PAMAMOS dendrimers

Figure 5.30. Illustration of a poly(amidoamine-organosilicon) (PAMAMOS) dendrimer, with two generations of each PAMAM and organosihcon units. Although the PAMAMOS represents a block copolymer, an unhmited number of other variations that contain a random copolymer array, or varying dendron subunits, may also be synthesized. Reproduced with permission from Dvornic, P. R. Owen, M. J. Synthesis and Properties of Silicones and Silicone-Modified Materials, ACS Symposium Series 838,2002,236.

Figure 5.31. Network (megamer) formation through the hydrolysis/crosslinMng of neighboring PAMAMOS dendrimer units. Hydrolysis of the C-O-Si bond may also be exploited for the controlled-release of entrained agents (e.g, cancer drugs, etc.). It should be noted that subsequent thermal annealing to remove the PAMAM cores results in a nanoporous network that has a dielectric constant (k) of ca. 1.5 - of extreme interest for next-generation IC interconnect applications. Reproduced with permission from Dvornic, P. R. Li, J. de Leuze-Jallouli, A. M. Reeves, S. D. Owen, M. J. Macromolecules, 2002, 35, 9323. Copyright 2002 American Chemical Society.

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

As already introduced in Section 2.4, angular dependent NEXAFS spectroscopy is a powerful tool in the study of self assembled systems, and was successfully applied to the investigation of self assembled monolayers and multilayers of molecular-based NLO materials. Among others, Bubeck and co-workers [70] investigated the interaction of Cu " with C, N and O moieties in nanocomposites containing copper ions complexed in poly(amidoamine-organosilicon), PAMAMOS (dendrimer molecular structure is shown in Fig. 4.19). 

Figure 3 Effect of the generation of PAMAM dendrimer reagent on the rate of the synthesis and composition of the resulting PAMAMOS dendrimer product...

As the degree of PAMAM dendrimer surface substitution by OS branch cells increases, the resulting PAMAMOS dendrimers cease to be water soluble. For example, this transition occurs at circa 50 % -NH substitution for the first OS branch cell layer around a generation 3 PAMAM interior. 

Surface activity of these PAMAMOS dendrimers was determined by the Wilhelmy plate method for the water soluble dendrimers and by the Langmuir trough technique for the insoluble ones. Some preliminary data on water soluble PAMAMOS have already been published (5,42), It was shown that the best of these materials lower the surface tension of water to just below 30 mN/m at 5 wt. %, with no break to a constant surface tension that would indicate micelle formation. Thus, these PAMAMOS behave more like considerably surface active water soluble polymers than surfactants. However, it is probable that their homologues with longer siloxane dendrons than the trimethylsilyl- groups studied so far will have considerably more surfactant-like behavior. 

PAMAMOS Dendrimer-Based Networks Containing Hydrophilic and Hydrophobic Nanoscopic Domains... 

Figure 6 Effect of the time of curing at room temperature on the glass temperature (Tg) of the resulting PAMAMOS dendrimer-based network...

Thermal and thermo-oxidative stability of the PAMAMOS dendrimer-based networks is predetermined by the stability of their less stable compositional parts, i.e., the PAMAM dendrimer interior. In nitrogen, thermal degradation occurs in two steps. 

However, when tested with NaCl and methylene blue water solutions, these PAMAMOS dendrimer-based networks behaved completely differently. For example, while permeability was excellent for sodium chloride dissolved in water, the films turned out practically impenetrable for methylene blue. The latter showed strong tendency towards complexation and encapsulation into the hydrophilic PAMAM domains, as manifested by coloration of the resulting samples. 

Radially layered copolymeric PAMAMOS dendrimers and their networks described in this chapter represent truly unique new materials which can be prepared in a variety of different chemical compositions. They consist of hydrophilic PAMAM and hydrophobic (i.e., oleophilic) OS nanoscopic domains, the sizes of which can be precisely controlled by the selection of the reagents used for the dendrimer synthesis and by the chemistry involved. Hence, this synthetic strategy is highly versatile and it enables close control of the resulting structure(s), permiting precise tailor-making of these unique new materials. 

The PAMAMOS dendrimer-based networks can be prepared in a variety of different chemical compositions as elastomers, plastomers or coatings, depending on the particularly desired end-application(s). They also contain well defined (in size and shape) hydrophilic PAMAM and oleophilic (hydrophobic) OS nanoscopic domains which are covalently interconnected in a three-dimensional network continuum. These... 

In conclusion, these unprecedented properties of PAMAMOS dendrimers and their networks, clearly result from the synergism of two distinctly different chemical compositions and unique dendrimer molecular architecture. These materials provide provoking new possibilities for applications in various fields, including optoelectronics, protective coatings, separation processes, etc. 

---