by\u00a0Shi-Qing Wang<\/a>, University of Akron, USA<\/p>\n Polymer processing suffers from a variety of rate-limiting difficulties.\u00a0In extrusion alone, we encounter surface roughness on extrudate (sharkskin), quasi-periodic extrudate distortion associated with pressure oscillation and gross melt fracture.\u00a0 To have better mechanical characteristics, polyolefin resins need to have sufficiently high molecular weight, and the same is true for rubbers.\u00a0 Consequently, useful polymers, in the annual amount of one hundred million tons, are always strongly entangled.\u00a0 Most of the melt processing instabilities of these polyolefin and rubbers are due to the presence of high entanglement.\u00a0 Our task is to understand and predict rheological responses of entangled polymeric materials.<\/p>\n 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.\u00a0 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].\u00a0The new molecular-level understanding unified the two fields of shear and extensional rheology.\u00a0 It not only addresses the concept of yielding in terms of chain disentanglement but also deals with the phenomena of strain localization.\u00a0 In simple shear, we can predict when, how and why polymeric liquids (solutions and melts) switches from wall slip to bulk shear banding.\u00a0 In global uniaxial extension, we can no longer expect to reach fully developed flow state before the specimen undergoes various forms of breakup.\u00a0 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.\u00a0 It appears all forms of \u201cmelt fracture\u201d arise from some form of strain localization due to localized yielding of the entanglement network via chain disentanglement.<\/p>\n [1] Nonlinear Polymer Rheology<\/em>, Shi-Qing Wang, Wiley (2018).<\/p>\n by\u00a0Shi-Qing Wang<\/a>, University of Akron, USA<\/p>\n In my lab, we focus on building a molecular-level understanding of polymer mechanics in both liquid and solid states.\u00a0 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.\u00a0 The latter two subjects can only be understood after the molecular foundation [2] for polymer melt rheology has been established.\u00a0In this talk, I will concentrate on subject b, exploring how we can understand the remarkable ductility of glassy polymers.\u00a0 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.\u00a0 The universally applied Eyring idea of activation alone is powerless to provide the foundation for the molecular mechanics of glassy polymers.\u00a0 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.<\/p>\n [2] Nonlinear polymer rheology: macroscopic phenomenology and molecular foundation, S. Q. Wang, Wiley (2018)<\/p>\n [3] A phenomenological molecular model for brittle-ductile transition and yielding of polymer glasses, S. Q. Wang et al., J. Chem. Phys.<\/em>141<\/strong>, 094905 (2014).<\/p>\n A third topic on\u00a0Ductility of glassy semicrystalline polymers<\/em>A<\/strong> unified framework to understand ductility in glassy polymers: from crazing,<\/strong> brittle-to-ductile transition to rubber-toughened polymers<\/strong><\/h5>\n
Location:\r\nMartin-Luther-Universit\u00e4t Halle-Wittenberg \r\nVon-Seckendorff-Platz 1, SR 1.30 06120 Halle (Saale)\r\nDate: October 14, 2019 \r\nTime: 9-11:30am<\/strong> (including break)\r\n\r\nLink to OpenStreetMap<\/a><\/pre>\n
\nwill be addressed in the Polymer and Soft Matter Seminar<\/a> on October 14, 2019 at 5 pm.<\/p>\n","protected":false},"excerpt":{"rendered":"