Viscous slippage

One example of this occurs with stress relaxation. If a polymer is deformed to a fixed strain at constant temperature the force required to maintain that strain will decay with time owing to viscous slippage of the molecules. One measure of this rate of decay or stress relaxation is the relaxation time 0, i.e. the time taken for the material to relax to 1/e of its stress on initial application of strain. 

A strain with increment of 2% is then applied axially to the tendon and the motion of the crosshead is stopped at that point. The initial (total) stress is then recorded as well as the stress after a period of time when it (the stress) no longer decreases with increasing time. This final stress value is the time-independent stress termed the elastic stress. The stress lost to viscous slippage (viscous stress) is the difference between the initial stress and the final stress. The elastic stress is just the stress at equilibrium and is plotted versus strain to get an elastic stress-strain curve. The elastic stress, as pointed out in Chapter 6, is the stress stored at the molecular level as a change in conformation of a helical or extended macromolecule. In theory, the slope of the elastic stress-strain curve is proportional to the molecular stiffness of the molecule being stretched. 

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

A deformation due to slippage of polymer molecules past one another (viscous deformation Dvisc)- H often assumed that such viscous deformation rates do not change with time if the applied stress is constant. However, in long-term deformations chemical and morphological changes may occur which affect the rate of chain slippage. 

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