Well-entangled monodisperse linear polystyrene melts
exhibit monotonic
thinning of the steady state elongational viscosity with increasing
the strain rate ε̇ even beyond the Rouse relaxation frequency,
τR
‑1. This behavior is quite different from the thinning followed by
hardening at ε̇ > τR
‑1 observed for entangled semidilute
solutions. We attempt to elucidate the molecular origin of this difference
by focusing on the concept of stretch/orientation-dependent monomeric
friction ζ recently proposed by Ianniruberto and co-workers.
Specifically, literature data of the stress relaxation after cessation
of transient elongational flow, reported for both PS melts and solutions,
are analyzed to evaluate the stretch/orientation-dependent decrease
of ζ. In our working hypothesis, ζ is expressed as a function
of the factor F
so = λ̃2
S̅, where λ̃ is the normalized
stretch ratio of entangled subchains defined with respect to the fully
stretched state, and S̅ is an average orientational
anisotropy of the components (polymer plus solvent if any) in the
system. The factor F
so was estimated from
the stress decay data after flow cessation. The resulting functional
form of ζ(F
so) was then used in
the primitive chain network (PCN) simulation including finite extensible
nonlinear elasticity (FENE) to examine the elongational behavior of
melts and solutions. For melts the simulation indicates that ζ
decreases significantly under fast elongation because the entangled
subchains are short and approach the fully stretched (and fully oriented)
limit rather easily. Hence, the steady elongational viscosity ηE follows this decrease of ζ to exhibit the monotonic
thinning even at ε̇ > τR
‑1. In contrast, for solutions,
the simulated ηE exhibits thickening at ε̇
> τR
‑1 because the average anisotropy S̅ is governed
by the solvent and remains small, thus overwhelming the increase of
the subchain stretch λ̃. The simulated results proved
to be in satisfactory agreement with the experiments.
For highly entangled cis-polyisoprene (PI) star polymers having more than 10 entanglements
in each arm, dielectric and viscoelastic properties were examined within a context of the generalized
tube model incorporating the dynamic tube dilation (DTD) mechanism. The star PI had the type A dipoles
parallel along the arm backbone, and the global motion results in the viscoelastic as well as dielectric
relaxation. The DTD relationship between the dielectric and viscoelastic relaxation functions Φ(t) and
μ(t), μ(t) ≅ [Φ(t)]2 (derived under an assumption of random displacement of the entanglement segment in
the dilated tube edge), was not valid for the star PI. Furthermore, the DTD model (Milner−McLeish
model) excellently described the viscoelastic data, but considerable differences were found for the dielectric
data, even if an effect of the segment displacement in the tube edge was considered in the model. These
results indicated a failure of the DTD molecular picture for a few entanglement segments near the
branching point. Thus, these segments near the branching point appeared to fully relax via the constraint
release (CR) mechanism before the expected tube dilation was completed. On the basis of this result, the
DTD model was modified by explicitly incorporating this CR process (though in a crude way). This
modification moderately improved the model prediction, suggesting a possible direction of further
refinement of the model.
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