Phillip M. Duxbury

General Strategies for Nanoparticle Dispersion

Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers.

The failure distribution in percolation models of breakdown

The probability of finding a largest defect cluster of size n in a percolation network is calculated analytically using a new distribution function scaling equation. From this result and the stress (voltage) enhancement at the tip of the most critical defect in the network, the probability of failure in percolation models of breakdown is calculated. For defect fractions less than the percolation point, this distribution is found to be of the form exponential of an exponential. Numerical simulations on the two-dimensional random fuse network confirm the new distribution function and convincingly distinguish between it and the Weibull (1951) form most often used in the fitting of breakdown data.

Ab initio determination of solid-state nanostructure

Advances in materials science and molecular biology followed rapidly from the ability to characterize atomic structure using single crystals. Structure determination is more difficult if single crystals are not available. Many complex inorganic materials that are of interest in nanotechnology have no periodic long-range order and so their structures cannot be solved using crystallographic methods. Here we demonstrate that ab initio structure solution of these nanostructured materials is feasible using diffraction data in combination with distance geometry methods. Precise, sub-ångström resolution distance data are experimentally available from the atomic pair distribution function (PDF). Current PDF analysis consists of structure refinement from reasonable initial structure guesses and it is not clear, a priori, that sufficient information exists in the PDF to obtain a unique structural solution. Here we present and validate two algorithms for structure reconstruction from precise unassigned interatomic distances for a range of clusters. We then apply the algorithms to find a unique, ab initio, structural solution for C60 from PDF data alone. This opens the door to sub-ångström resolution structure solution of nanomaterials, even when crystallographic methods fail.

Systems approaches and algorithms for discovery of combinatorial therapies

Effective therapy of complex diseases requires control of highly nonlinear complex networks that remain incompletely characterized. In particular, drug intervention can be seen as control of cellular network activity. Identification of control parameters presents an extreme challenge due to the combinatorial explosion of control possibilities in combination therapy and to the incomplete knowledge of the systems biology of cells. In this review paper, we describe the main current and proposed approaches to the design of combinatorial therapies, including the heuristic methods used now by clinicians and alternative approaches suggested recently by several authors. New approaches for designing combinations arising from systems biology are described. We discuss in special detail the design of algorithms that identify optimal control parameters in cellular networks based on a quantitative characterization of control landscapes, maximizing utilization of incomplete knowledge of the state and structure of intracellular networks. The use of new technology for high‐throughput measurements is key to these new approaches to combination therapy and essential for the characterization of control landscapes and implementation of the algorithms. Combinatorial optimization in medical therapy is also compared with the combinatorial optimization of engineering and materials science and similarities and differences are delineated. Copyright

Dynamics of Crystal Surfaces and Interfaces

From Atomic Diffusion to Step Dynamics B. Blagojevic, P.M. Duxbury. Atomic Steps in the Decay of 1- and 2-Dimensional Gratings J. Blakely, et al. Morphologies of Periodic Surface Profiles and Small Particles: A Source of Step and Step Interaction Energies H.P. Bonzel, S. Surnev. Anisotropy of Wetting of Pb Crystals by their Own Melt and by Liquid Ga-Pb Alloys D. Chatain, P. Wynblatt. Relaxation of Nanometer-Scale Surface Morphology S.J. Chey, D.G. Cahill. Smoothing of a Grooved Singular Surface Whose Neighboring Orientations are Unstable C. Duport, et al. Step Fluctuations: From Equilibrium Analysis to Step Unbunching and Cluster Diffusion in a Unified Picture T.l. Einstein, S.V. Khare. Kinetic Rate Law Issues in the Morphological Relaxation of Rippled Crystal Surfaces J.D. Erlebacher. Grain Boundary Motion in Aluminum Bicrystals G. Gottstein, et al. An Instability of Heteroepitaxial Interfaces via a Discrete Atom Methods J.K. Lee. Ab-Initio Simulations of the Si (100) Surface: Steps and Melting C.M. Roland, et al. Relaxation of Surface Steps Toward Equilibrium W. Selke. Coarsening of MBE Structures in 2+1 Dimensions E. Somfai, L.M. Sander. 5 Additional Articles. Index.

Island‐to‐percolation transition during growth of metal films

Metal films grown on nonwetting substrates evolve from an early stage of isolated compact islands to a later stage of elongated islands and percolation. Results are presented of a scanning electron microscopy study of Pb on SiO2 showing that the critical island radius Rc at which this crossover occurs is strongly dependent on temperature and weakly dependent on deposition rate. The experimental results are semiquantitatively described by a kinetic freezing model, in which the rate of island coalescence due to surface diffusion competes with the rate of island growth due to deposition.

Low-temperature series analysis of multilayer adsorption at surfaces

It is shown that a low-temperature series analysis, taken to all orders by selecting important graphs at each stage, can be used to demonstrate that an Ising system with an interface forced upon it exhibits an infinite sequence of first-order layering transitions as a function of applied magnetic field. A model with a short-range surface potential, the Abraham model (1980), is also discussed and it is shown that it exhibits at least one layering transition at low temperatures.

The mean field theory of the three-dimensional ANNNI model

The mean field equations of the simple cubic or tetragonal ANNNI model are studied on finite lattices. Structure combination branching processes are found which allow us to considerably refine previous mean field calculations on the model.

Stressed backbone and elasticity of random central-force systems.

We use a new algorithm to find the stress-carrying backbone of ``generic`` site-diluted triangular lattices of up to 10{sup 6} sites. Generic lattices can be made by randomly displacing the sites of a regular lattice. The percolation threshold is {ital p}{sub {ital c}}=0.6975{plus_minus}0.0003, the correlation length exponent {nu}=1.16{plus_minus}0.03, and the fractal dimension of the backbone {ital D}{sub {ital b}}=1.78{plus_minus}0.02. The number of ``critical bonds`` (if you remove them rigidity is lost) on the backbone scales as {ital L}{sup {ital x}}, with {ital x}=0.85{plus_minus}0.05. The Young`s modulus is also calculated. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

Improved polymer thin-film wetting behavior through nanoparticle segregation to interfaces

We report a systematic study of improved wetting behavior for thin polymer films containing nanoparticles, as a function of nanoparticle size and concentration, the energy of the substrate and the dielectric properties of the nanoparticles. An enthalpy matched system consisting of polystyrene nanoparticles in linear polystyrene is used to show that nanoparticles are uniformly distributed in the film after spin coating and drying. However, on annealing the film above its bulk glass transition temperature these nanoparticles segregate strongly to the solid substrate. We find that for a wide range of film thicknesses and nanoparticle sizes, a substrate coverage of nanoparticles of approximately a monolayer is required for dewetting inhibition. Cadmium selenide quantum dots also inhibit dewetting of polystyrene thin films, again when a monolayer is present. Moreover, TEM microscopy images indicate that CdSe quantum dots segregate primarily to the air interface. Theoretical interpretation of these phenomena suggests that gain of linear chain configurational entropy promotes segregation of nanoparticles to the solid substrate, as occurs for polystyrene nanoparticles; however, for CdSe nanoparticles this is offset by surface energy or enthalpic terms which promote segregation of the nanoparticles to the air interface.