The Molecular Structure and Properties of Rubber

The molecular structure of rubber resembles that of liquids, which have a characteristic distribution of linear-chain molecules, united by transverse van der Waals forces. The x-ray diffraction diagram of undeformed rubber shows a diffuse ring, the characteristic diffraction spectrum of liquids, and this is accepted as experimental proof of the similarity between rubber and liquids. The theory that the structure of rubber involves reactive bonds in the adjacent molecular chains at short distances apart is not in accord with the relatively free rotation of these chains, which is manifest by the elastic properties of rubber, and the theory in no way distinguishes unvulcanized rubber from straight-chain paraffins. A straight-chain molecular system in which most of the links of adjacent chains are cross-bridged by van der Waals forces is a fairly rigid system, with relatively little molecular motion. The rotation of the links is equivalent to rupture of the van der Waals bonds, and in no way corresponds to the observed high degree of motion of the molecular chains in rubber, as reflected in the high elasticity of the latter. The results of the experimental study which is described in the present paper and which is concerned with the x-ray diffraction spectra of amorphous rubber and of rubber which has become crystalline by stretching, lead to the conclusion that amorphous rubber contains fragments of molecular chains, the links of which do not scatter x-rays in the form of an amorphous ring. Futhermore, the experiments offer proof of the absolutely essential role of the above-mentioned molecular structure of rubber in determining a number of properties of the rubber.