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Title:Glassy viscoelasticity of dense suspensions of soft colloids and structure & miscibility of soft filler polymer nanocomposites
Author(s):Yang, Jian
Director of Research:Schweizer, Kenneth S.
Doctoral Committee Chair(s):Schweizer, Kenneth S.
Doctoral Committee Member(s):Granick, Steve; Trinkle, Dallas R.; Schroeder, Charles M.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Polymer Nanocomposites
Glassy Viscoelasticity
Effective Interaction
Multi-scale Modeling
Abstract:In this thesis, we studied two soft condensed matter systems. The first part of the thesis studied the glassy viscoelasticity of dense suspensions of soft colloids and the second part of the thesis studied the structure and miscibility of soft filler polymer nanocomposites (PNCs). In the first part of the thesis, we have presented the first microscopic theoretical study of activated glassy dynamics in dense fluids of finite range soft repulsive particles (many arm star polymers and microgel-like Hertzian spheres). The alpha relaxation time in the activated hopping regime is a rich function of volume fraction and temperature, including exhibiting a maximum value at ultra-high volume fraction due to a soft jamming crossover that signals local packing disorder due to particle overlap. A kinetic arrest diagram is constructed, and its qualitative features agree with the dynamic crossover (MCT) analog. The isothermal dynamic fragility varies over a wide range, and soft particles are predicted to behave as strong glasses. The highly variable dependences of the relaxation time on temperature and volume fraction are approximately collapsed onto two distinct master curves. We have applied NLE theory to study how particle softness influences the elastic shear modulus, the connections between the modulus (a short time property) and activated relaxation (a long time property), and the nonlinear rheological effects of stress-induced yielding, shear thinning of the relaxation time and viscosity, and stress versus shear rate flow curves of the repulsive Hertzian contact model of soft sphere fluids. In the second part of the thesis, we have presented representative results based on a new minimalist multi-scale model constructed for soft nanoparticle fillers. This is the first theoretical study of the role of nanoparticle morphology associated with surface corrugation and fluctuation at one and two particle limit in polymer melts with/without interfacial cohesion. Our results provide a physical basis for the unexpected ability to disperse chemically matched crosslinked polystyrene in linear polystyrene melt. The surface corrugation in the frozen surface regime results in a favorable entropic driving force for mixing which competes with unfavorable depletion, resulting in a major enhancement of nanoparticle dispersion, including a much larger spinodal solubility limit. Smooth hard sphere behavior is recovered when the number of beads are significantly large (N=282) and the relative corrugation size significantly reduced (from 20% to 10% of particle size). When surface fluctuation exists, the dependence of solubility limit on fluctuation magnitude is somewhat subtle, due to the relevance of multiple length scales. We find fluctuation suppresses dispersion for all particles sizes (or surface curvature) studied, and this effect is most pronounced when the monomer size is smaller than corrugation size. The extreme coherent fluctuation model shows dramatically enhanced dependence on fluctuation magnitude at large fluctuation magnitude and less miscibility. When interfacial cohesion exists, we develop a model for local bead-monomer level attraction, which explicitly connects to particle-monomer level attraction through potential mapping strategies. By varying the interfacial attraction strength, we can still observe all three regimes reported in prior PRISM studies of smooth HS filler PNCs. The steric stabilization regime is not sensitive to surface fluctuation magnitude, while the contact clustering and strong bridging regimes are very sensitive to surface fluctuation magnitude. Surface fluctuation smears out surface corrugation and reduces interfacial cohesion, leading to stronger depletion attraction and more negative second virial coefficients.
Issue Date:2013-02-03
Rights Information:Copyright 2012 Jian Yang
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12

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