by Isaac Asimov
Figure 11.3. The gutta percha molecule, a portion of which is shown here, is made up of thousands of isoprene units.The first five carbon atoms at the left (black balls) and the eight hydrogen atoms bonded to them make up an isoprene unit.
A simple sugar molecule, such as glucose, is a monosaccharide (Greek for “one sugar”); sucrose and lactose are disaccharides (“two sugars”); and starch and cellulose are polysaccharides (“many sugars”). Because two isoprene molecules join to form a well-known type of compound called terpene (obtained from turpentine), rubber and gutta percha are called polyterpenes.
The general term for such compounds was invented by Berzelius (a great inventor of names and symbols) as far back as 1830. He called the basic unit a monomer (“one part”) and the large molecule a polymer (“many parts”). Polymers consisting of many units (say more than a hundred) are now called high polymers. Starch, cellulose, rubber, and gutta percha are all examples of high polymers.
Polymers are not clear-cut compounds but are complex mixtures of molecules of different sizes. The average molecular weight can be determined by several methods. One involves measurement of viscosity (the ease or difficulty with which a liquid flows under a given pressure). The larger the molecule and the more elongated it is, the more it contributes to the internal friction of a liquid and the more it makes it pour like molasses, rather than like water. The German chemist Hermann Staudinger worked out this method in 1930 as part of his general work on polymers; and in 1953, he was awarded the Nobel Prize for chemistry for his contribution toward the understanding of these giant molecules.
In 1913, two Japanese chemists discovered that natural fibers such as those of cellulose diffract X rays, just as a crystal does. The fibers are not crystals in the ordinary sense but are microcrystalline in character: that is, the long chains of units making up their molecules tend to run in parallel bundles for longer or shorter distances, here and there. Over the course of those parallel bundles, atoms are arranged in a repetitive order as they are in crystals, and X rays striking these sections of the fiber are diffracted.
So polymers have come to be divided into two broad classes—crystalline and amorphous.
In a crystalline polymer, such as cellulose, the strength of the individual chains is increased by the fact that parallel neighbors are joined together by chemical bonds. The resulting fibers have considerable tensile strength. Starch is crystalline, too, but far less so than cellulose, and therefore lacks the strength of cellulose and its capacity for fiber formation.
Rubber is an amorphous polymer. Since the individual chains do not line up, cross-links do not occur. If heated, the various chains can vibrate independently and slide freely over and around one another. Consequently, rubber or a rubberlike polymer will grow soft and sticky and eventually melt with heat. (Stretching rubber straightens the chains and introduces some microcrystalline character. Stretched rubber, therefore, has considerable tensile strength.) Cellulose and starch, in which the individual molecules are bound together here and there, cannot undergo the same independence of vibration, so there is no softening with heat. They remain stiff until the temperature is high enough to induce vibrations that shake the molecule apart so that charring and smoke emission take place.
At temperatures below the gummy, sticky stage, amorphous polymers are often soft and springy. At still lower temperatures, however, they become hard and leathery, even glassy. Raw rubber is dry and elastic only over a rather narrow temperature range. The addition of sulfur to the extent of 5 percent to 8 percent provides flexible sulfur links from chain to chain, which reduce the independence of the chains and thus prevent gumminess at moderate heat. They also increase the free play between the chains at moderately low temperatures; therefore the rubber does not harden. The addition of greater amounts of sulfur, up to 30 percent to 50 percent, will bind the chains so tightly that the rubber grows hard. It is then known as hard rubber or ebonite.
(Even vulcanized rubber will turn glassy if the temperature is lowered sufficiently. An ordinary rubber ball, dipped in liquid air for a few moments, will shatter if thrown against a wall. This is a favorite demonstration in introductory chemistry courses.)
Various amorphous polymers show different physical properties at a given temperature. At room temperature, natural rubber is elastic, various resins are glassy and solid, and chicle (from the sapodilla tree of South America) is soft and gummy (it is the chief ingredient of chewing gum).
CELLULOSE AND EXPLOSIVES
Aside from our food, which is mainly made up of high polymers, probably the one polymer that man has depended on longest is cellulose. It is the major component of wood, which has been indispensable as a fuel and a construction material. Cellulose is also used to make paper. In the pure fibrous forms of cotton and linen, cellulose has been man’s most important textile material. And the organic chemists of the mid-nineteenth century naturally turned to cellulose as a raw material for making other giant molecules.
One way of modifying cellulose is by attaching the nitrate group of atoms (a nitrogen atom and three oxygen atoms) to the oxygen-hydrogen combinations (hydroxyl groups) in the glucose units. When this was done, by treating cellulose with a mixture of nitric acid and sulfuric acid, an explosive of until-then unparalleled ferocity was created. The explosive was discovered by accident in 1846 by a German-born Swiss chemist named Christian Friedrich Schonbein (who, in 1839, had discovered ozone). He had spilled an acid mixture in the kitchen (where he was forbidden to experiment, but he had taken advantage of his wife’s absence to do iust that) and snatched up his wife’s cotton apron, so the story goes, to wipe up the mess. When he hung the apron over the fire to dry, it went poof!, leaving nothing behind.
Schonbein recognized the potentialities at once, as can be told from the name he gave the compound, which in English translation is guncotton. (It is also called nitrocellulose.) Shonbein peddled the recipe to several governments. Ordinary gunpowder was so smoky that it blackened the gunners, fouled the cannon, which then had to be swabbed between shots, and raised such a pall of smoke that, after the first volleys, battles had to be fought by dead reckoning. War offices therefore leaped at the chance to use an explosive that was not only more powerful but also smokeless. Factories for the manufacture of guncotton began to spring up. And almost as fast as they sprang up, they blew up. Guncotton was too eager an explosive; it would not wait for the cannon. By the early 1860s, the abortive guncotton boom was over, figuratively as well as literally.
Later, however, methods were discovered for removing the small quantities of impurities that encouraged guncotton to explode. It then became reasonably safe to handle. The English chemist Dewar (of liquefied gas fame) and a co-worker, Frederick Augustus Abel, introduced the technique, in 1889, of mixing it with nitroglycerine, and adding Vaseline to the mixture to make it moldable into cords (the mixture was called cordite). That, finally, was a useful smokeless powder. The Spanish-American War of 1898 was the last war of any consequence fought with ordinary gunpowder.
(The machine age added its bit to the horrors of gunnery. In the 1860s, the American inventor Richard Gatling produced the first machine gun for the rapid firing of bullets; and this was improved by another American inventor, Hiram Stevens Maxim, in the 1880s. The Gatling gun gave rise to the slang term gat for gun. It and its descendant, the Maxim gun, gave the unabashed imperialists of the late nineteenth century an unprecedented advantage over the “lesser breeds,” to use Rudyard Kipling’s offensive phrase, of Africa and Asia. “Whatever happens, we have got / The Maxim gun and they have not!” went a popular jingle.)
“Progress” of this sort continued in the twentieth century. The most important explosive in the First World War was trinitrotoluene, familiarly abbreviated as TNT. In the Second World War, an even more powerful explosive, cyclonite, came into use. Both contain the nitro group (NO2) rather than the nitrate group (ONO2). As lords of war, however, all chemical explosives gave way to nuclear bombs in 1945 (see chap
ter 10).
Nitroglycerine, by the way, was discovered in the same year as guncotton. An Italian chemist named Ascanio Sobrero treated glycerol with a mixture of nitric acid and sulfuric acid and knew he had something when he nearly killed himself in the explosion that followed. Sobrero, lacking Schonbein’s promotional impulses, felt nitroglycerine to be too dangerous a substance to deal with and virtually suppressed information about it. But within ten years, a Swedish family, the Nobels, took to manufacturing it as a “blasting oil” for use in mining and construction work. After a series of accidents, including one that took the life of a member of the family, Alfred Bernhard Nobel, the brother of the victim, discovered a method of mixing nitroglycerine with an absorbent earth called kieselguhr or diatomaceous earth (kieselguhr consists largely of the tiny skeletons of one-celled organisms called diatoms). The mixture consisted of three parts of nitroglycerine to one of kieselguhr, but such was the absorptive power of the latter that the mixture was virtually a dry powder. A stick of this impregnated earth (dynamite) could be dropped, hammered, even burned, without explosion. When set off by a percussion cap (electrically, and from a distance), it displayed all the shattering force of pure nitroglycerine.
Percussion caps contain sensitive explosives that detonate by heat or by mechanical shock and are therefore called detonators. The strong shock of the detonation sets off the less sensitive dynamite. It might seem as though the danger were merely shifted from nitroglycerine to detonators, but it is not so bad as it sounds, since the detonator is only needed in tiny quantities. The detonators most used are mercury fulminate (HgC2N2O2) and lead azide (PbN6)·
Sticks of dynamite eventually made it possible to carve the American West into railroads, mines, highways, and dams at a rate unprecedented in history. Dynamite, and other explosives he discovered, made a millionaire of the lonely and unpopular Nobel (who found himself, against his humanitarian will, regarded as a “merchant of death”). When he died in 1896, he left behind a fund out of which the famous Nobel Prizes were to be granted each year in five fields: chemistry, physics, medicine and physiology, literature, and peace. Aside from the fabulous honor, a cash reward of about forty thousand dollars was involved, and the amount has risen since then. The first prizes were awarded on 10 December 1901, the fifth anniversary of his death, and these have now become the greatest honor any scientist can receive.
Considering the nature of human society, explosives continued to take up a sizable fraction of the endeavor of great scientists. Since almost all explosives contain nitrogen, the chemistry of that element and its compounds was of key importance. (It is also, it must be admitted, of key importance to life as well.)
The German chemist Wilhelm Ostwald, who was interested in chemical theory rather than in explosives, studied the rates at which chemical reactions proceed. He applied to chemistry the mathematical principles associated with physics thus becoming one of the founders of physical chemistry. Toward the turn of the century, he worked out new methods for converting ammonia (NH3) to nitrogen oxides, which could then be used to manufacture explosives. For his theoretical work, particularly on catalysis, Ostwald received the Nobel Prize for chemistry in 1909.
The ultimate source of usable nitrogen was, in the early decades of the twentieth century, the nitrate deposits in the desert of northern Chile. During the First World War, these fields were placed out of reach of Germany by the British Navy. However, the German chemist Fritz Haber had devised a method by which the molecular nitrogen of the air could be combined with hydrogen under pressure, to form the ammonia needed for the Ostwald process. This Haber process was improved by the German chemist Karl Bosch, who supervised the building of plants during the war for the manufacture of ammonia. Haber received the Nobel Prize for chemistry in 1918, and Bosch shared one in 1931. By the late 1960s, the United States alone was manufacturing 12 million tons of ammonia per year by the Haber process.
PLASTICS AND CELLULOID
But let us return to modified cellulose. Clearly, it was the addition of the nitrate group that made for explosiveness. In guncotton all of the available hydroxyl groups were nitrated. What if only some of them were nitrated? Would they not be less explosive? Actually, such partly nitrated cellulose proved not to be explosive at all. However, it did burn very readily; the material was eventually named pyroxylin (from Greek words meaning “firewood”).
Pyroxylin can be dissolved in mixtures of alcohol and ether—as was discovered independently by the French scholar Louis Nicolas Ménard and an American medical student named J. Parkers Maynard (and an odd similarity in names that is). When the alcohol and ether evaporate, the pyroxylin is left behind as a tough, transparent film, which was named collodion. Its first use was as a coating over minor cuts and abrasions; it was called new skin. However, the adventures of pyroxylin were only beginning. Much more lay ahead.
Pyroxylin itself is brittle in bulk. But the English chemist Alexander Parkes found that if it was dissolved in alcohol and ether and mixed with a substance such as camphor, the evaporation of the solvent left behind a hard solid that became soft and malleable when heated. It could then be modeled into some desired shape which it would retain when cooled and hardened. So nitrocellulose was transformed into the first artificial plastic, in the year 1865. Camphor, which introduced the plastic properties into an otherwise brittle substance, was the first plasticizer.
What brought plastics to the attention of the public and made it more than a chemical curiosity was its dramatic introduction into the billiard parlor. Billiard balls were then made from ivory, a commodity that could be obtained only over an elephant’s dead body—a point that naturally produced problems. In the early 1860s, a prize of 10,000 dollars was offered for the best substitute for ivory that would fulfill the billiard ball’s manifold requirements of hardness, elasticity, resistance to heat and moisture, lack of grain, and so on. The American inventor John Wesley Hyatt was one of those who went out for the prize. He made no progress until he heard of Parkes’s trick of plasticizing pyroxylin to a moldable material that would set as a hard solid. Hyatt set about working out improved methods of manufacturing the material, using less of the expensive alcohol and ether and more in the way of heat and pressure. By 1869, Hyatt was turning out cheap billiard balls of this material, which he called celluloid. It won him the prize.
Celluloid turned out to have significance away from the pool table. It was versatile indeed. It could be molded at the temperature of boiling water; it could be cut, drilled, and sawed at lower temperatures; it was strong and hard in bulk but could also be produced in the form of thin flexible films that served for shirt collars, baby rattles, and so on. In the form of still thinner and more flexible films, it could be used as a base for silver compounds in gelatin, and thus it became the first practical photographic film.
The one fault of celluloid was that, thanks to its nitrate groups, it had a tendency to burn with appalling quickness, particularly when in the form of thin film. It was the cause of a number of fire tragedies.
The substitution of acetate groups (CH3COO–) for nitrate groups led to the formation of another kind of modified cellulose called cellulose acetate. Properly plasticized, this has properties as good or almost as good as those of celluloid, plus the saving grace of being much less likely to burn. Cellulose acetate came into use just before the First World War; and after the war, it completely replaced celluloid in the manufacture of photographic film and many other items.
HIGH POLYMERS
Within half a century after the development of celluloid, chemists emancipated themselves from dependence on cellulose as the base for plastics. As early as 1872, Baeyer (who was later to synthesize indigo) had noticed that when phenols and aldehydes are heated together, a gooey, resinous mass results. Since he was interested only in the small molecules he could isolate from the reaction, he ignored this mess at the bottom of the flask (as nineteenth—century organic chemists typically tended to do when goo fouled up their glassware). Th
irty-seven years later, the Belgian-born American chemist Leo Hendrik Baekeland, experimenting with formaldehyde, found that under certain conditions the reaction would yield a resin that on continued heating under pressure became first a soft solid, then a hard, insoluble substance. This resin could be molded while soft and then be allowed to set into a hard, permanent shape. Or, once hard, it could be powdered, poured into a mold and set into one piece by heat and pressure. Very complex forms could be cast easily and quickly. Furthermore, the product was inert and impervious to most environmental vicissitudes.
Baekeland named his product Bakelite, after his own name. Bakelite belongs to the class of thermosetting plastics, which, once they set on cooling, cannot be softened again by heating (though, of course, they can be destroyed by intense heat). Materials such as the cellulose derivatives, which can be softened again and again, are called thermoplastics. Bakelite has numerous uses—as insulator, adhesive, laminating agent, and so on. Although the oldest of the thermosetting plastics, it is still the most used.
Bakelite was the first production, in the laboratory, of a useful high polymer from small molecules. For the first time the chemist had taken over this particular task completely. It does not, of course, represent synthesis in the sense of that of heme or quinine, where chemists must place every last atom into just the proper position, almost one at a time. Instead, the production of high polymers requires merely that the small units of which they are composed be mixed under the proper conditions. A reaction is then set up in which the units form a chain automatically, without the specific point-to-point intervention of the chemist. The chemist can, however, alter the nature of the chain indirectly by varying the starting materials or the proportions among them, or by the addition of small quantities of acids, alkalies, or various substances that act as catalysts and tend to guide the precise nature of the reaction.
