@article {589, title = {Role of Interfaces in Elasticity and Failure of Clay{\textendash}Organic Nanocomposites: Toughening upon Interface Weakening?}, journal = {Langmuir}, volume = {33}, year = {2017}, month = {Oct-24-2017}, pages = {11457 - 11466}, abstract = {

Synthetic organic-inorganic composites constitute a new class of engineering materials finding applications in an increasing range of fields. The interface between the constituting phases plays a pivotal role in the enhancement of mechanical properties. In exfoliated clay-organic nanocomposites, individual, high aspect ratio clay sheets are dispersed in the organic matrix providing large interfaces and hence efficient stress transfer. In this study, we aim at elucidating molecular-scale reinforcing mechanisms in a series of model clay-organic composite systems by means of reactive molecular simulations. In our models, two possible locations of failure initiation are present: one is the interlayer space of the clay platelet, and the other one is the clay-organic interface. We systematically modify the cohesiveness of the interface and assess how the failure mechanism changes when the different model composites are subjected to a tensile test. Besides a change in the failure mechanism, an increase in the released energy at the interface (meaning an increased overall toughness) are observed upon weakening the interface by bond removal. We propose a theoretical analysis of these results by considering a cohesive law that captures the effect of the interface on the composite mechanics. We suggest an atomistic interpretation of this cohesive law, in particular, how it relates to the degree of bonding at the interface. In a broader perspective, this work sheds light on the importance of the orthogonal behavior of interfaces to nanocomposites.

https://pubs-acs-org.libproxy.mit.edu/appl/literatum/publisher/achs/journals/content/langd5/2017/langd5.2017.33.issue-42/acs.langmuir.7b01071/20171025/images/medium/la-2017-01071k_0008.gif

}, issn = {0743-7463}, doi = {10.1021/acs.langmuir.7b01071}, url = {http://pubs.acs.org/doi/10.1021/acs.langmuir.7b01071}, author = {Gy{\"o}rgy Hantal and Brochard, Laurent and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Benoit A. Coasne} } @article {136, title = {Capturing material toughness by molecular simulation: accounting for large yielding effects and limits}, journal = {International Journal of Fracture}, volume = {194}, year = {2015}, month = {Aug-2015}, pages = {149 - 167}, abstract = {

The inherent computational cost of molecular simulations limits their use to the study of nanometric systems with potentially strong size effects. In the case of fracture mechanics, size effects due to yielding at the crack tip can affect strongly the mechanical response of small systems. In this paper we consider two examples: a silica crystal for which yielding is limited to a few atoms at the crack tip, and a nanoporous polymer for which the process zone is about one order of magnitude larger. We perform molecular simulations of fracture of those materials and investigate in particular the system and crack size effects. The simulated systems are periodic with an initial crack. Quasi-static loading is achieved by increasing the system size in the direction orthogonal to the crack while maintaining a constant temperature. As expected, the behaviors of the two materials are significantly different. We show that the behavior of the silica crystal is reasonably well described by the classical framework of linear elastic fracture mechanics (LEFM). Therefore, one can easily upscale engineering fracture properties from molecular simulation results. In contrast, LEFM fails capturing the behavior of the polymer and we propose an alternative analysis based on cohesive crack zone models. We show that with a linear decreasing cohesive law, this alternative approach captures well the behavior of the polymer. Using this cohesive law, one can anticipate the mechanical behavior at larger scale and assess engineering fracture properties. Thus, despite the large yielding of the polymer at the scale of the molecular simulation, the cohesive zone analysis offers a proper upscaling methodology.

}, issn = {0376-9429}, doi = {10.1007/s10704-015-0045-y}, author = {Brochard, Laurent and Gy{\"o}rgy Hantal and Hadrien Laubie and Franz-Josef Ulm and Roland Jean-Marc Pellenq and Benoit A. Coasne} } @article {84, title = {Atomic-scale modelling of elastic and failure properties of clays}, journal = {Molecular Physics}, volume = {112}, year = {2014}, month = {May-19-2014}, pages = {1294-1305}, type = {Article}, abstract = {

The elastic and failure properties of a typical clay, illite, are investigated using molecular simulation. We employ a reactive (ReaxFF) and a non-reactive (ClayFF) force field to assess the elastic properties of the clay. As far as failure is concerned, ReaxFF was used throughout the study; however, some calculations were also performed with ClayFF. A crack parallel to the clay layers is found to have low fracture resistance when submitted to a tensile loading perpendicular to the crack. The mechanism of both yield and fracture failures is decohesion in the interlayer space. In contrast, under shear loading, the nanoscale failure mechanism is a stick-slip between clay layers. No fracture propagation is observed as the clay layers slide on top of each other. The low fracture resistance in mode I and the stick-slip failure in mode II are both the consequence of the lack of chemical bonds between clay layers where the cohesion is provided by non-covalent interactions. This work, which provides a description of the failure of clays at the microscopic scale, is the first step towards describing the failure of clays at a larger scale where the polycrystalline distribution of clay grains must be taken into account.

}, keywords = {clay, elastic properties, fracture, reactive molecular simulation}, issn = {0026-8976}, doi = {10.1080/00268976.2014.897393}, author = {Gy{\"o}rgy Hantal and Brochard, Laurent and Hadrien Laubie and Ebrahimi, Davoud and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Benoit A. Coasne} } @article {32, title = {Surface Chemistry and Atomic-Scale Reconstruction of Kerogen-Silica Composites}, journal = {The Journal of Physical Chemistry C}, volume = {118}, year = {2014}, month = {Feb-6-2014}, pages = {2429-2438}, abstract = {
Abstract Image

Interest in gas shale, a novel source rock of natural gas, has increased tremendously in recent years. Better understanding of the kerogen\–rock interaction is of crucial importance for efficient gas extraction and, hence, asset management. In this study, we explore the possible chemical bonds between kerogen and silica, one of the most predominant mineral constituents of gas shale, by means of quantum chemistry. Energetically favorable bond formation reactions are found between alcoholic hydroxyl, carboxylate, and aldehyde groups, as well as aliphatic double bonds of kerogen and the silica surface. The performance of a reactive force field was also assessed in a representative set of chemical reactions and found to be satisfactory. The potential impact of bond formation reactions between the two phases on the actual kerogen\–silica interface is discussed as a function of the kerogen type, maturity, and density. Finally, a methodology aiming to reconstruct realistic kerogen\–silica interfaces is presented.

}, doi = {10.1021/jp406329n}, author = {Gy{\"o}rgy Hantal and Brochard, Laurent and Dias Soeiro Cordeiro, M Natalia and Franz-Josef Ulm and Roland Jean-Marc Pellenq} } @article {35, title = {Adsorption of Aromatic Hydrocarbon Molecules at the Surface of Ice, As Seen by Grand Canonical Monte Carlo Simulation}, journal = {The Journal of Physical Chemistry C}, volume = {117}, year = {2013}, month = {Apr-2013}, pages = {6719{\textendash}6729}, abstract = {
Abstract Image

The adsorption of four aromatic hydrocarbon compounds, benzene, naphthalene, anthracene, and phenanthrene, at the surface of Ih ice is investigated by grand canonical Monte Carlo (GCMC) computer simulation under tropospheric conditions at 200 K. By systematic variation of the value of adsorbate chemical potential in the simulations, the adsorption isotherms are determined. It is found that adsorption follows the Langmuir mechanism only up to a rather low relative pressure value in every case. In this range specific surface sites, called \α sites, to which adsorbate molecules can be bound particularly strongly in specific orientation, are occupied. In these \α sites, presumably the dangling OH bonds of the ice surface form O\–H....\π-type hydrogen bonds with the delocalized \π electrons of the adsorbed aromatic molecule lying parallel with the ice surface. Once these \α sites are saturated, lateral interactions become increasingly important, leading to large fluctuations of the lateral density of the adsorption layer and an increasing deviation of the adsorption isotherm from the Langmuir shape. The adsorption layer is found to be strictly monomolecular and even unsaturated in every case, as condensation well precedes the saturation of this monolayer for all four aromatic adsorbates considered in this study.

}, doi = {10.1021/jp401532x}, author = {M{\'e}sz{\'a}r, Zsuzsanna E and Gy{\"o}rgy Hantal and Picaud, Sylvain and Jedlovszky, P{\'a}l} }