Paper Number
SM18 

Session
Polymers Solutions, Melts and Blends

Title
Nature of steady-state fast flow in entangled polymer melts: Chain stretching, viscosity scaling, and shear-induced disentanglement

Presentation Date and Time
October 11, 2022 (Tuesday) 9:50

Track / Room
Track 2 / Sheraton 3

Authors (Click on name to view author profile)

1. Xu, Zipeng (Michigan State University, Chemical Engineering and Materials Science)
2. Sun, Ruikun (Michigan State University)
3. Lu, Wei (University of Tennessee)
4. Patil, Shalin (Michigan State University)
5. Mays, Jimmy (University of Tennessee)
6. Schweizer, Kenneth S. (University of Illinois at Urbana-Champaign)
7. Cheng, Shiwang (Michigan State University)

Author and Affiliation Lines (in printed abstract book)
Zipeng Xu¹, Ruikun Sun¹, Wei Lu², Shalin Patil¹, Jimmy Mays², Kenneth S. Schweizer³ and Shiwang Cheng^1
1Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824; ²University of Tennessee, Knoxville, TN 37996; ³University of Illinois at Urbana-Champaign, Urbana, IL 61801

Speaker / Presenter 
Cheng, Shiwang

Keywords
polymer melts

Text of Abstract
Understanding topologically entangled non-equilibrium polymer dynamics under strong external deformation has been a grand challenge in polymer science for more than half a century. Important 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, ?, 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 ?~? ?^(-0.7±0.1) at ?Wi?_R<1 to ?~(N/? ? )^0.50 at ?Wi?_R>1 where N is the degree of polymerization and ? ? the shear rate. Moreover, a remarkable unentangled melt scaling ?~N emerges under ultra-high constant stress conditions s/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 universal expression for the shear-thinning behavior controlled by purely dissipative considerations associated with orientational stress. This physical picture is in sharp contrast to state-of-the-art tube model predictions based on the widely adopted factorization approximation of the total stress into stretching and orientational parts, and also qualitatively differs from predictions of non-tube-based slip-link models based on a transient network perspective.