Macroporous structures

In the multistep swelling and polymerization method, polymerization time, temperature, and initiator concentration have marginal effects only. The ratio of monovinyl to divinyl monomer in the polymerization mixture, along with the composition of the porogenic systems, is known to be the most important factor in controlling the ultimate macroporous structure. The average molecular... 

The chemical nature of the packing has the largest influence on the retention of molecules and a big impact on the efficiency of the separation itself. The chemical and physical properties of the sorbent are determined by the choice of the comonomers for the copolymerization. The type of the copolymerization process employed by the synthetic chemist introduces the macroporous structure into the sorbent and determines the surface topology (accessibility, resolution) and the surface chemistry of the packing (4). 

Removal of diluent by an extraction process To obtain the final stable macroporous structure, the liquid organic diluents and the linear polymer are removed from the crosslinked structure by extraction with a good solvent for the inert diluents and particularly for the linear polymer. Toluene or methylene chloride are usually preferred for the removal of linear polystyrene from the divinylbenzene crosslinked macroporous polystyrene particles [125,128]. The extraction is carried out within a Soxhelet apparatus at the boiling point of the selected solvent over a period usually more than 24 h. 

Macroporous Structure of Neutral PHEMA Containing Networks... 

Fig. 6. Schematic representation of macroporous PHEMA hydrogel sponges. Interstitial spaces between polymer droplets create a macroporous structure 1-20 pm in size, whereas the polymer network creates a 1-100 nm mesh size in the polymer phase.

Physisorption measurements showed that carbon nanomaterials exhibit rather meso- and macroporous structures (maximum micropore fraction, 15% see Table 2.1). The lowest specific surface area was measured with the platelet fiber catalyst exhibiting slightly more than 100 m2/g. The Co/HB material offers 120 m2/g of surface area, and the highest BET value was determined with the Co/ MW catalyst featuring nearly 290 m2/g. Carbon nanomaterials, though, are not really porous, as the space between the graphene layers is too small for nitrogen molecules to enter. The only location of adsorption is the external surface of the nanomaterials and the inner surface of the nanotubes. 

The monolithic technology was used for CEC by Nilson et al. who introduced superporous imprinted monolithic capillaries in 1997 [125-127]. Isooctane was used as a porogen in order to produce a macroporous structure with large pores without interfering with the imprinting process. These imprinted monoliths were... 

While for macroporous structures the inner surface can be calculated from the geometry, meso and micro PS layers require other methods of measurement First evidence that some PS structures do approach the microporous size regime was provided by gas absorption techniques (Brunauer-Emmet-Teller gas desorption method, BET). Nitrogen desorption isotherms showed the smallest pore diameters and the largest internal surface to be present in PS grown on low doped p-type substrates. Depending on formation conditions, pore diameters close to, or in, the microporous regime are reported, while the internal surface was found to... 

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

The manufacture of a photonic crystal requires extreme process control because a deviation from perfect periodicity in the order of a few percent of the wavelength worsens the optical performance. Macroporous silicon is a potential candidate for the realization of such structures because of its photolithographic patterning. The precision of the macroporous structures is reflected in the transmission measurements along the T-M and T-K directions, which exhibit a photonic band-gap centered at 5 pm, as shown in Fig. 10.16. For measurement the macroporous... 

Designing a specific material architecture. 3D hierarchical carbon [79,80], 3D aperiodic [79,81,82] or highly-ordered hierarchical carbons are representative samples with multimodal pore structure to optimize the performance of the capacitors. The micropore, mesopore and macropore structure of such three-dimensional hierarchical carbons are generally perfectly interconnected. 

Monolithic stationary phases have to be regarded as the first substantial further development of HPLC columns, as they present a single particle separation medium, made up of porous polymer. As a consequence of their macroporous structure, they feature a number of advantages over microparticulate columns in terms of separation characteristics, hydrodynamic properties, as well as their fabrication ... 

The pore size distribution in highly activated carbon HSGD measured with low temperature nitrogen adsorption shows absence of the curve maximum in the range of 75-900 A (Fig. 29.6). In comparison with the pore distribution of SCN hemosorbent, HSGD has predominantly meso- and small macroporous structure, with some... 

An alternative explanation of the data presented in Figure 3 is that the majority of the macropore structure is available for reaction, and by breaking up the coal one is simply opening up hitherto sealed pores. However, this does not seem likely since—to explain the results in Figure 3— one would have to assume that from 2% to 90 mesh the exposed internal area of the coal increases by a factor of 12. Mercury penetration data on Coal C indicates that the macropore area increased only 7% going from 2% to 90 mesh (see Discussions). Also, Malherbe (16) found for three bituminous coals that in going from 5 to 270 mesh the BET area only increased by a factor of 2. Furthermore, the helium density of coal is independent of particle size indicating no sealed pores exist at least in respect to the accessibility of helium (14). 

Figure 1. SEM image of macroporous mesostructured Nb oxide showing (A and B) the tubular macropore structure and (C) a region of lower order.

In summary, we have shown how two very similar Mo and Nb oxide mesostructured systems leads to formation of either continuous macroporous structures with extended tube lengths or free-standing torroids with limited extension in the third dimension. We attribute this difference to relative rates of condensation versus nucleation in each system. A unique TEM-induced reaction involving evolution of shape was also discussed and demonstrates that many of the new mesostructures with order on the macroscopic level may also show dynamic behavior. 

Modification of NbMCM-41 sieves with Ni or Cu via a cation-exchange procedure causes the transformation of meso- to non porous or macroporous structure. 

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

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