Multiscale Materials Modeling at Rice University
Mechanics of Hybrid Organic-Inorganic Materials
By combining platelet-like ceramic building blocks and organic matrices, nature creates hybrid materials such as bone, teeth and mollusk shells that have outstanding balance of stiffness, strength and flaw-tolerance. This has inspired fabrication of several advanced human-made polymer-matrix composites with inorganic reinforcing materials such as cement, clays, glass, graphite, SiC, and mica. In all these natural and engineered composites, the issue of load transfer is a delicate, but critical problem that has a significant impact on the overall mechanical performance of composites such as strength, toughness and ductility. The origins of load transfer go back to the inherent, complex interactions at the subatomic level, which are not well understood. The complexities are even more pronounced across the interface of hybrid organic inorganic materials due to the mismatch in material properties and geometric characteristics.
To address this issue, we focus on hybrid polymer cementitious materials and map out the inherent, non-intuitive characteristics of interfacial bonding and shear load transfer in these hybrid materials. Due to filling effects of polymers in the porous regions of cement-hydrate and chemical bonding of polymers to the cement hydrate backbone, hybrid polymer cementitious materials are a promising solution to improve mechanical performance and durability in concrete systems. A prime example of synthetic polymer-cementitious composite is the introduction of polymers in Calcium-Silicate-Hydrate (C-S-H) gel, where the latter is the main product of cement hydration. With Zeolitic-type pores and interlaminar distances of a few angstroms, C-S-H provides an excellent system to host polymers in confined spaces and study their hybrid mechanical behavior.
Specifically, we will focus on load transfer in Polyvinyl alcohol (PVA)- C-S-H nanocomposites, and employ a combined ab-initio study, molecular dynamics modeling and rigorous analytical derivations. Together, these techniques provide a holistic understanding on the synergistic behavior of multiple length scales and material characteristics to optimize the mechanical responses of hybrid composites.
We have derived a generalized analytical framework that provides important physical intuitions and quantitative mechanistic information on interfacial shear load transfer for both natural and synthetic materials. Additionally, our computational results i) provides an “atomistic lens” on interfacial deformation-based mechanisms, and ii) shows that incorporation of polymers with more intra-molecular H-bonding increases the electronegativity of atoms across the interface, thus resulting in larger interfacial binding energy and adhesion. The latter is particularly important to improve the mechanical integrity of synthetic PVA – C-S-H composites, hence mitigating the intrusion of deleterious ions and improving durability.
Our results demonstrate a key fundamental step towards understanding the interfacial interactions between dissimilar building blocks, and open a new phase space to modulate such hybrid materials. Specifically, it may lay the foundation for the development of new unit processes that govern the mechanics of hybrid materials. A rich set of models for the deformation and fracture of ceramics and metallic systems have been developed over the past decades. However, similar advances for hybrid materials, especially at the interface of the constituent elements which is often the roadblock for improving the material properties, have thus far remained elusive. In analogy to dislocation nucleation and propagation in metals or shear band in metallic glasses, the breakage and formation of interfacial bonds in hybrid materials represents a basic unit process, which critically impact mechanical performance and failure.
1. Shahsavari R., Buehler M.J., Pellenq R., Ulm F.-J., First-principles study on elastic constants and interlayer interaction of complex hydrated oxides: the case study of tobermorite and jennite, Journal of American Ceramic Society, 92 (10) 2323-2330, (2009).
2. Pellenq R., Kushima A., Shahsavari R., Van Vliet K., Buehler M., Yip S., Ulm F.-J., A realistic molecular model of cement hydrates, Proceedings of National Academy of Sciences, 09021180106, 1-6 (2009).
3. Shahsavari R., Pellenq R., Ulm F.-J., Empirical force fields for complex caclio-silicate layered materials, Physical Chemistry Chemical Physics, 13, 1002-101, (2011).
4. Sakhavand N, Shahsavari R, Mechanistic investigation and origins of shear load transfer between dissimilar building blocks with a brittle interface, In Review, (2013)
5. M. A. Rafiee, T. N. Narayanan, D. P. Hashim, N. Sakhavand, R. Shahsavari, R. Vajtai, P. M. Ajayan, Hexagonal Boron Nitride and Graphite Oxide Reinforced Multifunctional Porous Cement Composites, Advanced Functional Materials, In Review, (2013).