Mayonnaise as Model for Solid Plastics
Intriguing Structural Strategy Aims at Making Designer Plastics Affordable
The future was supposed to be "plastics," according to advice given in a 1960s movie The Graduate. Many a company thought that future meant the gradual ascendancy of "designer" or specialty plastics, but almost 40 years later the market is still dominated by plastics that can be manufactured cheaply in bulk.
Six researchers from the University of California at Santa Barbara (UCSB) and one at Helsinki University of Technology in Finland report in the March 21 issue of Science a successful example of an intriguing strategy for combining the versatile properties afforded by expensive "designer" plastics with the favorable economics of the old standby, mass-produced plastics. They have done so, quite simply, by finding a way to combine the two types of plastics in one structure.
The resultant material is made mostly of the cheap plastic polystyrene or PS (the stuff of styrofoam®), but the material itself exhibits the properties of the designer plastic--the one (polyaniline) used by the researchers conducts electricity. One of the papers authors is UCSB physicist Alan Heeger, who shared the 2000 Nobel Prize in Chemistry for the discovery of conducting polymers such as polyaniline.
UCSB Chemical Engineering Professor Glenn Fredrickson got the idea that led to the reported research while listening at a professional meeting to a lecture on the liquid analogue of the solid material reported in Science. He figured that what could be done for liquids (as in mayonnaise), could be done for solids. Polymer and colloid scientists call such two-phase structures "high internal phase emulsions."
To explain the two-phase structure, Fredrickson begins with the easily envisioned gas-liquid, two-phase version or "foam," as in the head that forms on a newly poured glass of beer.
"This project," said Fredrickson, "is about trying to create a unique two-phase structure in a material like the structure of closed-cell-foam atop beer. That foam has gas bubbles of carbon dioxide entrained by a liquid. The spherical bubbles are close together, and there is a liquid film between them. And the spherical bubbles deform to a polyhedral shape, which enables denser packing than with rigid spheres.
"Imagine," he said, "freezing the beer foam and cutting through it. The honeycomb-like structure you would see is like the solid plastic structure we have produced."
In a standard high internal phase emulsion, the enveloping and the enveloped components are in the same state--liquid. For instance, the vast majority of mayonnaise consists of water (analogous structurally to the carbon dioxide bubble-contents of beer foam). The rest is oil plus surfactants or surface-active agents that stabilize the interfaces of the enclosed water droplets so that mayonnaise, left long in jars, doesnt flatten.
The researchers are the first to create a two-phase structure in which both components are solids; they therefore add the word "polymeric" to the standard nomenclature. As they state in the abstract of their paper, "The resulting cellular morphology can be viewed as a high internal phase polymeric emulsion."
What they have done is to adapt to solid plastics a morphology widely known in liquid form in the food industry.
The papers first author, Raffaele Mezzenga, who now works in that industry at the Nestlé Research Center in Lausanne, Switzerland, conceptualized and conducted the experiments as a postdoctoral fellow shared by Fredrickson and his UCSB collaborator Edward Kramer, a materials professor. Fredricksons initial idea evolved in conversations with Kramer. Funding for Mezzenga came from a start-up company PolyE Inc. Heeger and his colleague Daniel Moses provided guidance about and samples of the conducting polyaniline. Another of Fredrickson and Kramers postdocs, Janne Ruokolainen, and his thesis advisor at Helsinki, Professor Olli Ikkala, are also authors.
Fredrickson and Kramer are particularly interested in the morphology of two-phase structures. "Phase" in polymer science is a geometric term which pertains to the regions one polymer occupies with respect to another. If, for instance, little balls of one polymer dot the expanse of another (or are embedded), then the balled-up polymer is referred to as "discrete" because the aggregates are distinct from one another. And the polymer of the expanse is referred to as "continuous" because one point on it can be connected to any other point on it by tracing a route around the embedded aggregates.
"The notion of enhancing polymer properties by embedding one in another is standard practice of polymer engineering," said Kramer. "It is to polymers what alloys are to metals. As with alloys, polymer blends usually exhibit properties that differ from the properties of each of the constituents."
With the semiconducting polymer blend reported in the Science paper ("Templating Organic Semiconductors Through Self-Assembly of Polymeric Colloidal Systems"), the key desired transport property is a function only of the expensive designer polymer polyaniline, whose disposition in the polymer blend must be continuous in order to conduct electricity, but continuous in such a way (like the liquid film around the CO2 beer foam) that only a little of this expensive material is used.
In addition to transport (of electrons, molecules, and ions), this honeycomb morphology can also be adapted to perform the opposite function of providing a barrier, required for instance in packaging fresh food products, which decay with exposure to oxygen. The kind of plastic available today to provide that barrier would cost almost as much as or more than most of the packages contents. The Science technique would enable use of a little of that expensive plastic in combination with a cheaper plastic. The resultant material would cost a little more than the cheaper plastic alone, but would provide value by blocking spoilage that would be worth the cost.
The trick that Mezzenga figured out to assemble the polymers into the desired structure entails the use of another polymer in addition to PS and polyaniline. He designed a block copolymer of PS and poly(vinyl pyridine) or PVP that would complex appropriately with the other two components. Ruokolainen and Ikkala assisted this effort.
Block co-polymers are another standard tool of polymer engineering which, in effect, splices together two polymers such that the very long molecule which results has ends with usefully different properties.
As the authors write, "A route for producing semiconducting polymer blends is demonstrated in which a doped pi-conjugated polymer [polyaniline] is forced into a three-dimensionally continuous minor phase by the self-assembly of colloidal particles and block copolymers." (The colloidal particles are PS and the block copolymers, PS-PVP.)
Because this process requires a solvent, Fredrickson points out, "it will be better suited for paints and coatings rather than for bulk materials. Many paints are currently formulated with latex colloidal particles, so the present technology provides a way to introduce new functionality, such as conductivity that could have value in antistatic coatings."
Heeger said, "The concept is simple, but elegant--and the use of block copolymers to direct the formation of the two-phase structure and the cellular morphology provides a general approach to creating designer materials. Consequently, potential applications of our initial results in the science of new materials go well beyond conducting polymers."
Jacquelyn Savani | University of California, Santa