Networks molecularly interlocked

Fluorine, Fs, oxygen, 02, and nitrogen, N2, all form molecular crystals but the next member of this row of the periodic table, carbon, presents another situation. There does not seem to be a small molecule of pure carbon that consumes completely the bonding capacity of each atom. As a result, it is bound in its crystal by a network of interlocking chemical bonds. 

A molecularly interlocked IPN of epoxy and polyimide was developed by Gaw et al. to form molecular composites of ODA-PMDA polyimide and DGEBA epoxy [73]. In this system the epoxy monomers were homogeneously mixed with a fully polymerized precursor to the polyimide, polyamic acid, that contained reactive groups that could react with the epoxy forming the three dimensional network. This system overcame many of the problems of previous systems by the use of a novel solvent system. 

FIGURE 12.23 The basic structure of a tectosilicate is composed of an infinite network of interlocking silicate tetrahedrons. The example shown here is leucite, whose molecular formula is KAISijOg. 

The function of interpenetrating networks may be similar to that of chemical bonding in that molecular interlocking provides an effective connection between the silane layers and the matrix resin. However, no experimental evidence of interpenetrating networks has yet been reported. 

The essential property of a cementitious material is that it is cohesive. Cohesion is characteristic of a continuous structure, which in the case of a cement implies an isotropic three-dimensional network. Moreover, the network bonds must be attributed to attractions on the molecular level. Increasingly, recent research tends to show that cements are not bonded by interlocking crystallites and that the formation of crystallites is incidental (Steinke et al., 1988 Crisp et al., 1978). The reason is that it is difficult to form rapidly a mass which is both cohesive and highly ordered. 

This is a theoretical study on the entanglement architecture and mechanical properties of an ideal two-component interpenetrating polymer network (IPN) composed of flexible chains (Fig. la). In this system molecular interaction between different polymer species is accomplished by the simultaneous or sequential polymerization of the polymeric precursors [1 ]. Chains which are thermodynamically incompatible are permanently interlocked in a composite network due to the presence of chemical crosslinks. The network structure is thus reinforced by chain entanglements trapped between permanent junctions [2,3]. It is evident that, entanglements between identical chains lie further apart in an IPN than in a one-component network (Fig. lb) and entanglements associating heterogeneous polymers are formed in between homopolymer junctions. In the present study the density of the various interchain associations in the composite network is evaluated as a function of the properties of the pure network components. This information is used to estimate the equilibrium rubber elasticity modulus of the IPN. 

The magnets described in this work are among the very few two- or three-dimensional molecular structures with complete interlocking of independent infinite networks. Other examples are silver tricyanomethide [17], trimesic acid [18], dia-quabis(4,4 -bipyridine)zinc hexafluorosilicate [19], zinc bis(tricyanomethide) [20], and bis(l,2-di-(4-pyridyl)-ethylene-bis(thiocyanato)iron(H) [21]. Interlocking of rings in discrete supramolecular units is much more developed [22-25] and most of this book is devoted to this topic. 

In that respect, it is worth noting that in the sperm of the pentatomid bug Murgantia histrionica branched mitochondria fuse gradually to form a cluster of interlocked networks (Pratt 1968), a fusion that was seen as a mechanism to give the mitochondrial DNA a chance to recombine. Thus, an appreciable quantity of recombined paternal mitochondrial DNA is assumed to be transferred to the fertilized egg (Nass et al. 1965 Baccetti Afeelius 1976). However, it is often critical to detect an influence of the paternal mitochondrial DNA in a fertilized egg or embryo (Baccetti Afeelius 1976). Thus, by making a cross-fertilization between two Xenopus toad species, and using molecular hybridization techniques, Dawid (1972) failed to find any... 

Fig. 10. Interlocking of loops in irregular fold surfaces following folded-chain crystallization from suitably high molecular weight polymer solutions over a limited concentration range to form junctions in a network.

The continuing development of such systems exhibits great potential in the areas of sensing, molecular switches, and nanoscale machines capable of doing work. Realization of these ambitious goals will likely require new synthetic methods to construct more complex hierarchical ion pair networks containing multiple aligned mechanically interlocked assemblies for amplified functionality and macroscopic device incorporation. 

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