|Vol. 17, No. 27||April 16, 1998|
"This is a case where 1 plus 2 doesn't equal the expected 3, but rather 4, or even 5," says John F. Rabolt, materials science.
Thanks to modern high-tech materials characterization equipment and techniques, much is known about homopolymers, such as polyethylene, Teflon® and other typical plastics. But, few researchers worldwide are making so-called "microblock" copolymers, featuring multiple repeating chemical units, and little is known about their properties, Rabolt says.
When Rabolt's UD research team combined various homopolymers to make microblock copolymers, they "expected a hybrid effect," he says, but discovered that "the resulting properties are much better than would be predicted from looking at the properties of the two constituents."
Using pasta shapes as the basis for an analogy, Rabolt explains that a single strand of spaghetti would be like a homopolymer. A microblock copolymer, in contrast, could comprise a continuously repeating pattern of fusilli (thin spiral) and penne (tubular) pieces.
Merely pouring the two components together into a beaker has no effect on their properties because, like oil and water, they do not mix. Similarly, placing the different pasta shapes together in a container does not change them. Chemical processing, however, forms covalent or extremely strong bonds between the two polymers, and a phenomenon known as "conformational influence" brings about changes in homopolymers that come into contact with the microblock copolymer.
"To return to the pasta analogy," Rabolt explains, "if one piece of cooked penne was joined to two pieces of cooked fusilli, one on either end, the fusilli would probably bend the penne in ways that it would not bend if it was joined to two other pieces of penne. The end segments effect what's in the middle."
The enhanced properties that result from these novel combinations of polymers include higher tensile strength and temperature resistance as well as greater stability. "For example," Rabolt explains, "a polymer may be metastable as a homopolymer- that is, it can be extended when stretched from its coiled state but will spring back when the tension is removed. By making it a microblock copolymer component, we can change its properties so that it is extended and remains that way rather than coiling back up."
The researchers have found that in many cases a "supramolecular helix" results when microblock copolymers are formed. Rabolt has postulated that there is an attraction between like polymer segments, which means that when the material solidifies, it forms a coiled structure where like segments are lined up with each other (much like fusilli segments would line up with other fusilli segments, while the penne segments would line up with other penne segments).
This effect is probably at least partially responsible for the unexpectedly enhanced properties seen in these materials, Rabolt says.
The UD technique offers virtually limitless possibilities for materials design, Rabolt says, in terms of the types of polymer segments that can be combined into microblock copolymers as well as the lengths of the segments being 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 only two combinations of microblock copolymers: one type (ethylene-tetrafluoroethylene) supplied by Anselm Griffin's research group at the University of Southern Mississippi; and another (ethylene-ethylene oxide) provided by Greg Baker from Michigan State University.
"We've proven with measurements that the properties of microblock copolymers are not predictably hybrid but actually much better," Rabolt says. "What remains now is to determine which combinations of materials and segment lengths yield the properties that are desired for various applications."