Focus: A Crystal with a Twist
As in the famous line from the 1967 movie “The Graduate,” the importance of polymers in modern life can be expressed in one word: plastics. To develop new polymer-based materials for the future, researchers want to understand and manipulate polymer structures at the molecular level. The 29 November PRL describes the first synthetic polymer that crystallizes into a helical structure with twists along two different axes. The structure is reminiscent of the hierarchy of twists found in protein molecules, and the authors hope it will lead to new types of materials, perhaps using some of Nature’s old tricks.
Polymers are long chains made up of small, repeated molecular units, and they are responsible for both Styrofoam cups and plastic eyeglass lenses. Despite their tremendous success, synthetic polymer materials rarely take advantage of chirality–an asymmetrical structure of the basic molecular building blocks that can affect the material properties. Biopolymers such as DNA and protein molecules are built from chiral units, and the “twists” they impose are partly responsible for the wide variety of useful protein structures.
Taking a tip from Nature, Stephen Cheng of the University of Akron in Ohio and his colleagues wanted a polymer made from chiral units that crystallizes into a large helix. They used a long and narrow molecule called PET ( R*)-9 for the repeated unit and created only “right-handed” copies of it. When linking up these monomers, they assured that the units always connected head-to-tail. The team then crystallized the polymer and found the crystal structure with electron microscopy and electron diffraction.
Cheng and his colleagues found that the twisting nature of the molecular units was reflected in twists at two different levels in the crystal structure. The crystal is a 2-µm-wide helix made of vertically stacked films, or “lamellae,” like a deck of thousands of playing cards, where each card is rotated slightly compared to the one below it. Each lamella consists of about 2000 single units lying side-by-side, rather than end-to-end, so that the long polymer chain makes many S-curves. The second twist is within the lamellae: The team discovered that each monomer is not quite parallel to its neighbors, but rotated by about 0.01 degrees, which adds up to a 20-degree “warping” of each “card” in the stack.
Cheng points out that the symmetry of this crystal is completely new. It doesn’t fit into any of the conventional categories of crystal structures because a repeated unit can’t be slid sideways to overlap its neighbor in any direction without some rotation–it lacks “translational symmetry.”
The double twisted structure was “a total surprise–no one expected that,” says Andrew Lovinger of the National Science Foundation in Arlington, VA, and Bell Laboratories in Murray Hill, NJ. He says that a detailed understanding of polymer crystal structure is essential for developing new materials because that structure directly influences mechanical, electrical, and many other properties.