Molecular structure and rubber-like elasticity - III. Molecular movements in rubber-like polymers

The evidence of X -ray diffraction photographs shows that in gutta-perch a and in polychloroprene at the ‘melting ’-point, major changes of molecular configuration due to rotation round the single bonds occur spontaneously. Melting is ascribed to the onset of chain-bond oscillation between alternative, geometrically equivalent positions (‘molecular wriggling’), and the low melting-points of these substances (and also, by analogy, of rubber) as compared with polyethylene are ascribed to the greater freedom of rotation round the single bonds in the former substances (and also, by analogy, of rubber) as com pared with polyethylene are ascribed to the greater freedom of rotation round the single bonds in the former substances. It is postulated that the energy required for rotation round the single bonds is made up of two factors— the bond-orientation energy, and the interaction of the atoms or groups held by the bonds. The comparative freedom of rotation in rubber and polychloroprene is due to two causes: first, the presence of double bonds in the chains, which has the effect of making rotation round adjacent single bonds m ore free than in comparable saturated molecules; an d secondly, the small hindrance to rotation offered by the side substituents. In gutta-perch a the bond-orientation energy is presumably the same as in rubber an d polychloroprene, but geometrical hindrance is greater, an d it is suggested that this is the reason for its rather higher melting-point. The structure of amorphous rubber is considered. Starting from the known structures of crystalline polymers, which consist of crystals tied together by molecules each of which passes through a number of crystals, the minimum change on melting is assumed, and that change— the wriggling an d consequent disarrangement of the molecules— is well founded on experimental evidence; it is not necessary to assume that the molecules have a completely random configuration, an d there are reasons why this is unlikely. The mechanical properties above and below the melting-point are correlated on this basis. The statistical effects to which Mark has drawn attention operate through the ‘tying ’ portions of the molecules.