Porous and Cavity-Containing Structures

Both bpy and the extended bipyridyl ligand (9.10) have been used to produce square grid compounds, analogous to Hoffman-type inclusion compounds. The additional two carbon spacer in 9.10 apparently has the effect only of extending the grid dimensions in most cases with the exception of the NbO network 

While pyridyl-based coordination polymers are extremely synthetically versatile they are not generally particularly thermally robust as a consequence of the tendency of the pyridyl ligands to dissociate or decompose above ca. 250-300 °C. The volatility of the ligands is linked to the fact that they are neutral and hence stable in the free state. Moreover their neutrality means that even in the absence of interpenetration, coordination polymer frameworks are generally filled with counter-anions which. 

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The results presented here show that the PHEMA hydrogels prepared in the presence of water contain a porous network on a number of length scales and with varying structure. On the angstrom level, substantial free volume has been identified by both Xe NMR and positron lifetime annihilation spectroscopy. It is likely that the free volume cavities detected by these two techniques exists within relatively hydrophobic, i.e. non-hydrated domains. On the micron level and larger a network of water-filled pores was identified by H NMR relaxation time measurements. It is this porous network that is responsible for the transport properties of PHEMA confirmed in numerous previous studies. The results are consistent with previous NMR studies of bulk-polymerized PHEMA. 

Frequently we define a porous medium as a solid material that contains voids and pores. The notion of pore requires some observations for an accurate description and characterization. If we consider the connection between two faces of a porous body we can have opened and closed or blind pores between these two faces we can have pores which are not interconnected or with simple or multiple connections with respect to other pores placed in their neighborhood. In terms of manufacturing a porous solid, certain pores can be obtained without special preparation of the raw materials whereas designed pores require special material synthesis and processing technology. We frequently characterize a porous structure by simplified models (Darcy s law model for example) where parameters such as volumetric pore fraction, mean pore size or distribution of pore radius are obtained experimentally. Some porous synthetic structures such as zeolites have an apparently random internal arrangement where we can easily identify one or more cavities the connection between these cavities gives a trajectory for the flow inside the porous body (see Fig. 4.30). 

A porous material consists of at least two immiscible phases of which one is usually a continuous sohd material, the matrix, which surrounds the second phase of finely dispersed voids, the pores, containing a liquid, gas, or vacuum. If the void phase is discontinuous and comprises individually separated cavities filled with gas (bubbles), the material represents a foam structure. On the other hand, if both phases form two interpenetrating continua with the matrix as well as the pores being continuous, the material represents a sponge structure or a so-called porous network. Such porous networks with interconnected voids are the focus of this article as they may have funda-... 

MiHuStUe Structures Contiauous porous structures are the other popular type of alternatives to padced beds. They can be formed in many different forms, such as flat disks or as long rods or as anything in between. Flat disks have occasionally been called membranes and have been compared to stacked membranes (21). Monolithic rods have been compared to chromatographic columns (22). They are typically prepared in situ (although this is not a necessity) in the cavity that will also form the containment for use. 

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