Despite decades of studies of calcium-silicate-hydrate (C-S-H), the structurally complex binder phase of concrete, the interplay between chemical composition and density remains essentially unexplored. Together these characteristics of C-S-H define and modulate the physical and mechanical properties of this “liquid stone” gel phase. With the recent determination of the calcium/silicon (C/S = 1.7) ratio and the density of the C-S-H particle (2.6 g/cm³) by neutron scattering measurements, there is new urgency to the challenge of explaining these essential properties. Here we propose a molecular model of C-S-H based on a bottom-up atomistic simulation approach that considers only the chemical specificity of the system as the overriding constraint. By allowing for short silica chains distributed as monomers, dimers, and pentamers, this C-S-H archetype of a molecular description of interacting CaO, SiO₂, and H₂O units provides not only realistic values of the C/S ratio and the density computed by grand canonical Monte Carlo simulation of water adsorption at 300 K. The model, with a chemical composition of (CaO)_1.65(SiO₂)(H₂O)_1.75, also predicts other essential structural features and fundamental physical properties amenable to experimental validation, which suggest that the C-S-H gel structure includes both glass-like short-range order and crystalline features of the mineral tobermorite. Additionally, we probe the mechanical stiffness, strength, and hydrolytic shear response of our molecular model, as compared to experimentally measured properties of C-S-H. The latter results illustrate the prospect of treating cement on equal footing with metals and ceramics in the current application of mechanism-based models and multiscale simulations to study inelastic deformation and cracking.

International audienceThe understanding of the basic physical relationships between nano-scale structural variables and the macroscale properties of polymer nanocomposites remains in its infancy. The primary objective of this article is to ascertain the state of the art regarding the understanding and prediction of the macroscale properties of polymers reinforced with nanometer-sized solid inclusions over a wide temperature range. We emphasize that the addition of nanoparticles with large specific surface area to polymer matrices leads to amplification of a number of rather distinct molecular processes resulting from interactions between chains and solid surfaces. This results in a "non-classical" response of these systems to mechanical and electro-optical excitations when measured on the macroscale. For example, nanoparticles are expected to be particularly effective at modifying the intrinsic nano-scale dynamic heterogeneity of polymeric glass-formation and, correspondingly, recent simulations indicate that both the strength of particle interaction with the polymer matrix and the particle concentration can substantially influence the dynamic fragility of polymer glass-formation, a measure of the strength of the temperature dependence of the viscosity or structural relaxation time. Another basic characteristic of nanoparticles in polymer matrices is the tendency for the particles to associate into extended structures that can dominate the rheological, viscoelastic and mechanical properties of the nanocomposite so that thermodynamic factors that effect nanoparticle dispersion can be crucially important. Opportunities to exploit knowledge gained from understanding biomechanics of hierarchical biological protein materials and potential applications in materials design and nanotechnology are among future research challenges. Research on nanocomposites formed from block copolymers and nanoparticles offers huge promise in molecular electronics and photovoltaics. The surface functionalization of nanoparticles by the grafting of polymer brushes is expected to play important role in the designing of novel organic/inorganic nanocomposite materials. The formation of bulk heterojunctions at the nanometer scale leads to efficient dissociation of the charge pairs generated under sunlight. Based on the presentations and discussion, we make recommendations for future work in this area by the physics, chemistry, and engineering communities

It is shown that by controlling the relaxation of graphene adhered on a biaxially pre-stretched polymer substrate, graphene films can be reversibly crumpled and unfolded to form tailored hierarchical structures with tunable wettability and transmittance, and that the crumpled graphene–polymer laminates can be used as actuators. Crumpled graphene films are widely used, for instance in electronics1, energy storage2,3, composites4,5 and biomedicine6. Although it is known that the degree of crumpling affects graphene’s properties and the performance of graphene-based devices and materials3,5,7, the controlled folding and unfolding of crumpled graphene films has not been demonstrated. Here we report an approach to reversibly control the crumpling and unfolding of large-area graphene sheets. We show with experiments, atomistic simulations and theory that, by harnessing the mechanical instabilities of graphene adhered on a biaxially pre-stretched polymer substrate and by controlling the relaxation of the pre-strains in a particular order, graphene films can be crumpled into tailored self-organized hierarchical structures that mimic superhydrophobic leaves. The approach enables us to fabricate large-area conductive coatings and electrodes showing superhydrophobicity, high transparency, and tunable wettability and transmittance. We also demonstrate that crumpled graphene–polymer laminates can be used as artificial-muscle actuators.