@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 {56, title = {Poromechanics of microporous media}, journal = {Journal of the Mechanics and Physics of Solids}, volume = {60}, year = {2012}, month = {Apr-2012}, pages = {606{\textendash}622}, abstract = {

Microporous media, i.e., porous media made of pores with a nanometer size, are important for a variety of applications, for instance for sequestration of carbon dioxide in coal, or for storage of hydrogen in metal-organic frameworks. In a pore of nanometer size, fluid molecules are not in their bulk state anymore since they interact with the atoms of the solid: they are said to be in an adsorbed state. For such microporous media, conventional poromechanics breaks down.

In this work we derive poroelastic constitutive equations which are valid for a generic porous medium, i.e., even for a porous medium with pores of nanometer size. The complete determination of the poromechanical behavior of a microporous medium requires knowing how the amount of fluid adsorbed depends on both the fluid bulk pressure and the strain of the medium. The derived constitutive equations are validated with the help of molecular simulations on one-dimensional microporous media. Even when a microporous medium behaves linearly in the absence of any fluid (i.e., its bulk modulus does not depend on strain), we show that fluid adsorption can induce non-linear behavior (i.e., its drained bulk modulus can then depend significantly on strain). We also show that adsorption can lead to an apparent Biot coefficient of the microporous medium greater than unity or smaller than zero.

The poromechanical response of a microporous medium to adsorption significantly depends on the pore size distribution. Indeed, the commensurability (i.e., the ratio of the size of the pores to that of the fluid molecules) proves to play a major role. For a one-dimensional model of micropores with a variety of pore sizes, molecular simulations show that the amount of adsorbed fluid depends linearly on the strain of the medium. We derive linearized constitutive equations which are valid when such a linear dependence of the adsorbed amount of fluid on the strain is observed.

As an application, the case of methane and coal is considered. Molecular simulations of an adsorption of methane on a microporous realistic model for coal are performed with a flexible solid skeleton. The applicability of the set of linearized constitutive equations to this case is discussed and the results are shown to be consistent with swelling data measured during a classical adsorption experiment.

Example of molecular configuration. The fluid molecules can enter or leave the ...

Amount of fluid molecules adsorbed per unit length of the undeformed chain, in ...

Apparent tangent drained bulk modulus Kt of the crystalline chain.

}, doi = {10.1016/j.jmps.2012.01.001}, author = {Brochard, Laurent and Matthieu Vandamme and Roland Jean-Marc Pellenq} } @article {59, title = {Adsorption-induced deformation of microporous materials: coal swelling induced by co2{\textendash}ch4 competitive adsorption}, journal = {Langmuir}, volume = {28}, year = {2011}, month = {Dec-20-2011}, pages = {2659{\textendash}2670}, abstract = {
Abstract Image

Carbon dioxide injection in coal seams is known to improve the methane production of the coal seam, while ensuring a safe and long-term carbon sequestration. This improvement is due to the preferential adsorption of CO2 in coal with respect to CH4: an injection of CO2 thus results in a desorption of CH4. However, this preferential adsorption is also known to cause a differential swelling of coal, which results in a significant decrease in the reservoir permeability during the injection process. Recent studies have shown that adsorption in coal micropores (few angstr\öms in size) is the main cause of the swelling. In this work, we focus on the competitive adsorption behavior of CO2 and CH4 in micropores. We perform molecular simulations of adsorption with a realistic atomistic model for coal. The competitive adsorption is studied at various temperatures and pressures representative of those in geological reservoirs. With the help of a poromechanical model, we then quantify the subsequent differential swelling induced by the computed adsorption behaviors. The differential swelling is almost insensitive to the geological temperatures and pressures considered here and is proportional to the CO2 mole fraction in the coal.

}, doi = {10.1021/la204072d}, author = {Brochard, Laurent and Matthieu Vandamme and Roland Jean-Marc Pellenq and Fen-Chong, Teddy} }