Steady-State Shear and Extension

The uniaxial extensional viscosity rj(s) and the viscometric functions rj(y) and ki(y), predicted by the Doi-Edwards model for monodisperse melts, are shown in Fig. 3-32. The Doi-Edwards model predicts extreme thinning in these functions the high-shear-rate asymptotes scale as 17 oc oc y , and4 i oc The second normal 

Experimentally, melts of low polydispersity that do not overtly spurt, slip, or succumb to other material instabilities will typically show steeply decreasing values of the viscosity and first normal stress coefficient in the shear-thinning region. Menezes and Graessley (1980) reported that t] oc y ° y i- at large y. These dependencies 

The Doi— Edwards equation predicts that the ratio 2 is —2/7 = —0.29 at low shear rates. This changes to 4 2/ l i = —1/7 = —0.14 when the independent alignment approximation is dropped (Osaki et al. 1981). With or without the independent alignment approximation, the ratio —is predicted to decrease towards zero as the shear rate increases. The prediction of fof entangled solutions contrasts with that predicted 

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