Understanding the nonequilibrium dynamics of topologically entangled polymers under strong external deformation has been a grand challenge in polymer science for more than half a century. Important deformation-induced single-polymer structural changes have been identified, such as chain orientation and stretching. But how these changes impact the physical entanglement network and bulk viscoelasticity remains largely elusive in the fast flow regime that involves highly oriented and stretched polymer chains. Here, through new experimental and theoretical developments, we establish a unified understanding of the steady-state shear viscosity, η, of entangled polymer melts at high Rouse Weissenberg numbers, Wi R > 1. New capillary rheometry measurements in the absence of flow instabilities reveal a dramatic change in shear-thinning scaling from η ∼ γ̇− 0.7 ± 0.1 at Wi R < 1 to η ∼ (N/γ) 0.50 at Wi R > 1, where N is the degree of polymerization and γ̇is the shear rate. Moreover, the viscosity scaling exponent with polymer molecular weight decreases with applied shear stress, and a remarkable unentangled melt scaling η ∼ N emerges under ultrahigh constant stress conditions σ/G e ≥ 2, where G e is the equilibrium entanglement elastic modulus. These new observations are not consistent with existing molecular theories. We construct a dynamic scaling model based on tension blob concepts as extended to entangled polymers, resulting in a (near) universal expression for the shear-thinning behavior controlled by purely dissipative considerations associated with orientational stress. This physical picture is in sharp contrast to the predictions of various state-of-the-art tube-based models based on the widely adopted factorization approximation of the total stress into stretching and orientational contributions, and also qualitatively differs from predictions of non-tube-based slip-link models based on a transient network perspective.
Combining X-ray tomography and rheology, we investigate
the nanoparticle
(NP) clustering and viscoelastic properties of polymer nanocomposites
(PNCs) with non-attractive polymer–NP interactions. X-ray tomography
reveals linear and compact NP clusters with preferential orientations
rather than tenuous NP clustering structures, highlighting the non-equilibrium
nature of NP clustering and the influence of processing on morphology
of NP clusters. Detailed analyses show that the number density of
NP clusters of N NPs follows P(N) ≈ N
–k
exp(−N/N*) with N* as a constant and k ≈ 1.8 ±
0.2. Moreover, varying the polymer–NP interactions from strongly
repulsive to nearly athermal does not alter noticeably the clustering
and the percolation behavior of the NP phase. Linear viscoelastic
measurements reveal negligible dynamics slowing down and unexpectedly
weak mechanical reinforcement of PNCs following the classical Guth–Gold
relation, in sharp contrast to the strong NP clustering from structure
characterizations. These results point to qualitatively different
clustering behaviors and mechanical reinforcement of PNCs with non-attractive
polymer–NP interactions from those with strong attractive interactions.
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