Hierarchical phase structures

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Figure 5.24 Model of hierarchical self-assembly of chiral rodlike monomers.109 (a) Local arrangements (c-f) and corresponding global equilibrium conformations (c -f) for hierarchical selfassembling structures formed in solutions of chiral molecules (a), which have complementary donor and acceptor groups, shown by arrows, via which they interact and align to form tapes (c). Black and the white surfaces of rod (a) are reflected in sides of helical tape (c), which is chosen to curl toward black side (c ). (b) Phase diagram of solution of twisted ribbons that form fibrils. Scaled variables relative helix pitch of isolated ribbons h hh /a. relative side-by-side attraction energy between fibrils eaur/e. Reprinted with permission from Ref. 109. Copyright 2001 by the National Academy of Sciences, U.S.A.

These three different approaches are distinguished by the type of pore formers that are introduced in each case leaving particulates, molecules or functional groups in the former, self-organized entities (mainly micelle and lamellar structure formers) in the second approach, and a continuous polymeric phase in the latter approach. The three different approaches also yield, respectively, very different gel morphologies microporous or macroporous material mesoporous materials and hierarchical pore structures with macro- or mesoporosity as well as nanoscale pores within the same material domain. 

In this chapter, I outlined recent developments of a new polymerization techniques using a asymmetric liquid crystal reaction field of the N -LC phase, focusing on helical polyacetylene with a super-hierarchical spiral structure (H-PA). Chiral binaphthyl axially chiral derivatives are added as a chiral dopant to nematic liquid crystal to produce the chiral N -LC phase. 

Another important consideration involves the hybridization of porous carbon with hierarchical 3D architectures, such as fibers or arrays. Wet chemical techniques are often useless as the mandatory solvent removal/drying typically results in the at least partial collapse of the nanocarbon pore structure. Gas phase deposition is a... 

The thermal properties of C3 materials at high temperatures are most remarkable if protected from oxidation. This issue is discussed below in more detail. If they are not oxidized, the C3 materials exhibit similar stability data as ceramics [22], in particular at temperatures above 1500 K where protective coatings applied behave like a plastic and close developing surface cracks against air attack. C3 materials expose the advantages of their hierarchical structure being present in both filler and binder phase and develop wood-like properties under ambient conditions. A descriptive pa-... 

There are thus multiple effects by which the properties of the nanocarbon-semiconductor hybrid material can be different from the simple physical mixture of the two components [1] The nanocarbon offers an effective way for an efficient dispersion of the semiconductor, thus preventing agglomeration, but also providing a hierarchical structure [15] for efficient light harvesting and eventually easy access from gas/liquid phase components (in photocatalytic reactions) or electrolyte (in DSSC). 

Figure 15. Hierarchical structure formed by combining liquid-phase templating with micromolding. (Reprinted with permission from ref 166. Copyright 1998 American Association for the Advancement of Science.)...

De Mori et al. have taken a different approach to take advantage of MC simulations. They used a coarse-grained Hamiltonian to presample phase space in an approximate manner. This is followed by MD simulations starting from representative structures from the most dominantly populated clusters within the MC ensemble. Such a hierarchical strategy was employed to study the folding of a small protein [144] and the oligomer formation of short, amyloidogenic peptides [145]. 

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