Designing Novel Microstructured Materials via Molecular Simulation

摘要cts - 0553719；康奈尔大学;F. escobedo通过分子模拟设计新型纳米和微结构材料智力优势：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形状的刚性胶体纳米颗粒的系统的液晶相行为和流变性。这项研究的动机是实验生产几乎任何可以想象的形状的纳米和微粒子的能力不断增长，这一目标属于纳米技术的范围，旨在实现对纳米级物体的位置组装的更好控制；在这种情况下，通过确定有希望的构建块，可以导致新的自组装结构。在这种情况下，粒子形状的互补性起着“熵键”的作用，帮助粒子以规则的模式定向和定位（即使在没有化学选择性的情况下）。要研究的粒子形状包括四方平行六面体或“长方体”，它们可能导致立方体和其他新的双轴中间相。这些粒子类型的选择二元和多分散混合物将被考虑，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的共存），以及互补形状粒子的混合物（这可能导致微组装相）。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）的粗粒度表示，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。还将模拟立方体悬浮液的稳态剪切流动流变。可以设想，形成的中间相可能表现出不寻常的剪切响应，包括强流动方向性（例如，对剪切方向的弱依赖性）和屈服应力行为。要追求的方法发展是：(i)扩展集成方法的扩展，以模拟纯组分和非对称二元系统中的中间相转变；（ii）开发一个总体框架，以在任何合适的顺序参数上绘制自由能景观，这些参数可能需要在中间相之间协商大障碍；（iii）耗散粒子动力学的适应，以模拟新型胶体中间相的剪切流变。因此，该项目可以被视为具有双重范围。主要目标是阐明模型刚性聚合物和胶体系统的行为，这些系统在自组装纳米技术中具有潜在的用途；例如，通过研究熵力和剪切流对有序中间相的开始和持续的影响。第二个目标是制定具有潜在广泛应用的新型数值统计力学技术。更广泛的影响：这个项目是对合作者的实验努力的补充，他们将试图实现预测的新相，并测试它们的机械、光学和流变特性。从长远来看，该研究结果可能会影响陶瓷、塑料和半导体行业，帮助拓宽高颗粒负载的强纳米复合材料、具有规则拓扑和孔径的筛子、用于光子材料光控制的胶体基介晶、以及对应力方向敏感的传感器和润滑剂的开发方法。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改善产品性能。参与该项目的研究生和本科生将获得对胶体性质的良好理解，同时获得分子和介观建模方面的重要专业知识。科学成果将通过专业会议和由康奈尔材料研究中心（CCMR）组织的工业外展计划进行传播。本研究结果将至少用于三门课程：一门新开设的分子模拟课程，一门新开设的基于问题的研究生核心课程，以及一门高级热力学核心课程。教育推广工作将包括指导本科生研究人员和参与CCMR教育项目办公室的工作。

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.