Efficiency of various lattices from hard ball to soft ball: theoretical study of thermodynamic properties of dendrimer liquid crystal from atomistic simulation.
Efficiency of various lattices from hard ball to soft ball: theoretical study of thermodynamic properties of dendrimer liquid crystal from atomistic simulation.
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DOI：
10.1021/ja038617e
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发表时间：
2004-02
影响因子：
15
通讯作者：
Youyong Li;Shiang‐Tai Lin;W. Goddard
中科院分区：
文献类型：
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作者：
Youyong Li;Shiang‐Tai Lin;W. Goddard
自组装超分子有机液晶结构在分子电子学、光子学和多孔纳米材料等领域具有潜在的应用前景。这些结构大多数是由软球形超分子聚集而成的，它们具有软电晕，在堆积过程中相互重叠。我们的主要重点是在原子水平上通过分子动力学模拟来研究可能的堆积机制。我们考虑由软树枝状聚合物球填充的各种晶格的相对稳定性，所述软树枝状聚合物球首先由Percec等人合成并表征（J. Am. 1997，119，1539）。形成软树枝状聚合物球的树枝状分子具有从锥尖到边缘的具有C（12）烷烃“毛发”的硬芳香区域的特征。树枝状分子组装成球后，球的核心具有坚硬的芳香基团，而表面则覆盖着C（12）烷烃的“毛发”。在我们的研究中，我们提出了三种方法来组织球上的头发，光滑/Valentino球，粘性/爱因斯坦球和不对称/朋克球，这导致三种不同的包装机制，Slippery，粘性和各向异性，分别。我们进行了一系列的分子动力学（MD）研究三个合理的晶体结构（A15，FCC和BCC）作为密度的函数和分析的MD的基础上的振动态密度（DoS）方法提取的焓，熵，和这些系统的自由能。我们发现各向异性堆积的A15晶格比FCC，BCC晶格更有利。我们预测的最佳结构的X射线强度与实验非常一致。本文提出的“各向异性球堆积”在熵有利的“盘堆积”和熵有利的“各向同性球堆积”之间起着中间作用，它解释了不同温度下的相变。不同密度下的各种晶格的自由能基本上是相同的，这表明优选的晶格在填充过程中没有确定。焓和熵都随着密度的增加而减小。自由能随体积的变化显示两个稳定相：凝聚相和孤立胶束相。软树枝状聚合物球之间的相互作用被发现是晶格依赖时，描述的两体势，因为软球自我调整其形状和在不同的晶格中的相互作用。自由能势的形状类似于“方肩势”。本文从几何角度提出了一个解释理想软球在各种晶格中的填充效率的模型。
Self-assembled supramolecular organic liquid crystal structures at nanoscale have potential applications in molecular electronics, photonics, and porous nanomaterials. Most of these structures are formed by aggregation of soft spherical supramolecules, which have soft coronas and overlap each other in the packing process. Our main focus here is to study the possible packing mechanisms via molecular dynamics simulations at the atomistic level. We consider the relative stability of various lattices packed by the soft dendrimer balls, first synthesized and characterized by Percec et al. (J. Am. Chem. Soc. 1997, 119, 1539) with different packing methods. The dendrons, which form the soft dendrimer balls, have the character of a hard aromatic region from the point of the cone to the edge with C(12) alkane "hair". After the dendrons pack into a sphere, the core of the sphere has the hard aromatic groups, while the surface is covered with the C(12) alkane "hair". In our studies, we propose three ways to organize the hair on the balls, Smooth/Valentino balls, Sticky/Einstein balls, and Asymmetric/Punk balls, which lead to three different packing mechanisms, Slippery, Sticky, and Anisotropic, respectively. We carry out a series of molecular dynamics (MD) studies on three plausible crystal structures (A15, FCC, and BCC) as a function of density and analyze the MD based on the vibrational density of state (DoS) method to extract the enthalpy, entropy, and free energies of these systems. We find that anisotropic packed A15 is favored over FCC, BCC lattices. Our predicted X-ray intensities of the best structures are in excellent agreement with experiment. "Anisotropic ball packing" proposed here plays an intermediate role between the enthalpy-favored "disk packing" and entropy-favored "isotropic ball packing", which explains the phase transitions at different temperatures. Free energies of various lattices at different densities are essentially the same, indicating that the preferred lattice is not determined during the packing process. Both enthalpy and entropy decrease as the density increases. Free energy change with volume shows two stable phases: the condensed phase and the isolated micelle phase. The interactions between the soft dendrimer balls are found to be lattice dependent when described by a two-body potential because the soft ball self-adjusts its shape and interaction in different lattices. The shape of the free energy potential is similar to that of the "square shoulder potential". A model explaining the packing efficiency of ideal soft balls in various lattices is proposed in terms of geometrical consideration.