Chicken-wire networks

However, various attempts during the last 17 years to prepare crystals having empty "chicken-wire networks and large guest molecules in the postulated cylindrical channels have failed. The various TMA polymorphs and complexes examined so far were found to be characterized by structures in which the channels formed by the TMA sheets are not available for accommodating guest molecules due to the remarkable phenomenon of mutual interlacing or triple catenation of the TMA networks [3>4>5]. 

Fig. 2. The basic chicken wire motif in hydrogen-bonded TMA (i) is a two-dimensional network of six-molecule rings, the hydrogen bonds between carboxyl groups being represented by dashed lines. This network is planar in y-TMA and folded (pleated) in a-TMA (taken from Ref )...

In y-TMA the hexagonal networks (the chicken wire ) are all essentially planar and thus the two interlaced sets of parallel triplets cannot fill space efficiently but leave channels with axes along c. The overall arrangement is shown schematically... 

Fig. 10. Stereophotographs of a space-filling model of part of the a-TMA (5) structure, showing how the three-dimensional structure is built up by triple catenation of two pleated chicken-wire TMA networks. The arrangement in (a) is directly comparable with the schematic diagram of Fig. 8 however the model comprises only that portion of Fig. 8 lying between the eentral and right-hand two-fold axes, (b) Two interlaced TMA networks. This part of the diagram is directly comparable to Fig. 9. (c) Three networks (d) Four networks. This shows the interpenetration of one network by parallel portions of three other networks, (e) Six networks, showing the mutual interlaeing of three parallel networks with three others in the second orientation. (Taken from Ref.

Different theories exist about the thickening mechanism of fumed silicas. One of the first was the so-caUed chicken wire structure. That means the silica particles interact with each other via their silanol groups and form a three dimensional structure, which reduces the mobility of the hquid molecules. Under mechanical impact like shearing or shaking the structure is destroyed and the viscosity of the system decreases. After the end of the mechanical impact, the three dimensional network re-establishes itself and the viscosity increases again as a function of time. This mechanism may be valid in simple nonpolar liquids. In liquid mixtures or polymer solutions it is much more complicated and the adsorption pattern on the fumed silica surface seems to play an important role [76]. 

Let us now introduce the concept of degree of continuity of a phase. In the beginning of the IPN synthesis, polymer network I obviously exhibits continuity of both the network structure and its phase. When monomer II is uniformly swollen in, before polymerization of II, one phase also exists. Polymer network I is continuous (the sample is usually a swollen elastomer), and the monomer II is also distributed everywhere. Upon polymerization of II, phase separation takes place. Polymer network I is still continuous, but is partially or wholly excluded from some regions of space. Assuming the previous even distribution of monomer II, we have reason to believe polymer network II will exhibit some degree of chain continuity. Sometimes, polymer network II also appears to exhibit a degree of phase continuity. Usually, polymer network II has less continuity than polymer network I. A simple example of greater and lesser phase continuity in everyday life is chicken-wire in air. 

Figure 1 Open network arrangements of trimesic acid on graphite under UHV at low temperature, (a) and (c) The chicken wire structure and (b) and (d) the flower arrangement. (Reproduced from Ref. 18. Wiley-VCH, 2002.)...

Figure 6.15 Chicken-wire motif in TMA. Interpenetration of three such networks results in the generation of a close-packed structure. 

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