In order to study the mechanical properties of high polymer on the basis of their molecular structure, it is necessary to know the elemental structure of polymeric substances. Recently it was found that the single crystal is a fundamental structure in addition to the original fringe micell model. Here the growing process and the structure of the single crystals are discussed briefly, on the basis of which the elastic properties, the viscoelastic absorption and the drawing processes are explained.
Screw dislocations with a spiral structure often grow on a single crystal. The Burgers vector of the screw dislocation in the crystal of metal is of the order of atomic distance, while that of polymer single crystal is equal to the thickness of the crystal itself, that is about 100A. This may be due to the much lower elastic modulus of single crystal of high polymer. In metal and other inorganic salts, the plastic properties as well as the growth mechanism are explained in term of the screw dislocation. On the other hand, in the case of polymer, the roles played by dislocation on their mechanical properties are not yet clear.
The fold conformation of the polymer chain induces a stress in the surface layer of the lamellae and this stress can be released by twisting the lamellae. The critical stress inducing the twisting and the period of the twist are represented in terms of the thickness, the width, the rigidity, the Young's modulus of the lamellae, the strain in the surface and the thickness of the strain layer.
The elasticity of polymer along the main chain in crystalline part is calculated to be about 2.5×109g/cm2 from the measurement of the change in X-ray diagram of a polymer sample under stress. This value is several times as large as those of extended fibers and about one hundred times as large those of unoriented crystalline polymers. The elasticity of the direction normal to the molecular chain is determined to be about 2×107g/cm2. This is very close to those obtained by the direct measurement of unoriented crystalline polymers, and suggests that the elasticity of unoriented crystalline polymers is the value correspond to the van der Waals force in the direction normal to the main chain folded in the lamellar structure in solid polymers.
The best way of illustrating the relation between the fringe micell model and the new concept of the folded conformation model may be to observe the process by which the fold molecules in a single crystal are reoriented into a drawn fiber. Some electron micrographs obtained seem to illustrate such processes. This may be called the“necking”of a single crystal. The fold plane (110) cleaved sharply when the crystal was torn along it, while molecules are drawn into a fiber when the crystal was torn along a plane crossed with a large angle to (110) plane.
