Some Differences in the Behavior of Effective Viscosity Under Extension

6 Some Differences in the Behavior of Effective Viscosity under Extension 

The major difference between dependencies of ct/x upon t, in case of polyethylene, and the similar curves in case of polyisobutylene (see Fig. 4) was that even at very low values of tensile strain velocities x in case of polyethylene, the flow failed to remain linear during the entire time interval of the experiment. Dependencies of effective viscosity a(t)/x at low values of x reached a constant value (stationary flow) exceeding 3r during times t, significantly exceeding the similar time under shear. With further increase in strain velocity x, the value t started to decrease in the same way as it was in experiments with polyisobutylene (PIB). 

The effective viscosity under stationary flow, with increasing x, was growing significantly at first and then reduced. Reduction in the viscosity of PIB 23,26) also was not observed, apparently, because of limitations of instrumentation. 

Some regularities, similar to Refs. 1718 21 , of viscosity variation in time under conditions of x = const were observed by the authors of Refs. 23,24,32-341 in extension of polyethylene and butadiene rubber (BR). Note that in Ref.34) the linear region of strain reaching the stationary flow was attained in extension of butadiene rubber. With further step-wise increase of x the effective viscosity grew and the time during which the stationary flow was attained increased significantly. References 23,24) will be discussed in more detail below. 

on the basis of facts discussed in this section, we may conclude that effective viscosity ct/x, in extension of some polymers, passes the maximum with increasing strain velocity x. The first time t measured reliably in the nonlinear strain region exceeds significantly the respective time 0 in the linear area. Dependency x decreases with further increase in t(x). 

---