Liquid-crystal template pathway

Soft template method by using block copolymers was reported for first time by Liang et al. [72] in 2004. After that, a significant progress on the fabrication of carbon with a well ordered mesopores was achieved [32, 73-77]. Zhao and coworkers performed a widespread study of soft template via the triblock poly (ethylene oxide) (PEO) and poly(propylene oxide) (PPO) based systems, PEO-PPO-PEO [65, 78]. Several ordered pore stmctures corresponding to various surfactant liquid crystal phases were synthesized by liquid crystal template pathway, a schematized synthesis procedure is shown in the Pig. 7.11. 

As mentioned previously, mesoporous inorganic materials can be formed by one of two reaction pathways, liquid-crystal templating and cooperative self-... 

Moreover, diffusion pathways can be individually designed by templating particular phase structures. Above all, the pore system of a macroscopic object is exclusively determined by the pore system, whereas particulate powders show a significant contribution to the surface area caused by the nonstructured particle surface. The direct liquid crystal templating approach was also used to prepare monolithic silica from TMOS or TEOS in block copolymer-water mixtures mixed with alcohol cosurfactants and hydrophobic swelling agents [39-41]. 

Besides cooperative pathways, also tme liquid crystal templating (TLCT) and the hard template route (Section 9.3.7) have been developed for the synthesis of ordered mesoporous materials. In the case of the TLCT, a preformed surfactant liquid crystalline mesophase is loaded with the precursor for the inorganic materials (140). The nanocasting route, on the other hand, is a clearly distinct method (141). Here, no soft surfactant template is used but, instead, the pore system of an ordered mesoporous solid is used as the hard template serving as a mold for preparing varieties of new mesostructured materials, for example, metals, carbons, or transition metal oxides. 

Figure 9.18 Formation of ordered mesoporous MCM-41 silica (liquid crystal templating mechanism (LCT) according to Beck et al. (137). The pathway 1 is liquid crystal-initiated and the pathway 2 is silicate-initiated. (Adapted and reproduced with permission from J. S. Beck et al. J. Am. Chem. Soc. 1992,114,10834. Cop5rright 1992 American Chemical Society.)...

All authors conclude that carbons with adjusted pore size distribution in the entire range of the nanopores can be obtained, depending the synthesis conditions and cationic surfactants used as templates. Despite this, it is an effective pathway to obtain porous carbons, even though the pore formation mechanism is not well understood. Hence, different mechanisms are used to explain its effect emerged, such as liquid crystal templating mechanism, cooperative self-assembly, electrostatic interaction between cationic surfactant molecules and the anionic RF polymer chain and micelles as nanoreactors to produce RF nanoparticles [51, 70]. In these cases, the simple mold effect from the globular form and the RF polymerization around it is insufficient to explain the structuring of the material by the template, where spherical closed pores would be expected. 

Figure 1. Schematic pathway for preparing surfactant-templated mesoporous silicas, illustrating a formation mechanism based on preformed liquid crystal (LC) mesophase (route A) or a cooperative process (route B). Reprinted from [20], Copyright (2008) WILEY-VCH Verlag GmbH Co.

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