A Coherent Description of Nonlinear Flow Behavior of Entangled Polymers as Related to Processing and Numerical Simulations

Abstract

Feature ArticleThis Feature Article describes a plausible picture of entangled polymer shear flow behavior, resulting from a decade of efforts in our laboratory to unravel the origins of a host of intricate and interconnected rheological phenomena. The first report of spurt flow by Bagley (1958) raised several key questions: Does the spurt flow reflect a constitutive failure as suggested by Vindogradov et al. (1972), Doi-Edwards (1979) and McLeish-Ball (1986)? How is it related to sharkskin, ''melt fracture'' and/or wall slip? The subject remained unsettled and ''paradoxical' ' [Denn (1990)] until the 1990's when a first quantitative description of polymer slip was provided by Brochard and de Gennes (1992). The experimental elucidation [Rheol. Acta 1998, 37, 415] of the interfacial origin of the spurt flow led to the conclusion that the stress maximum in the Doi-Edwards theory (1979) was a theoretical artefact and to a decade of efforts to derive a constitutive description of polymer flow by incorporating Marrucci's idea of convective constraint release (1996). Ongoing studies (2003-present) of constitutive flow and interfacial slip behavior of entangled polymer solutions have revealed something surprising: the entangled polybutadiene solutions appear to respond differently to rate-controlled vs. stress-controlled shear. The results [Macromolecules 2004, 37, 9083] imply that there would not necessarily be a homogeneous shear field in a simple-shear apparatus in the rate-controlled mode. An effective particle tracking velocimetric (PTV) method was developed to allow determination of the velocity profile across the gap in various shear apparatuses including cone-plate and linear sliding-plate setups for startup shear, large amplitude oscillatory shear (LAOS) and step strain experiments. Our PTV observations show an initial linear velocity variation across the gap and a nonlinear velocity profile beyond the shear stress overshoot [Phys. Rev. Lett. 2006, 96, 016001], growth of shear banding in LAOS [Phys. Rev. Lett. 2006, 96, 196001] and elastic breakup of several entangled solutions after step strain [Phys. Rev. Lett. 2006, 97, 187801]. These results present great challenges to both the prevailing theoretical description of entangled polymer flow that is based on tube models and the conventional rheometric protocols used to experimentally determine constitutive flow behavior, and questions the validity to perform numerical simulations of polymer processing based on the available constitutive models.

From Wall Slip to Bulk Shear Banding in Entangled Polymer Solutions

“…The EDT appeared to imply yielding of the entanglement network via chain disentanglement and raised the question of whether breakdown of the entanglement network could result in shear strain localization on macroscopic scales. Such a question motivated the design of particle‐tracking velocimetry for commercial rotational rheometers that could be used to verify whether shear banding would occur in various modes of simple shear. The decade from 2004 to 2015 witnessed extensive PTV observations of shear banding upon startup shear, large amplitude oscillatory shear, and after large step shear…”

Homogeneous Shear, Wall Slip, and Shear Banding of Entangled Polymeric Liquids in Simple-Shear Rheometry: A Roadmap of Nonlinear Rheology

“…To verify whether this is the case, we have recently developed an in situ particle-tracking velocimetric (PTV) method as shown in the middle figure of the TOC graphic . The combination of rheometric and PTV measurements revealed a clear violation of each of the three basic assumptions listed in the first paragraph.…”

Basic characteristics of uniaxial extension rheology: Comparing monodisperse and bidisperse polymer melts

“…Our recent studies on a series of monodisperse linear SBR melts [Wang et al (2007b); Wang and Wang (2008)] have suggested that certain failure behavior of highly entangled polymers in rapid uniaxial extension is analogous to the shear inhomogeneity revealed by particle-tracking velocimetry [Tapadia and Wang (2006); Wang (2007)], and can be understood in terms of the disintegration of the chain entanglement network.…”