Stress relaxation upon cessation flow

Stratton,R. A., Butcher,A.F. Stress relaxation upon cessation of steady flow and the overshoot effect of polymer solutions. J. Polymer Sci. Polymer Phys. Ed. 11,1747-1758 (1973). 

For isotropic polymer fluids, stress relaxation upon cessation of flow reflects the relaxation time scales during the previous flow. As the relaxation spectrum is determined by the microstructure such measurements can be used to probe the effect of shear on the structure. This turns out to be a rather insensitive technique in polymer fluids because of changes which already occur during the relaxation (161. 

Figure 11.17 presents the relaxation of normalized (a) shear stress and (b) the first normal stress difference upon cessation of steady shear flow at y = 0.5 s for PSHQ9 at 130 °C. Here, experimental data are shown by dots and fitting curve by dashed line. The normalizing values Oq and Ni are the steady values of shear stress... 

Figure 10 shows that upon cessation of shear flow of the melt, shear stress relaxation of LLDPE is much faster than HP LDPE because of the faster reentangle-... 

Figure 10 Relaxation of shear stress with time upon cessation of steady flow. (O) Resin A, ( ) Resin B, (O) Resin C, (A) Resin D, (A) Resin E. (Refer to Table 2 for symbol code.) Source Ref. 56.

Two liquid crystalline polybenzylglutamate solutions, adjusted to the same Newtonian viscosity, have been investigated Theologically. The steady state shear properties and the transient behaviour are measured. For the same kind of polymer, the dynamic moduli upon cessation of flow can either increase or decrease with time. This change in dynamic moduli shows a similar dependency on shear rate as the final portion of the stress relaxation but no absolute correlation exists between them. By comparing the transient stress during a stepwise increase in shear rate with that during flow reversal the flow—induced anisotropy of the material is studied. 

Figure 3. Comparison of the characteristic time scales for the stress relaxation and the evolution of the dynamic moduli (t(c)) upon cessation of flow for the PBLG sample ...

The relaxation behavior, or the transient behavior of cellulosic liquid crystalline solutions upon the cessation of steady flow, is unique for LCPs. There are two kinds of relaxation. The bulk stresses relax quickly while the structures relax over a much longer time. Mewis and Moldenaers suggested that two levels of structures exist.Stress relaxation reflects fast relaxation at the molecule level and is independent of the previous shear rate. Structure relaxation reflects the gradual change of the textures. This slow process is unique in... 

Figure 11.15 Relaxation of shear stress Oi2 for Titan at 340°C upon cessation of steady flow at shear rate y = 6 s experimental data are shown by dots and simulated curve by dashed line.

Figure 11.20 Relaxation of normalized (a) shear stress, a y,t)/a, and (b) first normal stress difference, Ny y,t)/Nijn, upon cessation of steady shear flow aty = 1 s for PI-14-5CN at 70°C notations are the same as in Figure 11.17.

---