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Films with tiny particles could have many uses

MIT researchers have discovered a method for creating transparent films with special properties that could prove useful in a variety of different industrial applications, such as magnetic "watermarks" to safeguard security papers or as a coating for window glass that would absorb the heat from the sun's rays.

Robert Cohen, St. Laurent Professor of Chemical Engineering, and B.H. Sohn, a former graduate student in Professor Cohen's lab, published a paper earlier this year that describes this novel method of embedding specially created particles into transparent films resembling cellophane food wrap. The embedded particles, made from one of several different elements, become a permanent part of the polymer film and confer new properties which allow the film to perform practical functions.

"The breakthrough in this research is not in the creation of the small particles -- that had already been done. What's important is that we're actually building them inside the film so that they're permanent and keep to the regions we've put them," said Professor Cohen, whose research was published in the January issue of the Chemistry of Materials.

The positioning of the particles would be especially important in an application such as the magnetic watermark, which would require the particles' placement into a permanent pattern.


The particles, called nanoclusters or nanoparticles, are tiny bits of common compunds or elements such as iron oxide, copper or silver. Unlike larger portions of the materials, the nanoclusters remain transparent because of their tiny size -- roughly one to 10 nanometers in diameter. (A nanometer is one-billionth of a meter.)

Professor Cohen's research group has a long-standing interest in a category of transparent polymer films known as block copolymers. The chain molecules which make up these films naturally organize into exceedingly small and uniformly arranged structures, which Professor Cohen refers to as nanoreactors. His group uses these nanoreactors as a kind of three-dimensional scaffold in which the nanoclusters are synthesized. He calls this method a "nanoreactor approach to cluster synthesis." He and his students have been learning how to load the nanoreactors with a wide variety of metal ions for subsequent transformation into nanoclusters.

"When you use these exceedingly small nanoparticles of materials, they exhibit unusual behavior. The properties are not the same as in the smaller molecular or atomic ranges, but they're also not the same as with bulk quantities. For instance, the opaqueness and color of silver remain the same until you get down to the nanometer size -- then it becomes transparent and yellow," explained Professor Cohen.

But some properties of the materials do not entirely disappear with decreasing size. Iron and iron oxide remain magnetic, although with some variation in their magnetism. Professor Cohen credits Professor Georgia Papaefthymiou, an expert in magnetic characterization of materials at the Francis Bitter Magnet Lab, with helping to make the precise measurements necessitated by the project on magnetic transparent films.

Those magnetic films could eventually be used as an invisible magnetic watermark in security papers. The watermark would have iron oxide nanoclusters embedded in it, sequestered in several nanoreactors which would form a pattern. These little regions would give the watermark a particular magnetic signature which would actually amount to stored information. That signature would be permanent and nonerasable, or paramagnetic, unlike the magnetic strips on credit cards which, when exposed to a powerful magnet, lose their signature and therefore the information they store.


The generality of the nanoreactor approach to synthesis (nanoreactors can synthesize nanoclusters of many different metals), coupled with the particles' transparency, give these new polymer films significant potential for novel applications. They could even keep a car's interior from heating up so much in the summer.

"Because of the infrared absorption capabilities of copper oxide, we could cover window glass with copper oxide nanocluster-embedded film, or use the particles as an additive to plastics used instead of glass. This could prevent some of the heat from building up in cars or buildings," said Professor Cohen. He added that a way must also be found to dissipate the heat so that the glass or plastic does not become hot to the touch.

"Because of environmental concerns, we're trying to focus on familiar, noncontroversial metals such as silver, gold, copper and iron. We 're also trying to do much of our polymer synthesis and nanoreactor processing in aqueous solutions, thereby eliminating the need for the handling of volatile organic solvents," Professor Cohen said.

The research is supported by a grant from the National Science Foundation's Division of Materials through the MIT Center for Materials and Engineering.

A version of this article appeared in MIT Tech Talk on October 8, 1997.

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