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Wall slip of polydisperse linear polymers : effects of molecular weight characteristics, and surface conditions Ebrahimi, Marzieh

Abstract

The classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of complex fluids including polymer melts. Since the slip velocity of polymer melts complicates analysis of rheological data and it is needed for simulation of polymer processes and process optimization, a comprehensive predictive slip velocity model should be developed. In this thesis, the slip behavior of monodisperse and polydisperse linear polymers including high-density polyethylenes (HDPEs), polybutadienes (PBDs), and polystyrenes (PSs) is studied to fully understand the effect of molecular weight (MW) and molecular weight distribution (MWD). Concepts from double reptation mixing rule are used to develop an expression for slip velocity of polydisperse polymers based on their MW and MWD. Very good agreement between experimental data and predictions of proposed model is observed, validating the applicability of the model. Surface enrichment of short chains next to solid boundaries due to entropic effects complicates slip analysis specially in the case of bimodal polymers. To address surface segregation, the slip behavior of several HDPEs with broad range of molecular weight including bimodals is studied. Moreover, the developed slip model coupled with a model of surface molecular weight fractionation is used to predict the slip velocity of the studied polymers. It is observed that surface fractionation has a minor effect on slip of narrow to moderate MWD polymers (particularly unimodal), but its role is significant for broad bimodal polymers. Moreover, the dynamic slip behavior of a polymer melt was investigated by performing dynamic shear experiments using the stress/strain controlled rotational rheometer equipped with parallel partitioned plate geometry. The multimode integral Kaye-Bernstein-Kearsley-Zapas (KBKZ) constitutive model is applied and it is found that a dynamic slip model with a slip relaxation time is needed to adequately predict the experimental data at large shear deformations. Finally, the effects of surface topology and energy on slip velocity of high-density HDPEs was studied using treated and untreated smooth and patterned slit dies. It was found that the slip velocity is decreased by roughness and is increased by silanization. These effects have been incorporated into the slip velocity model.

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