Core-and-shell

Spray Drying. Spray-dry encapsulation processes (Fig. 7) consist of spraying an intimate mixture of core and shell material into a heated chamber where rapid desolvation occurs to thereby produce microcapsules (24,25). The first step in such processes is to form a concentrated solution of the carrier or shell material in the solvent from which spray drying is to be done. Any water- or solvent-soluble film-forming shell material can, in principle, be used. Water-soluble polymers such as gum arable, modified starch, and hydrolyzed gelatin are used most often. Solutions of these shell materials at 50 wt % soHds have sufficiently low viscosities that they stiU can be atomized without difficulty. It is not unusual to blend gum arable and modified starch with maltodextrins, sucrose, or sorbitol. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Fig. 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm. The emission band of CdS-ZnS core-shell nanoparticles is higher dian that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticl is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9], Althou the sur ce treatment conditions are different, the ranission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchan d by different surfaKs treatment. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

T divided by the viscosity of the solvent r s. For n-octane this number is 837 K/cP at T = 323 K. The results of the fitting process are all below this theoretical value. This is not surprising, since even in the case of dilute solutions of unattached linear chains, the theoretical values are never reached (see Sect. 5.1.2). In addition the experimental T/r s values differ considerably for the different labelling conditions and the different partial structure factors. Nevertheless, it is interesting to note that T/r s for the fully labelled stars is within experimental error the arithmetic mean of the corresponding core and shell values. 

Wooley and coworkers have cross-linked the micellar corona and obtained the so-called shell cross-linked knedellike micelles [83,84]. This strategy was further applied to a wide variety of block copolymer micelles. Armes and coworkers have used a similar approach for the preparation of shell cross-linked micelles with hydrophilic core and shell [85]. Many other related examples can be found in the literature. 

Fig. 18 Schematical representation of different types of micelles formed by ABC triblock copolymers. Core-shell-corona micelles with insoluble core and shell (a), core-shell-corona micelles with radially compartmentalized corona (b), and Janus micelles with laterally compartmentalized corona (c)...

Figure 27.2 (a) Symmetry properties for core-shell structures where r,/r2 < 1.20 (b) sterically induced stoichiometry (SIS) based on respective radii (r,) and (r2) core and shell dendrimers respectively (c) Mansfield-Tomalia-Rakesh equation for calculation of maximum shell filling when r,/r2 < 1.20... 

Peleg-Raibstein D, Feldon J (2006) Effects of dorsal and ventral hippocampal NMDA stimulation on nucleus accumbens core and shell dopamine release. Neuropharmacology 51 947-957 Pettit HO, Ettenberg A, Bloom FE, Koob GF (1984) Destruction of dopamine in the nucleus accumbens selectively attenuates cocaine but not heroin self-administration in rats. Psychopharmacology 84 167-173... 

Harrod et al. (2004) studied DA transporters and Dl, D2, and D3 receptors following repeated intravenous nicotine administration in male and female rats. No sex differences were depicted in the number of Dl or D2 receptors in either the striatum or the NAC, but female rats had increased number of DA transporters in both the core and shell of the NAC and decreased density of D3 receptors in the shell of NAC, compared to males. Nicotine-induced changes of DA transporters and D3 receptors are reported to be partly responsible for increased behavioral sensitization measured by locomotor activity in female rats. 

Fig. 2 Hydrogen bond geometry (not to scale) + and - are positions 5/2 and - 5/2, respectively. P and S are P04 core and shell is the potassium ion dotted lines indicate the contributions to C(5)...

In the case of Walde et al. (1994a) the synthesis of poly(A) - which can be viewed as a simple form of RNA - proceeded simultaneously with the self-reproduction of vesicles, thus providing a core and shell reproduction, as schematically illustrated in Figure 10.3. 

Silica has been used both as a core and shell material. For example, monodis-persed silica spheres were coated with titania by decomposing titanyl sulfate, TiOS04, in acidic solutions at 90°C (146). The particles so produced showed good hiding power, to be useful as paper whiteners (147). Due to the uniformity of the cores and shells, the optical properties of such dispersions were predictable and reproducible, as shown in Figure 1.1.22, which compares the scattering coefficient,... 

Another generator that was successfully used in the formation of polymer colloids, particles of mixed composition, and coated particles is shown in Figure 1.5.3f, in which the falling film tube is replaced by a boiler (38). The complete apparatus described in Figure 1.5.3 makes it possible to produce cores and shells in a continuous process (39). 

As a result of the dependence of universal calibration on column elution behavior (i.e., anomalous behavior due to adsorption or exclusion), the contribution of the polymer core and shell components (33,34) to hydrodynamic behavior must be fully understood if competent analysis of block copolymers and branched heteropolymers is to be made. It is hoped that with the advent of appropriate MW, composition, and branched polymer standards, the limits of fit of universal calibration to biopolymers such as lignin can be judged. 

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