AIChE Report: Polymers Gain from Self-Assembly
Annelise E. Barron, a chemical engineering professor at Northwestern University, says researchers are learning to design molecules that can assemble themselves into much larger objects using natural systems with specific functionalities, much the way genetically encoded proteins and cells grow and precisely arrange themselves into functioning entities.
Already, self-assembling peptide systems are being developed to produce a biological scaffolding necessary to support the regeneration of human tissue, says Shuguang Zhang, a biomedical engineering researcher at MIT (Cambridge, MA) working with Convatec, a Bristol-Myers Squibb company. Zhang and his colleagues are now looking at ways to dramatically reduce the cost of making the biological materials, estimated at between $1 and $2 million/kg. Because the individual peptides can be molecularly engineered easily through self-assembly, this provides improved biological compatibility. These new assemblies are different from current organic polymers, he says, because they can be used in the body and have no toxic side effects.
And, Bruno Michel, a researcher for IBM in Ruschlikon, Switzerland, who is working with Self-Assembled Monolayers (SAMs), believes that such materials are close to commercialization for coatings for computer hard disks to prevent crashes, and for several corrosion-protection products. He expects SAMs to make a major impact on single-layer lithography, and in biosensors. He adds that the materials have great potential longer term for the production of computer chips.
Nitash P. Balsara, a professor of chemical engineering, chemistry, and materials science at the Polytechnic Institute of New York (Brooklyn) says that self-assembled polymers also show promise for photonics.
Other researchers are working on a myriad of potential applications, including drug delivery and drug development processes, and more-stable and stronger polymers.
A leading role for ChEs?
Although self-assembly has been largely the domain of chemists and biochemists, this is beginning to change. While more chemical engineering teams have become involved with nanostructures (see Chem. Eng. Prog., Apr. 1996, p. 15, and Apr. 1998, p. 13), recent research is focusing on the larger self-assemblies that polymers can create, notes Balsara.
In fact, several researchers contacted by CEP have gone so far as to predict that this may be the next new frontier for chemical engineers. Northwestern's Barron, for example, says that "chemical engineers should be able to eventually design a wide variety of functional polymers based on their ability to fold and self-assemble." (Several sessions on self-assembling polymers and nanostructures are scheduled for the <%=company%> Annual Meeting in Miami Beach, FL, Nov. 15-20.)
Others, however, such as Polytechnic's Balsara, are more cautious, emphasizing that much more research is needed before substantial advances will be realized. "It may take another five to ten years before this field is fully developed." But, he, too, believes this is an area "where contributions of chemical engineers will be important."
A golden opportunity
At the Ruschlikon, Switzerland, research center of IBM, Michel and his associates are working with SAMs of alkanethiols and disulfides on gold. SAMs form organic interfaces with properties largely controlled by the end groups of the molecules comprising the film. According to Michel, SAMs provide a link "between the science of organic surfaces and technologies that seek to exploit their adaptable character."
SAMs are a model system for the study of organic and biological interfaces and are of interest for the fabrication of sensors, transducers, protective and lubricating layers, and as patternable materials that can be used as templates on computer chips, he notes.
"Increasing evidence demonstrates that these organic monolayers provide the necessary resists for methods of fabrication like microcontact printing. This type of manipulation provides a basis for solving problems in the manufacturing of nanoscale devices and their use in a plethora of derivative technologies with the attendant advantages of characteristically smaller size," Michel notes.
Bigger is better
Chemical engineers at the University of Rochester, (Rochester, NY) have created large microstructures, measuring up to 50 micrometers long, using polymers that self-assemble after some prodding. These hollow spheres, cylinders, solid rings and flat disks, which fluoresce, are 1,000 times larger than other synthetic self-assembled structures, says X. Linda Chen. Each is made up of millions of molecules that have grouped themselves together.
Chen and Samson Jenekhe were able to create large objects because they started out with a large building block: poly(phenylquinoline)-block-polystyrene, a block copolymer, or macromolecule, that forms plastics. Once the polymer is prepared, it takes the molecules just minutes to organize themselves into discrete, microscopic objects.
"Most researchers have used small molecules, believing it would be impossible to tame polymers into self-assembling," notes Jenekhe, whose work is supported by the National Science Foundation and the Office of Naval Research.
Jenekhe says the key to these larger assemblies is the incorporation of hydrogen bonds into the polymer structures, "giving them the same source of stability that helps DNA and self-assembled proteins in nature arrange themselves into functioning objects."
He believes that drug delivery systems may be one of the first self-assembly applications to reach commercialization. Already, self-assembled hollow spheres have been used as containers to carry buckyballs, stuffing each sphere with billions of them. It has been found that the buckyballs, molecular arrangements of 60 atoms of carbons in sphere-like structures resembling geodesic domes with large cavities, can shield certain cells from many different types of damage, "but their usefulness is limited because they are not very soluble. The self-assembled spheres solubilize buckyballs by sequestering them in their interior cavities. Moving buckyballs using a self-assembled shell might prove a convenient means of delivery," Jenekhe says. (An article about buckyballs and their applications appeared in CEP, Mar. 1997, p. 17.)
Jenekhe also predicts that the new self-assembly technique may be useful for producing adhesives, pesticides, biomaterials, sensors, composites, coatings and paints, photographic and imaging media, catalysts and microelectronics. He also says that "since some of the objects are composed mainly of a hollow cavity surrounded by a fluorescing shell, they may even be useful in making microscopic lasers."
Darwinian drug development
Using either zinc or cadmium atoms as "molecular matchmakers," another University of Rochester researcher, Benjamin Miller, has developed a method of creating potential drugs using self-assembled facilitators that "would offer a faster way for chemists to create and screen potential new drugs."
"With our technique, we try to find molecules to bind receptors in much the same way nature has for millions of years," says Miller. "We take a receptor we want to target, add many little molecules to it, and see which ones are best at binding the receptor."
These metal matchmakers, he adds, "are versatile, allowing molecules to come apart as readily as they form. The two monomers can easily break away from their metal mooring if the resulting compound doesn't succeed in binding the target molecule."
Miller's work is funded by the National Science Foundation, and Research Corporation Technologies, Tucson, AZ.
Stanford University's Paul Wender believes Miller's technology strategy might also find a niche in the production of next-generation chemicals other than drugs. "It's easy to imagine ultimately producing a variety of materials with superior properties through such an evolutionary process."
Making stronger bonds
At the University of Delaware, Newark, another group of engineers is looking at the strengths of the bonds formed by self-assembling complexes. John F. Rabolt, a professor of materials sciences and engineering, says the properties of materials synthesized by strongly bonding together segments of different "homopolymers" - long-chain plastics composed of a single repeating chemical unit - are significantly better than would be predicted. "This is a case where 1 plus 2 doesn't equal the expected 3, but rather 4, or even 5."
The enhanced properties that result from these novel combinations of polymers include higher tensile strength and temperature resistance, as well as greater stability.
According to Rabolt, this technology offers virtually limitless possibilities for materials design in terms of the types of polymer segments that can be combined into microblock copolymers. Different lengths of the segments also can be combined. "The segments usually fall into the 10- to 20-unit range, but can vary from three to 50 chemical units. The properties of two microblock copolymers will differ with different segment lengths, even if the same two polymer chemical architectures are combined," he notes.
Thus far, Rabolt has examined two combinations of microblock copolymers: ethylene-tetrafluoroethylene and ethylene-ethylene oxide. "We've shown that the properties of microblock copolymers are not predictably hybrid but can actually be much better," Rabolt says. "What remains now is to determine which combinations of chemical architectures and segment lengths yield the properties that are desired for which applications."
By Claudia M. Caruana,
associate editor, CEP
This article appears in the May issue of Chemical Engineering Progress
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