Internal Voids

Compared to optical photography. X-rays have the distinct advantage that they see through opaque clouds of smoke and debris. Most materials can also be penetrated by the appropriate high-energy X-rays, making internal voids in materials, such as incipient spalls, clearly visible. 

Another common problem on moulding is voids. These occur when a moulding or extrudate cools and hardens rapidly on the surface. On further cooling the moulding or extrudate cannot shrink inward because the outside dimensions are fixed and therefore the molecules are pulled towards the surface of the product, generating internal voids. At the same time molecules in the inner... 

Controlling the extraction rate is vital because the shape and texture of the resultant fiber is directly influenced by the solvent removal rate. As the solvent is extracted from the surface of the fiber, significant concentration gradients can form. These gradients may result in a warping of the desired eircular shape of the fiber. For example, if the solvent is removed too quickly, the fiber tends to collapse into a dog-bone shape. Additionally, the solvent extraction rate influences the development of internal voids or flaws in the fiber. These flaws limit the tensile strength of the fibers. 

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Dynamic tensile failure, called spall, is frequently encountered in shockloading events. Tension is created as compression waves reflect from stress-free surfaces and interact with other unloading waves or release-wave profiles. Spall has been widely studied by authors such as Curran, Ivanov, Dremin, and Davison and there is considerable data. As shown in Fig. 2.19, the wave profiles resulting from spall are characterized by an additional loading pulse after release of pressure. The late pulse is caused by wave reflection from the internal void of the tensile fracture. Analysis of such wave profiles yields appropriate spall stress values. 

Harder to retrofit to appearance 5. Possible internal voids 5. Poorer high-volume... 

Higher tool costs than 6. Possible internal voids available for various... 

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

In order to distinguish between isolated silicon-hydrogen bonds in a dense network and other bonding configurations, such as clustered monohydride and dihydride bonds, bonds on internal void surfaces, and isolated dihydride bonds, Mahan et al. [60] have defined the microstructure factor R as... 

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

Such a methodology is also useful for the chemoselective functionalization of internal voids of dendrimers. This can be accomplished for example by adding 2 equiv. of allyl, propargyl or phosphonate trifluoromethane sulfonate on the dendrimer of generation 1 71-lGj] (Scheme 34). Functionalization occurs on the sulfur atom of the two P=N-P(S) units with the quantitative formation of the... 

At this stage two strategies (one of them outlined in Scheme 37) can be developed to start the construction of dendrimers within the cascade structure of 88-[G3]. Each of them allows the synthesis of six internal dendrimers of generation 4 into the internal voids of 88- [G3]. 31P NMR constitutes an extraordinary and unique tool for monitoring the construction of these controlled polydendritic structures (see Fig. 9 for illustration). Chemical shifts of phosphorus groups are different from one generation to another and the intensities of signals are of... 

This work demonstrates that functionalization of the internal cavities of various dendrimers can be done via a post modification of the skeleton. Various functional groups can be selectively introduced aminophosphite, aldehyde, hydrazone, dichlorophosphane sulfide. Therefore all the chemistry reported on the surface of dendrimers can be now envisaged to be done into the cavities and it is demonstrated for the first time that a macromolecular chemistry can be performed into the internal voids of a dendrimer. 

Phosphorus also permits the postmodification of the backbone of dendrimers. Indeed charges, and also various functional groups, can be selectively introduced into the internal voids. Such a transformation can be performed where and when required. 

The facile functionalization of the cavities allow the development of a macro-molecular chemistry within the cascade structure of dendrimers. As an example six dendrimers of generation 4 were built into the internal voids of a dendrimer of generation 3. 

In conclusion, in the limited study undertaken, the visual defects were indicated as being due to the presence of internal voids in the molded material. 

A typical plot of torque versus amount of liquid (di-butyl-phthalate, DBT) added is given in Fig. 15 for a very porous sodium carbonate powder mixture. The increase in measured torque at around 25% by volume DBT and again at around 65 %, can easily be seen (the final decrease in torque upon the formation of the wet cake is not shown). These two points correspond to the condition under which the continuous network of bridges forms, at about 25% liquid present, and the formation of the dry paste, at around 65%, where enough binder is available to fill most internal voids. The amount of liquid in Fig. 15 is given as a fraction of the total volume of powder, instead... 

A catalytic fixed bed reactor is a (usually) cylindrical tube that is randomly filled with porous catalyst particles. These are frequently spheres or cylindrical pellets, but other shapes are also possible. The use of rings or other forms of particles with internal voids or external shaping is on the increase. During single-phase operation, a gas or liquid flows through the tube and over the catalyst particles, and reactions take place on the surfaces, both interior and exterior, of the particles. 

Fig. 26. View of velocity contours down internal voids or holes of the center 4-hole particle.

Fio. 27. (a) Near-wall temperature map for the 1-hole particles (b) radial temperature profiles for solid cylinders and cylinders with two different sizes of internal void. 

The extension of this work to include catalyst particles with internal voids is more complex, as there are regions of catalytic activity adjacent to the internal holes, complicating the testing procedure. A comparison of several different catalyst configurations of internal voids has recently been completed, and a description of the method, its verification, and the results obtained will be the subject of a future publication. 

A very important question in the context of dendrimers and their utility as host molecules relates to the existence of cavities within these macromolecules. The presence of internal voids in dendrimers is closely related to their conformational behaviour and to the degree of back-folding of the terminal branches into the interior of the dendrimer. The issue of back folding was already briefly touched upon in section 16.2.1. Next to the purely theoretical calculations mentioned there, several calculations have been performed on specific dendrimer types. 

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