Paper Number
SM25 

Session
Polymer Solutions and Melts

Title
Edge fracture and shear banding in a highly entangled polystyrene solution

Presentation Date and Time
October 13, 2015 (Tuesday) 3:10

Track / Room
Track 2 / Constellation D

Authors (Click on name to view author profile)

1. Li, Yanfei (Texas Tech University)
2. McKenna, Gregory B. (Texas Tech University, Chemical Engineering)

Author and Affiliation Lines (in printed abstract book)
Yanfei Li and Gregory B. McKenna
Chemical Engineering, Texas Tech University, Lubbock, TX 79409

Speaker / Presenter 
Li, Yanfei

Text of Abstract
The possibility of shear banding has become an important problem. Does it exist? If it does, is it a steady-state or a transient phenomenon? What are the mechanisms for its occurrence? The answers to these questions would help rheologists better evaluate the validity of the Doi-Edwards tube model [1]. Experimentally, if shear banding had occurred, many observational rheological measurements [2] would need to be reevaluated since these experiments were based on the assumption of a homogeneous strain field. The visualization of steady-state shear banding was first reported in 2006 [3] using particle tracking velocimetry with a polybutadiene solution. On the other hand, other findings suggested shear banding is transient and its occurrence is not embedded in the constitutive equation [4]; instead, it could be attributed to edge effects. In the present work, to evaluate the banding phenomenon, we focused on a highly entangled polystyrene solution with entanglement density Z=61 and PDI=1.14. Using particle tracking velocimetry, startup shear tests were carried out deep into the nonlinear regime where the Weissenberg number Wi reaches up to 255. With wall slip prevented by surface roughening, edge fracture occurs at Wi > ~ 3. We found that the velocity profiles are linear, except at Wi = 255, where transient weak banding occurs. These findings suggest that banding is transient, and its occurrence could relate to the edge fracture, which serves as a stress concentrator that either perturbs the monotonic constitutive response or that directly induces shear banding due to, e.g., strong secondary flows that propagate into the sample from the free edge.

[1] Doi M, Edwards SF, J Chem Soc Farad T 2 75:38 (1979). [2] Shamim N, McKenna GB, J Rheol 58:43 (2014). [3] Tapadia P, Wang SQ, Phys Rev Lett 96:016001 (2006). [4] Hu YT, J Rheol 54:1307 (2010).

Acknowledgement: We thank John R. Bradford Endowment at Texas Tech University for the support.