterogeneous nucleus”). The lamellae are again chain-folded, somewhat like a single crystal, although not as perfectly. In spherulites, however, there are interlamellar links that make it stronger, and there are branch points that allow the spherulite to be a three-dimensional object. Nevertheless, the basic structural unit is similar, but not exactly equal, to that of the single crystal. The lamellar nature of polymer spherulites in melt-crystallized polymer was first recognized by Eppe, Fischer, and Stuart9 in Germany shortly after Keller’s original work in England2 on single crystals from dilute solution. The bands seen in Figure 4D arise from the cooperative twisting of adjacent lamellae.10 This lamellar nature is more clearly seen in Figure 6,11 which is an electron micrograph of a microtomed section of a spherulite of polyethylene. The lamellae are being viewed edge-on and have the appearance of the edges of a fanned deck of cards.12 The lamellae in Figure 6 are about 30 nm thick, and the polyethylene sample is a good polymer fraction with about the same molecular weight as the single crystal of Figure 2. (A “fraction” is a polymer specimen for which special techniques have been employed to ensure that the polymer chains are about the same length; most polymers, as synthesized, have a broad distribution of lengths.) Impurities and the shorter polymer chains are normally excluded from such lamellae.13 Some of the shorter chains may subsequently crystallize; the remainder of the

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Advancing Materials Research FIGURE 4 Spherulites in specimens of intermediate-molecular-weight polyethylene at ∆T>17.5°C, optical micrograph, crossed Nicol prisms. A, B, and C show coarsegrained nonbanded spherulites resulting from isothermal growth, ∆T>17.5°C. Micrograph D shows typical banded spherulites obtained in specimen 30.6 K by rapid quenching (30.6 K means molecular weight=30,600). From Hoffman et al.8 Reprinted with permission.

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Advancing Materials Research FIGURE 5 Schematic diagram of polymer spherulite with chain-folded lamellae. The spherulite consists of chain-folded lamellae radiating from a central point. The polymer chain axes in lamellae are more or less perpendicular to the radius of the spherulite. Branching causes the spherulite to become spherical in shape after sufficient growth. Noncrystallizable material (not shown) when present accumulates between lamellae and at the outer boundary. Twist of the lamellae when present causes rings in optical extinction pattern (compare Figure 4D). Interlamellar links and entanglements can cause incomplete crystallization in high-molecular-weight polymers. From Hoffman, Davis, and Lauritzen.4 Reprinted with permission. excluded material contributes to the noncrystalline (amorphous) component of the spherulite. Spherulites from branched polyethylene, i.e., polyethylene with adventitious pendant—CH3 and—CH2CH3 groups, do not display as clear a lamellar picture. Polyethylene spherulites, then, present an interesting morphology in which 15 to 20 percent of the material in the objects in Figure 4 is amorphous and the remainder is like that of a single crystal. The amorphous material is largely between the lamellae, and part of it is in the form of interlamellar links. Since composites are discussed later, note that the polymer spherulite is a self-assembled composite. That is one of the reasons polymers such as polyethylene and nylon are useful. They are natural composites, held together in part by interlamellar links, the reinforcement coming from the crystal lamellae. The composite is formed by the process of crystallization itself, an interesting result. Self-assembly is mentioned again below. Another mode of solution crystallization is possible while still holding the chemistry constant. Solutions, such as those used to prepare polyethylene

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Advancing Materials Research single crystals, can be crystallized under shear. Vigorous stirring is sufficient, and this process works best if the chains are long. Here a high molecular weight is desirable, and a more concentrated solution (approximately 0.1 to 1.0 percent) is to be used. Under these conditions a strikingly different morphological entity is obtained, as shown in Figure 7 (lying on a graphite substrate).14 The central thread is a very strong fiber. Work of this nature was first performed by Pennings and Kiel15 in Holland (which highlights the international character of the significant advances in this field). A break in the central thread can be seen roughly in the middle of the lower unit—it was stressed too much while being subjected to the beam in an electron microscope. The shear direction during crystallization was parallel to the central threads. Such entities are called “shish kebabs” and display the structure shown in Figure 816—a tremendously strong central fiber of primarily extended-chain conformation decorated with (again) chain-folded lamellae. The chain-folding part is not as perfect as it is in a single crystal, but the extended-chain perfection in the central fiber is high. The important point here is that shish kebabs are enormously strong—at least half the strength of the carbon-carbon bond and, in relation to their weight, stronger FIGURE 6 Electron micrograph of microtomed section of spherulite in melt-crystallized polyethylene showing lamellar nature. Lamellae are about 30 nm thick. Sample was stained with a chlorosulfonic acid/uranyl acetate treatment. From Keller.11

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Advancing Materials Research FIGURE 7 Electron micrograph of polyethylene “shish kebabs” formed by shear of a moderately concentrated solution. Note the break in the central thread in the lower unit. From Clark.14 than steel. The strength, of course, is due to the “shish.” Although shish kebabs themselves are not commercially important, they have been the impetus behind worldwide efforts to produce fibers, sheets, and rods commercially whose strengths take advantage of the molecular orientation of the shish kebab structure. This area of processing is known to polymer scientists as stress-induced crystallization (SIC). Briefly, one the approaches used to exploit the shish kebab effect is that of solid-state extrusion of polymers in the absence of solvents (for a recent review, see note 17). Figure 9 shows the results for polyethylene.18 The thick, opaque rod at the upper right of this figure consists of common, spherulitic, melt-crystallized polyethylene, as discussed earlier. The extrusion process breaks up the spherulitic structure and produces optically clear fibers, as depicted in the lower portion of the figure. Such processes are being commercialized in laboratories in many countries, for example, by the Allied Corporation in the United States. The structure of such extruded polymers proposed by Zachariades and Porter19 is shown schematically in Figure 10 (others have presented similar pictures). The resemblance to the

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Advancing Materials Research FIGURE 8 Models showing molecular nature of “shish kebabs” of polyethylene produced by stress-induced crystallization in solution. Strong core fibril is extended-chain; “kebabs” are imperfect chain-folded crystals. From Pennings, van der Mark, and Kiel.16 Reprinted with permission. FIGURE 9 Example of the optical clarity achieved by solid-state extrusion of a normally opaque spherulitic rod of semicrystalline polyethylene. From Porter.18

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Advancing Materials Research FIGURE 10 Schematic representation of extended-chain, lamellar block crystal structure produced by extrusion of spherulitic melt-crystallized polyethylene. From Zachariades and Porter.19 Reprinted with permission. structure of the shish kebab in Figure 8 is clear—mostly concatenated chains in the core fibril (the “shish”) with perhaps some residual folds. Whatever the fine details, the important point is that the structure consists primarily of long molecules that are parallel over long distances. Only in that way can the enormous tensile strength and high modulus of such materials be explained. A considerable amount of the progress cited here depended on electron microscopy. Polymer science has made its definite contributions, but it owes a considerable debt to those scientists and engineers who made the electron microscope a practical laboratory instrument. Table 1 lists the mechanical properties (modulus) of several common materials (the modulus of a material is the initial slope of its stress-strain curve). The first material is a soft metal (aluminum), followed by glass fibers, and so on, ending with polyethylene in different forms. Moduli vary from 70 to 420 GPa, with that of extruded polyethylene fibers being essentially equal to that of steel. Theoretically, the modulus of polyethylene should be about 300 GPa; experimentally, the best achieved to date is about 220 GPa. Also shown in Table 1 is the specific modulus, which is the modulus divided by the density. On a weight basis, then, the polymeric fibers are impressive,

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Advancing Materials Research FIGURE 22 Schematic representation of the effect of composition on properties of rubber-modified thermoplastics. S=polystyrene and B=polybutadiene. From Meier.20 As already indicated (Figure 23), phase separation of the polymeric species occurs. The phase-separated blend is in effect a self-assembled composite but of a different origin than that in the semicrystalline polymers mentioned earlier. This phase separation is of a most unusual type because one part of a given giant molecule is in one phase and the other part is in a second phase. The phase domains are thus of molecular dimensions, typically 10 to 30 nm in diameter. This is a curious situation, but thermodynamics is not violated. TABLE 2 Influence of Molecular Architecture on Tensile Strength Type Molecular Architecture Tensile Strength (kg/cm2) Random (SB) -S-B-B-S-B-S-S-S-B-B- ≃0 Diblock (S-B) -S-S-S-S-S-B-B-B-B-B- ≃0 Triblock (S-B-S) -S-S-S-[B-B…B-B]-S-S-S- 300 Triblock (B-S-B) -B-B-B-[S-S…S-S]-B-B-B- ≃0 Multiblock (S-B)n -([S-S…S-S] [B-B…B-B])n- ≃0 (n>3) NOTE: Polymers: 25 percent styrene (S), 75 percent butadiene (B), molecular weight=100,000. SOURCE: D.J.Meier.20

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Advancing Materials Research Thus, the whole question of phase diagrams becomes as important for polymer blends as it is for metals and ceramics. It is also necessary to know how fast phase separation occurs and whether it is spinodal decomposition or ordinary phase separation. The nature and properties of the interfaces are important. There is greatly increasing emphasis in both fundamental and applied work in this field. Many commercial applications have arisen with gratifying improvements in properties. Much basic work is going on in studies involving phase diagrams, dynamic mechanical behavior, and attempts to understand impact strengths and the size and nature of interfaces. Progress is already highly significant, but much remains to be done. Some of the needs for new knowledge in the field of blends are Theoretical basis of compatibility Methods of measuring compatibility Establishment of phase diagram Theory of kinetics of segregation Measurement methods for kinetics of segregation FIGURE 23 Schematic representation of the structure of styrene-butadiene diblock (S-B) and triblock (S-B-S and B-S-B) copolymers: solid line, polybutadiene; dashed line, polystyrene. Only in the S-B-S triblock copolymer do the polystyrene domains “tie” the structure together over the entire sample. From Meier.20

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Advancing Materials Research Theory of interfacial zones Mechanism of compatibilizers Dependence of properties on segregation Durability of properties Morphology and structure in segregated phases Composites The second area in which much is being done is composites. Nature invented composites—wood (cellulose and lignin) and bone (the polymer collagen and the mineral hydroxyapatite) are specific examples. A composite can and often does have much more desirable properties than do the individual “pure” or “virgin” materials from which it was made. Man-made composites have also been successful, as in the case of the “rubber” tire, which in its most common modern form is a composite of vulcanized rubber (the synthetic or natural polymer), carbon filler, and steel or polymeric fibers. One reason for interest in other man-made composites is indicated in Figure 24, which compares specific strengths (tensile strength/density). The high strength-to-weight ratio of composites is more favorable than their ratio of strength to size. The high strengths of the aramid Kevlar and graphite composites justify commercial interest in them. Glass composites combine a slight sacrifice in properties with a significant drop in cost. Current commercial aircraft use substantial amounts of nonstructural composites and about 37 percent by weight of composites in primary structure. Composites are an absolutely essential component in modern military aircraft. The current in-service airplane contains alloy steel in the engines, aluminum over the body, some titanium, and various types of composites. Current commercial aircraft design FIGURE 24 Specific strengths of various engineering materials (ultimate tensile strength/ density).

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Advancing Materials Research contemplates an increased use of composites; for future subsonic (not supersonic) aircraft, the potential composite use in primary structure exceeds 50 percent by weight of the aircraft. The net result will be a considerable weight saving, with a concomitant increase in fuel efficiency, as well as highly satisfactory durability, strength, and corrosion resistance. For many aircraft structural members, composites have become the materials of choice. In a related field, the body of the popular and impressive Pontiac Fiero automobile is made of composites. Lest one believe that the use of polymers for automobile parts and bodies is entirely new, one of the authors had in his possession a photograph taken in 1942 showing Henry Ford wielding an axe on a plastic trunk door made from soybeans (there is no photograph of the sequel). The science of polymers was in its infancy in those days. The idea, strongly espoused by Staudinger,37 that polymers were giant molecules had not yet taken full hold. (It now seems almost incredible that Staudinger, rightly deemed the father of polymer science, had an extremely difficult time convincing many of the scientists of his day that giant molecules existed. Some of the resistance undoubtedly arose, apart from the natural conservatism of most scientists, from the then strong presence of influential scientists who refused to believe even in atoms, because thermodynamics, mechanics, and optics had been developed so successfully without them.) The current scientific basis is much improved, and more rapid progress can now be made. The increased use of composites in automobile manufacture is a virtual certainty. Although not emphasized up to this point, most of the current polymer formulations employed in composites in aircraft and automobiles are three-dimensional networks, the so-called epoxy compounds being typical. These networks are similar to those shown in Figure 23, except that the cross-links involve chemical bonds. These materials (“thermosets”) are converted from the monomer-fiber-filler mixture (which is like a slurry) by the introduction of a chemical catalyst and application of heat in a curing cycle. The result is a tough composite consisting of high-molecular-weight polymer bonded to the fiber and the filler. Voids can be a problem. Because of the time required for the procedure, thought is currently being given to using thermoplastic polymers (i.e., polymers that are softened by heat and thereby rendered rapidly moldable) in composites. Such thermoplastic polymers might be of either the crystallizable or amorphous type; if crystalline, the crystals themselves can act as cross-links between chains. Rapid manufacture of a finished part having uniform high quality and predictable properties is the overall goal. Current problems and issues for further study concerning composites certainly include their processing and manufacturability and the manner in which they fail. Damage to an object made of metal (such as a car) results in a visible dent, and repair is relatively simple. A composite can be damaged

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Advancing Materials Research badly and frayed on the inside but show little sign of it on the outside.38 Repair tends to be difficult. The detection of flaws with nondestructive evaluation is a major issue in composites. A basic scientific understanding of the mechanism of failure and of the lifetime durability of composites is a high-priority subject. In this regard, a research briefing39 prepared at the request of the Office of Science and Technology Policy and the National Science Foundation (under the joint chairmanship of D.W.McCall of AT&T Bell Laboratories and R.Pariser of E.I.du Pont de Nemours & Co.) recommended directions for research. Among other concerns, it is necessary to obtain a better understanding of the relationships between molecular structure and physical properties of fibers and how these relationships can be translated into the behavior of a fiber in a composite. Since composites are subject to long-term cyclical loading, an understanding of the fatigue behavior of fibers under such conditions is necessary. Other key issues for further study are the fundamentals of the fiber-polymer interface as related, for example, to adhesion and load bearing; methods for joining or fastening composites to like or different materials; and control of creep under load (desired also for metals). The best way to resolve these issues is not clear. Efforts have been made, with some success, but there remains a great challenge. A common current practice is to pick a specific system and then explore that system in depth. But can broad generalizations be made? The answer, it is hoped, is “yes,” but the information must be sought out with vigor. The issues involved in the processing of polymer composites include the chemistry of the composite itself (what materials are chosen), the chemical interactions at the interfaces with the filler and the fiber, the flow and other problems associated with manufacture, and the nature of the structure finally achieved. Fundamental scientific and applied technological attacks on all aspects are required. It should come as no surprise that some of the flow properties encountered in the “slurries” that make up pre-composites before molding or curing are related to concepts developed by soil scientists, who have studied the behavior of moist earth under stress for a long time. For both blends and composites there is a clear need to understand chemical, physical, processing, and lifetime behavior. Approaches must be interdisciplinary, based both on theory and on experiments in organic chemistry, physical chemistry, rheology, and solid-state physics. They must include the fields of metals, polymers, ceramics, interfaces, and nondestructive evaluation. This breadth of endeavor itself poses a large and difficult challenge to scientists and engineers. Cooperation and interaction among industrial, academic, nonprofit, and government organizations will be important. Japanese and European competition is already evident, but the future holds sufficient promise to make the effort well worthwhile. The universities have made a significant response to the need for education

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Advancing Materials Research and research in composites. Some of the institutions involved are the University of Massachusetts, Washington University, the University of Delaware, Virginia Polytechnic Institute, and the Case Western Reserve-University of Akron dyad. This list is, of course, not exhaustive. Certain universities, such as Michigan State University, Michigan Technological University, and the University of California, Santa Barbara, have serious intentions in the field. Doubtless, there will be more. The authors’ organization, Michigan Molecular Institute, plans to cooperate with universities on both education and research efforts in the areas of blends and composites. We believe that a strong materials science base is an absolute prerequisite for such an activity. Industrial research laboratories are also highly active in the area—the aerospace industries heading the list— and it is clear that great interest and activity have also developed in the automotive company laboratories. Governmental involvement is not lacking either: the highly significant programs at Wright-Patterson Air Force Base and by the National Aeronautics and Space Administration are well known, and recently the National Bureau of Standards developed a response. The National Science Foundation is fully aware of the issues involved and has provided significant support to some of the universities mentioned. POLYMER SCIENCE The final topic of this review concerns aspects of the newer theories of polymer science currently stimulating the field: reptation, phase transitions, and some unusual behavior and generalizations concerning polymer melts. Reptation has already been mentioned and will be discussed again shortly. The “trajectory” of a flexible polymer molecule in the molten state is not unlike the path of an inebriated bumblebee, except that, because the chain atoms occupy space, the chain cannot cross itself. Consequently, the chain is slightly expanded and is called the excluded-volume coil (Figure 25, left). If there were no self-exclusion, the chain would be somewhat smaller (Figure 25, center); certain solvents (called theta solvents) permit the chain to act like this. If the pH or the thermodynamic driving force of the solvent is changed FIGURE 25 Schematic representation of the trajectory of an “amorphous” polymer molecule under various conditions: (left) with excluded volume; (center) random coil (theta conditions); (right) collapsed. The single molecule may undergo an abrupt phase transition.

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Advancing Materials Research (i.e., the “goodness” of the solvent is changed), a collapsed coil can appear abruptly (Figure 25, right). In other words, there is a true phase transition within a single molecule! This possibility has been of great interest to theoreticians. This “collapse transition” was originally predicted by Stockmayer,40 Monte Carlo studies suggested it,41 and the newer renormalization group and scaling theories26 have been able to deal with it effectively. The problem is similar—practically identical mathematically in one limit—to the magnetic spin problem in ferromagnets.42 This for a linear, flexible polymer! Recently, the collapse was observed experimentally.43 Figure 26 summarizes the theories and observations. The mean-square radius of gyration, is related to the length, N, of a polymer chain raised to some power, γ, where γ is a measure of the solvent power. The schematic shows an abrupt change in size with solvent character, with the transition actually occurring over a few tenths of a degree. In other words, polymer chains in solution behave differently than do low-molecular-weight compounds. One fascinating side effect of all this is that articles relating to the aforementioned phase transition in polymers are now occasionally found in the Physical Review, a journal that has in the past carried very few articles on polymers. It must be pointed out, however, that neither the American Chemical Society nor the American Physical Society ignores the topic of polymers; on the contrary, both strongly support active divisions for polymer science. Polymer melts also show unusual behavior when compared with normal fluids. For example, an ordinary liquid pumped out of a tube will exhibit the profile left-hand part of Figure 27.44 A polymer liquid (without confinement) pumped out of a tube swells up on exit, as shown on the right-hand FIGURE 26 Schematic mean-square radius of gyration of a polymer molecule as a function of temperature, T, or of solvent character. The value of is a measure of the effective size or “extent” of the polymer molecule.

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Advancing Materials Research FIGURE 27 Illustration of the difference in flow behavior between polymeric and simple Newtonian liquids. From Bird and Curtiss.44 Reprinted with permission. side of Figure 27; i.e., it exhibits “memory.” In many systems, such as metals, the deviation from perfect elasticity is usually small. In polymeric systems, by contrast, mechanical behavior is frequently dominated by such viscoelastic behavior with its pervasive memory effects. Bird and Curtiss44 have illustrated many other differences in the behavior of simple and polymeric liquids. (We have already mentioned one—namely, the behavior of the fluidity ϕ with temperature.) The unusual mechanical behavior of polymer melts is governed in general by nonlinear viscoelastic theory. To understand manufacturability and processing in polymers better, one must understand not only such simple behavior but also many complex phenomena, of which the example of Figure 27 is but a premonitory hint. One wants a molecular view of these effects that could be reduced to practice. However, the polymer engineers and phenomenologists prefer to think in terms of continuum equations, called constitutive equations. An example of a generalized simple linear constitutive equation is

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Advancing Materials Research FIGURE 28 A reptation“tube” in which the polymer chain is confined. The arrow indicates the direction of reptation. From de Gennes.26 Copyright © 1979 by Cornell University. Used by permission of the publisher, Cornell University Press. where γ21 is the shear strain at time t′ relative to that at time t. In a sense this approach is similar to thermodynamics in that there is no presence of the molecule in such equations—i.e., no molecular parameters. This equation implies that the shear stress, σ12, is obtained by an integration of the product of some kind of memory function, m, and the shear strain. This, in a general sense, is the sort of formulation required to understand phenomena such as that illustrated in Figure 27. There is no lack of inspired constitutive-based equations to deal with such situations; an example is the useful Bernstein-Kearsley-Zapas (BKZ) theory.45 But, as we have said, we would like to know about the role of the molecules. The reptation model (which is molecular in character) discussed earlier is thought to be applicable here and is shown with the subject molecule and its confining tube in an isolated state in Figure 28.26 Recently, Doi and Edwards,27 de Gennes,46 Curtiss and Bird,44 and Graessley47 have begun to modify the reptation model by, for example, letting the tube diffuse around on its end. On the basis of this type of molecular model, in one simplistic form, they have been able to derive an elementary constitutive equation from molecular considerations. It is hoped that such an approach, if it is not mathematically intractable, will lead to a much-improved molecular picture of what is happening when polymers are processed. The ultimate goal is to relate chain (i.e., molecular) properties to stress-strain-time relations in a polymer melt so that processing may be understood in basic terms. A special need is perceived for what may indeed be a new language that will allow scientists and engineers (and project managers) to communicate better concerning polymer properties as they relate to practice. Quoting the modulus is no longer enough, if indeed it ever was. Neither is a computer simulation that slurs over material properties. The need is great enough for polymers in conventional use and surely reaches its zenith when composites of polymers with metals or ceramic bodies are considered. The field of polymer science is displaying a tremendous vitality and energy, coupled with high-quality science. It now involves basic scientists from seemingly distant fields: theoretical physics (phase transitions); solid-state physics (piezoelectric, conductive, and semiconductive polymers); statistical mechanics; quantum mechanics; continuum mechanics; biophysics; and bio-

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Advancing Materials Research chemistry. In addition, rheology and viscoelasticity are its special province. There is plenty of scope for intellectual curiosity and creativity, with many unsolved problems even in the “conventional” parts of the field. In these, as well as in some of the new directions we have noted, there is a high probability that the science will lead to practical results and benefits. However, progress will depend in considerable degree on broad and fundamental knowledge and training in materials science as a whole; as we have illustrated repeatedly, a single type of material no longer stands in total isolation from the others, and the basic disciplines are still fundamental to every aspect of materials science. ACKNOWLEDGMENTS In preparing this review the efforts of numerous others were invaluable, specifically C.M.Guttman, G.T.Davis, L.Smith, and D.Huntston of NBS; M.Jaffe and R.M.Mininni of Celanese Corporation (in cooperation with DARPA and Dow Corning); K.Bowen of MIT; DARPA; R.E.Hefner, consultant to Michigan Molecular Institute; D.J.Meier of MMI; D.W. McCall of AT&T Bell Laboratories; J.T.Quinlivan of Boeing Company; R.S.Porter of the University of Massachusetts; and R.K.Eby of the Johns Hopkins University. NOTES 1.   H.-G.Elias, New Commercial Polymers 1969–1975 (Gordon and Breach, New York, 1977); H.-G.Elias and F.Vohwinkel, Neue polymere Werkstoffe für die industrielle Anwendung, 2nd ed. (Hanser, Munich, 1983). 2.   A.Keller, Philos. Mag. 2, 1171 (1957). 3.   D.C.Bassett, F.C.Frank, and A.Keller, Philos. Mag. 8, 1753 (1963). 4.   J.D.Hoffman, G.T.Davis, and J.I.Lauritzen, Jr., in Treatise on Solid State Chemistry, edited by N.B.Hannay (Plenum, New York, 1976), Vol. 3, Chap. 7. 5.   T.C.Chean and S.Krimm, J.Polym. Sci., Polym. Phys. Ed. 19, 423 (1981); X.Jing and S.Krimm, ibid. 20, 1155 (1982). 6.   C.M.Guttman, E.A.DiMarzio, and J.D.Hoffman, Polymer 22, 597 (1981). 7.   D.M.Sadler, “Neutron scattering by crystalline polymers: Molecular conformations and their interpretation,” in Structure of Crystalline Polymers, edited by I.H.Hall (Elsevier, London, 1984), p. 125. 8.   J.D.Hoffman, L.J.Frolen, G.S.Ross, and J.I.Lauritzen, Jr., J. Res. Natl. Bur. Stand., Sect. A 79, 671 (1975). 9.   R.Eppe, E.W.Fischer, and H.A.Stuart, J. Polym. Sci. 34, 721 (1959). 10.   H.D.Keith and F.J.Padden, Jr., Polymer 25, 28 (1984). 11.   A.Keller, University of Bristol (private communication). 12.   G.C.Claver, Jr., R.Buchdahl, and R.L.Miller, J. Polym. Sci. 20, 202 (1956). 13.   H.D.Keith and F.J.Padden, Jr., J. Appl. Phys. 35, 1270, 1286 (1964). 14.   E.J.Clark, National Bureau of Standards (unpublished). 15.   A.J.Pennings and A.M.Kiel, Kolloid Z.Z. Polym. 205, 160 (1965). 16.   A.J.Pennings, J.M.A.A.van der Mark, and A.M.Kiel, Kolloid Z.Z. Polym. 237, 336 (1970).

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Advancing Materials Research 17.   I.M.Ward, Adv. Polym. Sci. 70, 1 (1985). 18.   R.S.Porter (private communication). See also R.S.Porter, in Cutting Edge Technologies (National Academy Press, Washington, D.C., 1984), pp. 109–116. 19.   A.E.Zachariades and R.S.Porter, in The Strength and Stiffness of Polymers, edited by A.E.Zachariades and R.S.Porter (Marcel Dekker, New York, 1983). 20.   D.J.Meier, Michigan Molecular Institute (private communication). 21.   K.Ziegler, E.Holzkamp, H.Breil, and H.Martin, Angew. Chem. 67, 426, 541 (1955). 22.   G.Natta, P.Pino, P.Corradini, F.Danusso, E.Mantica, G.Mazzanti, and G.Moraglio, J. Am. Chem. Soc. 77, 1708 (1955). 23.   W.F.Graessley, Adv. Polym. Sci. 47, 67 (1982). 24.   P.J.Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, N.Y., 1953). 25.   E.A.DiMarzio, National Bureau of Standards (unpublished). 26.   P.G.de Gennes, Scaling Concepts in Polymer Physics (Cornell University Press, Ithaca, N.Y., 1979). 27.   M.Doi and S.F.Edwards, J. Chem. Soc., Faraday Trans. 2, 74, 1789, 1802, 1818 (1978). 28.   J.D.Hoffman, “Golden jubilee conference polyethylene, 1933–1983,” Proceedings of the Plastics and Rubber Institute (June 1983), p. D3.1. 29.   J.D.Hoffman, SPE Trans. 4, 315 (1964). 30.   J.H.Gibbs and E.A.DiMarzio, J. Chem. Phys. 28, 373 (1958). 31.   J.H.Gibbs (private communication). 32.   M.L.Williams, R.F.Landel, and J.D.Ferry, J. Am. Chem. Soc. 77, 3701 (1955). 33.   G.T.Davis, J.E.McKinney, M.G.Broadhurst, and S.C.Roth, J. Appl. Phys. 49, 4998 (1978). 34.   A.J.Lovinger, Science 24, 3 (1983). 35.   R.L.Miller, in Polymer Handbook, 2nd ed., edited by J.Brandrup and E.H.Immergut (Wiley, New York, 1975). 36.   W.J.Bailey, R.L.Sun, H.Katsuki, T.Endo, H.Iwama, R.Tsushima, K.Saigo, and M.M.Bitritto, in Ring-Opening Polymerization, ACS Symposium Series No. 59), edited by T.Saegusa and E.Goethals (American Chemical Society, Washington, D.C., 1977), p. 38. 37.   H.Staudinger, Chem. Ber. 53, 1073 (1920). 38.   M.D.Rhodes and J.G.Williams, “Concept for improving the damage tolerance of composite compression panels,” presented at DOD/NASA Conference on Fibrous Composite Structures, New Orleans, La., 27–29 June 1981. 39.   “Report of the research briefing panel on high-performance polymer composites,” in Research Briefings 1984 (National Academy Press, Washington, D.C., 1984), pp. 45– 56. 40.   W.H.Stockmayer, Makromol. Chem. 35, 54 (1960). 41.   F.L.McCracken, J.Mazur, and C.M.Guttman, Macromolecules 6, 859 (1973); A.T. Clark and M.Lal, Br. Polym. J. 9, 92 (1977). 42.   P.G.de Gennes, J. Phys. (Paris) 36, L-55 (1975). 43.   I.Nishio, S.-T.Sun, G.Swislow, and T.Tanaka, Nature (London) 281, 208 (1979); S.-T.Sun, I.Nishio, G.Swislow, and T.Tanaka, J. Chem. Phys. 73, 5971 (1980). 44.   R.B.Bird and C.F.Curtiss, Phys. Today (January 1984), p. 36. 45.   B.Bernstein, E.A.Kearsley, and L.J.Zapas, Trans. Soc. Rheol. 7, 391 (1963); 9, 27 (1965). 46.   P.G.de Gennes and L.Léger, Ann. Rev. Phys. Chem. 33, 49 (1982). 47.   W.W.Graessley, J. Polym. Sci., Polym. Phys. Ed., 18, 27 (1980).