Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India.
Soft Matter. 2014 Mar 21;10(11):1823-30. doi: 10.1039/c3sm52216d.
The aggregation and dispersion of two anisotropic nanoparticles (NPs), cubes and tetrahedrons, in a polymer matrix are studied in this work using coarse-grained molecular dynamics simulations. We present the phase diagrams of NP-polymer composites, depicting microscopically phase-separated, dispersed, and bridged cubes and tetrahedrons in a polymer matrix, which depend on the interaction between the NPs and polymer (εnp), along with the NPs' volume fraction (ϕ). The microscopic phase separation occurs at very low εnp, where NPs self-organize into multidimensional structures, depending on ϕ. In particular, for tetrahedrons, a cross-over from an ordered spherical aggregate to a disordered sheet-like aggregate is observed with increasing ϕ. In the case of cubes, a transition from cubic array → square column → square array (sheet) is identified with increasing ϕ. The clusters of NPs are characterized by their asphericity and principal radii of gyration. The free energy profile for a structured assembly is estimated, which clearly shows that the successful assembly of NPs is energetically favorable at a lower temperature. However, there exists an energy barrier for the successful assembly of all the NPs in the system. At intermediate εnp, a transition from a clustered state to a state comprising dispersed cubes and tetrahedrons in a polymer matrix is observed. At higher εnp, a further transition takes place, where gas-like dispersed NPs form a liquid-like aggregate via polymer layers. Therefore, the findings in this work illustrate that the effective interaction between anisotropic NPs in a polymer matrix is very diverse, which can generate multidimensional structured assemblies, with the disordered clustering, dispersion, and bridging-induced aggregation of NPs.
本文采用粗粒化分子动力学模拟研究了聚合物基体中两种各向异性纳米粒子（NPs）——立方体和四面体的聚集和分散。我们提出了 NP-聚合物复合材料的相图，描绘了在聚合物基质中微观相分离、分散和桥接的立方体和四面体，这取决于 NP 与聚合物（εnp）之间的相互作用，以及 NPs 的体积分数（ϕ）。在非常低的 εnp 下发生微观相分离，其中 NPs 根据 ϕ 自组织成多维结构。特别是对于四面体，随着 ϕ 的增加，从有序的球形聚集体到无序的片状聚集体发生交叉。对于立方体，随着 ϕ 的增加，从立方阵列→方柱→方阵列（片）的转变被识别出来。NPs 簇的特征在于它们的非球形度和转动惯量主半径。估计了结构化组装的自由能曲线，清楚地表明在较低温度下，NP 的成功组装在能量上是有利的。然而，在系统中所有 NP 的成功组装存在能量障碍。在中等 εnp 下，观察到从聚集态到聚合物基质中分散的立方体和四面体的状态的转变。在较高的 εnp 下，进一步发生转变，其中气态分散的 NPs 通过聚合物层形成液态聚集。因此，这项工作的结果表明，聚合物基质中各向异性 NPs 之间的有效相互作用非常多样化，可以产生多维结构组装，包括无序聚集、分散和桥接诱导的 NPs 聚集。
