Ordered structures of nanorods induced by the helixes of semiflexible polymer chains
YF Hua and D Zhang and LX Zhang, ACTA PHYSICA SINICA, 64, 088201 (2015).
Self-assembly of nanoparticles, such as nanospheres, nanorods (NRs), and nanotubes, in polymer systems is one of the most prominent and promising candidates for the development of novel materials with high mechanical, optical, and electrical performances. A most concerned topic on the nanoparticle/polymer composites is the spatial arrangement and distribution of nanoparticles in the nanocomposites, which is controlled by the competition between the entropic packing constraints related to the incompatibility between species with different sizes and geometries, and the enthalpic consequences of a variety of polymer-nanoparticle interactions. The studies on the nonspherical nanoparticles, such as NRs, are of more challenging than on spherical nanoparticles, because both positional and orientational ordering of anisotropic nanoinclusion have an important influence on the morphology of nanocomposition, while those studies are necessary for applications of nanoscopic anisotropic objects in photovoltaic and filled emission devices. When low-volume fractions of NRs are immersed in a binary, phase-separating blend, the rods can self-assemble into needle-like, percolating networks and this special structure can enhance the macroscopic electrical conductivity and mechanical property of the material. When an electric field is applied, the phase separations of ligand-functionalized NRs in a polymer matrix and densely packed hexagonal arrays of NRs are produced. In this paper, by employing the coarse-grained model and molecular dynamics simulation, we explore the structures of nanocomposites in which a small number of NRs bind with semiflexible polymer chain. The morphology of NRs/polymer mixture is greatly affected by the bending energy b of semiflexible polymer and the binding energy D-0 between NRs and semiflexible polymer. If the binding energy D-0 is less than 1.1 k(B) T, the NRs are almost free and a gas-like phase is observed. For a suitably large value of D-0, three completely different morphologies of NRs/polymer mixtures are identified, namely, the side-to-side parallel aggregation of NRs, the end-to-end parallel aggregation of NRs, and the dispersion of NRs. For the flexible polymer chain (i.e., small bending energy b), the side-to-side parallel aggregation structure of NRs and the disordered conformation of adsorbed polymer chain are observed. In general, a typical equilibrium conformation of free flexible polymer chain is random coil, the binding energy between NRs and polymer can lead to the collapse of a random coil for flexible polymer chain, and the NRs aggregate in the manner of the side-to-side parallel to each other because the enthalpy is maximized through sharing the more polymer monomers between neighbor NRs. That is to say, the local aggregation of NRs can be found because the orientational entropy can make the aggregated NRs arrange in the side-to-side parallel manner. In the rigid polymer chain limit (very large bending energy), the rigid polymer chain is stretched and the NRs are well dispersed. As the rigid polymer holds a long persistence length, the NRs can move freely along the stretched polymer chain, and the dispersed conformation of NRs is formed. For the semiflexible polymer chain with a moderate bending energy, the NRs are aggregated in the end-to-end parallel arrangement. Meanwhile, the polymer monomers wrap around those NRs in a well-defined helical structure. The above discussion indicates that the morphologies of NRs are closely related to the conformations of polymer chains. In fact, when a semiflexible polymer chain binds with a large rigid surface, such as nanotube, the helical structure will be formed and it is driven by entropy. The formation of helical structures for a semiflexible polymer chain can induce NRs to form an end-to-end parallel aggregation. The formation of end-to-end parallel arrangement of NR aggregation is driven by the helical structure of semiflexible polymer chain. For the moderate binding energy, the entropy can drive the semiflexible polymer chain to form local helical structure around the NRs. When more NRs are added to the semiflexble polymer chain/NR mixtures, more local helical structures around NRs are formed. Because the movements of NRs binding with the semiflexible chain are nearly free and an end-to-end parallel arrangement of NRs can form more helical structures than the dispersed NRs, the self-assembly of NRs into an end-to-end parallel structure is expected. That is to say, the formation of end-to-end parallel aggregation of NRs is induced by the helix of semiflexible polymers because it can gain more entropies. The self-assembly of a small number of NRs can be well controlled by varying the stiffness of adsorbed polymer chain. This investigation may provide a new pathway to develop "smart" medium to manipulate the aggreagtion behavior of a few NRs and to construct novel materials with high performance.
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