The molecular dynamics theories need to make a proper combination to describe the
rheological behaviors of polymer melt in various regions of shear rates (Bent et al.
2003 ). Above 1/tt, the convective constraint release dominates the rheological
behaviors of polymers in shear flow, and thus explains the shear-thinning phenome-
non. Beyond 1/tR, the extensional deformation reaches saturation, and the shear flow
becomes stable, entering the second Newtonian-fluid region, as demonstrated in
Fig.7.6.
From the practical viscosity with respect to the shear rate of polymer melts, the
shear-thinning phenomenon can also be observed, as illustrated in Fig.7.7b. The
viscosity in the first Newtonian–fluid region is called zero-shear viscosity 0 , while
the viscosity in the pseudo-plastic region is called apparent viscositya. The
unstable flow occurs often at the beginning of the second Newtonian-fluid region,
and the viscosity 1 at the end of this region is not easy to reach. The shear-
thinning phenomenon offers a larger flow velocity upon extruding and injection
molding process, and hence the efficiency of production can be raised, meanwhile
the energy cost can be reduced. Different processing methods operate within
various regions of shear rates. Figure7.7ademonstrates the curve of viscosity
with respect to the shear rate of polypropylene at 230C, as well as the applicable
processing methods in various shear-rate regions, such as roto-molding, compres-
sion molding, blow molding & thermoforming, pipe & profile extrusion, film
extrusion, injection molding, fiber spinning, and coating (Gahleitner 2001 ). The
main factors that influence the viscosity of polymer are molecular weights and their
distribution, chain branching, temperature, pressure, additives and fillers. Fig-
ure7.7bsummarizes these factors and their corresponding effects. Higher molecu-
lar weights, filler addition and higher pressure lead to higher viscosities, while more
chain branching, more plasticizers and higher temperature favor lower viscosities.
Polymer melts with a broader distribution of molecular weights often show more
significant shear-thinning phenomenon.
Shear flow can be roughly separated into the inclined extensional flow brought
by the transverse velocity gradient, and the rotating flow, as illustrated in Fig.7.8.
The extensional flow leads to the major deformation of polymer coils, while the
rotating flow makes tumbling of polymer coils upon flow. The latter realizes the
Fig. 7.6 Illustration of
convective constraint release
responsible for the shear-
thinning region between
ttandtR
134 7 Polymer Flow