FIGURE 3: Chemical bonds acting as spacers are generating large cavities in a lattice.
This empty space does not contribute to the structural stiffness of the material, being filled with solvent molecules, which of course are too fluid to achieve significant mechanical strength. It would be very desirable to bring the cores of the MBBs together much closer, so that their mutual vdW-interactions are able to contribute to the material stiffness. This is the case in the interior of proteins, where most of the stability comes only from weaker-than-covalent interactions. But then on the other hand, the internal design of proteins does not adhere very much to well organized rasters, which renders their design much more difficult. A strategy that might be pursued to reduce the spacer-effect is illustrated symbolically in figure 4.
FIGURE 4: Reducing the adverse separation effect of the spacers.
The idea is to fold back the link-chemistry onto the surface of the MBBs, so that the links do not stand off like the spines of a hedgehog. An actual implementation might look like figure 5.
FIGURE 5: A pair of MBBs with three-fold connectivity. The two components trindan-diene and iceane-tridiph can be retrieved for viewing in a molecular modelling program.
Note that this proposed design also incorporates the separation of the link-polarities to circumvent the storage problem alluded to above. That is why there are two distinct types of MBBs here. Only the one on the left side bends back the link-functionalities somewhat. This example demonstrates the new problem with this approach. To bend back the links, it is necessary to introduce more angles in the skeleton, which have to be rigid to avoid floppy structures. This leads to polycyclic structures which are increasingly difficult to synthesize (without already having access to molecular manufacturing methods :-).
Surprisingly, a tendency in current research seems to have turned the cavity problem into a virtue. Design of zeolites has become very popular, because of the hope that the large internal surface areas might be used for catalytic or separational purposes. But the zeolite designers are encountering a recurring problem which is again related to the emptiness of the very cavities they would like to create. Very often not one lattice is generated, but several, which are independent but mutually interlocked, and so all of the space is filled out nevertheless. An instructive example can be found in [Erm88]. This can be avoided by the following trick. During the growth of the zeolite crystals, one has to provide template molecules which are able to claim the cavity-space, to prevent the formation of the interpenetrating lattices. One of the most successful attempts and a concise description of the zeolite phenomenon can be found in [HosRob90]. But the issues involved in getting useful zeolite designs to work, add an additional twist to the already significant challenge of designing good MBBs.
It seems as if the real challenge in constructing designer solids lies in achieving dense packing. Airy zeolite-like structures have been prepared several times, accidentally and deliberately. But the systematic elimination of the empty spaces is the difficult task. This situation is reminiscent of the challenges in the protein engineering community, where the first step of getting artificial proteins to fold up into roughly the right way, including secondary structures, seems to have been easier than commonly imagined, but the last step of getting the amino acid side chains to organize properly in the interior to obtain really good tight packing, seems to be unexpectedly hard to accomplish [Ric92].