Systems Manifesting Superposed Elastic and Viscous Behavior

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Abstract
Actual substances exhibit a very complicated behavior under mechanical stresses which cannot be described by classical elasticity theory nor by the classical theory of the hydrodynamics of viscous fluids. A general molecular theory describing the behavior of matter under stress is discussed and related to previous investigations and to experimental observations. Particular attention is devoted to rubberlike substances for which the classical theories are definitely inadequate. Experimental results on relaxation and creep of rubbers are interpreted in terms of modern structural concepts. It is found that these substances exhibit three regions of stress-temperature-time dependence. At intermediate temperatures there is a region of relative stability in which the statistical-thermodynamic theory of rubber elasticity is valid. At elevated temperatures, relaxation and creep are caused by chemical changes involving the rupture and formation of primary valence bonds. These chemical changes, which are responsible for the aging of rubber, can be isolated and studied by appropriate experimental techniques. At low temperatures relaxation and creep are caused by the slipping of secondary interchain bonds which are breaking and reforming in times comparable to experimental times of measurement. Theories are advanced to explain the observed stress-temperature-time behavior of rubbers over the entire temperature range studied.

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