In Part I of this work, the crystal structures of β-gutta-percha, rubber and polychloroprene (deduced from x-ray diffraction photographs) were described. In pursuance of the idea that rubberlike properties are due to molecular flexibility which result from the swivelling of the chain units around single chain bonds, it is necessary to consider which chain-bond positions are the most stable and what hindrances there are to rotation away from these positions. The question of the most stable bond positions was considered in Part II. I now consider the evidence for the occurrence of rotation in rubberlike substances, the hindrances to rotation in different molecules, their effect on the crystallization or melting temperature, and the explanation of the mechanical properties of these substances in terms of structure and molecular movement, both above and below the crystallization temperature. Rubber is noncrystalline and elastic at room temperature; but on cooling below 0° C it crystallizes, and in that condition has the mechanical properties of gutta-percha; i.e., a frozen specimen no longer has enormous elasticity, but it can be drawn or rolled out irreversibly, whereby the crystals become oriented. Conversely, if gutta-percha is wanned to 70° C, it becomes amorphous (transparent, optically isotropic and noncrystalline), and its mechanical properties are then like those of rubber—it is soft and elastic. The difference between the two substances thus appears to be essentially a difference of crystallization temperature. These crystallization temperatures are remarkably low; e.g., polyethylene, (—CH2—CH2—)n, of comparable molecular length, crystallizes at 115–125° C. In attempting to understand rubberlike elasticity in terms of molecular behavior, the first question is why are the crystallization points of such enormous molecules so low, and why is the crystallization point of the cis-form of polyisoprene (rubber) lower than that of the trans-form (gutta-percha).

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