This property, described as "plastic flow", allows many plastics to change shape to absorb energy rather than breaking apart, says University of Wisconsin-Madison chemistry professor Mark Ediger. For example, one type of bulletproof glass stops a bullet by flowing around it without breaking. Regular window glass, unable to flow in this way, would simply shatter.
"This is an odd combination of properties... These materials shouldn't be able to flow because they're rigid solids, but some of them can," he says. "How does that happen?"
Ediger's research team, led by graduate student Hau-Nan Lee, has now described a fundamental mechanism underlying this stiff-but-malleable quality. In a study appearing Nov. 28 in Science Express, they report that subjecting a common plastic to physical stress - which causes the plastic to flow - also dramatically increases the motion of the material's constituent molecules, with molecular rearrangements occurring up to 1,000 times faster than without the stress.
These fast rearrangements are likely critical for allowing the material to adapt to different conditions without immediately cracking.
Plastics are a type of material known to chemists and engineers as polymer glasses. Unlike a crystal, in which molecules are locked together in a perfectly ordered array, a glass is molecularly jumbled, with its constituent chemical building blocks trapped in whatever helter-skelter arrangement they fell into as the material cooled and solidified.
While this atomic disorder means that glasses are less stable than crystals, it also provides molecules in the glass with some wiggle room to move around without breaking apart.
"Polymer glasses are used in many, many different applications," including polycarbonate, which is found in popular reusable water bottles, Ediger says. Aircraft windows are also often made of polycarbonate. "One of the reasons polymer glasses are used is that they don't break when you drop them or fly into a bird at 600 miles per hour."
However, their properties can change dramatically under different physical conditions such as pressure, temperature, and humidity. For example, many polymer glasses become brittle at low temperatures, as anyone knows who has ever dropped a plastic container from the freezer or tried to work on vinyl house siding in cold weather.
As plastics become more and more prevalent in everything from electronics to airplanes, scientists and engineers face questions about the fundamental properties and long-term stability of these materials over a range of conditions.
For example, next-generation commercial aircraft are trending toward including less metal in favor of higher proportions of lightweight polymer materials - roughly 50 percent in the new Boeing 787 compared to only 10 percent in the Boeing 777 - and engineers need to know how these materials will respond to different stresses: a hard landing, strong winds, or changes in temperature or humidity.
"How is it going to respond 20 years from now when it gets twisted, or stretched, or compressed? Is it going to respond by absorbing that energy and staying intact, or is it going to respond by breaking bonds and flying apart into pieces?" asks Ediger.
The Wisconsin team examined the mechanics of a common plastic called polymethylmethacrylate - also known as Plexiglas or acrylic - and found that a pulling force had a pronounced effect on the molecules within the material, speeding up their individual movements by more than a factor of 1,000. The team observed internal molecular rearrangements within 50 seconds that would have taken a full day without the force applied. They believe this increased motion allows the material to flow without breaking.
"When you pull on it, you increase the mobility in the material," Ediger says. "The act of pulling on it actually transforms the glass into a liquid that can then flow. Then when you stop pulling on it, it transforms back to a glass."
The work has benefited from collaboration between chemists and engineers in a Nanoscale Interdisciplinary Research Team (NIRT) supported by the National Science Foundation (NSF), which includes UW-Madison chemical and biological engineering professor Juan de Pablo and groups at the University of Illinois and Purdue University.
"From the most fundamental perspective, we're trying to understand why pulling on a glass allows it to flow," Ediger says. "The answer to that question will help us to better model the behavior of real materials in real applications."
In addition to Ediger and Lee, the paper is authored by Keewook Paeng and Stephen Swallen. The work was funded by NSF.Mark Ediger, (608) 262-7273, email@example.com
Jill Sakai | Newswise Science News
Barely scratching the surface: A new way to make robust membranes
13.12.2018 | DOE/Argonne National Laboratory
Topological material switched off and on for the first time
11.12.2018 | ARC Centre of Excellence in Future Low-Energy Electronics Technologies
What if, instead of turning up the thermostat, you could warm up with high-tech, flexible patches sewn into your clothes - while significantly reducing your...
A widely used diabetes medication combined with an antihypertensive drug specifically inhibits tumor growth – this was discovered by researchers from the University of Basel’s Biozentrum two years ago. In a follow-up study, recently published in “Cell Reports”, the scientists report that this drug cocktail induces cancer cell death by switching off their energy supply.
The widely used anti-diabetes drug metformin not only reduces blood sugar but also has an anti-cancer effect. However, the metformin dose commonly used in the...
A research team from the University of Zurich has developed a new drone that can retract its propeller arms in flight and make itself small to fit through narrow gaps and holes. This is particularly useful when searching for victims of natural disasters.
Inspecting a damaged building after an earthquake or during a fire is exactly the kind of job that human rescuers would like drones to do for them. A flying...
Over the last decade, there has been much excitement about the discovery, recognised by the Nobel Prize in Physics only two years ago, that there are two types...
What if a sensor sensing a thing could be part of the thing itself? Rice University engineers believe they have a two-dimensional solution to do just that.
Rice engineers led by materials scientists Pulickel Ajayan and Jun Lou have developed a method to make atom-flat sensors that seamlessly integrate with devices...
12.12.2018 | Event News
10.12.2018 | Event News
06.12.2018 | Event News
13.12.2018 | Life Sciences
13.12.2018 | Physics and Astronomy
13.12.2018 | Earth Sciences