Dale W. Schaefer

Sol-gel transition in simple silicates II☆

Abstract Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

Size, Volume Fraction and Nucleation of Stober Silica Nanoparticles

29Si NMR, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) are used to monitor the synthesis of silica nanoparticles from the base-catalyzed hydrolysis of TEOS in methanol and ethanol. The reactions are conducted at a [TEOS] =0.5 M, low concentrations of ammonia ([NH(3)] =0.01-0.1 M), and [H(2)O] =1.1-4.4 M to resolve the initial size of the first nuclei and to follow their structural evolution. It is found that after an induction period where there is a buildup of singly hydrolyzed monomer, the first nuclei are fractal and open in structure. Interestingly, the nuclei are twice as large in ethanol (R(g) approximately 8 nm) as those in methanol (R(g) approximately 4 nm). The data suggest that the difference in primary particle size is possibly caused by a higher supersaturation ratio of the singly hydrolyzed monomer in methanol than in ethanol if it is assumed that the surface energy of the first nuclei is the same in methanol and ethanol. The particle number concentration and the volume fraction of the silica particles are calculated independently from the SAXS, DLS, and 29Si NMR results. Finally, the rate of nucleation is obtained from the particle number concentrations.

Polymers, Fractals, and Ceramic Materials

Concepts borrowed from polymer science have been applied to tailor the properties of inorganic materials, especially those derived from amorphous precursors. Fractal geometry can be used to characterize macromolecular precursors and to relate their structures to kinetic growth processes. Within the silica system, for example, it is possible to manipulate the conditions of solution polymerization to yield a variety of macromolecules from branched chains to smooth colloidal particles.

Structural studies of complex systems using small-angle scattering: a unified Guinier/power-law approach☆

A unified analysis method for small-angle scattering data is demonstrated by surveying complex systems that display multiple size-scale structures. Using this approach the relationship between micro- and nano-structures can be ascertained. The method uses a function that is general enough to adequately describe systems ranging from particulates with fractally rough interfaces to mass fractals such as polymer coils. Additionally multiple Guinier and power-law regimes can be treated. The unified method can distinguish Guinier regimes buried between two power-law regimes. Data from particulate filled systems, low crystallinity polymers and low density polymer foams are analyzed.

Colloidal suspensions as soft core liquids

The structure of a suspension of highly charged macromolecules indicates both solidlike and liquidlike phases separated by a distinct melting point. Here the structure factor of the liquidlike phase measured by optical techniques is analyzed using methods previously developed for simple fluids. The hypernetted‐chain approximation is found to be superior to the Percus–Yevick and Kirkwood‐superposition approximations.

Growth and Structure of Combustion Aerosols: Fumed Silica

A model is presented for kinetic growth processes in flames, based on neutron, X-ray, and light-scattering data on commercial fumed silica powders. The model and data are consistent with the existence of fractally rough primary particles aggregated into highly ramified clusters. By mapping onto kinetic models, four processes are identified that determine the structure of the powders: kinetic nucleation, ballistic polymerization, sintering, and diffusion-limited aggregation.

Synthesis, structure, and properties of hybrid organic–inorganic composites based on polysiloxanes. I. Poly(dimethylsiloxane) elastomers containing silica

Various synthetic protocols were used to prepare several classes of poly- siloxane-silica filler systems. The structures of these fillers and their interactions with the polysiloxane matrices were studied using small-angle X-ray and neutron scattering. In addition, the mechanical properties of the composites were characterized using equi- librium stress-strain isotherms in elongation. The results indicated that manipulation of the chemical reactions used to generate the filler can lead to a wide range of complex structures and unusual properties. Some of the observed mechanical properties were correlated with information on the composite structures and on elastomer-filler inter- actions. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1167-1189, 1998

Morphology of dispersed carbon single-walled nanotubes

Using scattering methods, we determine the morphology of carbon nanotube suspensions over length scales from 1 nm to 50 μm. We find no evidence of rod-like character at any length. Rather, a network structure of aggregated tubes, similar to that seen in dry samples, is found. These observations have significant implications regarding the use of single-walled nanotubes as a composite reinforcing filler since the network structure has significantly lower modulus than fully dispersed tubes. We also show that it is possible to isolate a rod-like fraction from the aggregated suspension using intense sonication, providing a potential route to fully dispersed nanotubes.

Fractal Models and the Structure of Materials

Science often advances through the introdction of new ideas which simplify the understanding of complex problems. Materials science is no exception to this rule. Concepts such as nucleation in crystal growth and spinodal decomposition, for example, have played essential roles in the modern understanding of the structure of materials. More recently, fractal geometry has emerged as an essential idea for understanding the kinetic growth of disordered materials. This review will introduce the concept of fractal geometry and demonstrate its application to the understanding of the structure of materials.Fractal geometry is a natural concept used to describe random or disordered objects ranging from branched polymers to the earths surface. Disordered materials seldom display translational or rotational symmetry so conventional crystallographic classification is of no value. These materials, however, often display “dilation symmetry,” which means they look geometrically self-similar under transformation of scale such as changing the magnification of a microscope. Surprisingly, most kinetic growth processes produce objects with self-similar fractal properties. It is now becoming clear that the origin of dilation symmetry is found in disorderly kinetic growth processes present in the formation of these materials.

Spectrum of Light Quasielastically Scattered from Tobacco Mosaic Virus

We present measurements of the spectrum of light scattered from solutions of tobacco mosaic virus. The spectrum, which is generally not Lorentzian in shape, was measured at a series of angles ranging from 20° to 170° in solutions of 0.02% and 0.04% by weight. We have analyzed this data in terms of a theory which includes the effect on the spectrum of both rotational and anisotropic translational diffusion of rodlike molecules of length L. By calculating the angular dependence of the spectra both in tails and at the line center, it is possible to provide graphically and analytically simple means of estimating the average translational diffusion constant, D¯, and a linear combination of the rotational constant, DR, and the translational anisotropy, D∥‐D⊥. We find D¯=(0.390±0.01)×10−7 cm2/sec, and −6.65 (D ∥‐D⊥)+0.84DRL2= (3.10±0.30)×10−7 cm2/sec at 25°C for both tobacco mosaic virus solutions.