Concentrated entangled and semidilute entangled
polystyrene solutions and the second normal stress difference
J.J. Magda and S.G. Baek
Steady shear flow values of the normal stress ratio Y
= - N2/N1 are compared for monodisperse
polystyrene solutions in the concentrated entangled and semidilute
entangled regimes. In both regimes, the zero shear rate limiting
values are the same: Y0
= 0.275 ±0.005. Surprisingly, the molecular weight required
for full entanglement (on the basis of Y0
) scales with concentration as C -1.5±0.1
, rather than C -1.0 as commonly supposed. The value
of Y0 is insensitive to
fractional free volume and to solvent affinity for polystyrene,
even at semidilute concentrations. However, Y
depends on the shear rate, with shear-thinning behavior in all
solvents. Shear thinning of Y has
not been widely reported, probably because of the 'edge fracture'
flow instability in rotational rheometers. In concentrated solutions
or melts, edge fracture usually occurs before Y
shear thins by a significant amount.