The essence of nonlinear polymer rheology: everything you must know
by Shi-Qing Wang, University of Akron, USA
Polymer processing suffers from a variety of rate-limiting difficulties. In extrusion alone, we encounter surface roughness on extrudate (sharkskin), quasi-periodic extrudate distortion associated with pressure oscillation and gross melt fracture. To have better mechanical characteristics, polyolefin resins need to have sufficiently high molecular weight, and the same is true for rubbers. Consequently, useful polymers, in the annual amount of one hundred million tons, are always strongly entangled. Most of the melt processing instabilities of these polyolefin and rubbers are due to the presence of high entanglement. Our task is to understand and predict rheological responses of entangled polymeric materials.
This presentation summarizes more than one decade of intensive research carried out at Akron that has completely changed our worldview of the essence of nonlinear rheology of entangled polymers. In about sixty publications, we have collected coherent and comprehensive phenomenology and proposed a molecular network paradigm for polymer rheology and processing, summarized in a recently published book [1]. The new molecular-level understanding unified the two fields of shear and extensional rheology. It not only addresses the concept of yielding in terms of chain disentanglement but also deals with the phenomena of strain localization. In simple shear, we can predict when, how and why polymeric liquids (solutions and melts) switches from wall slip to bulk shear banding. In global uniaxial extension, we can no longer expect to reach fully developed flow state before the specimen undergoes various forms of breakup. In extrusion, well-entangled melts suffer sharp shear strain discontinuity upon entry into extrusion dies that varies both spatially and temporally, leading to the so called gross melt fracture. It appears all forms of “melt fracture” arise from some form of strain localization due to localized yielding of the entanglement network via chain disentanglement.
[1] Nonlinear Polymer Rheology, Shi-Qing Wang, Wiley (2018).
A unified framework to understand ductility in glassy polymers: from crazing, brittle-to-ductile transition to rubber-toughened polymers
by Shi-Qing Wang, University of Akron, USA
In my lab, we focus on building a molecular-level understanding of polymer mechanics in both liquid and solid states. This is a journey that involves three episodes or steps: a. Phenomenology and conceptual foundation of polymer melt rheology, b. Molecular mechanics of polymer glasses, c. Brittleness and ductility of semicrystalline polymers. The latter two subjects can only be understood after the molecular foundation [2] for polymer melt rheology has been established. In this talk, I will concentrate on subject b, exploring how we can understand the remarkable ductility of glassy polymers. In a pedagogical way, I will explain why a valid theory to explain yielding of glassy polymers must address when the polymers fail to remain ductile, i.e., unable to yield and undergo brittle fracture. The universally applied Eyring idea of activation alone is powerless to provide the foundation for the molecular mechanics of glassy polymers. Rich experiences with melt rheology have provided us the crucial ingredients to formulate the basis [3] for all aspects of mechanical behavior of polymeric glasses including brittle-ductile transition, crazing and rubber-toughening.
[2] Nonlinear polymer rheology: macroscopic phenomenology and molecular foundation, S. Q. Wang, Wiley (2018)
[3] A phenomenological molecular model for brittle-ductile transition and yielding of polymer glasses, S. Q. Wang et al., J. Chem. Phys.141, 094905 (2014).
Location: Martin-Luther-Universität Halle-Wittenberg Von-Seckendorff-Platz 1, SR 1.30 06120 Halle (Saale) Date: October 14, 2019 Time: 9-11:30am (including break) Link to OpenStreetMap
A third topic on Ductility of glassy semicrystalline polymers
will be addressed in the Polymer and Soft Matter Seminar on October 14, 2019 at 5 pm.
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