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Introduction

1.1 Milestones in the Development of Polymer Science

Regarding its subject of study, polymer science belongs to the oldest fields of science (existing since the times when living cells appeared), while as a separate, well-defined science it was formulated less than 100 years ago - between 1920 and 1930 - thanks to the pioneering works of the German chemist Hermann Staudinger (Figure 1.1). Polymer materials have always been used by human beings without having any idea what differs these materials from the others. The situation did not change even when the first synthetic polymer material was prepared around 1910 by Leo Baekeland -Bakelite.1 First, Baekeland produced soluble phenol-formaldehyde shellac called "Novolak", but later he succeeded in preparing a hard plastic material.

Figure 1.1 H. Staudinger (1881-1965), the "father of polymer science" who received the Nobel Prize in 1953 for his studies.

Bakelite continues to be used for wire insulation, brake pads and related automotive components, and industrial electric-related applications. Bakelite stock is still manufactured and produced in sheet, rod and tube form for industrial applications in the electronics, power generation, and aerospace industries, and under a variety of commercial brand names.

Baekeland was a very talented chemist with an extremely strong feeling for commercially important products. But as a matter of fact, he did not contribute to the creation of polymer science in any theoretical aspect such as, for example, trying to answer such questions as how does the molecular structure of polymeric substances differ from that of other substances.

It must be stressed that his target was completely different - to create a new material with a well-defined combination of properties needed for a particular application. And he completed his task in an excellent way! Today we know that even if he tried to perform theoretical research, he could hardly have the same success he achieved in material synthesis. This is because his polymer, Bakelite, belongs to a special group of polymers characterized by a rather dense molecular cross-linking, that is, the single molecular chains are connected via covalent bonds, forming in this way a giant three-dimensional (3-D) molecular network, where separate linear molecules with a strictly defined start and end no longer exist. What is more, the cross-linked polymers do not display the typical and unique properties of polymers as the non-cross-linked ones do. So, the answer to the question regarding why the molecular structure of polymeric substances differs from that of other substances was given by Hermann Staudinger, who was working around the same time.

Staudinger had been studying natural products as cellulosic derivatives, natural rubber, and others, and particularly their solutions. Thus he got the impression that in these solutions one deals with larger "particles" than in solutions of low molecular weight substances. By the way, the same observation had been made much earlier, around the mid-nineteenth century when the colloid chemistry was formulated and such solutions were called "lyophilic colloids." Staudinger went further, suggesting that these "particles" are giant molecules comprised of a large number of atoms bonded via covalent bonds; and this met a negative reaction from the scientific world. The leading chemists and physicists of that time were against him, stating that it was not possible that such a large number of atoms could be bonded in one molecule.2

Staudinger did not abandon his idea, and step by step, he gathered arguments in favor of his hypothesis which, around 1930, was accepted world-wide. As a matter of fact, he formulated a new science, polymer science suggesting and proving that polymers are comprised of large molecules consisting of a huge number of atoms connected to each other by covalent bonds. In this way he demonstrated the basic difference in the molecular structure of polymers and low molecular weight substances.

The most serious support for Staudinger's concept was made in 1930 by the American chemist Wallace Carothers (Figure 1.2). Carothers was a group leader at the DuPont Experimental Station laboratory, near Wilmington, Delaware, where most polymer research was done. Carothers was an organic chemist who, in addition to first developing Nylon, also helped lay the groundwork for Neoprene. After receiving his PhD, he taught at several universities before he was hired by DuPont to work on fundamental research. In his research, Carothers showed even at this time the high degree of originality which marked his later work. He was never content to follow the beaten path or to accept the usual interpretations of organic reactions. His first thinking about polymerization and the structure of substances of high molecular weight began while he was at Harvard.

Figure 1.2 W. Carothers (1896-1937), the inventor of nylons and neopren.

As a matter of fact, the systematic studies at Du Pont on the condensation process with the aim to prepare a synthetic macromolecules allowed Carothers to start on polyesters using diacids and diols. The obtained products were not of commercial interest because of their low melting temperatures and problems with their low hydrolytic stability. The commercial success came with polyamides - Nylon 6 and Nylon 66 in the mid- to late 1930s.

The commercially important polyester, the poly(ethylene terephthalate), was developed in the United Kingdom around 1940 in a programme initiated by J. R. Whinfield. Its commercialization was handled by Du Pont and ICI (UK) since they had a current agreement for joint work on this topic.

Polyesters and polyamides, being the subject of Carothers' studies, are examples of condensation polymers formed by step-growth polymerization. Carothers worked out the theory of step-growth polymerization and derived the Carothers equation which relates the average degree of polymerization to the fractional conversion (or yield) of monomer into polymer. This equation shows that, for a high molecular weight, a very high fractional conversion is needed (for step-growth polymer only).

Carothers had been troubled by periods of depression since his youth. Despite his success with Nylon, he felt that he had not accomplished much and had run out of ideas. His unhappiness was compounded by the death of his sister and on the evening of 28 April, 1937 he checked into a Philadelphia hotel room and committed suicide by drinking a cocktail of lemon juice laced with potassium cyanide.

In this way, after the successful synthesis of linear polyesters and polyamides, the further establishment and development of polymer science involved many talented chemists from Europe and the United States. One of the first achievements was the demonstration that the macromolecules are not rigid, non-flexible, rod-like formation, as wrongly suggested by H. Staudinger3 but are extremely flexible formations.

The application of physics and physical chemistry to macromolecular systems dates back to the early attempts made in the late nineteenth century to understand the unusual properties of natural polymers, such as rubber, polysaccharides, and proteins. However, it was not until the 1920-1930 period that Meyer and Mark in Germany began to establish the structure of cellulose and rubber with the use of X-ray diffraction techniques. Explanations of rubbery elasticity in terms of polymer conformations were put forward by Kuhn, Guth, and Mark between 1930 and 1934. Kuhn, in particular, was the first to apply statistical methods to the study of macromolecules.

The application of light scattering to macromolecular systems was made by Debye during World War II. It was also during this period that Flory began a series of investigations into the applications of statistical methods, conformational analysis, and other fundamental physicochemical techniques to polymer science. During the early 1950s, Watson, Crick, Wilkins, and particularly Franklin successfully applied X-ray diffraction analysis to the structure determination of biological polymers, such as deoxyribonucleic acid (DNA), hemoglobin, and insulin.

Single crystals of polyethylene were first reported by Keller, Fischer, and Till4 independently from each other in the same year, 1957. This discovery was possible thanks to the development of electron microscopy. The more recent introduction of solid-state nuclear magnetic resonance (NMR) methods in the 1980s has had a major impact on polymer structural analysis.

Regarding the synthesis of polymers it has to be noted again that a number of important new polymers were prepared and commercialized in the period between 1890 and 1930, many of which were based on the use of chemical reactions (acetylation, nitration) carried out on cellulose. It is perhaps astonishing to realize that most of these technological developments occurred during a time when the polymeric nature of these products was not recognized or believed.

The 15 years between 1930 and 1945 represent the springboard for the development of modern synthetic polymer chemistry, as can be concluded from Table 1.1, which summarizes the development of polymer synthesis.

Table 1.1...