(6cv) Computational Crystallization and Assembly of Polymers and Soft Matter

Complex structural and dynamical behaviors can lead to various assembled structures, and in turn govern the final material properties of polymers and soft matter. To predict the structure-property relations for these soft materials, a fundamental understanding of these behaviors at molecular level is essential. My future research will focus on predicting the assembly and properties of a variety of soft materials, including polymers, liquid crystals, colloids, surfactants and peptides, from their molecular structures and processing conditions using statistical mechanics theories and multi-scale simulations.

During my graduate training at the Pennsylvania State University, I studied the properties and behaviors of conjugated polymers from theoretical perspectives under the guidance from Professors Enrique Gomez and Scott Milner. By combining multi-scale simulations, ranging from density functional theory to coarse grained molecular dynamics simulations, with statistical mechanics theories such as self-consistent field theory, we developed various methods to predict material parameters, such as persistence length, Maier-Saupe parameter and Flory-Huggins χ from chemical structures for conjugated polymers. We have also demonstrated that orientational order of polymer backbones can impose molecular weight dependence on charge transport for conjugated polymers.

As a postdoctoral fellow at University of Michigan, I work with Professor Ronald Larson on colloidal assembly and polyethylene (PE) crystallization. We developed a theoretical tool to predict the effective interactions between colloidal particles induced by telechelic polymers that can form loops on a single particle or bridges between two colloids. The effective interactions can be then used in a novel Brownian dynamics method to predict the shear rheology of these colloid/polymer solutions. We have also applied atomistic simulations and analytical theories to predict the effects of flows and side-chain branching on PE crystallization. Combining our crystallization simulations with rheology modeling, we aim at predicting the crystallization behaviors and the resulting morphologies and properties for blown PE films.

Moving forward, my independent research will focus on bulk and interfacial crystallization and assembly of soft materials. Using multi-scale simulations and analytical theories, we aim at predicting assembly pathways, and final morphologies and properties from molecular structures and processing conditions for model materials. We will also seek collaborations with experimentalists to validate our computational approaches and obtain macroscopic material properties that are difficult to measure in simulations. Using both simulations and experimental material properties as training data, we can then apply artificial neural networks to estimate structure-property relations for rational design of novel high-performance soft materials. My initial research directions are:
1. Polymer crystallization at interfaces and in flows;

2. Hierarchical assembly of peptides and peptide amphiphiles;

Over years, I have been taught, trained and advised by excellent teachers and supervisors, from whom I have learned that clarity in teaching and student engagement are the keys to deliver high quality education. The clarity in presentation echoes the logic nature of science, and thus can greatly improve the efficiency in the knowledge delivery process. The participation and actual practice from students are indispensable and learning is most effective when students are highly motivated. From my own learning and previous mentoring experience, both the joy of discovery and credited achievements are positive incentives that can drive students to improve themselves constantly. My goal is to incorporate this philosophy in teaching in order to train the next generations of excellent chemical engineers.

I have put my teaching philosophy in practice and observed successful outcomes. At the Pennsylvania State University, I mentored an undergraduate student in an independent research project and, at the end, he published a peer-reviewed paper as a first author. At the University of Michigan, I instructed junior Ph.D. students on molecular simulations and polymer physics. In addition to mentoring, I also worked as a teaching assistant in an undergraduate fluid mechanics class and as a guest lecturer on Monte Carlo simulations for an undergraduate elective course.

Given my educational background (B.S. and Ph.D in chemical engineering) and experience, I feel confident in teaching several chemical engineering courses at both undergraduate and graduate levels. For undergraduate courses, I am interested in teaching mass and energy balances, thermodynamics, reaction kinetics, mass and heat transfer, and fluid mechanics. Among the graduate level courses, I would like to teach statistical mechanics and polymer physics.

11. "A metastable nematic precursor accelerates polyethylene oligomer crystallization as determined by atomistic simulations and self-consistent field theory", Zhang, W.; Larson, R.G.,Journal of Chemical Physics, in press.

10. "Thermal fluctuations lead to cumulative disorder and enhance charge transport in conjugated polymers", Zhang, W.; Bombile, J.H.; Weisen, A.R.; Xie, R.; Colby, R.H.; Janik, M.J.; Milner, S.T.; Gomez, E.D., Macromolecular Rapid Communications, in press.

9. "Tension-induced nematic phase separation in bidisperse homopolymer melts", Zhang, W.; Larson, R.G., ACS Central Science, 2018, 4, 1545-1550.

8. "Direct all-atom molecular dynamics simulations of the effects of short chain branching on polyethylene oligomer crystal nucleation", Zhang, W.; Larson, R.G., Macromolecules, 2018,51, 4762-4769.

7. "Nematic order imposes molecular weight effect on charge transport in conjugated polymers", Zhang, W.; Milner, S.T.; Gomez, E.D., ACS Central Science, 2018, 4, 413-421.

6. "Predicting Flory-Huggins χ from simulations", Zhang, W.; Gomez, E.D.; Milner, S.T., Physical Review Letters, 2017, 119, 017801.