Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H14: Focus Session: Organic / Inorganic Hybrid Nanomaterials |
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Sponsoring Units: FIAP DPOLY Chair: John Kieffer, University of Michigan Room: LACC 403B |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H14.00001: Synthesis Mechanisms and Properties of POSS Compounds Invited Speaker: Polyhedral oligomeric silsesquioxanes (POSS) compounds have generated considerable interest because of their many interesting properties, not the least of which is their resistance to extreme environments. Among their many applications are as viscosity modifiers and pre-ceramics. Despite their importance, the mechanisms by which POSS compounds form has not not been well understood. Therefore, we have undertaken a comprehensive study of the possible formation mechanisms for POSS compounds, including the effects of substitutions at various positions and of the solvent. In addition, the possibility of passing small molecules through POSS, or storing small molecules inside POSS cages had been of interest, so we have begun studies of the possibility of passing H$_{2}$, N$_{2}$, and O$_{2}$ into POSS cages of varying size. Finally, alternative mechanisms for adsorbing POSS compounds onto the Si(100) surface are being explored. \newline \newline Co-authors are Baudilio Tejerina, Iowa State University and Takako Kudo, Gunma University. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 9:12AM |
H14.00002: How Does POSS Influence Polymer Properties? Invited Speaker: Research into the influence of polyhedral oligosilsesquioxane (POSS) incorporation within polymers on microstructural and physical properties, including thermal, fracture, and rheological properties, is beginning to reveal the mechanisms by which such properties are enhanced. This talk will first survey the structure and properties of POSS-based polymers where POSS is incorporated via various architectural approaches, including random copolymers, block copolymers, telechelics, and dispersions. A significant sensitivity of the rheological, thermal, and solid mechanical properties on the POSS vertex ``R'' groups indicates the importance of molecular-level interactions, while sensitivity to the incorporation architecture demonstrates the role of nanometer-scale assembly. After giving tentative conclusions on the ways in which POSS influences polymer properties, applications exploiting the novel thermal and mechanical properties will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H14.00003: Simulation of Self-Assembly of Functionalized Silsesquioxane Molecules John Kieffer, Feng Qi, Jinhua Zhou, Murat Durandurdu, Xi Zhang, Sharon Glotzer, Cheng-Ying Lee, Matt Neurock We have developed a computational framework for the simulation of structural assembly in Polyhedral oligomeric silsesquioxane (POSS) nanocomposites, by combining computational techniques ranging from ab initio quantum mechanical calculations to molecular dynamics simulations to coarse-grained mesoscale modeling methods. Using ab initio calculations we predict the inherent properties of molecular building blocks. Using large- scale molecular dynamics simulations we reproduce the transport, reaction, and microphase evolution processes that occur during nano-assembly, and thereby generate realistic models that serve to establish structure-property-processing relationships for these materials. In this presentation the general approach taken with our the computational framework will be outlined and results from two representative materials simulation studies will be discussed. One example details the strategies pursued for the design of materials with photonic applications. The other one describes the peculiar amphiphilic behavior of mono-tethered POSS that causes nano-phase separation and can be exploited for pattern formation at this scale. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H14.00004: Simulation of Organic-tethered Silesquioxane Nanocube Assemblies Xi Zhang, Elaine Chan, Lin Ho, Sharon Glotzer Polyhedral oligomeric silesquioxane (POSS) based materials are a class of organic/inorganic hybrid nanomaterials with superior properties. Recent experiments have demonstrated that the self-assembly of tethered POSS ``nanocubes'' is a promising route to synthesis novel materials with highly ordered and sophisticated nanostructures. We have developed a coarse-grained model and performed molecular simulations for organic-tethered POSS molecules, to complement the ongoing experimental studies and advance our understanding of the fundamental assembly principles. We systematically explore the parameters that control the assembly process and the resulting equilibrium structures, including concentration, temperature, tethered POSS molecule topology, and solvent conditions. We report conventional lamellar and cylindrical structures that are typically found in block copolymer and surfactant systems, but with interesting modifications of the phase diagram caused by the bulkiness and cubic geometry of the POSS nanocubes. Our computational methodology not only provides insight to the assembly process of existing structures, but also facilitates the rational design of novel POSS based organic/inorganic hybrid materials. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H14.00005: Nanostructured Organosilicates from Self-assembled Block Copolymers/Silsesquioxanes Mixtures Ho-Cheol Kim, Erik Freer, Jennifer Cha, James Hedrick, Robert Miller Block copolymers have been used as a structure-directing agent for generating nanostructures in inorganic materials. Domain-selective chemical reactions have been studied using pre-assembled block copolymer films or mixtures of block copolymer and ceramic precursor for sol-gel reaction. We report a simple route to nanostructured organosilicates from binary mixtures of amphiphilic block copolymers and silsesquioxanes. Ordered microdomains of spheres, cylinders and lamellae were generated by varying mixing ratio which determines the volume fraction of each microdomain phase. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) were used to image the nanostructures. Small angle x-ray scattering (SAXS) was used to characterize the microstructures. Potential applications of the nanostructures will be discussed in correlation with microdomain morphologies of the thin films of silsesquioxanes. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H14.00006: Density Functional Theory and MD Simulations of Properties of PDMS near Silica Surfaces Shyamal K. Nath, Amalie L. Frischknecht, John G. Curro, John D. McCoy Polydimethylsiloxane (PDMS) is a very important polymer for its application in the adhesives, sealants, coatings and biomedical industries. In this work, we study the properties of PDMS melts confined between silica surfaces. We used two different approaches: First, a density functional theory (DFT) that uses the structure of the homogeneous liquid as input; and, second, full-scale molecular dynamics simulations. We used a fully detailed, realistic model of PDMS within the united atom framework in both the DFT and simulation studies. In solving the DFT equations, the direct correlation functions were obtained from PRISM theory with attractions treated via the random-phase approximation. Separate scaling factors were employed for the PRISM and RPA parts of the direct correlation functions. Good agreement was observed between DFT and MD simulations for density profiles, stress profiles, and surface tensions. We also made direct connection to experimental results by estimating the forces on the silica surface from DFT. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H14.00007: Atomistic Simulations of Acene Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS) Molecules Feng Qi, Murat Durandurdu, John Kieffer Ab initio quantum mechanics calculations and classical molecular dynamics (MD) simulations were performed to study a series of polyhedral oligomeric silsesquioxanes (POSS) molecules functionalized with acene groups. Using ab initio calculations we first determine the electronic properties of these novel molecules. When we identified molecules with the desired electronic properties, we used classical MD simulations to predict the structures that result from self-assembly of these molecules. These structures were characterized in terms of their phase behavior and mechanical properties. Our simulations reveal that these acene functionalized POSS configurations have a similar band gap as pure acene molecules, but in bulk configuration have superior thermal and mechanical properties. Our results indicate that these novel acene-functionalized POSS oligomers are potential new candidates for semiconducting organic/inorganic hybrid molecular materials with tunable band gap energies, that can be synthesized based on nano building block self-assembly. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H14.00008: Quantitative Determination of the Chemical Composition of Silica-Poly(Norbornene) Nanocomposites Thomas Seery, Mark Jordi Hybrid materials consisting of a silica nanoparticle surrounded by a grafted poly(norbornene) brush have been prepared by ring opening metathesis polymerization (ROMP). A quantitative determination of each stage of composite formation has been accomplished including a determination of the density of surface bound functional groups, catalyst molecules, and polymer chains. This analysis has enabled the determination of the reaction efficiency between the catalyst and the surface bound functional groups as well as the determination of the fraction of metal mediating species that initiate a polymer chain. Control of the chain density was demonstrated by two methods: the use of controlled reaction times between the catalyst and the surface, and the variation of the surface functional group density. Polymer chain densities resulting from composites prepared with different tether structures will also be reported. The resulting brush densities were found to span a wide range including those previously reported for polymer layers formed by adsorption, grafting of preformed polymer chains, and surface initiated polymerization (SIP). [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H14.00009: Branch Content in Hybrid Materials using Small-Angle Scattering Greg Beaucage Inorganic/organic hybrid materials often display ramified mass- fractal structures characterized by primary particle size, aggregate size, and mass-fractal dimension. Physical properties, such as mechanical and dynamic mechanical properties and electrical conductivity (in carbon composites for instance), can not be predicted using only these structural features since such properties are intimately tied to the degree and type of branching as shown by Witten [1]. Witten suggested the use of the minimum dimension, or the related connectivity dimension, to calculate mechanical response in these hybrid systems. A viable technique to quantify the minimum dimension and connectivity dimension in hybrid materials has, until recently, been absent from the literature. This presentation will discuss the use of small-angle x-ray and neutron scattering to describe branch content in hybrid materials [2] and will outline an approach to use the minimum dimension and connectivity dimension to predict static and dynamic mechanical properties for hybrid materials based on structure [1, 3]. 1. Witten TA, Rubinstein M, Colby RH Reinforcement of Rubber by Fractal Aggregates J Phys II 3 (3): 367-383 (1993). 2. Beaucage G Determination of branch fraction and minimum dimension of mass-fractal aggregates Phys Rev E 70 (3): art. no. 031401 Part 1 (2004). 3. Kohls DJ, Beaucage G Rational design of reinforced rubber Curr Opin Solid St M 6 (3): 183-194 (2002). [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H14.00010: Nanoscale Ordered Structure in High Temperature Ceramics Azar Alizadeh, Julin Wan, Patrick Malenfant, Seth Taylor, Sergio Loureiro, Mohan Manoharan Many natural ceramic structures, such as nacre, exhibit fracture toughness values much higher than those of the constituent ceramic material. There are many factors contributing to this enhancement, with the ordered microstructure playing a critical role. A key challenge in making bio-inspired, structural ceramics based on these observations is to create and preserve nanoscale order in high temperature ceramic materials. Towards this goal, a bottom-up approach was explored to build nanometer scale structures with long-range order in a Si-C-N system. Polymeric precursors that lead to the desired ceramic composition were self-assembled using block copolymers as structure-directing agents. It will be shown that the ordered nanoscale structure created in the block copolymer/precursor hybrids can be preserved through the pyrolyzation process, thereby leading to ordered nano-structures in the final dense ceramic. The structural and compositional evolution of this process was characterized by SAXS and TEM. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H14.00011: Mechanical and electronic properties of organic–inorganic hybrid semiconductor ZnSe(C2H8N2)1/2 Amjad Nazzal, Huaxiang Fu Organic--inorganic semiconductor, ZnSe(C$_{2}$H$_{8}$N$_{2})_ {1/2}$, under hydrostatic pressure is studied by using a first-principles pseudopotential method with mixed-basis set[1], aiming to understand its mechanical and electronic properties. With respect to the parental inorganic bulk, the hybrid is found to be nearly as twice as flexible, with a bulk modulus smaller than most known semiconductor bulks. Our calculations also demonstrate that the pressure dependence of the band gap in the hybrid semiconductor shows much pronounced nonlinearity than in bulk ZnSe. Further, our results reveal an interesting pressure-induced isoelectronic phase transition by which the in-plane hole mobility along a given direction can be drastically varied. (This work was supported by NSF-MRSEC). [1] H. Fu and O. Gulseren, PRB 66 (02) 214114. [Preview Abstract] |
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