In a finding that could help answer fundamental questions about how glass forms, scientists have discovered that the structure of amber barely changes even after tens of millions of years.
"What we found was that in 20 million years, the amber changed density by only 2.1 percent. What we found challenges the way we look at glasses," said Gregory McKenna, a professor of chemical engineering at Texas Tech University.
The findings, detailed in a recent issue of the journal Nature Communications, are also further evidence that—contrary to what many students are taught in first-year chemistry courses—the stained glass windows in medieval cathedrals aren't thicker at the bottom because glass flows like a liquid and moves over time.
"Those windows aren't flowing," McKenna said. "The glass makers were just smart enough to put the thicker ends at the bottom."
In their new study, McKenna and his team focused on amber—fossilized tree resin—because its atoms are not arranged in any regular order.
"In a crystal, everything is periodically arranged. If you know what's happening in one little bit, you can predict where the atoms are going to be everywhere else. In glass, things are much more disordered," explained Mark Ediger, an experimental chemist at the University of Wisconsin—Madison who was not involved in the study.
Amber's noncrystalline nature thus makes it a good analog for studying glasses, which also have unordered atoms.
McKenna and his team were particularly interested in a phenomenon called the glass transition, which refers to the temperature at which a material transforms from a soft and flexible rubber-like state to a hard and brittle one.
Despite decades of study, many aspects of the glass transition are still not well understood. For example, "what causes a liquid to slow so rapidly as it becomes a glass?" Ediger said. "And what's the best way to think about it?"
The answers to these questions are of more than just academic interest. Glass transition is related to the performance of materials, and its properties are important for the design and manufacture of a whole host of glassy materials.
Beyond Window Glass
When laypeople talk about glass, they usually think of window glass, which is transparent and made primarily of silicate. But for scientists, any noncrystalline, or amorphous, solid is considered a glass. Under this broader definition, plastics can be transformed into glass, as can metals.
"The current planes are probably fine because they have relatively high glass-transition temperatures and the airplanes don't get very hot, but imagine if you are building a supersonic transport and the whole airplane gets hot and remains hot for several hours," McKenna said.
"At that point, you're pushing the limits of the materials, and working fairly close to the glass-transition temperatures. As the material changes, it could get more and more brittle, and you could conceivably have issues if you don't model them properly."
To better understand glass transitions, McKenna, along with colleagues Sindee Simon and Jing Zhao, experimented on 20-million-year-old Dominican amber.
One of the tests they performed was called a stress-relaxation experiment, which involved taking strips of amber, stretching them out at different temperatures, and measuring the rate at which they relaxed back to their original states. The findings from this experiment provided clues about how the molecules inside the amber behave.
Because it takes a certain amount of force to distort the amber strips, measuring "the time it takes that force to go away tells you how fast the molecules inside the material can move," Ediger explained.
The ancient amber provided a rare opportunity to study the glass transition in slow motion and at ambient temperatures. That's because the temperature at which a material gets frozen into the glassy state depends in part on how long it has to cool.
"When you cool a liquid, the reason it becomes a glass is because the molecules are moving so slowly that at some temperature they get stuck, and then they cannot reach the state they should have at such low temperatures," Ediger said.
The longer a material takes to cool, the lower the temperature at which it turns to glass. "If I cool a liquid ten times more slowly, I'll get to a slightly lower temperature before I get stuck," Ediger said.
With the fossil amber, McKenna and his team essentially had a glass that had cooled over a period of 20 million years—something impossible to replicate in a lab experiment.
That allows scientists to "get way the heck down there in temperature"—down even to ambient air temperatures when amber would normally be frozen in a glass-like state—"and still have a liquid," Ediger said. "If we understood the properties of that material, then we would know a lot more about how glass formation occurs."
"To Be Continued"
Ediger said using fossil amber to study the glass transition was a creative idea and called the experiments by McKenna's team "beautifully done."
"I'm not sure there's another lab in the world that could do the experiment with the needed precision," he added.
McKenna and his team are already preparing to perform the same experiments with even older, 220-million-year-old amber from the Triassic period.
"We are in the very early stages," McKenna said in a statement. "However, our research definitely is 'to be continued.'"
The research was funded by the Division of Materials Research at the National Science Foundation.