Nature of Steady-State Fast Flow in Entangled Polymer Melts: Chain Stretching, Shear Thinning, and Viscosity Scaling

Abstract

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.

Polymeric Dynamic Crosslinker for Upcycling of Fragile Low-Molecular-Weight Polypropylene

“…We would like to note that the SAOS measurement of polymers and filled polymers can be conducted to modulus of ∼10 9 Pa that is at least 1 order of magnitude higher than the shear modulus of semi-crystalline polymers in this study. 39 In our systems, all PP v samples had high fractions of the amorphous region (∼80 v %) that facilitate the SAOS measurements. To examine the influence of wallslip in the SAOS measurements for the PP v samples, we varied the testing gap from 0.80 to 0.95 mm and observed negligible changes in the shear modulus (Figure S1).…”

“…The strain amplitude was chosen to be 5% at 160 °C and gradually reduces to 0.05% for the temperature of 80 °C. We would like to note that the SAOS measurement of polymers and filled polymers can be conducted to modulus of ∼10 9 Pa that is at least 1 order of magnitude higher than the shear modulus of semi-crystalline polymers in this study . In our systems, all PP v samples had high fractions of the amorphous region (∼80 v %) that facilitate the SAOS measurements.…”

3D Printable Modular Soft Elastomers from Physically Cross-linked Homogeneous Associative Polymers

“…We heated the extruder to 170 °C, slightly lower than the melting point, T m ≈ 200 °C, but much higher than the glass-transition temperature, T g ≈ 54 °C, of the glassy nodules in the dual-cross-linked network. To avoid typical die swell in polymer extrusion, in which the extrudate diameter is usually greater than the channel size, 39 we used a relatively low extrusion rate of 0.3 mm 3 /s to ensure that the extruded filament has a diameter comparable to the nozzle diameter. Synchronizing with the extrusion rate, we used a translation speed of the printhead at 100 mm/min.…”

Scaling Perspective on Dynamics of Nanoparticles in Polymers: Length- and Time-Scale Dependent Nanoparticle–Polymer Coupling

“…The onset of polymer shear thinning (reduction in the shear viscosity η) coincides with the onset of the alignment of the rod (reduction in the average angle ⟨θ⟩ with the shear direction). Investigating the nanoparticle–polymer coupling under out-of-equilibrium flow conditions will help design particle-based experiments to investigate nonlinear polymer rheology , as well as control the processing of nanoparticle–polymer composites under strong nonequilibrium flows such as encountered in 3D additive manufacturing.…”