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The experimental results of the Viscoelastic properties of linear polymers of narrow molecular weight distribution (MWD) and of their mixtures have been analyzed and generalized. Based on the study of the properties of polymers of narrow MWD, we propose a classification of high molecular weight compounds. It specifies a distinct boundary between oligomers and polymers, assuming that the most important feature of polymers is the manifestation of large high‐elastic recoverable deformations of entropy character. For polymers to be characterized, not the absolute molecular weight is essential, but the molecular weight referred to the boundary values. The corresponding state for polymers is attained at temperatures 100°C away from the glass temperature. The transition from the fluid to the high‐elastic state with increasing deformation rate (or frequency for cyclic deformation) has been studied. Transition to the high‐elastic state takes place over a narrow stress range (0.1‐1.0 dynes/cm2), independent of molecular weight, whereas the critical deformation rates (frequencies), like viscosity, depend greatly on molecular weight. An increase in the amount of deformation shifts, to u certain extent, this transition to lower Kites of deformation (frequencies). In the region of deformation rates (frequencies) corresponding to the high‐elastic state, the effect of large deformations during shear manifests itself largely in the tear‐off of polymers Iron, the confining surfaces and in specimen rupture. Polydispersity has a strong effect on the properties of polymeric systems. As the rate of deformation is increased, the transition proceeds successively from the higher molecular weight components. This relaxational transition is tantamount to a change of the structure for polymeric systems. It is responsible for non‐linear, particularly, non‐Newtonian behavior of such systems. The transition to the high‐elastic state and all the related phenomena are observed also in concentrated solutions of high molecular weight polymers. The long‐term durability of un‐cured rubbers in the high‐elastic state is described by the same relationships.

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