Relaxation of Stress by Reptation

The segmental distribution along the primitive chain has reached the equilibrium state at t = Teq. In this state, by using Eq. (8.27), Eq. (8.32) 

By absorbing the isotropic part of the integrand into the pressure term, Eq. (8.39) can be expressed as 

Assume in a short time step At the primitive chain moves a curvilinear distance As. Then 

As As can be positive or negative and is random, its distribution is characterized by the moments 

At both ends of the chain, the distribution of u is random so that 

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