Creep Entanglements

Berge, j. W., P. R. Saunders, and J. D. Ferry Mechanical properties of poly-n-octyl methacrylate at low frequencies and in creep entanglements in methacrylate polymers. J. Colloid Sci. 14, 135 (1959). 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

Selective bond rupture at entanglement points, or other such sites of stress concentration, could magnify the effect of a chain scission in the presence of an external stress, but it seems unlikely that this is occurring since the sol-gel data actually indicated a (slightly) lower ratio of scissions to crosslinks with an imposed stress. It also is difficult to visualize how the formation of free radicals, scissions, and crosslinks could directly cause the radiation expansion noted under no stress. Therefore, the mechanism of accelerated creep is probably not caused by the formation and reaction of macromolecular free radicals in the polymer specimens. 

Laws are generally found to be nets of such a texture as the little creep through, the great break through, and the middle-sized are alone entangled in. ... 

The loss of entanglements (and the decrease in molecular weight due to chain scission) adversely impacts fibril stability. Fibril breakdown by localized creep should occur more rapidly in polymer crazes with low entanglement densities and small diameter fibrils. 

Certain dynamic mechanical measurements are thought to reflect the pseudo-network formed by the entanglements and yield a quantity Z f presumably equal to Z (7, 18). For example, the pseudo network creep compliance observed as a plateau in the creep compliance response is believed to be related to Z through the expression (7)... 

It is common to compare the behavior of polymers with the behavior of metals and to use similar types of experiments to evaluate their performance under mechanical deformation. It is, therefore, important to highlight any qualitatively significant differences between their behavior and the fundamental physical reasons for these differences. In metals, creep is neither linearly viscoelastic nor recoverable, since (unlike polymer chains) metals do not have entanglements. Furthermore, creep is significant only at very high temperatures in metals. 

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