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 Title: Elastic stiffening in PVA-Borax studied with experimental medium amplitude oscillatory shear Author(s): Carey-De La Torre, Olivia Advisor(s): Ewoldt, Randy H. Department / Program: Mechanical Sci & Engineering Discipline: Mechanical Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: M.S. Genre: Thesis Subject(s): Stiffening Asymptotic nonlinearity Medium amplitude oscillatory shear (MAOS) Abstract: In this thesis, we seek to understand the mechanisms of strain-stiffening and shear-thickening often observed in transient or associative polymer hydrogels; specifically, Poly(vinyl) alcohol (PVA) crosslinked with Sodium tetraborate (Borax), or PVA-Borax. We use medium amplitude oscillatory shear (MAOS) as a tool to measure asymptotically nonlinear viscoelastic material functions across a range of compositions of PVA-Borax with storage moduli ranging from $G_0 \approx 100-3000$~Pa. The material functions can be related to model parameters to gain physical insight into the structure of the material system. We demonstrate that with a "good" structure-rheology model \cite{AshwinModel}, a single nonlinear parameter scales the asymptotically nonlinear material functions, and we argue that finite-extensibility is the sub-dominant cause of nonlinearity ($<5$\%) compared to stretch-induced crosslinking. Furthermore, we validate the model by using first-harmonic MAOS measures to predict third-harmonic MAOS via a single nonlinear model parameter. Finally, we define a universal nonlinear parameter to compare the strength of nonlinearities across a range of strain-stiffening materials. Issue Date: 2017-09-20 Type: Text URI: http://hdl.handle.net/2142/99288 Rights Information: Copyright 2017 Olivia Carey-De La Torre Date Available in IDEALS: 2018-03-13 Date Deposited: 2017-12
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