Morphology Particle

The particle morphology can have important ramifications for the latex product performance. Because multi-lobed particles have a larger hydrodynamic volume than a spherical particle of equal polymer mass, such types of latexes have been used to raise the viscosity in coatings applications. Hollow particles are used in paper coatings to improve the optical properties and surface smoothness. Particles with core-shell morphologies or with domains have been developed for impact modification. In addition, various microencapsulation techniques have been employed to enclose a wide variety of materials (47, 97,239) for pharmaceutical, agricultural and cosmetic applications. 

Other techniques are available to characterise particle morphology, including infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) (184, 243), electron spectroscopy for chemical analysis (ESCA), and various scattering methods. 

TEM micrograph of monodisperse polystyrene latex particles produced by emulsion polymerisation 

The size of a cubic particle is uniquely defined by its edge length. The size of a spherical particle is uniquely defined by its diameter. Other regular shapes have equally appropriate dimensions. With some r ular particles more than one dimension is necessaiy to specify the geometiy of the particle as, for example, a cylinder, which has a diameter and a length. With irregulariy shaped particles, many dimensions 

The shape of the gold particles is an important factor for catalytic activity since it determines the relative amount of corners and edges, i.e., of low-coordinated atoms. It depends on the support through the interfacial energy this is well known from works performed on model samples (Table 15.1). The higher the energy, the flatter the gold particles. 

The shape of the gold particles may also depend on the activation temperature of the gold catalysts. This was observed with Au/Ti02 prepared by deposition- 

Spherical gold particles are also less active than hemispherical ones because of the smaller interface perimeter (Table 15.2). This result has been correlated to the perimeter length of the gold-support interface [8], and has contributed to the establishment of the mechanism reported in Fig. 15.2. 

2 Influence of the Oxide Support on the Electronic Properties of Cold Particles 

This section of the chapter is divided into two parts. The first part discusses the instruments and methods used to evaluate particle morphology. It pays particular attention to nomenclature since the words used in this field are often ambiguous. The second part deals with the details of particle size analysis by microscopy. It pays particular attention to sampling issues and to the use of image analysis. 

The microscope is often best used as a problem-solving tool. As such, the ideal situation occurs when the requestor of the information (engineer, analyst, or pharmaceutical scientist) can be present during the microscopical examination. Photomicrographs only present a few selected fields-of-view and these 

A clear goal of a particle examination is the attempt to relate some property of the individual particles to some bulk physical property of the powder. For example, the dissolution rate of drug substance in the body can often be related to the size of the dmg substance particles [22]. A microscopical examination has the maximum utility if it can relate some measured property of the particles to some physical property of the bulk. Establishing this sort of relationship nearly always requires a numerical analysis of size. While it is possible to measure or estimate particle size using only a microscope and microscopist, the operation is tedious and prone to operator error. The use of a digital image analyzer can, in some cases, completely automate the analysis, and in other cases, automate some of the more tedious and error-prone operations of the work. 

The following paragraphs discuss each of these steps. 

Sampling is by far the most important part of particle size analysis by microscopy (and probably all particle size techniques). A kilogram of drug substance will contain many millions of particles. Since, at most, the particle size analysis samples a few thousand particles, the measured particles must be selected with care. Allen [20] presents an extensive discussion of bulk sampling issues relevant to all particle size analysis, irrespective of the particular technique. Our interest, though, is primarily directed toward sampling as it relates to the specimen used for particle size analysis by microscopy. We will assume that the 50 mg or so of sample dehvered to the laboratory is truly representative of the bulk powder. 

Latex made out of composite polymer particles, that is, particles containing different phases, present definitive advantages in many applications. Thus, particles formed by an elastic core and a hard shell are used as impact modifiers for polymer matrices. Hard core-soft shell particles are particularly useful for paints because they have a low minimum film formation temperature and are not sticky at higher temperatures. Hollow particles are efficient opacifiers, and hybrid polymer-polymer particles, for example, epoxy-acrylic polymer particles, combine the properties of the constituent polymers in a synergetic way. The properties of these materials largely depend on the particle morphology. Batch and 

The motion of the clusters is ruled by the balance between the van der Waals attraction-repulsion forces and the resistance to flow that arises from the viscous drag. 

Although LRP techniques are well understood in bulk or solution, in heterogeneous polymerisations the already complex kinetics are further complicated by partitioning of the activating species in the various environments and by the rate of transportation of these 

In general, crystal shape can be modified by changing the conditions of crystallization such as the solvent used, supersaturation, temperature, pH, 

The purity of the limestone or dolomite is crucial to the purity of the end product, MgO. The major impurities derived from this source are silica, Fe203, A12C 3, and CaO. Low levels of other impurities may also be present in the raw stone, such as MnO and B203, and may result in some degree of contamination of the MgO end product. Many of the impurities present in the raw stone are difficult to reduce economically and, therefore, a high-purity limestone or dolomite source is essential to producing a high-purity product. 

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In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

Additional information on elastomer and SAN microstmcture is provided by C-nmr analysis (100). Rubber particle composition may be inferred from glass-transition data provided by thermal or mechanochemical analysis. Rubber particle morphology as obtained by transmission or scanning electron microscopy (101) is indicative of the ABS manufacturing process (77). (See Figs. 1 and 2.)... 

Particle Morphology, Size, and Distribution. Many fillers have morphological and optical characteristics that allow these materials to be identified microscopically with great accuracy, even in a single particle. Photomicrographs, descriptions, and other aids to particle identification can be found (1). 

Two modifications of the duidized-bed reactor technology have been developed. In the first, two gas-phase duidized-bed reactors coimected to one another have been used by Mobil Chemical Co. and Union Carbide to manufacture HDPE resins with broad MWD (74,75). In the second development, a combination of two different reactor types, a small slurry loop reactor followed by one or two gas-phase duidized-bed reactors (Sphetilene process), was used by Montedision to accommodate a Ziegler catalyst with a special particle morphology (76,77). This catalyst is able to produce PE resins in the form of dense spheres with a diameter of up to 4—5 mm such resins are ready for shipping without pelletization. 

Emulsion polymeriza tion of ABS (241) gives a mbber-phase particle morphology which is mostly deterrnined by the mbbet-seed latex. Since the mbber particle size, polydispersity, and cross-linking ate estabhshed before the preparation, the main variables relate to grafting, molecular weight... 

Impression Plasters. Impression plasters are prepared by mixing with water. Types I and II plasters are weaker than dental stone (types III and IV) because of particle morphology and void content. There are two factors that contribute to the weakness of plaster compared to that of dental stone. First, the porosity of the particles makes it necessary to use more water for a mix, and second, the irregular shapes of the particles prevent them from fitting together tightly. Thus, for equally pourable consistencies, less gypsum per unit volume is present in plaster than in dental stone, and the plaster is considerably weaker. 

The initial studies on nickel-aluminide synthesis defined a number of important issues in shock-induced solid state synthesis. This work was extended to the influence of powder particle morphology in recent work of Thadhani and... 

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

Sample 5 is close to an H2-type hysteresis, whereas 6 and 7 can be tentatively assigned to H3- and Hi-type hystereses, respectively [27]. The hystereses are caused by capillary condensation in interparticle pores and the shape is an indication of a particular particle morphology. Sample 7 has a more regular narrow mesopore size distribution, whereas sample 5 is more complex with pores of... 

TEM investigations support the interpretation of the nitrogen physisorption isotherms (Fig. 19.3). They show particles 5-20 run in diameter, whereas the particle size estimated from the specific surface area, assuming spherical particle morphology, is 7 nm. Indeed, the particle morphology for sample 7 is mostly spherical, but for some crystallites edges are discernible. 

The dynamics of particle morphology can be used to an advantage, to counteract the effect of sintering ofthe copper particles. As Fig. 8.13 shows, a Cu/ZnO catalyst slowly loses activity, which is attributed to sintering. Exposing the catalyst for a short time to a highly reducing mixture of C02-free synthesis gas restores the activity. 

There are a number of concepts concerning the structure of small particles which have a bearing upon geometrical catalytic effects (e.g. 41-43). These follow both from the surface imaging results, and a detailed experimental (13-15) and theoretical (44-47) study of particle morphologies. 

Figure 7.5 Two topologically distinct types of mesoporous gold sponge, each with 50 volume % gold, (a) Swiss-cheese morphology produced by de-alloying, (b) aggregated particle morphology produced by sintering of nanoparticles.

A subtle control of the particle morphology and size can also be brought about using Fievet s Polyol process for catalyst preparation. The main advantage here is the easy access to nanoparticulate metals at any normally equipped laboratory bench without the restrictions imposed by anaerobic working conditions [224]. It is clear... 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

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