Designing Novel Microstructured Materials via Molecular Simulation

CTS-0553719；康奈尔大学；F.埃斯科贝通过分子模拟设计新型纳米和微结构材料智力价值：这个相关项目的目标是开发和应用新的分子模拟方法来研究含有非典型几何结构的刚性胶体纳米粒子的体系的液晶相行为和流变学。这项研究的动机是通过实验生产几乎任何可以想象到的形状的纳米和微粒子的能力不断增强，这一目标位于纳米技术的范围内，该技术寻求实现对纳米级物体的位置组装的更大控制；在这种情况下，通过确定可以导致新型自组装结构的有前途的构建块来实现。在这种情况下，颗粒形状的互补性起到了“熵键”的作用，有助于在规则的模式中定向和定位颗粒(即使在没有化学选择性的情况下)。要研究的颗粒形状包括四方平行四面体或“长方体”，可能导致立方和其他新的双轴中间相。将考虑这些颗粒类型的选定的二元和多分散混合物，包括大长方体和小长方体的混合物(这可能导致多个液晶相共存)，以及互补形状的颗粒的混合物(可能导致微组装相)。所使用的模型是胶体颗粒(有机聚集体或表面功能化的无机颗粒)的粗粒表示，其有效的颗粒间相互作用可以通过溶剂介质的组成来调节。并对立方体悬浮液的稳态剪切流变性进行了数值模拟。可以预见，形成的中间相可能表现出不寻常的剪切响应，包括强烈的流动方向性(例如，对剪切方向的弱依赖性)和屈服应力行为。方法学上的发展是：(I)扩展系综方法以模拟纯组分和不对称二元体系的中间相转变，(Ii)开发一个通用框架来绘制任何适当的有序参数上的自由能图景，这些参数可能是谈判中间相之间的大势垒所必需的，以及(Iii)采用耗散粒子动力学来模拟新型胶体中间相的剪切流变学。因此，该项目可以被视为具有双重范围。主要目的是阐明模型刚性聚合物和胶体体系的行为，这些体系在纳米自组装技术中具有潜在的应用；例如，通过研究熵力和剪切流对有序中间相的开始和持续时间的影响。第二个目标是建立具有潜在广泛应用的新的数值统计力学技术。广泛的影响：这个项目是对合作者的实验工作的补充，他们将试图实现预测的新相，并测试它们的机械、光学和流变性。从长远来看，这一结果可能会影响陶瓷、塑料和半导体行业，因为它有助于拓宽现有方法，以开发具有高颗粒负载量的坚固纳米复合材料、具有规则拓扑和孔径大小的筛子、用于光子材料光控制的胶体介晶以及对应力方向性敏感的传感器和润滑剂。模拟方法的进步也应该有助于工业材料建模者通过预测和利用中尺度现象和层次结构来努力改善产品性能。参与这个项目的研究生和本科生将对胶体的性质有一个很好的了解，同时在分子和介观建模方面获得重要的专业知识。科学成果将通过专业会议和康奈尔材料研究中心(CCMR)组织的工业推广计划进行传播。这项研究的结果将至少用于三门课程：一门关于分子模拟的新课程，一门最近开发的以问题为基础的核心研究生课程，以及高级热力学核心课程。教育外展工作将包括指导本科生研究人员和参与CCMR教育方案办公室的工作。

ABSTRACT CTS- 0553719; Cornell University; F. EscobedoDesigning Novel Nano- and Micro-structured Materials via Molecular SimulationIntellectual Merit: The goal of this related project is to develop and apply novel molecular simulation methods to study the liquid-crystalline phase behavior and rheology of systems containing rigid colloidal nano-particles of atypical geometry. The motivation for this research is the growing ability to experimentally produce nano- and micro-particles of almost any imaginable shape, This goal lies within the scope of nanotechnology that seeks to achieve greater control of positional assembly of nanoscale objects; in this case, by identifying promising building blocks that can lead to novel self-assembled structures. In this context, particle-shape complementarity plays the role of an "entropic bonding" that helps orient and position particles in regular patterns (even in the absence of chemical selectivity). The particle shapes to be investigated include tetragonal parallelepipeds or "cuboids" that may lead to cubatic and other novel biaxial mesophases. Selected binary and polydisperse mixtures of these particle types will be considered, including mixtures of large and small cuboids (which may lead to the coexistence of multiple liquid-crystal phases), and mixtures of complementarily shaped particles (which may lead to micro-assembled phases). The models used are coarse-grained representations of colloidal particles (organic aggregates or surface-functionalized inorganic particles) whose effective inter-particle interactions can be tuned by the composition of the solvent media. The steady shear-flow rheology of cuboidal suspensions will also be simulated. It is envisioned that the mesophases formed may exhibit unusual shear responses including strong flow directionality (e.g., a weak dependence on shear direction) and yield stress behavior. The methodological developments to be pursued are: (i) Extension of expanded ensemble methods to simulate mesophase transitions in pure components and asymmetric binary systems, (ii) development of a general framework to map out free-energy landscapes over any suitable order-parameters that may be needed to negotiate large barriers between mesophases, and (iii) adaptation of dissipative particle dynamics to simulate the shear rheology of novel colloidal mesophases. The project can thus be seen as having a dual scope. The primary goal is to elucidate the behavior of model rigid polymer and colloidal systems that have potential uses in the nanotechnology of self-assembly; e.g., by studying the effect of entropic forces and shear flow on the onset and endurance of ordered mesophases. The secondary goal is to formulate novel numerical statistical mechanics techniques that have potentially widespread applications.Broader Impacts: This project is complementary to experimental efforts by collaborators who will try to realize the predicted novel phases and test their mechanical, optical, and rheological properties. In the long term, the results could impact the ceramic, plastics, and semiconductor industries by helping broaden the approaches available to develop strong nanocomposites with high particle loadings, sieves with regular topology and pore-sizes, colloid-based mesocrystals for light control in photonic materials, and sensors and lubricants sensitive to stress directionality. The advances in simulation methods should also help industrial materials modelers in their efforts to improve product properties by predicting and exploiting meso-scale phenomena and hierarchical structure. The graduate and undergraduate students involved with this project will gain a good understanding of the properties of colloids while acquiring a significant expertise on molecular and mesoscopic modeling. The scientific results will be disseminated through professional meetings and an industrial outreach program organized by the Cornell Center for Material Research (CCMR). Results of this investigation will be used in at least three courses: a new course on molecular simulations, a recently developed problem-based core graduate course, and the advanced thermodynamics core course. Educational outreach efforts will include mentoring of undergraduate researchers and participation in the work of the Educational Programs Office of CCMR.