Introduction to Nanotechnology

11.5. SUPRAMOLECULAR STRUCTURES 295
nanostructure is called a “hairy nanosphere,” whereas if the sphere is small and
the projections are long, it is called a “star polymer”. If the nanosphere is hollow,
then fibers of the polymers (B), can also project inward from the inner surface of the
spherical shell. Copolymers can be used to construct structures that resemble the
micelles discussed in Section 12.4.2.
Star polymers are used in industry to improve the melt strength, that is, the
mechanical properties of molten plastic materials. Hairy nanospheres have been
employed for the removal of organic compounds from water, both in a dispersed
form and as solid microparticles. Polymer brushes are effective for dispersing latex
and pigment particles in paint. Nanostructures consisting of block copolymers
hnction as catalysts, are utilized in the production of nanosized electronic devices,
and find applications for water reclamation. Otsuka et al. (2001) pointed out that
block copolymers adsorbed on surfaces in brush or micelle forms, or self-assembled
into micelles, provide a powerful tool for manipulating the characteristics of surfaces
and interfaces. An example from this article is described in Section 11.5.4.
Block copolymers are expected to have novel applications, especially of the
biomedical type.
11.5. SUPRAMOLECULAR STRUCTURES
11 5 1. Transition-Metal-Mediated Types
Supramolecular structures are large molecules formed by grouping or bonding
together several smaller molecules. In this section we will follow Stang and Olenyuk
(2000) and discuss the assembly of supramolecular structures containing transition
metals in the form of molecular squares with a high degree of symmetry. Analogous
patterns have been synthesized in the shapes of equilateral triangles, pentagons,
hexagons, and even three-dimensional octahedra. These configurations can often be
constructed by the process of self-assembly. The use of self-assembly procedures in
industry could lead to lower manufacturing costs for chemical products.
A square supramolecular structure can be fabricated by starting with an angular
subunit and combining it with either a linear subunit, or another angular subunit, in
the manner sketched in Fig. 1 1.1 1. The former process, outlined in Fig. 1 1.12, was
used to produce the assembly shown in Fig. 1 1.13 in which either palladium (Pd) or
platinum (Pt) is the transition metal. Eight nearby singly charged counterions
-OSO,CF, compensate for the +2 charges on each of the four metal ions M2+.
The latter process for forming an approximately square molecule, outlined in
Fig. 1 1.14, produced the molecular square with the structure sketched in Fig. 1 1.15.
This figure lists the bond lengths and bond angles, and indicates that the Pd-Pd and
Pt-Pt separation distances are 1.4 nm and 1.3 nm, respectively. Once again, the +2
charge on each Pd ion is balanced by four nearby -OSO,CF, counterions. The
overall geometry of the center square is nearly flat, with only minor deviations from a
perfect plane. The stacking diagram of Fig. 1 1.16 clarifies how adjacent molecules fit
together in space.