Classification Scheme for Molecules

A better classification makes use of the sizes of molecules, dividing them into small molecules and macromolecules, as is indicated in Fig. 1.6 [3]. The importance of large molecules, or macromolecules, was shown by Staudinger about 80 years ago. Following his suggestion, any molecule with more than 10(X) atoms or a molar mass of more than 10,000 Da should be called a macromolecule. This operational definition is quite useful since there are not many molecules that are known with about 1000 atoms. Most are either much larger or much smaller. 

T0 complete this classification of Fig. 1.6, a further subdivision into flexible and rigid macromolecules is advantageous. Flexible macromolecules are of main interest 

The glass transition is a solidification or liquifaction without change in molecular order. The crystallization and melting involves a change in order which causes the glass transition. 

Chemistry is the science dealing with synthesis, reactions, and properties of all substances, but traditionally small molecules have become the main focus. After the discovery of X-ray diffraction, rigid macromolecules have become the primary substances of interest to material science and solid state physics. The new, flexible macromolecules fell between these fields and have only slowly been accepted into the various academic disciplines. Presently polymer science is most prominently taught in material science courses, less in chemistry, and least in physics. 

No large molecules can be evaporated thermally without decomposition. If one tries to place flexible macromolecules into the gas phase by evaporation of the solvent molecules from a dispersion of droplets of a solution with only one macro-molecule per droplet, the macromolecules become solid microphase particles and collect at the bottom of the container. Typical examples of single polymer glass phases and crystals are shown in Chap. 5. 

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A preliminary, operational definition of the solid state is given within the box of Fig. 1.6. It will be expanded upon and linked to the material properties throughout the book. For materials, the transitions between solid and liquid are basic and determine their utility. Similarly, the evaporation characteristics need to be known to choose a molecule for a given application. The new classification scheme for molecules of Fig. 1.6 is, thus, much more useful than the earlier, arbitrary distinction that rehed upon the abihty or inability of living organisms to synthesize a particular substance. The bottom brackets give a rather unique explanation of the glass transition which is detailed in Sect. 2.5. 

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