Cold‐drawing of glass‐like and crystalline polymers

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Abstract
Solid polymers—both glass‐like and crystalline—can be cold‐drawn. In addition to studies of the specific aspects of this process in individual cases, as governed by structural features, the development of a theory of cold‐drawing requires an investigation of the general laws of this process. According to the experimental conditions used, cold‐drawing may be carried out either isothermally (at low rates of drawing and with good heat dispersion) or non‐isothermally. A number of specific features of the non‐isothermal process may be explained by a temperature increase in the stressed region; this has in fact been done in published work on this subject. However, the basic features of this phenomenon persist, as has been shown experimentally, in the isothermal cold‐drawing of both glass‐like and crystalline polymers. These features include: (a) the possibility of cold‐drawing, with large residual deformations; (b) the character of the stress—deformation curves; (c) a displacement at the beginning, and a homogeneous or inhomogeneous stretching over the further course of the cold‐drawing (both possibilities being realized in either type of polymer—glass‐like or crystalline); (d) the effect of temperature and the rate of drawing on the initial stress σB. These phenomena must be interpreted in a way which does not involve the assumption of a temperature rise. The values of σB depends on the rate of drawing v, this dependence being expressed by the relation σB = A + B log v, valid for both glass‐like and crystalline polymers. The two types differ only in the value of the constant B, which is lower for crystalline polymers (polyethylene, polyamide, crystalline silicone rubber) than for glass‐like polymers (polyvinyl chloride, amorphous silicone rubber, polymethyl methacrylate, butadiene‐acrylonitrile copolymer, etc.). Crystalline and glass‐like polymers differ in their mechanisms of cold‐drawing. Crystalline polymers, as is well known, undergo recrystallization on deformation; in either type, however, there is a rupture of intermolecular bonds by stresses. Hence follows the kinetic, relaxational character of cold‐drawing. The dependence of σB on the temperature and rate of drawing may be explained on the assumption that the activation energy of the molecular rearrangement is decreased with increasing stresses. Uniform isothermal stretchig is, generally speaking, unstable. The fact that it may be realized in practice, and also the stabilization of the block in non‐uniform drawing, makes it necessary to assume that the mechanical relaxation properties of the polymer change on orientation.

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