Why plastic bends rather than break when put under stress

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Washington, Nov 28 : Researchers at the University of Wisconsin-Madison have discovered the mechanism behind plastic's most useful but puzzling quality - the ability to bend rather than break when put under stress.

University of Wisconsin-Madison chemistry professor Mark Ediger, says this property, described as "plastic flow", allows many plastics to change shape to absorb energy rather than breaking apart.

"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. How does that happen?" he said.

Ediger's research team, led by graduate student Hau-Nan Lee, has now described a fundamental mechanism underlying this stiff-but-malleable quality.

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.

The 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.

According to the researchers, this increased motion allows the material to flow without breaking.

"When you pull on it, you increase the mobility in the material. 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," Ediger said.

The study is appearing Nov. 28 issue of Science Express.

ANI

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