Interparticle spacing

In this section, we focus on the strategies of controlling nanoparticle assemblies through functionalized polymer scaffolds, starting from interparticle spacing in bulk aggregates to 3-D morphologically controlled hierarchical nanostrucmres. 

X-ray scattering (SAXS) plots of nanoparticle assembly indicating systematic control of inter-particle distance by dendrimer generations. Reprinted with permission from Frankamp, Boal, et al. (2002). Copyright 2002 American Chemical Society. 

By applying the same principle, Rotello s group also showed that dendrimer-mediated assembly of magnetic nanoparticles (-y-Fe203) can directly modulate 

In a similar fashion, the Rotello group reported several protein-mediated assemblies of nanoparticles (Srivastava, Verma, et al. 2005 Verma et al. 2005). Unstable proteins such as chymotrypsin are readily denatured upon prolonged interaction with functionalized nanoparticles because of the exposure of proteins to the hydrophobic layer (Srivastava, Verma, et al. 2005). Addition of a hydrophilic portion to the monolayer by inserting a short tetraethylene glycol between the charged terminal group and hydro-phobic aliphatic chain circumvented this denaturation problem (Hong et al. 2004). 

A ferritin-FePt nanoparticle bionanocomposite was fabricated in later studies, which shows the integrated magnetic behavior of the synthetic and biological components (Fig. 6.5 Srivastava, Samanta, Jordan, et al. 2007). A transmission electron 

Modified polymer shell does not overlap. Matrix is continuous phase and dominates properties 

Modified polymer shells overlap and become the dominant factor in determining properties 

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Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Transition metal colloids can also be prevented from agglomeration by polymers or oligomers [27,30,42,43]. The adsorption of these molecules at the surface of the particles provides a protective layer. In the interparticle space, the mobility of adsorbed molecules should be reduced decreasing the entropy and thus increasing the free energy (Fig. 2). 

The steps displayed in the porosimetry curves are attributed to the pores of the sample. Very low pressures will fill the interparticle spaces when the sample is a powder. Increasing pressures will cause the mercury to penetrate into the pores, with higher pressures corresponding to smaller pores. For each step in the intrusion curve, there will be a corresponding step in the extrusion curve at a lower pressure. 

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

Following the intrusion branch with increasing pressure (Fig. 1.16A), the steep initial rise at low pressures is caused by the filling of interparticle spaces. The breakthrough pressure, i.e. the pressure when the voids between the particles are filled, follows in principle the theory of Mayer and Stowe [94], and is inversely proportional to the particle size [95]. The demarcation between interparticle spaces and actual intraparticle pores may be unclear for microparticles, but in the case of polymer beads from suspension polymerization having particle sizes between 50-500 pm, usually no interference occurs. The second rise of the intrusion branch is caused by pores inside the particles. Shown in Fig. 1.16A is a porous material of rather narrow pore size distribution. 

Fig. 1.18A shows the pore size distribution for nonporous methacrylate based polymer beads with a mean particle size of about 250 pm [100]. The black hne indicates the vast range of mercury intrusion, starting at 40 pm because interparticle spaces are filled, and down to 0.003 pm at highest pressure. Apparent porosity is revealed below a pore size of 0.1 pm, although the dashed hne derived from nitrogen adsorption shows no porosity at aU. The presence or absence of meso- and micropores is definitely being indicated in the nitrogen sorption experiment. 

To measure the force-extension law of a small biomolecule, these authors employed a two-step strategy. First, the background repulsive force-distance profile, in the absence of biomolecules, Fbg(h), is measured, h being the interparticle spacing. Then, once the biocomplexes have been properly attached within each interval between colloids, the same measurement is repeated, allowing determination of the force-distance profile of this irreversible assembly The force / >(/t)... 

Frankamp BF, Boal AK, RoteUo VM. Controlled interparticle spacing through self-assemhly of Au nanoparticles and poly(amidoamine) dendrimers. J Am Chem Soc 2002 124 15146-15147. 

Verma A, Srivastava S, Rotello VM. Modulation of the interparticle spacing and optical behavior of nanoparticle ensembles using a single protein spacer. Chem Mater 2005 17 6317-6322. 

One important class of particulate composites is dispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2-0.3 tim between particles usually helps optimize properties. As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFes also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten. 

LSP near-field. It was demonstrated recently by theoretical calculations that at interparticle spacings of about 2 nm, the field is almost completely concentrated in the gap between the particles, and its intensity is about 10 that of the incident wave [76]. 

Bulk density, or packing density, includes all pores and voids (interparticle spaces) in its calculation. This value depends on the form of the particle (powder, tablets, or extmdates) and the packing procedure. It is extensively used in reactor designing since this value connects the solid volume with that of the reactor. 

Some additional complexity arises from the possibility of different adsorption sites and the presence of pores, which reflect in nonideal adsorption isotherms and mass-transfer problems. The mass transport can be relatively slow in pores and interparticle spaces [13], as it is the case of P25, for which, in suspension, there are particles ranging from 0.2 to 2 p,m, formed by 30-nrn-sizcd primary particles. In such spaces, the diffusion coefficient is comparable to liquid diffusion in zeolites. 

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