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Nanostructured chemicals: A new era in chemical technology

Joseph D. Lichtenhan is president and cofounder of Hybrid Plastics (18237 Mt. Baldy Circle, Fountain Valley, CA 92708; 714-962-0303; lichtenhan@hybridplastics.com). He received his B.S. degree in chemistry from Kansas State University, Manhattan, and his Ph.D. in materials chemistry from the University of California, Irvine. He worked for the University of Dayton Research Institute and Hughes Aircraft before becoming a technical and business area director for the Polymeric Component Applications Program at the Air Force Research Laboratory at Edwards AFB, CA. In 1998, he cofounded Hybrid Plastics in order to commercialize POSS Nanostructured Chemicals. As a result of his efforts, Hybrid Plastics’ POSS Molecular Silicas recently received an R&D 100 Award as one of the 100 most technologically significant new products of 2000.

Joseph J. Schwab is director of R&D and cofounder of Hybrid Plastics (jjschwab@hybridplastics.com). He received B.S. degrees in chemistry and biology from Northeastern Illinois University, Chicago, and a Ph.D. in chemistry from the University of Illinois, Urbana–Champaign. Following postdoctoral work at the University of California, Irvine, he joined Hughes/Raytheon STX Corp. as a principal scientist at the Air Force Research Laboratory, Edwards AFB, CA. In 1998, he and Joe Lichtenhan cofounded Hybrid Plastics.

William A. Reinerth, Sr., is a principal scientist at Hybrid Plastics (reinerth@hybridplastics.com). He received his B.S. degree in chemistry from Ursinus College, Collegeville, PA, and his Ph.D. in chemistry from the University of Illinois, Urbana– Champaign. Following postdoctoral work at the University of Kentucky, Lexington, and the University of South Carolina, Columbia, he joined the research staff at Hybrid Plastics in 1998.

The beginnings of research on nanotechnology can be traced back over 40 years, but the past decade has seen nanotechnology cut a large swath across a variety of disciplines. From chemistry to biology and from materials science to electrical engineering, scientists are creating the tools and developing the expertise to bring nanotechnology out of the research labs and into the marketplace. It is only a matter of time before nanotechnology will have a dramatic impact on practically all areas of our technologically dependent society.

Chemists, by the very nature of their work, are accustomed to manipulating matter on the nanoscale. Given this, and the fact that chemical technology already affects every aspect of society, the first significant applications of nanotechnology and its effect on everyday life are likely to be attained through chemistry.

Whereas chemical technology touches every area of society, the pool of feedstocks from which most chemical products and, in particular, polymeric materials are derived is limited. Therefore, opportunities abound for new chemical feedstocks that are compatible with existing chemical systems yet provide distinctive value that is not attained with conventional technology. Because they are hybrids, three-dimensional, and nanoscopic, our nanostructured chemicals represent the only entirely new chemical feedstock technology to be developed within the past 50 years.

Nanostructured chemicals are defined by the following four features.

• They are single molecules and not compositionally fluxional assemblies of molecules such as hydrogen-bonded networks. 
• Their sizes range from ~0.7 to ~50 nm, which makes them larger than small molecules but smaller than macromolecules. 
• Their systematic chemistries make it possible to control functionality, stereochemistry, reactivity, and physical properties. 
• They possess polyhedral geometries with well-defined 3-D shapes. 

Fullerenes and metal clusters are good examples, whereas planar hydrocarbons, dendrimers, particulates, and carbon nanotubes are not considered nanostructured chemicals.

Figure 1. The unique hybrid (organic–inorganic) compositions available using nanostructured chemicals.

The Hybrid Age

It has been said that the 19th century was the Ceramic Age, and the 20th century was the Polymer Age (1). The 21st century may well become the Hybrid Age. Nanostructured chemicals represent a merger between traditional organic chemicals and inorganic materials, resulting in compositions that are truly hybrid. Thus nanostructured chemicals occupy a unique property “space” relative to traditional material types (Figure 1).

Deca- and pentacarboranes, the first examples of chemical systems that satisfy the definition of nanostructured chemicals were commercialized in the mid-1960s. Decacarboranes were used primarily in the preparation of specialty silicones, which were used as high-temperature coatings for gas chromatographic supports. These materials are still available today under the trade name Dexsil (Dexsil Corp., Hamden, CT). Despite their outstanding physical properties and the identification of niche military markets for carborane-based polymers, their manufacturing costs remain high, as does the expense of the borane components. Current nanostructured chemical technology is based on polyhedral oligomeric silsesquioxanes (POSS). POSS are produced cost-effectively on a multiton scale directly from silicones, silanes, and even silica. POSS nanostructured chemicals can be thought of as the smallest particles of silica possible. However, unlike silica, silicones, or fillers, each POSS molecule contains nonreactive organic functionalities that make the POSS nanostructure compatible with polymers, biological systems, and surfaces. In addition, POSS nanostructured chemicals can contain one or more covalently bonded reactive functionalities suitable for polymerization, grafting, surface bonding, or other transformations (see Figure 2)

Figure 2. The anatomy of a POSS nanostructured chemical. The hybrid organic–inorganic framework is thermally and chemically robust. R represents unreactive organic groups for solubilization of the molecule and compatibility with other organic species. X represents reactive groups for grafting or co-polymerization. The Si–Si distance is ~0.5 nm, and the R–R distance is ~1.5 nm.

Figure 3. The evolution of nanocomposite plastics. Left (1975–1985): Composites were prepared by dispersing particulates within a polymer matrix or via sol–gel routes. Center (1985–1995): Composites were prepared by dispersing a clay filler within a polymer matrix via intercalation and exfoliation. Right (1995–2005): Composites are prepared by dissolving, copolymerizing, or grafting nanostructured chemicals onto polymer chains.

(2). Unlike traditional organic compounds, POSS nanostructured chemicals release no volatile organic components, so they are odorless and environmentally friendly. More than 120 POSS reagents and POSS monomers are commercially available (as solids or oils) for grafting onto polymer chains. Molecular Silicas (POSS cages with eight unreactive organic groups) and POSS polymers are designed to be compounded into traditional polymers.

The demand for advances in performance and properties of polymeric and composite materials has driven the search for new technologies and methods for upgrading the properties of existing plastics. As a result, the field of nanocomposites has been developed with the goal of reinforcing polymer chains at the molecular level in much the same way that fibers reinforce composites at the macroscopic level.

Nanocomposite plastics (Figure 3) had their origins in the mid-1970s with the use of sol–gel technology to form homogeneous dispersions of inorganic domains throughout a polymeric matrix. In such systems, the inorganic phase may or may not be chemically attached to the organic phase. This first-generation approach gained some commercial value in coating applications in which its complex processing and limited material strength are most easily tolerated.

In the early 1980s, second-generation nanocomposite technology emerged with the resurgence in the use of minerals and clay fillers that were treated with various surfactants to modify their surface (or interfacial) properties. The critical technical obstacle for second-generation nanocomposites continues to be homogeneous dispersion (via exfoliation or intercalation) of these organically modified fillers into common polymer systems.

The second-generation approach has been of interest because of the strong precedent in the industry for using filled polymers. However, it has suffered from limited compatibility between the filler and matrix, as well as complex, costly processing requirements. In 1995, third-generation nanocomposite technology was developed based on nanostructured, silicon-based chemical feedstocks. In this technology, standard polymerization, compounding, and coating techniques are used to incorporate nanostructured chemicals easily and in a well-controlled manner into all known polymeric materials. The third-generation approach is broadly applicable to all classes of polymers, and these nanocomposites can be further reinforced with traditional fillers (e.g., organoclays, carbon, glass, and fibers).

Blanski and co-workers demonstrated the unique compatibility of POSS nanostructured chemicals with polymers (3). Their results indicate that not all nanostructured chemicals are created equal. The principle of “like dissolving like” was key in their achieving optimal alloying with retention of optical quality for nanoreinforced polystyrene (Figure 4).

They used a solution blending process to incorporate (cyclo-C₅H₉)₈Si₈O₁₂ and (PhCH₂CH₂SiO_1.5)_n, in which n = 8, 10, or 12, into monodisperse 2 × 10⁶-Da molecular weight polystyrene at the molecular level. Despite the transparency of the polymer and both Molecular Silicas while in solution, evaporation of the solvent upon film casting resulted in phase separation of the composition containing (cyclo-C₅H₉)₈Si₈O₁₂. This result is not surprising given the characteristic solubility differences expected between the aromatic-based polystyrene and the aliphatic cyclopentyl groups on (cyclo-C₅H₉)₈Si₈O₁₂. However, full compatibility at 50 wt% levels was obtained for (PhCH₂CH₂SiO_1.5)_n, which was expected because both polystyrene and (PhCH₂CH₂SiO_1.5)_n contain phenethyl groups.

Nanostructured chemical technology provides new tools for chemical and materials research. The technology emerged from the desire to control the physical and biological function of materials at the molecular level and to radically improve the physical properties of traditional materials. The chemical diversity of POSS nanostructured chemicals (the first affordable, commercialized class of nanostructured chemicals) is vast and parallels that of traditional organic systems, yet incorporates it onto a robust and precisely defined inorganic (silicon–oxygen) nanostructure. When incorporated into polymers, POSS nanostructured chemicals provide nanocomposite properties while maintaining or improving processability, thereby offering turnkey utility. Nanostructured chemicals offer the potential to revolutionize not only the plastics industry, but also our entire technological society from automobiles to medical devices to computers to sports equipment.

References

1. Paint Research Association International Centre for Coatings Technology. Brochure for Organic–Inorganic Hybrids Conference, Surrey, Guildford, U.K., June 12–14, 2000. 
2. Lichtenhan, J. D.; Schwab J. J.; Feher, F. J.; Soulivong, D. U.S. Patent 5,942,638, 1999. 
3. Blanski, R. L.; Philips, S. H.; Lee, A.; Lichtenhan, J. D.; Chaffee, K. P.; Geng, H. P. MRS Proceedings, in press.