Historians like to define periods of human history based on our ancestor’s relationship with materials. This naming convention is convenient because materials are often the only permanent records left for anthropologists to study a particular civilization. It can also be viewed in a more esoteric fashion as a measure of our ability to modify our environment for our advantage. Animals have always been limited to seeking out venues which best meet their needs. Humans are the first beasts able to substantially modify their proximate surroundings to promote their own survival, as well as comfort and aesthetics. This has only been possible due to our relationships with materials.
The Stone Age dates back to the first divergence of the direct ancestors of modern humans from other primates, at that critical stretch of grassland in Ethiopia which first led us from the jungle (or so the current anthropological evidence suggests). During this period, about 2.5 – 2.9 million years ago, this scraggly band of pre-humans began to stretch its legs and tour the savannah, evolving into homo erectus, which is known as the tool-equipped savanna dweller.
If define this transitory period as the effective beginning of human history, then the following Stone Age makes up about 99.75% of our history.
The First Materials Scientists
The first metal that caught our attention was gold. It has been used since before the beginning of recorded history as coinage and jewelry, but is far too soft and ductile to be of practical use apart from currency. We next stumbled upon meteoric iron, which comes from meteorites containing nickel-iron alloys and provided the first source of usable iron to humans.
The first practical use of copper wasn’t until about 10,000 years ago. To put this into perspective, if all of human history (beginning with our first step into the savannah) was condensed to a single year, that’s only about 1 day and 6 hours ago. Copper gave us experience with metallurgy, and we developed cold working of native copper, annealing, and finally smelting. The latest discovery, smelting, is a major milestone in our history. Among the seven metals which we have had some relationship with since antiquity, gold is the only one which occurs regularly in native form in the natural environment. Copper, lead, silver, tin, iron, and mercury occur predominantly as minerals and must be smelted to be used. We discovered smelting around 8,000 years ago (24 hours ago), by mixing copper and arsenic to form bronze, which had much higher hardness than copper and greater strength. We subsequently found that replacing arsenic with tin provided stronger and easier to cast materials without the toxicity of arsenic, and began the period of ancient history known as the Bronze Age.
The Bronze Age lasted for about 2,000 years, and enabled massive trade networks to form because it was quite rare for individual sites to contain copper and tin ores. The Bronze Age did not end because of the superiority of iron, but because iron was easier to find and to process. A serious disruption in the tin trade is thought to have precipitated the transition to the Iron Age, which occurred about 3,200 years ago in the Ancient Near East, India, and Europe, 2,600 years ago in China, 2,400 years ago in Nigeria and Korea, and 2,100 years ago in Japan. The true value of iron was not realized until the discovery and perfection of steel, which is stronger than bronze and holds a sharper edge longer.
The Polymer Age
Although modern engineering disciplines have brought about a much higher degree of understanding and applications of the classical materials (ceramics and metals), for all intensive purposes we now live in a polymer age. Since World War II, plastics, fibers, elastomers, rubbers, proteins, and cellulose have permeated our world in every way and fulfill a variety of applications from the routine (milk bottles, packaging, toys, non-stick cookware, adhesives, diapers), to the extraordinary (aromatic polyamide fibers in lightweight bulletproof vests, fluorinated phosphazene elastomers for flexible applications in arctic environments, carbon fiber reinforced epoxies in aircraft structures, artificial hip joints, resorbable sutures). Polymers may be the main ingredient, or even additives which play a crucial role in properties of the final material, such as in asphalt (suppress brittle fracture at low temperature and flow at high temperature), shampoos (impart body), automobile windshields (prevent shattering), and motor oil (prevent temperature-dependence of viscosity and inhibit crystallization). In order to be good engineers and scientists, we must understand the physical reasons why polymers have become such a ubiquitous part of our daily life, and with this understand continue to progress knowledge to fulfill ongoing needs and demands in various fields.
The first “synthetic” polymers used were really just modified natural polymers, such as nitrated cellulose, although we didn’t know it at the time. The polymerization of styrene was first reported in 1839, and poly(ethylene glycol) and poly(ethylene succinate) were synthesized in 1860. Despite these advances, chemists still had no idea about the structure involved of these materials.
The first synthetic polymer used on a commercial scale was a phenol-formaldehyde resin known as Bakelite, developed in the early 1900s by chemistry Leo Baekeland. Other polymers were introduced around the same time, such as polyester paints and polybutadiene rubber, but the prevailing theory remained that polymers were essentially aggregated small molecules held together by a mysterious secondary force.
German chemistry Hermann Staudinger was the first to attribute the remarkable properties of polymers to ordinary intermolecular forces between molecule of very large molecular weight, with his “Macromolecular Hypothesis.” He coined the term macromolecule (Staudinger, H. (1933). "Viscosity investigations for the examination of the constitution of natural products of high molecular weight and of rubber and cellulose". Trans. Faraday Soc. 29 (140): 18–32), and also made several other significant findings in organic chemistry. For example, Staudinger discovered ketenes, which are a family of molecular intermediates for production of (yet undiscovered) antibiotics such as penicillin and amoxicillin.
In the 1930s, American chemist Wallace Hume Carothers experimentally verified the future Nobel Laureate Staudinger’s theory, and subsequently developed neoprene rubber and polyamide (nylon) fibers. He also provided the theory of step-growth polymerization and proposed what we now know and love as the Carothers’ equation, which relates the average degree of polymerization, N, to the amount of monomer consumed, x, with the simple relation: N = 1/1-x. Unfortunately, he was a bit depressed and drank some potassium cyanide shortly before World War II, which provided a substantial acceleration in resources devoted to development of his findings. The main drivers were the development of synthetic rubbers to hedge against the rubber-growing regions of the far East becoming inaccessible, and nylon for use in parachutes in the event that Asian silk farms were taken by the Axis Powers.
Around this time, Karl Ziegler of Germany discovered new coordination catalysts for initiation polymerization reactions, and Giulio Natta of Italy subsequently applied these theories to develop polymers with controlled stereochemistry. Their findings revolutionized the polymer industry because the resulting stereoregular polymers exhibit remarkably higher mechanical properties than most non-stereoregular varieties.
To summarize the work of Paul Flory briefly would be to say that he contributed to our understanding of most all physical behavior of polymers. Flory was a remarkably prolific researcher in polymer science and actually had his first job at DuPont with Wallace Carothers. His work established a quantitative basis for polymer behavior from physical properties of macromolecules in solution or in bulk. He also made major strides in understanding chemical phenomena such as crosslinking and chain transfer. He introduced the term, “Excluded volume,” in which two macromolecules cannot occupy the same space,” which also led to the concept of the “theta point,” where the effects of excluded volume are effectively neutralized under a specific set of condition. Numerous other physical chemists such as Kuhn, Guth, and Mark applied statistics and crystallography to describe conformations of polymers and macromolecules. No description of polymer history can be complete without some mention to the pioneering works of de Gennes, who theorized that polymer chains would reptate, or move with a snake-like motion through a landscape with boundaries. Doi and Edwards adapted this formulation to make predictions on bulk polymer motion with their concepts of constraint release and primitive tube, and extended the theory to describe rigid rod behavior, such as in liquid crystals.
Recent important advances in polymer science:
Polymers with excellent thermal and oxidative stability, for use in high-performance aerospace applications
Engineering plastics – Polymers designed to replace metals
High-strength aromatic fibers, some based on liquid crystal technology, for use in variety of applications from tire cord to cables for anchoring oceanic oil-drilling platforms
Non-flammable polymers, including some which emit a minimum of smoke or toxic fumes
Non-flammable polymers, which not only help reduce the volume of unsightly plastics waste but also allow controlled release of drugs or agricultural chemicals
Polymers for broad spectrum of medical applications from degradable sutures to artificial organs
Conducting polymers – Polymers that exhibit electrical conductivities comparable to those of metals
Polymers that serve as insoluble supports for catalysts or for automated protein or nucleic acid synthesis.