Linked Network Assemblies

Growing from simple polymeric assemblies, more complex structures arise from the interactions between polyvalent boronic acids interacting with poly-functional diols. These products may be dynamic, highly cross-linked polymer networks, analogous to slime. Alternatively, these assemblies have taken the form of highly ordered frameworks with persistent pores. Regardless of the degree of order inherent in these systems, the key assembly motif still relies on boronate ester formation. 

As discussed earlier, linear assemblies can cause gelation via boronate ester formation. Multidimensional or network systems have also been 

FIGURE 30. (a) Poly(lysine) appended boronic acids interact with fructose to form helix-rich super-structure architectures, (b) Alternatively ditopic glucose intramole-cularly cross-links to the polymer to introduce P-turns. 

One of the unique architectural motifs that truly relies on the covalent yet reversible nature of boronate ester formation is the creation of complex hierarchical structures referred to as COFs. If boronate ester formation can be used to obtain discrete macrocyclic structures, it is then reasonable to expect that linked macrocyclic networks may be generated. These networks assemble 

FIGURE 31. Amorphous assemblies form from boronate functionalized polysaccharides as well as in layer-by-layer assembhes between boronic acid functionalized poly(aniline) and RNA. 

---

From several applications, it is convenient to describe much of the above systems as resulting from the modification of the parent porous materials by a second component. In this sense, one can separate network modification, network building, and network functionalization processes. Network modification exists when the final structure of the parent material is modified as a result of its combination with the second component, thereby resulting in the formation of a new system of links. Network building occurs when the material is formed by assembling the units of both components. Finally, functionalization involves the attachment of selected molecular groups to the host porous material without modification of its structure. 

Mechanochemical transduction aided by sonication can also be achieved at a supramolecular level. Given the dynamic character (self-assembly and disassembly) of non-covalent interactions, reversible transformations to create new responsive polymers may potentially be accomplished. As shown in Fig. 16, a cross-linked network containing a europium(III) salt undergoes metal-ligand dissociation upon... 

Organic template structures derived by self-assembly processes of amphiphiles are imprinted into cross-linked networks, mostly of the inorganic type. The template morphology determines the structure and morphology of the entire pore system. Furthermore, detailed investigation of the pore structure allows inferences to be dl-awn about the template, and therefore, about the self-assembly process. 

In solution, peptide molecules adopt a specific secondary conformation, like P-sheet, P-hairpin, a-helix, or the coiled coil (Fig. 5.26). The secondary structures then self-assemble to form nanofibers or physically cross-linked networks. Elongation of the nanofibers in 3D space leads to thicker and longer fibers, which further assemble to fibrillar networks capable of entrapping water (Fig. 5.25) [55]. P-Structured peptides dominate the literature of self-assembled systems, either natural or designed. 

Wool and hair have the most complex structures of any textile fibres. In the paper by Viney, fig. 1 shows how keratin proteins, of which there are more than one type, all having a complicated sequence of amino acids, assemble into intermediate filaments (IFs or microfibrils). But, as shown in Fig. 5a, this is only one part of the story. The microfibrils are embedded in a matrix, as shown in Fig. 5b. The keratin-associated proteins of the matrix contain substantial amounts of cy.stine, which cross-links molecules by -CH2-S-S-CH2- groups. Furthermore, terminal domains (tails) of the IFs, which also contain cystine, project into the matrix and join the cross-linked network. At a coarser scale, as indicated in Fig. 5c, wool is composed of cells, which are bonded together by the cell membrane complex (CMC), which is rich in lipids. As a whole, wool has a multi-component form, which consists of para-cortex, ortho-cortex, meso-cortex (not shown in Fig. 5a), and a multi-layer cuticle. In the para-and meso-cortex the fibril-matrix is a parallel assembly and the macrofibrils, if they are present, run into one another, but in the ortho-cortex the fibrils are assembled as helically twisted macrofibrils, which are clearly apparent in cross-section.s. 

Similarly P-P, meso-meso, P-P triply linked dimers were obtained by Diederich and CO workers. They showed that such dimers led to weU-organized self-assembled networks on silver surfaces [162, 163]. Similar self-organized networks were observed for hybrid Ceo-triply linked dimer assemblies [162, 163]. Hybrid assemblies between single-walled carbon nanotubes (SWNTs) and triply linked trimers were also investigated and allowed the formation of stable supramolecular carbon nanotubes-porphyrin complexes [164]. 

---