Molecular Order and Mechanical Properties of Polymer Plastics

Abstract

The main difference between polymer materials and low-molecular solids originates in the enormous difference of molecular size and shape in both cases. The linear structure of macromolecules introduces an unusually high degree of anisotropy in mechanical forces. In the amorphous plastics the long chainlike molecules are randomly coiled. Below the glass-transition temperature the chain mobility is so small that the polymer solid is quite rigid and behaves like glass. Above this temperature it exhibits rubberlike elasticity. The elastic forces are a consequence of entropy increase on stretching. Crystals act as permanent cross-links. The larger the ratio of crystals in a polymer plastic, the higher the mechanical strength, but also the greater the brittleness. Therefore a proper equilibrium between crystalline and amorphous areas is needed for obtaining useful properties of material under consideration. In addition, in many cases, particularly in fibers, a high alignment of crystals and of molecular chains in amorphous areas is desirable. This can be achieved by irreversible mechanical deformation (cold-drawing).