Core-shell microparticles

PP-PA6 Blends Containing Dispersed Core-Shell Microparticles. In the third type of PP-PA6 blend system, the PP-g-MA blend com-patibilizer and rubber was completely substituted by maleic anhydride-grafted rubbers such as EPR-g-MA and SEBS-g-MA. As reported previously (22, 23) and schematically represented in Figure 4, imide-coupling at the PP-PA6 interface, and surface-tension gradient and immiscibility between PP, PA6, and rubber are responsible for the accumulation of the rubber at the PA6 microparticle surface, which results in microparticles with a PA6 core and a rubber shell. Like PP-g-MA blend compatibilizers, maleic anhydride-grafted rub-... 

Reprinted with permission from Thermochimica Acta, Microthermal analysis of rubber-polyaniline core-shell microparticles using frequency-dependent thermal responses by Changshu Kuo, Chien-Chung Chen and William Bannister. 403, 1, 115-127. Copyright... 

Similar structures using a silica-in-carbon core-shell structure have also been synthesized [76], which afford new possibilities for nanoencapsulation. Carbon can be removed by calcination, leaving silica, and (if the positions of silica and carbon are reversed) a carbon shell can be created using NH4OH solution to dissolve the silica. The formation of silica microparticles using silanol-functionalized polystyrene latexes proceeds along similar lines (Scheme 5.16) [77]. 

A scaled-up version of this central template-concentric sphere surface assembly approach has been demonstrated for the growth of multi-layer core-shell nano- and microparticles, based upon the repeated layer-by-layer deposition of linear polymers and silica nanoparticles onto a colloidal particle template (Figure 6.8) [60]. In this case, the regioselective chemistry occurs via electrostatic interactions, as opposed to the covalent bond formation of most of the examples in this chapter. The central colloidal seed particle dictates the final particle... 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Another procedure for the preparation of modified thermosets consists of introducing preformed particles in the initial formulation. This technique is also well documented for modified thermoplastics (Paul and Bucknall, 2000). In Chapter 7 different macromolecular architectures such block copolymers, crosslinked microparticles, hyperbranched polymers, and den-drimers, were presented (Fig. 7.11). All these compact molecules can be used as thermoset modifiers. Thermoplastic powders and core-shell polymers are the more accessible preformed molecules. Some examples are given below. 

A Models to describe microparticles with a core/shell structure. Diametrical compression has been used to measure the mechanical response of many biological materials. A particular application has been cells, which may be considered to have a core/shell structure. However, until recently testing did not fully integrate experimental results and appropriate numerical models. Initial attempts to extract elastic modulus data from compression testing were based on measuring the contact area between the surface and the cell, the applied force and the principal radii of curvature at the point of contact (Cole, 1932 Hiramoto, 1963). From this it was possible to obtain elastic modulus and surface tension data. The major difficulty with this method was obtaining accurate measurements of the contact area. 

Figure 4. In situ formation of core-shell-type microparticles, in which a PA6 core is surrounded by a SEBS-g-MA shell.

The process has been extensively explored by Barbe and coworkers in Australia (where the same scientists established the spin-off company Ceramisphere), and can be viewed as an emulsification of a sol-gel solution in which gelation takes place concomitantly. Depending on the order of addition of the different chemicals, furthermore, the porous microparticles prepared from interfacial hydrolysis and condensation of TEOS in W/0 emulsion will be full porous matrix particles or core-shell capsules. Normally, if the emulsification of the sol-gel solution takes place concomitantly with gelation, full microparticles are formed with the dopant molecules homogeneously distributed within the inner huge porosity of the particles (Figure 18.3). 

The sol-gel process to make doped silica-based materials has evolved from encapsulates in irregular Si02 xerogel particles, to sophisticate core-shell particles capable to encapsulate high amounts of functional organic species, and effectively release the entrapped species under small load. Sol-gel microcapsule and microparticle delivery systems will soon be introduced by numerous industries. 

A few systems that do not follow any of the procedures described so far are detailed below. Lee and Senna [162] described the synthesis of magnetic PS microparticles of the core-shell type prepared by emulsion polymerization of styrene in the presence of PS seed microspheres and magnetite coated with a bilayer of sodium oleate. 

Y.D. Liu, EE Fang, H.J. Choi, Core-shell structured semiconducting PMMA/ polyaniline snowman-like anisotropic microparticles and their electrorheology, Langmuir, 2010, 26, 12849. 

Organic spheres are predominantly polymeric, consisting of synthetic or natural polymers. The field of polymeric nano- and microparticles is vast, comprising, for instance, latex particles for coatings, hollow particles for syntactic foams, and microcapsules for foaming and additive release. In addition, there are core-shell microbeads and coated polymeric particles, where the particles can exhibit multiple functionalities, thanks to the individual features of their different layers 1]. As fillers in thermosets and thermoplastics, hollow microspheres and expandable microcapsules are among the most frequently used in commercial applications. 

Figure 5.3 Various designs of textile structures loaded with active agents (a) active agent incorporated in fibers/filaments (core/shell or matrix structure), (b) active agent incorporated into coating/impregnation film, and (c) active agent incorporated in microparticles attached to the textile surface (by grafting or via binder).

Membrane and microfiuidic devices have also been adopted for the precision manufacture of solids from double-emulsion templates. To date, several different types of particles have been successfully produced by incorporating use of various membrane and microfiuidic devices in processes of polymerization, gel formation, crystallization, and molecular or particle self-assembly. Membrane emulsification is more suited to the fabrication of less sophisticated particulates, such as solid lipid micro-Znanoparticles, gel microbeads, coherent polymeric microspheres, and inorganic particles such as silica microparticles. Microfiuidic devices allow more sophisticated particle designs to be created, such as colloidosomes, polymerosomes, 3D colloidal assemblies, asymmetric vesicles, core-shell polymer particles, and bichromal particles. 

Fig. 1.4 Drug distribution in microparticles. Left dissolved in polymer matrix. Center dispersed in polymer matrix. Right as a solid core in a polymer shell.

The morphology of microcapsules depends mainly on the core material, how it is distributed within the system, and the deposition process of the shell. Similarly, the morphology of the internal structure of a microparticle depends largely on the selected shell materials and the microencapsnla-tion methods that are employed. The microcapsules may be categorized into several arbitrary and overlapping classifications such as... 

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