The tip of iceberg in nonlinear polymer rheology: Entangled liquids are “solids”

Abstract

It is very timely to be asked to provide a personal perspective in my field of research-nonlinear rheology of entangled polymers that is at a cross-road and may undergo profound transformation if one road proves to lead us much further as judged by evidence-based experimentation but not necessarily by the conventional criterion of obtaining quantitative agreement between experiment and theory. Although it may be very early to draw convincing conclusions, a vivid phenomenological picture has emerged.Polymer dynamics and rheology, 1,2 along with the subjects of polymer crystallization and polymer glass formation, is a core study in polymer physics. A central theme in the past four decades has been to describe chain entanglement, respectively, in absence and presence of external deformation. 3 Pioneers in the field include Beuche, Lodge, Ferry, Graessley, Edwards, de Gennes, and Doi, to name a just few most familiar. Since 1970s, the focus has shifted to exploring how chain entanglement gives rise to sluggish polymer dynamics far above the glass transition temperature. The first idea to account for the origin of chain entanglement in terms of a packing concept emerged almost simultaneously between 1985 and 1987 from Rault, Heymans, Lin, Kavassalis, and Noolandi. A competing notion of percolating network was subsequently introduced by Wool in 1993. The readers can get a comprehensive review of the relevant literature in a recent publication. 4 A great number of experiments have accumulated for more than three decades to indicate that the de Gennes-Doi-Edwards tube model 3,5 is very successful in describing polymer dynamics in the linear response regime, including diffusion and linear viscoelastic behavior in bulk and at surfaces, and that much is reliably known about linear viscoelasticity at a phenomenological level, although some recent theoretical activities indicated 6-8 that the tube theory may have serious limitations even in the simplest case of stress relaxation after small step strain.In sharp contrast, phenomenology of nonlinear polymer rheology has been elusive and largely assumptive although large deformation and flow behavior of polymeric liquids have been rheometrically studied for several decades. Entangled polymer solutions and melts have been regarded as liquids, capable of undergoing homogeneous flow even during sudden large deformation. 9 Various theoretical descriptions 3,10,11 of constitutive behavior of entangled polymers have compared with experiment on the basis of homogenous shear. Standard rheological methods that work well for linear viscoelastic measurements have been extended to explore steady-state nonlinear rheological behavior, recognizing that entangled polymer solutions and melts are liquids at long times.Based on an effective application of a particle-tracking velocimetric (PTV) method 12 shown in Figure 1,

Phase separation in dilute polymer solutions at high‐rate extension

“…Turning to the kinetics of phase separation, the quantitative theory should be reconsidered due to the presence of entanglements. We also expect qualitatively new effects under high‐rate elongation, like formation of mostly tight (rather than loose) entanglements between the chains due to their strong tension . This effect may render the entanglement network nearly permanent giving rise to an interesting superposition of flow‐induced phase separation and gelation.…”

“…We also expect qualitatively new effects under high-rate elongation, like formation of mostly tight (rather than loose) entanglements between the chains due to their strong tension. 26,27 This effect may render the entanglement network nearly permanent giving rise to an interesting superposition of flow-induced phase separation and gelation. Clearly, a separate study is invited to investigate these and other effects of strong extension in entangled systems.…”

Perspective: Ionomer Research and Applications

“…All of those groups used capillary viscometry in their investigations, so no information on elasticity of the melt was obtained, and none of the studies provided a detailed analysis of the effects of temperature and/or cation on the material functions. It is also questionable whether a viscometric flow was really established in those studies or whether slip within the melt occurred due to plastic deformation or yielding of the ionic network structure …”

Challenges and opportunities: a hopeful future of polymer rheology in China

“…This elastic yielding phenomenon clearly points to the necessity to depict an entangled polymer in terms of a multiple-chain picture. The single-chain mean-field description based on the tube theory appears to be inadequate [7]. It is minimal to point out that the tube theory did not anticipate elastic yielding, nor did it truly expect shear banding to emerge during startup shear.…”