(3q) Computational and Theoretical Studies of Amorphous Polymeric and Molecular Materials

The kinetically arrested amorphous solid state is found in a surprisingly wide range of materials, from ”hard” silicate and metallic glasses, to ”soft” colloids, polymers (thermoplastics, thermosets), physical gel networks, biological scaffolds, granular materials, and even living cells. Understanding the complex behavior of these materials, especially the slow dynamics, mechanical and other functional properties and their relation with chemical structure, intermolecular packing and thermodynamics across different time and length scales, not only presents a grand challenge in statistical mechanics, but is crucial for molecular engineering new materials with high industrial impact and in diverse biological contexts (e.g., tissue engineering).

My PhD work with Prof. Scott Milner at Penn State focused on the structure and dynamics of various glass-forming systems from simple hard-sphere and purely repulsive WCA fluids, to more realistic polymer thin films and silica networks. To avoid the unwanted crystallization issue in monodisperse colloidal fluids, arguably the simplest model glass former, we developed a novel crystal-avoiding simulation method based on hybrid Monte Carlo to suppress crystallization while preserving the dynamics. This allowed us to probe with simulation, for the first time, the ultra-dense deeply metastable states of colloidal fluids. We then characterized the structural and thermodynamic properties of simulated systems using non-numerical computational methods, such as rigorous geometric calculation of local free, and graph theory based analyses for structural motifs and entropy, to explore their connections to the slow dynamics and confront with existing glass theories. We also investigated the effects of confinement on glassy dynamics via random particle pinning or thin film geometry. For example, we performed atomistic simulations of free-standing polystyrene thin films to probe the local glass transition temperature (T_g) shift. We studied the cooperative dynamics between polymer segments and small molecules in di-2-ethlyhexyl phthalate (DEHP) plasticized polyvinyl chloride (PVC) with molecular dynamics simulations, which provided a better understanding of plasticization on a microscopic level that is important for searching alternative non-phthalate plasticizers due to health and environment concerns. I also collaborated with experimentalists to explore dielectric properties of silica glasses under thermal and mechanical stress using core-shell atomistic simulations, which shed light on the origin of infrared spectrum band shifts in silica glasses upon chemical strengthening or thermal tempering.

My current postdoctoral research with Prof. Kenneth Schweizer at University of Illinois focuses on developing and applying microscopic statistical mechanical theories to understand and predict the phase behavior, structure, slow dynamics, and mechanical properties of colloidal suspensions, polymer melts, polymer nanocomposites (PNCs) and other soft materials. These questions often involve extremely long time, length and/or energy scales that are impossible to access with brute force simulation. We improved the closure approximation for computationally convenient integral equations theories (IET) to predict the structure and phase behavior of binary mixtures with high size asymmetry and strong interfacial attractions, and validated the ideas with our own benchmark simulations. This methodology advance is broadly relevant to colloidal mixtures, polymer nanocomposites, and other complex liquids and materials. Combined with the advanced Elastically Collective Nonlinear Langevin Equation (ECNLE) theory for cooperative activated glassy dynamics, we systematically explored the roles played by multiple experimental control parameters, including total density, particle size, loading, and interfacial attraction, in determining the dynamics and mechanical properties of PNCs and their relationship to statistical microstructure. Our recent collaborative work with experimentalists (small angle scattering, rheology, x-ray photon correlation spectroscopy) at Oak Ridge and Stony Brook on silica-P2VP nanocomposites revealed the key roles played by interfacial cohesive energy, particle loading and adsorbed layers on the bridging-controlled network microstructure, which in turn strongly affects the collective motions of nanoparticles.

My future independent research will synergistically combine novel computational and theoretical techniques to facilitate both fundamental understanding and rational design of amorphous polymeric and molecular materials that involves large time, length and energy scales in a manner that retains chemical predictability. I will also seek collaborations with experimentalists to validate our modeling, interpret specific experimental results, and make testable predictions that can guide new experimental efforts. Initial research topics involve:

1. Predicting and manipulating glassy dynamics to control bulk properties and molecular permeability in plasticized polymers and thin films. This problem has broad relevance in engineering the mechanical properties of thermoplastics and also creating novel separation membranes.

2. Characterizing the structure and dynamics of active glassy colloids and polymers. As a novel class of nonequilibrium soft matter, active glassy matter combines external energy-driven motion and slow activated intrinsic dynamics, which offers a new paradigm for creating functional synthetic and living materials.

3. Developing novel methods for atomistic simulation of polar molecules near metallic substrates. By efficiently handling induced charges on the metal surface, large scale simulations can be rendered tractable to address questions such as structure and kinetics at electrode/electrolyte interfaces and the growth of metal nanoparticles or wires.

Teaching Interests

I believe teaching is essential for a successful academic career. I think inquiry-based active learning is important for teaching science and engineering courses, since it is not about remembering specific equations or formulas, but the way of thinking and the skills to identify and solve scientific problems that are most valuable for students. Creating an inclusive environment is also crucial for effective learning. In my PhD studies at Penn State, I worked as a teaching assistant in the undergraduate level thermodynamics course as well as mentored an undergraduate student on his thesis project about local structure in simulated amorphous silica. Given my formal undergraduate and PhD background and training in Chemical Engineering, plus my extensive research experience in Materials Science, I can teach a wide range of courses. I would be particularly interested in teaching thermodynamics, statistical mechanics, polymer physics, chemical reaction kinetics, mathematical and numerical methods.

Selected Publications

Y. Zhou, K. S. Schweizer, ” Theory of Microstructure-Dependent Glassy Shear Elasticity and Dynamic Localization in Melt Polymer Nanocomposites”, submitted, July 2020.

Y. Zhou, B. M. Yavitt, Z. Zhou, V. Bocharova, D. Salatto, M. K. Endoh, A. E. Ribbe, A. P. Sokolov, T. Koga, K. S. Schweizer, ”Bridging Controlled Network Microstructure and Long Wavelength Fluctuations in Silica-Poly(2-vinylpyridine) Nanocomposites: Experimental Results and Theoretical Analysis ”, Macromolecules, submitted, June 2020.

B. Mei, Y. Zhou, K. S. Schweizer, ”Thermodynamics-Structure-Dynamics Correlations and Nonuniversal Effects in the Elastically Collective Activated Hopping Theory of Glass-Forming Liquids”, J. Phys. Chem. B, in press, July 2020

Y. Zhou, K. S. Schweizer, ”Local structure, thermodynamics, and phase behavior of asymmetric particle mixtures: Comparison between integral equation theories and simulation”, J. Chem. Phys., 2019, 150, 214902.

Y. Zhou, S. T. Milner, ”Average and local T_g shifts of plasticized PVC from simulations”, Macromolecules, 2018, 51(10) 3865-3873.

Y. Zhou, S. T. Milner, ”Short-time dynamics reveals T_g suppression in simulated polystyrene thin films”, Macromolecules, 2017, 50(14) 5599-5610.

J. Luo*,Y. Zhou*, S. T. Milner, C Pantano, S. Kim, ”Molecular dynamics study of correlations between IR peak position and bond parameters of silica and silicate glasses”, J. Am. Ceram. Soc., 2017, 00 1-11. *First authorship shared

Y. Zhou, S. T. Milner, ”T1 process and dynamics in glass-forming hard-sphere liquids”, Soft Matter, 2015, 11, 2700-2705.