Printer Friendly

Model growth: simulations expose branching nature of polymer crystals.

The intricate shapes of snowflakes have long fascinated scientists, but water isn't the only fluid that freezes into elaborate crystal structures. Polymers and metal alloys do the same. Now, scientists in the United States and Hungary have uncovered previously unknown facets of the physics underlying such crystal growth.

Using computer models, the researchers simulated a polymer liquid as it solidifies into a crystal. In one scenario, the researchers added increasing amounts of dirt particles, representing impurities, to the liquid and confirmed previous observations that the more dirt present, the more complex the resulting polymer crystal.

Normally, a polymer in solution begins solidifying by forming tiny needle-shaped crystals. These grow along one direction, eventually packing into a polycrystalline structure. However, when one of these needle crystals encounters an impurity, "it has to figure out how to incorporate the particle into its structure," says James Warren of the National Institute for Standards and Technology in Gaithersburg, Md.

He and his colleagues at the Research Institute for Solid State Physics and Optics in Budapest found that in their model, the position and orientation of a particle can make the polycrystal growth veer in a new direction, creating a branch point. Therefore, the more dirt particles it encounters, the greater the randomness of the branching structure. The researchers dub polycrystals that follow these branching growth patterns "dizzy dendrites."

Warrens group also reports a surprising discovery: Dirt isn't required for branching structures. When the team modeled a dirt-free solution and simulated it being chilled below its normal freezing point, the polycrystals that formed were as highly branched as were the dizzy dendrites produced from solutions with impurities.

Warren notes that many small crystals form independently in a polymer solution. Generally, a small crystal rotates to line up with a growing polycrystal before joining it. However, in a supercooled solution, the small crystals rotate so slowly that they become incorporated in whatever orientation they happen to be in when they encounter the polycrystal. The researchers describe their simulations in the September Nature Materials.

"The simulations employed in this paper are truly state of the art," says Mark Asta, a computational materials scientist at Northwestern University in Evanston, Ill. Because a material's microstructures determine its mechanical properties, the findings could help scientists design improved materials--for use in products ranging from plastic grocery bags to airplane wings--by controlling the way a material crystallizes, researchers contend.
COPYRIGHT 2004 Science Service, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2004, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

 Reader Opinion

Title:

Comment:



 

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:This Week
Author:Goho, Alexandra
Publication:Science News
Geographic Code:1USA
Date:Sep 11, 2004
Words:397
Previous Article:Cool harvest: frost on sea ice may boost atmosphere's bromine.
Next Article:An exploitable mutation: defect might make some lung cancers treatable.
Topics:


Related Articles
Dramatizing life's chemical prelude.
Shaping synthetic metals: dendrimers branch out into the electronic world.
Knotting weakens a polymer molecule.
Foamy polymers hit goal right on the nose.
Anyone want to knit a microscopic sweater?
National Institute of Standards and Technology synchrotron radiation facilities for materials science.
New material responds to growing tissue. (Bone Fix).
New polymer ink writes tiny structures.
Infrared vision: new material may enhance plastic solar cells.
Shape shifter.

Terms of use | Copyright © 2014 Farlex, Inc. | Feedback | For webmasters