The Macromolecular Hypothesis

The cross-linking of rubber with sulfur is called vulcanization. Cross-linking bonds the chains together to form a network. The resulting product is called a thermoset, because it does not flow on heating. 

Plasticizers are small molecules added to soften a polymer by lowering its glass transition temperature or reducing its crystallinity or melting temperature. The most widely plasticized polymer is poly (vinyl chloride). The distinctive odor of new vinyl shower curtains is caused by the plasticizer, for example. 

Fillers may be of two types, reinforcing and nonreinforcing. Common reinforcing fillers are the silicas and carbon blacks. The latter are most widely used in automotive tires to improve wear characteristics such as abrasion resistance. Nonreinforcing fillers, such as calcium carbonate, may provide color or opacity or may merely lower the price of the final product. 

All those sticky, mucilaginous, resinous, tarry masses which refuse to crystallize, and which are the abomination of the normal organic chemist those substances which he carefully sets toward the back of his cupboard. .., just these are the substances which are the delight of the colloid chemist. 

Indeed, those old organic colloids (now polymers) and inorganic colloids such as soap micelles and silver or sulfur sols have much in common (11)  

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Polymer solutions represent the most convinient systems for studying the properties of the macromolecules. In effect, almost the all information that we have now about the properties of macromolecules comes from the characterization realized in solution. This is the state in which linear chains are characterized. Osmotic pressure measurements in polymer solutions revealed for the first time the existence of high molecular masses and this result confirmed the macromolecular hypothesis. The development of our knowledge of the polymer solutions reflects to some extention the development of the Polymer Chemistry itself. 

In 1920, Staudinger proposed the macromolecular hypothesis polymers are molecules made of covalently bonded elementary units, called monomers. In this view, the colloidal properties of polymers were attributed entirely to the sizes of these large molecules, called macromolecules or... 

In fact, the decisive experimental proof of the macromolecular hypothesis has been given by osmotic pressure measurements in very dilute solutions. A second fundamental contribution is more recent. It deals with the study of systems of strongly overlapping chains. The observations made by Noda and his collaborators (1980) definitely showed that, in this case, the osmotic pressure dependence of the solute concentration follows a universal law the crossover with the dilute state is also universal. These results gave an experimental proof of the principles which constitute the basis of the modern theory introduced by de Gennes and des Cloizeaux. 

Each expansion of maximal degree q is valid in an interval 0 < p < pmi in which one tries to represent the osmotic pressure, and the larger q is, the larger is pmat.. Actually, for a given q, one must to take care to choose for pmin a value which is not too large otherwise, systematic errors occur and they lead to an underestimate of M when q = 2, and an overestimate of Mn when q = 3. According to Flory, this kind of error has prevented the macromolecular hypothesis from being accepted sooner. 

By about 1930 Staudinger and others had accumulated much evidence in favour of the macromolecular hypothesis. The final part in establishing the concept was carried out by Wallace Carothers of the Du Pont company in the USA, He began his work in 1929 and stated at the outset that the aim was to prepare polymers of definite structure through the use of established organic reactions. Though his personal life was tragic, Carothers was an excellent chemist who succeeded brilliantly in his aim. By the end of his work he had not only demonstrated the relationship between structure and properties for a number of polymers,... 

Since Herman Staudinger proposed the macromolecular hypothesis in 1926 [29], the 20th century has wimessed significant development of macromolecular chanistry. Three major macromolecular architectures have evolved since then, namely linear (class I), crosshnked (class II), and branched types (class HI), as shown in Figure 11.2. These three classes of traditional synthetic polymers are produced by largely statistical polymerization processes. 

In 1920, Hermann Staudinger formulated the macromolecular hypothesis That there was a special class of organic substances of high viscosity which were composed of long chains (1,2). This revolutionary idea was argued throughout important areas of chemistry (3). Finally, it became accepted and formed the most important cornerstone in the development of modern pol3uner science. 

The existence of dilute solutions of macromolecules was denied by many experts until the macromolecular hypothesis was largely accepted in the time period from 1930 to 1940. The dilute-solution state is still the basis for characterizing individual macromolecules and the interactions of pairs of macromolecules and the solvent. The structural, thermodynamic, and hydro-dynamic properties of polymer solutions are explained in terms of the random-coil model developed by Kuhn, Debye, Flory, Kirkwood, Yamakawa, and deGennes. While this subject alone could easily be the basis for a one-semester course, the topics are developed so that the material could be presented as part of a complete development of the subject. 

Aylsworth s patent (97), however, does not mention IPNs and polymer networks, or even use the term polymer. Note that that date was six years prior to H. Staudinger s emmciation of the macromolecular hypothesis, which was, of course, the first statement that certain colloids had the structure of long chains. 

In 1920 Herman Staudinger (13,14) enunciated the Macromolecular Hypothesis. It states that certain kinds of these colloids actually consist of very long-chained molecules. These came to be called polymers because many (but not all) were composed of the same repeating unit, or mer. In 1953 Staudinger won the Nobel prize in chemistry for his discoveries in the chemistry of macro-molecular substances (15). The Macromolecular Hypothesis is the origin of modern polymer science, leading to our current understanding of how and why such materials as plastics and rubber have the properties they do. 

The first truly synthetic polymer was a densely cross-linked material based on the reaction of phenol and formaldehyde see Section 14.2. The product, called Bakelite, was manufactured from 1910 onward for applications ranging from electrical appliances to phonograph records (16,17). Another early material was the General Electric Company s Glyptal, based on the condensation reaction of glycerol and phthalic anhydride (18), which followed shortly after Bakelite. However, very little was known about the actual chemical structure of these polymers until after Staudinger enunciated the Macromolecular Hypothesis in 1920. 

Ever since Hermann Staudinger developed the macromolecular hypothesis in the 1920s (41), polymer scientists have wondered about the spatial arrangement of polymer chains, both in dilute solution and in the bulk. The earliest models included both rods and bedspring-like coils. X-ray and mechanical studies led to the development of the random coil model. In this model the polymer chains are permitted to wander about in a space-fiUing way as long as they do not pass through themselves or another chain (excluded-volume theory). 

According to H. Mark (A5), the story of the development of the random coil began with the X-ray work of Katz on natural rubber in 1925 (A6-A9). Kalz studied the X-ray patterns of rubber both in the relaxed state and the extended or stretched state. In the stretched state, Katz found a characteristic fiber diagram, with many strong and clear diffraction spots, indicating a crystalline material. This contrasted with the diffuse halo found in the relaxed state, indicating that the chains were amorphous under that condition. The fiber periodicity of the elementary cell was found to be about 9 A, which could only accoimnodate a few isoprene units. Since the question of how a long chain could fit into a small elementary cell is fundamental to the macromolecular hypothesis, Hauser and Mark repeated the Katz effect experiment and, on... 

From the time of Hermann Staudinger s enunciation of the macromolecular hypothesis in 1920, polymer science and engineering has had many fundamental advances leading to the understanding of plastics, rubber, adhesives, coatings, and fibers of today. The discoveries that these Nobel Prize winners made are summarized in Table C5.3.1 (C1,C2). These people have revolutionized life in the modern world. 

Historically the study of crystallinity of polymers was important in the proof of the Macromolecular Hypothesis, developed originally by Staudinger. In the early 1900s when X-ray studies were first applied to crystal structures, scientists found that the cell size of crystalline polymers was of normal size (about several Angstoms on a side). This was long before they developed an understanding of the chain nature of polymers required to completely characterize the cell contents. In the case of ordinary sized organic molecules, each cell was... 

The history of polymer science (169,170), takes the reader back to before the macromolecular hypothesis. Natural rubber, civ-polyisoprene, was investigated around the turn of the twentieth century. Morawetz (169) writes of the destructive distillation of rubber to isoprene, and of the ozonolysis of rubber to levulinic aldehyde. These studies led to the dimer ring structure of dimethyl-cyclooctadiene for ds-polyisoprene ... 

Once the macromolecular hypothesis of Staudinger was accepted, a basic understanding of the molecular structure was possible. Before cross-linking, rubber (natural rubber in those days) consists of linear chains of high molecular weight. With no molecular bonds between the chains, the polymer may... 

X-ray confirmation of the Macromolecular Hypothesis Synthesis of nylon (polyamide) Mark-Houwink-Sakurada equation... 

If you had listened to Staudinger expound on the Macromolecular Hypothesis in the year 1920, what would you have suggested for a research project to prove or disprove his ideas (Assume you have all the modern instruments available at your disposal, however.)... 

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