Paper Number
SM13 

Session
Polymers Solutions, Melts, and Blends

Title
Exploring the origins of the distinct relaxation times measured in shear and extensional rheometry for concentrated polymer solutions

Presentation Date and Time
October 11, 2021 (Monday) 5:00

Track / Room
Track 1 / Ballroom 5

Authors (Click on name to view author profile)

  1. Du, Jianyi (Massachusetts Institute of Technology)
  2. Ohtani, Hiroko (Ford Motor Company)
  3. Ellwood, Kevin (Ford Motor Company)
  4. McKinley, Gareth H. (Massachusetts Institute of Technology, Mechanical Engineering)

Author and Affiliation Lines (in printed abstract book)
Jianyi Du1, Hiroko Ohtani2, Kevin Ellwood2 and Gareth H. McKinley1
1Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA; 2Ford Motor Company, Dearborn, MI

Speaker / Presenter 
Du, Jianyi

Keywords
applied rheology; polymer melts; polymer solutions; rheology methods

Text of Abstract
Concentrated polymer solutions exhibit distinct rheological properties from their dilute counterparts, due to the enhanced importance of inter- and intramolecular interactions. A number of previous experimental studies have reported notably lower extensional relaxation times obtained from capillary breakup extensional rheometry than those from steady or oscillatory shearing flows. However, the origin of such differences has not been considered in depth, and a physical model that incorporates the structural complexity of entangled polymer systems is absent, hampering the extraction of accurate constitutive parameters from experiments. To address this limitation, we analyze the capillarity-driven thinning dynamics of entangled polymer solutions described by the Doi-Edwards-Marrucci-Grizzuti model and the Rolie-Poly model. Both models incorporate the key features of polymer reptation and contour length fluctuation, while differing slightly due to convective constraint release. Numerical calculation of the filament thinning profiles for parameters representative of entangled systems reveals three distinct regimes of dynamics: a long initial tube-reorientation regime, followed by a brief intermediate elasto-capillary regime, and finally a finite-extensibility regime very close to the pinch-off singularity. An apparent extensional relaxation time obtained from fitting the elasto-capillary regime using an exponential-thinning formula gives values close to half the Rouse time of the entangled chains. By contrast, the much longer disengagement time controlled by the tube reorientation dynamics sets the filament lifetime, as well as the rheological responses in shear flow. Finally, we develop a universal expression with no additional fitting parameters for the ratio of the shear and extensional relaxation times. This formula, expressed as a function of the number of entanglements and the polymer concentration, agrees well with available experimental data from previous studies for a range of entangled polymer solutions.