W. Xia et al., Science Advances (2019), https://advances.sciencemag.org/content/5/4/eaav4683?utm_source=TrendMD&...

Multiscale coarse-grained (CG) modeling of soft materials, such as polymers, is currently an art form because CG models normally have significantly altered dynamics and thermodynamic properties compared to their atomistic counterparts. We address this problem by exploiting concepts derived from the generalized entropy theory (GET), emphasizing the central role of configurational entropy sc in the dynamics of complex fluids. Our energy renormalization (ER) method involves varying the cohesive interaction strength in the CG models in such a way that dynamic properties related to sc are preserved. We test this ER method by applying it to coarse-graining polymer melts (i.e., polybutadiene, polystyrene, and polycarbonate), representing polymer materials having a relatively low, intermediate, and high degree of glass “fragility”. We find that the ER method allows the dynamics of the atomistic polymer models to be faithfully described to a good approximation by CG models over a wide temperature range.

Wenjie Xia, Xin Qin, Yao Zhang, Rober Sinko, Sinan Keten

Macromolecules (2018), https://pubs.acs.org/doi/abs/10.1021/acs.macromol.8b02243

Understanding and designing nanoscale interfaces are essential to advancing the thermomechanical performance of polymer nanocomposites reinforced by nanocellulose. In this context, it remains to be understood how disorder introduced on the surfaces of crystals as filler materials during extraction and processing influences interfacial adhesion with glassy polymers. Using atomistic molecular dynamics (MD) simulations, here we systematically explore the interfacial adhesion between nanocellulose and poly(methyl methacrylate) (PMMA) by comparing an ordered cellulose nanocrystal (CNC) interface to a disordered amorphous cellulose (AC) interface. Using a bilayer system that consists of a cellulose underlayer and a polymer upper layer, our simulations show that the AC–PMMA interface can achieve about 50%–60% greater interfacial adhesion energy than that of the CNC–PMMA interface. We uncover that the improved adhesion primarily arises from a larger number of hydrogen bonds formed between the cellulose and polymer chains. Remarkably, the greater adhesion energy and smaller filler–filler surface energy achieved by the AC lead to significantly improved dispersive capability of nanofiller in polymer matrices in comparison with the CNC. Further analyses reveal that while the polymer chain configurations are characteristically different near the two interfaces, where stronger ordering and denser packing of chains are observed near the CNC, their relaxation dynamics are quite similar for the two interfaces. We attribute this observation to the competing effects between the interfacial adhesion and chain packing on polymer relaxation. Our study provides fundamental insights into the interfacial mechanisms of polymer–nanocellulose interfaces at a molecular level and reveals that surface disorder inevitably introduced during production may serve to improve interfacial adhesion energy with the polymer matrix while also enhancing nanofiller dispersion within polymer nanocomposites.

Wenjie Xia, Fernando Vargas-Lara, Sinan Keten, Jack F. Douglas

ACS Nano (2018), https://pubs.acs.org/doi/abs/10.1021/acsnano.8b00524

We explore the structural and dynamic properties of bulk materials composed of graphene nanosheets using coarse-grained molecular dynamics simulations. Remarkably, our results show clear evidence that bulk graphene materials exhibit a fluid-like behavior similar to linear polymer melts at elevated temperatures and that these materials transform into a glassy-like “foam” state at temperatures below the glass-transition temperature (Tg) of these materials. Distinct from an isolated graphene sheet, which exhibits a relatively flat shape with fluctuations, we find that graphene sheets in a melt state structurally adopt more “crumpled” configurations and correspondingly smaller sizes, as normally found for ordinary polymers in the melt. Upon approaching the glass transition, these two-dimensional polymeric materials exhibit a dramatic slowing down of their dynamics that is likewise similar to ordinary linear polymer glass-forming liquids. Bulk graphene materials in their glassy foam state have an exceptionally large free-volume and high thermal stability due to their high Tg (≈ 1600 K) as compared to conventional polymer materials. Our findings show that graphene melts have interesting lubricating and “plastic” flow properties at elevated temperatures, and suggest that graphene foams are highly promising as high surface filtration materials and fire suppression additives for improving the thermal conductivities and mechanical reinforcement of polymer materials.

One or two fully funded PhD positisons (tuition plus reasearch or teaching assistantships) are immediately available in the Computational Dynamics and Materials Laboratory at North Dakota State University (NDSU) during the academic year 2018~2019. Our research aims to advance the design and development of high-performance multifunctional engineering materials (including polymers, composites, granular/soft matter and biomaterials) through computation, multiscale modeling and machine learning.

Qualified candidates with a strong interest and background in computational mechanics and materials, atomistic and coarse-grained modeling, polymer physics, civil & environmental engineering, mechanical engineering or other relevant fields are welcome to apply.

Interested candidates can send a CV with 2~3 contact references to Dr. Wenjie Xia at wenjie.xia@ndsu.edu for further inquiry. The candidates should also fulfill the admission requirement by the Graduate School at NDSU, including GRE and TOFEL (for international applicants only).

Dear Colleagues,

We hope this message finds you well. We are organizing a symposium entitled "Mechanics of Biological and Bioinspired Materials" at The Society of Engineering Science (SES) 53rd Annual Technical Meeting at University of Maryland, College Park, from October 2–5, 2016. The deadline abstract submission is June 15, 2016.

This symposium will focus on multiscale mechanics in biomolecular materials as found in natural and living systems as well as bioinspired engineered materials. The subject materials can be materials with biological origin (bacterial and plant based materials, extracellular matrix, nucleic acids, cell and subcellular components), as well as synthetic macromolecules that aim to mimic biological structure and functionality.

Topics range from experimental approaches, theoretical and computational modeling, as well as new tools for characterization and design of biomolecular materials. The symposium aims to bring together researchers investigating various aspects of multiscale mechanics of materials for application in science and engineering. Relevant topics include and are not limited to:

- Multiscale experiments, simulation and theory with a particular focus on predictive and materials-by-design approaches for the mechanics of biomolecular materials.

- Synthesis, characterization and modeling of hybrid materials that incorporate biological building blocks into engineered materials with mechanical functions.

- Fundamental investigations relevant to self-assembly, folding, micro- and nano-structure prediction in biological applications.

- Nanoscale physics of interfaces in biology and engineered systems.

- Development of experimental tools and computational methods for modeling, imaging, and mechanical characterization of biomolecular materials.

- Bioinspiration approaches with mechanics-based design principles.

We look forward to your submission!

Sinan Keten, Northwestern University

Wenjie Xia, Northwestern University