Extension of Molten Polymers and Molecular-Kinetic Theories

3 Extension of Molten Polymers and Molecular-Kinetic Theories 

Viscoelastic properties of molten polymers conditioning the major regularities of polymer extension are usually explained within the framework of the network concept according to which the interaction of polymer molecules is localized in individual, spaced rather far apart, engagement nodes. The early network theories were developed by Green and Tobolsky 49) and stemmed from successful network theories of rubber elasticity. These theories were elaborated more fully in works by Lodge50) and Yamamoto S1). The major elasticity. These theories is their simplicity. However, they have a serious drawback the absence of molecular weight in the theory. 

From this point of view the Doi-Edvards reptation theory can be regarded as the most perfect network theoryS2). In a molten polymer, macromolecules can not move notable in lateral direction since that is impeded by other polymer chains. This circumstance in the Doi-Edvards theory is taken into account by means of introduction of a 

In description of effects observed in extension of molten polymers, the determinant is the phenomenon of anisotropy of the mobility of macromolecules. In the Doi-Ed-vards reptation theory the anisotropy of the mobility of macromolecules is taken into account topologically by means of placing a macromolecule into a certain hypothetical tube. In this case large-scale movements are allowed only along the macromolecule and are totally inhibited in the lateral direction. This, indeed, is a limiting case of mobility anisotropy. 

It has been demonstrated 54) that the dependency of viscosity upon elongation velocity varies significantly in its character with alteration of the lateral (transverse) mobility of macromolecules. In case there is no lateral mobility of macromolecules, the longitudinal viscosity strictly decreases under tension (see Fig. 11) which corresponds to prognostications proceeding from the Doi-Edvards theory. In case there is a lateral mobility of macromolecules, an insignificant maximum or monotonous increase in longitudinal viscosity of the medium with increase in extension velocity is observed. 

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