Session A2: Multiscale Modeling of Materials

Multiscale modeling of salt-responsive polyelectrolyte morphologies

Responsive materials, which react to changes in the surrounding environment through specific property adjustments, will play an increasingly important part in a diverse range of applications. However, the mechanisms of responsiveness is difficult to characterize due to its inherent complexity and multiscale nature: stimuli triggers atomistic-level molecular changes that cause macroscopic response in physical and chemical properties of material. Modeling a responsive material presents a challenge with a large number of unknown variable parameters, such as chemical reactions kinetic or conformational changes as a function of environment, that is hard to measure directly. We have recently developed a method which is parameterized based on a single set of parameters, which allows for large-scale simulations of self-assembling polyelectrolytes materials and their morphological response to the changes in salt concentration. Polyelectrolyte block copolymers, which combine structural features of polyelectrolyte, block copolymers and surfactants, can self-assemble in a variety of nanoaggregates in aqueous environment, such as micelles, vesicles, lamellar mesophases or micellar aggregates. The morphology and size of formed aggregates are determined by the characteristically complex equilibrium of noncovalent forces and depends on variations in ionic strength or/and pH in the aqueous solution. In this talk, I will illustrate our recent progress in prediction of responsive morphologies of polyelectrolytes on the example of the DNA-based materials. Our methodology permit us to construct a morphological diagram of polyelectrolyte block copolymers and evaluate the size of aggregates obtained along with their responsive morphological transitions and scaling relation.   

Isotropic-Nematic Behaviour of Semiflexible Polymers in the Bulk and under Confinement

Semiflexible polymers in solution under good solvent conditions can undergo an isotropic-nematic transition. This transition is somewhat reminiscent of the well-known entropically driven transition of hard rods described by Onsager's theory, but the flexibility of the macromolecules causes specific differences in behaviour, such as anomalous long wavelength fluctuations in the ordered phase, which can be understood by the concept of the deflection length. A brief review of the recent progress in the understanding of these problems is given, summarizing results obtained by large scale Molecular Dynamics simulations and Density Functional Theory. These results include also the interaction of semiflexible polymers with hard walls, and the wall-induced nematic order, which can give rise to capillary nematization in thin film geometry. Various earlier theoretical approaches to these problems are briefly mentioned, and an outlook on the status of experiments is given. It is argued that in many cases of interest it is not possible to describe the scaled densities at the isotropic-nematic transition as functions of the ratio of the contour length and the persistence length alone, but the dependence on the ratio of chain diameter and persistence length also needs to be considered.   

Multiscale Modeling in the Development of Light Weight, High Strength Carbon Nanotube Composites for Space Applications

The implacable arithmetic of the rocket equation tells us that the initial mass of a rocket increases exponentially with the change in velocity (delta-v) required to reach a target destination. As NASA contemplates manned missions to Mars and potential visits to other high delta-v deep space locations, reducing non-propellant mass will be critical in making these missions achievable, affordable, and scientifically productive. Because of the constant demand to increase the mass allocated to scientific payloads and life support equipment, vehicle designers are looking for ways to reduce the mass of both structural and non-structural components. While carbon fiber composites will certainly play an important role, their specific strength and stiffness are not sufficient to meet the mass reduction requirements for the mission designs mentioned above. To address this gap, a multi-institution collaborative team has been working for the past few years to advance the properties of carbon nanotube yarn-based composites, and recently demonstrated CNT composites with uniaxial specific strengths and moduli nearly equivalent to those of state-of-the-art aerospace-grade carbon fiber composites, despite a far from optimal microstructure. A modeling program, closely coupled with the experimental work, is also being established to provide insight into the structure and properties of this hierarchically structured material and to guide experimental efforts focused on improving the mechanical properties by a factor of three relative to current aerospace carbon fiber composites. This presentation will describe the current status of the modeling program, which spans the scales from atomistic molecular dynamics to systems analysis.   

Mesoscopic simulations of mechanical and thermal transport properties of carbon nanotube films

The behavior of carbon nanotube (CNT) materials subjected to mechanical loading exhibits a number of fascinating effects caused by collective interactions among individual CNTs. This work is aimed at revealing the microstructural mechanisms of deformation of CNT films and variation of their thermal transport properties in simulations based on a recently developed mesoscopic model. The mesoscopic model accounts for stretching, bending and buckling of individual nanotubes, van-der Waals interaction and existences of cross-links between CNTs, their intrinsic thermal conductivity, contact heat transfer between nanotubes, and finite conductance of bending buckling kinks. Mechanical properties of CNT films with continuous networks of bundles subjected to compressive and tensile loading are studied in dynamic simulations. The deformation-induced variations of structure are analyzed depending on the CNT length, material density, density of cross-links, and structural parameters of the CNT networks. The scaling laws of the effective thermal conductivity of CNT films with respect to the nanotube length and material density are established and compared with theoretical predictions.