Morphology hollow structures

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]. 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

It appears from the TEM investigation that both the hollow structure and the morphology of SnOi nanoislands depend strongly on the average thickness of initial Sn layer and the thermal treatment ambiance. Fig. 2 shows BF TEM images of the SnOa islands after MBE deposition of Sn to epy thickness of 2.5 nm (a), 7 nm (b) and 20 nm (c) followed by thermal oxidation at 650°C for... 

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. 

Polymer vesicles are considered to form in a two-step process. First, the polymer chains form a bilayer-type membrane, which then subsequently closes to form a hollow structure (Figure 9). This process involves an interfacial curvature change, which can correspond to a change in the packing parameter for the polymer and hence a change in the resultant morphology. However, theoretical calculations have revealed that some vesicle formation process may be more complicated than the above two-step procedure. These results can be summarized as two different proposed mechanisms for the spontaneous formation of vesicles from the homogeneous state. 

The product obtained under acid conditions was an ellipsoid nanoparticle, and that obtained under alkaline condition was hollow silica ensembles. The study revealed that [bmimJBF played a critical role in the process of formation of the mesoporous silica hollow structure. It was thus evident that nonaqueous IL microemulsions can contribute to the synthesis of materials with various structures by providing a reaction environment that is able to control the size and morphology of the materials. 

N. Widjojo, T.S. Chung, and W.B. Krantz, A morphological and structural study of Ultem/P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology. Journal of Membrane Science 294 (2007) 132-146. 

By changing the ultrasound power, changes in the mesoporosity of ZnO nanoparticles (average pore sizes from 2.5 to 14.3 nm) have been observed. In addition to the changes in mesoporosity, changes in the morphology have also been noted [13]. Recently, Jia et al. [14] have used sonochemistry and prepared hollow ZnO microspheres with diameter 500 nm assembled by nanoparticles using carbon spheres as template. Such specific structure of hollow spheres has applications in nanoelectronics, nanophotonics and nanomedicine. 

In addition to chemical composition, as discussed in Sect. 3, the route of self-assembly also significantly affects the resulting structure because different kineti-cally folded structures may be formed. For example, subtilisin-triggered formation of Fmoc-Ls via ester hydrolysis gives rise to hollow nanotubular structures [22], whereas Fmoc-Ls gel formed by the thermolysin-catalysed reversed hydrolysis of the Fmoc-L/L2 system gives rise to nanofibrillar morphology [21]. 

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

---