Morphology hollow particles

Significantly different seemed intiaUy the crystal morphology of polyethylene, polybutene-1, polypropylene, polystyrene, poly(4-methyl pen-tene-1), and polyisoprene polymerized with varying solvents and at varying temperatures (114, 123). Discrete hollow particles with a fibrous texture could be observed. The fibrils had an appearance similar to polyethylene crystallized from solution sheared by rapid stirring (118). A closer analysis of this similarity was carried out by Wikjord and Manley (124), Keller and Willmouth (117), and Ingram and Schindler (125) for polyethylene. 

The most important parameter affecting the morphology of powders is the process temperature. This is because the temperature has a great influence on the solvent evaporation rate. As discussed by Jayanthi et al. [2], for a particular precursor, at given operating conditions, the morphology of particles and whether they are solid and fully-filled, or hollow and disrupted, depends on the concentration distribution within the droplet. The concentration distribution is a strong function of the process temperature. 

Figure 37.8 summarizes different possible final morphologies for hollow particles formed in SP and SD [13]. If a relative velocity between the air and the droplet exists, a boundary layer will form around the droplet, which results in a non-uniform solvent evaporation and, therefore, a non-uniform crust may form. Depending on the nature of the solute and the process conditions, the crust could be either permeable or impermeable. Pressure buildup in impermeable particles may cause several final particle morphologies. If the crust is uniform and contains no defects, the uniform stress applied on the internal wall of the shell may cause a uniform particle disruption and the particle will be cut into several pieces (type a). If the particle wall thickness is not uniform, it may break from the weakest part, and a particle with a small hole may form (type b). As another scenario, if the crust is strong enough to resist the internal pressure, the vapor trapped inside the particle... 

Fig. 18 TEM and SEM images of extracted chiral hollow particles with differeht morphology, Reprinted with permission from Crudden et al. Copyright 2012 Americah Chemical Society.

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. 

McDonald et al found that the modification of an emulsion polymerization with a water-miscible alcohol and a hydrocarbon nonsolvent for the polymer can influence the morphology and enables the formation of monodisperse particles with a hollow structure or difiuse microvoids [58]. Both kinetic and thermodynamic aspects of the polymerization dictate particle morphology. Complete encapsulation of the hydrocarbon occurs, provided that a low molecular-weight polymer is formed initially in the process. Monodisp>erse hollow particles with diameters ranging from 0.2 to 1 pm were obtainable, and void fractions as high as 50% are feasible. 

Okubo et al. examined the penetration/release behavior of various solvents in-to/from the interior of micron-sized monodisperse cross-linked polystyrene/poly-divinylbenzene composite particles [63]. The hollow particles were produced by the seeded polymerization utilizing the dynamic swelling method [64], Itou et al. prepared crosslinked hollow polymer particles of submicron size by means of a seeded emulsion polymerization [65]. The morphology of the particles depends on the composition of divinylbenzene and methyl methacrylate. 

A specific core-shell morphology is given when the core is a void (hollow particles), that is, the particle is a porous material. In the dried state these particles... 

Fig. 3 Confocal microscopy images of gel particles composed of a polyelectrolyte (cationically modified hydroxyethyl cellulose (JR-400 ) and two different types of surfactants (cetyltri-methylammonium bromide, CTAB, and sodium perfluorooctanoate, FC7). The resulting gel particles can assume two types of morphologies depending on the surfactant concentration a smooth hollow particle with dense thin gel shell for lower concentrations, and b gel particle with sparse thick shell, and a gel corona for higher concentrations. Reprinted from Lapitsky and Kaler [51]. Copyright (2004), with permission firom Elsevier...

Recently, Jung et al. (2000) described the polymerisation in vesicles, leading to different types of morphologies including hollow particles. 

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 benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

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