Hierarchical Crystalline Structures

3 Crystalline Structures of Polymers 10.3.1 Hierarchical Crystalline Structures 

Most of crystalline polymer materials exhibit multi-scale hierarchical structures. At the scale of 0.1 nm, the polymer chains contain regular sequences. At the scale of 0.5 nm, they form stable helical conformations, which then pack together in a compact parallel fashion to make the periodic lattice structure, with the unit cell at the scale of 1 nm. At the scale of 10 run, the folded-chain lamellar crystals are formed for the flexible polymer chains. At the scale of micrometers or larger, the lamellae further assemble into spherulites. Such hierarchical structural characteristics at varying length scales of polymer morphologies are illustrated in Fig. 10.7. 

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Bone is an extremely dense connective tissue that, in various shapes, constitutes the skeleton. Although it is one of the hardest structures in the body, bone maintains a degree of elasticity owing to its structure and composition. It possesses a hierarchical structure and, as most of the tissues, is nanostructured in fact, it is a nanoscaled composite of collagen (organic extracellular matrix) and hydroxycarbonate apatite, (HCA, bone mineral). This nanostructure is in intimate contact with the bone cells (several microns in size), which result (at the macroscopic level) in the bone tissue. Figure 12.2 shows the bone hierarchical ordering from the bone to the crystalline structure of HCA. 

The first step in any chemical approach to crystalline structure is to determine the short-range order, i.e. which atoms are bonded. The most convenient way of doing this is by means of the bond graph described in Section 2.5. In many cases all or most of the bond graph can be determined from first principles, since, except for the weakest bonds created in the post-crystallization stage, the bond graph is determined by the rules of chemistry, particularly the hierarchical principle (Rule 11.5), the valence matching principle (Rule 4.2), and the principle of maximum symmetry (Rule 3.1). 

The mesogen structures may be formed not only by covalent bonds, but also by non-covalent interactions, such as hydrogen bonds, ionic interactions, and metal coordination [71]. A recent example [72] of this concept comprised the self-assembly of complex salts into stable hierarchical aggregates with a dense core and a diffuse shell. These materials were made from diblock copolymers poly(acrylic acid)-block-poly(acrylamide) and the cationic surfactant dodecyltrimethylammonium. Due to non-covalent interactions the surfactant/polymer aggregates exhibited a liquid crystalline structure of cubic symmetry. 

One may now ask whether natural systems have the necessary structural evolution needed to incorporate high-performance properties. An attempt is made here to compare the structure of some of the advanced polymers with a few natural polymers. Figure 1 gives the cross-sectional microstructure of a liquid crystalline (LC) copolyester, an advanced polymer with high-performance applications [33]. A hierarchically ordered arrangement of fibrils can be seen. This is compared with the microstructure of a tendon [5] (Fig. 2). The complexity and higher order of molecular arrangement of natural materi-... 

Up to this point, we have summarized the NMR parameters of some selected crystalline compounds. Model crystalline compounds of calcium phosphates are important structural reference for the study of biominerals because the highly complex hierarchical structures of bones and teeth contain different phosphorus species such as unprotonated orthophosphate (PC>43 ) and protonated orthophosphate (HP042 ).70,77 It is beyond doubt that 31P chemical shifts are the most important spectroscopic parameters for the characterization of calcium phosphates. While the 31P chemical shift can reflect the protonation state of a phosphate species, 31P CSA is a sensitive measure of the symmetry of the electronic... 

Silk is a fibrous protein produced by several insect species. Commercially, silk is produced from the cocoon stage larvae of the moth caterpillar Bombyx mori, as it has been, in China, for some 4500 years. A single cocoon produces a continuous thread up to 1 km in length, and the protein fibroin contains large amounts of glycine, alanine, tyrosine, proline and serine The peptide chains are arranged in anti-parallel P-sheets which make up the hierarchical structure of the crystalline silk fibres. A number of spiders also produce silk webs, although the fibroin structure is rather different to that from silk worms. 

Silks are partially crystalline protein fibers. Animal tendons consist of collagen, another fibrous protein with a complex hierarchical structure. 

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

As with other natural fibres, silk has a hierarchical microstructure - about five anti-parallel (f-sheets, each with around 12 chains, aggregate to form parallel, crystalline microfibrils (approximately 10 nm in diameter), bundles of which make up fibrillar elements (roughly 1 p,m across), which in turn associate to comprise the individual fibroin filaments (7-12 xm) at each level of organisation, the ordered elements are embedded within amorphous matrices derived from the non-crystalline components. Once again, then, the behaviour of the structural composite can be understood in terms of the semi-crystalline array of its component parts. 

First, the lyotropic phase is used as a template for the preparation of a bicontinuous silica structure, from which the polymer is removed by calcination or extraction. In the second step the porous inorganic structure is filled with monomer and crosslinker which is polymerized to form a bicontinuous organic polymer network from which the silica template is removed by treatment with hydrofluoric acid. An example for the preparation of hierarchical structures is the synthesis of bicontinuous pore structures by using two templates simultaneously [115]. In this case a liquid crystalline lyotropic phase of an amphiphilic block copolymer is used as a template together with suspended latex particles. The sol-gel process with subsequent calcination leads to a bicontinuous open pore structure with pores of 300 nm and 3 nm. 

Zeolite materials with tunable size and volume of mesopores can be prepared by using dispersed carbon black particles with narrow distribution of their sizes as inert mesoporous matrix or as secondary template. In such confined space for synthesis the crystallization of zeolite gel occurs inside the interparticle voids of carbon matrix [10,11,12]. In the case of generation of mesopores by secondary templating by means of addition of carbon black into the reaction mixture, zeolite crystals are formed around carbon particles [13]. After burning off a carbon matrix or carbon particles, zeolite crystals with a controlled pore size distribution and a crystalline micro-mesoporous hierarchical structure are prepared. 

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