Efficiency of various lattices from hard ball to soft ball: theoretical study of thermodynamic properties of dendrimer liquid crystal from atomistic simulation.
ABSTRACT 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.
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ABSTRACT: The polyamidoamide (PAMAM) class of dendrimers was one of the first dendrimers synthesized by Tomalia and co-workers at Dow. Since its discovery the PAMAMs have stimulated many discussions on the structure and dynamics of such hyperbranched polymers. Many questions remain open because the huge conformation disorder combined with very similar local symmetries have made it difficult to characterize experimentally at the atomistic level the structure and dynamics of PAMAM dendrimers. The higher generation dendrimers have also been difficult to characterize computationally because of the large size (294,852 atoms for generation 11) and the huge number of conformations. To help provide a practical means of atomistic computational studies, we have developed an atomistically informed coarse-grained description for the PAMAM dendrimer. We find that a two-bead per monomer representation retains the accuracy of atomistic simulations for predicting size and conformational complexity, while reducing the degrees of freedom by tenfold. This mesoscale description has allowed us to study the structural properties of PAMAM dendrimer up to generation 11 for time scale of up to several nanoseconds. The gross properties such as the radius of gyration compare very well with those from full atomistic simulation and with available small angle x-ray experiment and small angle neutron scattering data. The radial monomer density shows very similar behavior with those obtained from the fully atomistic simulation. Our approach to deriving the coarse-grain model is general and straightforward to apply to other classes of dendrimers.The Journal of chemical physics 05/2009; 130(14):144902. · 3.09 Impact Factor
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ABSTRACT: This review introduces the extensive key factors of the design of mesostructured monoliths with two- and three-dimensional (2D and 3D) geometries and large particle morphologies. Simple strategy in terms of fabrication time (within 10min) and composition phase domains was achieved by using an instant direct-templating method of lyotropic and microemulsion phases of a variety of triblock copolymer (EOm-POn-EOm) surfactants, as we recently reported in  and . The synthetic strategy provides realistic control over a wide range of mesophase geometries, yet maintains the long-range structural ordering, and thus improved the simplicity, significant periodicity, and high uniformity of the resultant silica monoliths. Cubic mesophases, in particular, exhibit a wide variety of mesostructured geometries when the block copolymers were used as a structure-directing agent under acidic synthesis conditions. For example, triblock copolymer (P123, EO20PO70EO20) was used to fabricate 2D hexagonal (P6mm). Our synthesis protocol revealed that ordered 3D cubic (Fd3m), (Im3m), and (Ia3d) silica monoliths were also fabricated in large domain sizes by templating P123 copolymers. In general, key factors such as the degree of solubilization of the hydrocarbons (co-solvent), the copolymer concentrations used in the phase domains, and the copolymer molecular nature, such as EO/PO ratio, significantly affect the formation of mesostructured phases and their extended long-range ordering in the final replicas of the silica monolith frameworks. The remarkable structural findings of 2D and 3D frameworks, transparent monoliths, and micropores combined with large cage- and cylindrical-like mesopores might show their desirability in many applications.Journal of Porous Materials 07/2008; 15(4):369-387. · 1.35 Impact Factor
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ABSTRACT: The development of the nanoscale structures and their integration into components, systems, and natural architectures (such as monoliths), and large-scale devices, is one of the most promising areas in the emerging field of nanotechnology. We believe that it is time to write a review that focused on the rapid synthesis and the functional properties of HOM mesoporous monoliths. Thus, we here introduce comprehensive and up-to-date reports on the instant synthesis (within minutes) of a range of mesoporous silica monoliths (HOM-type, High-Order-Monolith) by means of a direct-templating method of lyotropic and microemulsion liquid crystalline phases. A number of nonionic n-alkyl-oligo(ethylene oxide), namely, Brij-type (C x EO y ), and Triton- and Tween-type and cationic alkyl trimethylammonium bromide or chloride (C n TMA-B or -C, where n=12, 14, 16 and 18) surfactants were used as soft templates. A variety of 1D, 2D and 3D mesostructure geometries were successfully fabricated by using this simple, fast and yet reproducible design strategy. This is the first and detailed review of using rapid synthesis to fabricate disordered and ordered silica/surfactant mesophases with supermicro- and meso-pore engineering systems. In this review, we also addressed the prominent factors affected the formation of the large-scale ordered and worm-like structures (HOM): (1) the phase composition of domains, (2) the extent of solubilization of hydrocarbons, and (3) the nature of surfactant molecules (corona/core features). Significantly, due to large morphological particle sizes, these HOM monolithic structures exhibited considerable structural stability against longer hydrothermal treatment times. Such retention is crucial in industrial applications. KeywordsHOM monoliths–Mesopores–Cationic and nonionic surfactants–Instant synthesis–Hydrothermal stabilityJournal of Porous Materials 04/2012; 18(3):259-287. · 1.35 Impact Factor