Abstract
Ferry and Stratton proposed a relation between a shift factor and a strain. They assumed compressibility of free volume on the basis of a sample volume. On the other hand, we defined the compressibility on the basis of the free volume, and derived a new relation between the shift factor and the strain (or stress). The equation we obtained was different in the formula from the Ferry-Stratton equation, but the properties of the equations were quite similar. Our equation could satisfactorily approximate a relation between the shift factor and the strain for relaxation curves of a polyethylene film, and also the relation between the shift factor and the stress for creep curves of monofilaments of nylon and poly (ethyleneterephthalate). Fractions of free-volume were estimated from these equations.
