Nonlinear Modulus and Damping Function

Doi and Edwards (1978a, 1979) explained this strain softening using an extension of the tube model of de Gennes. Suppose that the melt is subjected to a step strain and 

Retraction moves a strand from one part of the tube to another hence the strand s orientation is determined not by the orientation of the part of the tube it originally occupied, but by the orientation of the part of the tube into which it moves. To simplify the problem, however, Doi and Edwards invoked the independent alignment approximation, which assumes that after retraction each strand is oriented independently of the others, and the change in orientation produced by retraction is neglected. 

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