Crystalline networks

Fig. 16.5. Glass formation. A 3-co-ordinoted crystalline network is shown at (a). But the bonding requirements are still satisfied if o random (or glassy) network forms, as shown at (b). The network is broken up by adding network modifiers, like NojO, which interrupt the network as shown at ( ).

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

J 8 Explain the role of chain length, crystallinity, network formation, cross-linking, and intermolecular forces in determining the physical properties of polymers (Section 19.12). 

Covalently crosslinked siloxane containing liquid crystalline networks with elastic properties were prepared 349). In all of the networks liquid crystalline phases of the linear precursors were retained. For low degrees of crosslinking the phase transition temperatures remained nearly unchanged, whereas higher degrees of crosslinking reduced the phase transition temperatures. 

Figure 4. The quasi-cubic units of crystalline network for 1,3,5,7- tetrahydroxyadamantane. Molecules are shown as spheres and hydrogen bonds as solid linking hnes. This crystalline structure is similar to that of CsCl. Taken from Ref. [8] with permission.

We have touched briefly on three simple ionic lattices, but there are many others. Moreover, the stmctures of many crystalline network solids can also be described by the methods we have introduced here for NaCl, CsCl, and CaF2. 

Similar behavior can occur when a crystalline network is disassembled by adding a solvent rather than by heating. These mesogens are called lyotropic liquid crystals and the mesophase formation shows temperature and concentration dependence. They are very important in biological systems, but have been much less studied in materials science. 

Two additional features of a semi-crystalline network should be considered one is the presence of some completely amorphous chains, and the other is the displacement of crosslinks by growing crystallites. As a network crystallizes the crystallites upset the balance of forces about the crosslinks, which are then forced to new positions of equilibrium. These problems have recently been attacked (7) by consigning the chains to their most probable positions. 

The elastic free energy AFe causes difficulty because of its sensitivity to the crystallization model assumed. To estimate AFe for lamellar morphology, consider first an important property of a network, amorphous or crystalline. Network crosslinks are considerably restricted in their fluctuations. Fluctuations of crosslinks several chains removed from a particular chain are therefore inconsequential for that chain. A chain in the interior of a path traced through several sequentially connected chains behaves as if the path ends are securely anchored at fixed positions ( 7). If Gj chain vectors make up the path, then... 

Both appear to be acceptable. And indeed they lead to quite acceptable solutions for amorphous networks. But neither of these constraints appear satisfactory for semi-crystalline networks, at least a proper solution has not yet been found. Our choice of vectois then is not a choice at all but rather an Imposition. 

We can assert outright that a semi-crystalline network in a continuous state of equilibrium might be satisfactorily described by a representative chain of average contour length and average crystallization. This is, after all, the same assumption that is invariably applied to amorphous networks. The results in this instance are exactly the same as those presented herein. 

Given the range of host molecules that may be that be put into network structures and strong current interest in the inclusion properties of crystalline network materials, embedding molecular hosts into hydrogen bonded network structures will continue to be a fruitful and exciting area of inclusion and structural chemistry. 

In other words, Aiatf/°(LiOCH3) is the energy required to destroy the crystalline network, yielding the gas-phase ions. As the cation and the anion are infinitely separated, the gas phase can be described by the ideal gas model and the enthalpy corresponding to the same process is given by... 

Figure 4.12 Doping profiles of 60 keV Al ions implanted in 4H-SiC with different alignments of the beam direction with respect to the crystalline network. Parts (a) and (b) refer to wafers and part (c) refers to the-20 ones. The beam-to-crystal alignment per profile is given in the inset of each picture. Parts (a) and (c) are SIMS measurements and part (b) is an MC-BCA simulated profile. The concentration scale is normalized to the implantation dose. (From [23]. 2003 American Institute of Physics. Reprinted with permission.)...

In a system based largely on self-assembly through mesogenic interactions such as the above example by Percec, it can also be beneficial to incorporate hydrogen bonding as well for an added element of organizational control. For instance, Frechet and coworkers demonstrated sequential, hierarchical self-assembly of various levels of order in a small molecule-based system which ultimately resulted in a hydrogen-bonded liquid crystalline network (Fig. 7.9) [53]. 

Fig. 7.9 Self-assembly arising from both hydrogen bonding and mesogenic behavior leading to a smectic liquid crystalline network.

Stoddart et al.l<,lb have reported the self-assembly of branched [njrotaxanes in an investigation aimed at the preparation of larger, dendritic rotaxanes. Kato et al. 9lcl have reported the preparation of supramolecular liquid crystalline networks based on self-assembly of carboxylic acid-based, trigonally branched, //-bonding donors and bipyridine-type //-bonding acceptors. 

There are two basic and complementary approaches to building crystalline networks that are constituted from organic or metal-organic components ... 

Wuest has demonstrated that the pyridone moiety also generates a hydrogen-bonded supramolecular synthon that is suitable for building extended arrays.67 Remarkably, methanetetra(6-phenylethynyl-2-pyridone) exhibits a diamondoid network, sevenfold interpenetration and cavities large enough to enclathrate butyric or valeric acid.27 Wuest introduced the concept of tectons to describe molecules that inherently possess the molecular structure and intermolecular recognition features to predictably self-assemble into crystalline networks. He followed this study with several other examples of diamondoid networks sustained by the pyridone moiety 27c d... 

---