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, WiR > 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 WiR N/γ̇)0.50 at WiR > 1, where N is the degree of polymerization and γ̇ is 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/Ge ≥ 2, where Ge 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