The remarkable properties of pure bulk polymers

Macromolecules can exist in pure condensed states. One of the most remarkable natural substances is Hevea rubber, obtained from the rubber tree (Hevea brasiliensis). Samples of this material were brought to Europe soon after Columbus made contact with South America. Faraday purified it sufficiently to obtain an elemental analysis that showed it was a pure hydrocarbon. 

When rubbery materials are cooled, two typical outcomes are obtained (1) the material partially crystallizes to become a flexible semicrystalline substance or (2) the material does not crystallize and, at sufficiently low temperature, becomes an amorphous glassy solid. One of the most confusing aspects of the behavior of macromolecules during the 19th century was the observation of crystallization. It was believed that polymers could not crys- 

A long controversy arose over the extent to which any particular chain was confined to a single microcrystal. It is now understood that one chain can be part of many microcrystals and that the tensile modulus of a semicrystalline polymer sample is related to the modulus of the tie chains that connect individual microcrystals. The number of different space groups observed in crystalline polymers and the overwhelming niunber of different morphologies that are found are a reminder that polymer science is an unending frontier of research. 

The existence of pure amorphous bulk polymers has been a controversial issue since the beginning of polymer science. Natural rubber yields an X-ray pattern tiiat contains only amorphous halos, typical of any liquid. Nevertheless, it was difficult for many scientists to believe that molecules with a polymeric chain structure could pack in a truly amorphous way. There are still papers that are submitted for publication that assert that amorphous rubbery polymers are actually composed primarily of microcrystalline domains. This issue has been clarified by the incisive theoretical and experimental work of Flory. It is now understood that there are polymers that exhibit liquid crystalline phases upon melting of the crystals. The nature of the noncrystalline state of pure bulk polymers depends on tiie detailed local structure of the chain and the ratio of die persistence lengtii of die chain to the diameter of the mer. Molecules that are conformationally dexible enough to have a small persistence length can exist in the amorphous liquid state. 

There are many macromolecules that are amorphous liquids at high temperature and that do not have a crystalline phase. These systems are actually copolymers of various types. When these liquids are cooled, their viscosity increases enormously. Eventually they exhibit shear moduli in the 

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