Everything you always wanted to know about SDPD⋆ (⋆but were afraid to ask)

An overview of the smoothed dissipative particle dynamics (SDPD) method is presented in a format that tries to quickly answer questions that often arise among users and newcomers. It is hoped that the status of SDPD is clarified as a mesoscopic particle model and its potentials and limitations are highlighted, as compared with other methods

Water nano-diffusion through the Nafion fuel cell membrane

International audienceNafion, an amphiphilic polymer based on fluorocarbon backbones and acid groups, is probably the most widely used fuel cell membranes. According to its hydration level it self-organizes leading to nano-cavities through which water diffuse. The diffusion of water that controls the protonic transport is then central in the conversion of chemical energy to the electrical one. Recently, a sub-diffusive and heterogeneous dynamics of water were experimentally evidenced paving the way for more efficient fuel cell membranes. Fundamentally, this dynamics which occurs at the nanoscale needs to be microscopically understood. Molecular dynamics simulations are thus performed to locally examine the water dynamics and its relation with the water and Nafion structure. The sub-diffusive regime is numerically corroborated and two sub-diffusive to diffusive transitions are found. The first is time dependent whereas the second is rather water uptake dependent. The sub-diffusive regime is ascribed to the water molecules strongly anchored close to the acid groups. We show that the sub-diffusive to diffusive time transition is the result of the elapsed time before to escape from the attractive interactions of the acid groups. The diffusive regime is recovered far from the acid groups in a homogeneous water phase that is the result of the percolation of the hydrogen bonding network. The progressive transition between sub-diffusive to diffusive regime as a function of the hydration level is due to the respective weight of diffusive dynamics that increases with respect to the sub-diffusive regime given the increase in diffusive pathways as the expense of the localized dynamics. Close to the fluorocarbon backbones the dynamics of water is also sub-diffusive but faster whereas time dynamical transition is not observed. Furthermore we highlight the existence of water corridors based on the hydrogen bonds between molecules forming single file structure in line with a sub-diffusive dynamics. These water corridors are thus possible conducting pathways of protons in a Grotthuss mechanism. Structurally we depict an interdigitated structure where the sulfonate are interleaved. Eventually, at high water uptake, we exhibit the self-organizing of Nafion leading to a phase separation between water and the Nafion membrane. We establish a specific interfacial organization of the hydrophilic sulfonate groups involving a water/Nafion interface

Role of MOF surface defects on the microscopic structure of MOF/polymer interfaces: A computational study of the ZIF-8/PIMs systems

International audienceThe influence of defects at the metal-organic framework (MOF) surface on the microscopic structure of a MOF/polymer composite has been studied by a computational methodology that combines density functional theory calculations with force field-based molecular dynamics simulations. This has been applied to composites formed by ZIF-8 and two different polymers of intrinsic microporosity: PIM-1 and PIM-EA-TB. Analysis of the MOF/polymer interactions, surface coverage, polymer conformation/stiffness and a full characterization of the interfacial voids are provided. We found that, although the nature of the MOF/polymer interactions changes in the presence of defects, the coverage and conformation of the polymer, as well as the morphology of the "interfacial microvoids" remain practically unchanged from a microscopic point of view. These results suggest that there is no microscopic evidence that defective MOF surfaces drastically change the geometry of the MOF/polymer interface and the strength of the physisorption-type interactions in play. (C) 2017 Elsevier Inc. All rights reserved

Microscopic Model of the Metal-Organic Framework/Polymer Interface: A First Step toward Understanding the Compatibility in Mixed Matrix Membranes

International audienceAn innovative computational methodology integrating density functional theory calculations and force field-based molecular dynamics simulations was developed to provide a first microscopic model of the interactions at the metal-organic framework (MOF) surface/polymer interface. This was applied to the case of the composite formed by the polymer of intrinsic microporosity, PIM-1, and the zeolitic imidazolate framework, ZIF-8, as a model system. We found that the structure of the composite at the interface is the result of both the chemical affinity between PIM-1 and ZIF-8 and the rigidity of the polymer. Specifically, there is a preferential interaction between the -CN groups of PIM-1 and the NH terminal functions of the organic linker at the ZIF-8 surface. Additionally, the resulting conformation of the polymer gives rise to interfacial microvoids at the vicinity of the MOF surface. The porosity, rigidity, and density of the interfacial polymer were analyzed and compared to those for the bulk polymer. It was shown that the polymer still feels the impact of the MOF surface even at long distances above 15-20 Å. Further, both the polydispersity of the polymer and the flexibility of the MOF surface were revealed to only slightly affect the properties of the MOF/interface. This work, which delivers a microscopic picture of the MOF surface/polymer interactions at the interface, would lead, in turn, to the understanding of the compatibility in MOF-based mixed-matrix membrane