“Ordinarily, glasses are very far from equilibrium, and not very dense,” says Forrest, a professor of physics and astronomy at Waterloo. “If you wait long enough though, the molecules in the solid glass slowly change over time, and will try to get to equilibrium — to an ideal state. However, the aging times necessary to get a glass close to equilibrium are geological timescales… millions and billions of years!”
Unlike other solid states of matter, however, scientists don’t know if there is actually a “best” or ideal state for glasses yet. Therefore, by studying densely packed glasses, researchers may be able to answer this question. Samples of ancient glasses are the closest materials to “ideal glass” that physicists can study, but it is impractical to wait for the needed geological timescales. Also, for newer materials such as polymers (long, repeating chain-like molecules found in plastics), there simply are no ancient glassy samples.
In their recent study published in the prestigious journal Nature Materials, and highlighted in a commentary article in the same journal, Forrest and his colleagues demonstrated that it is possible to make near-ideal polymer glasses in just minutes, using the polymers polystyrene and polymethyl methacrylate and a technique called vapour deposition.
After extensively testing the glass they made, the research team was able to say that the lab-made glass is comparable to stable glasses created by waiting more than three million years.
“You’re accessing a type of material that takes nature millions of years to make! Nature doesn’t know how to make very good glass,” Forrest says, while describing his excitement about this project. “We’re trying to make a material that nature can’t make, to figure out the way that nature behaves.”
The traditional method of making glass is to quickly cool a liquid material, in order to avoid the crystal phase when the material solidifies. However, this method creates glasses that are very loosely packed at the molecular level. Conversely, vapour deposition creates high density glasses by slowly building up the glass layer by layer — similar to the blocks in the game Tetris. The slower the blocks are deposited, the better the blocks can be fit together, and the closer to ideal a glass becomes.
Researchers have been making stable glasses using vapour deposition for the past ten years, but there hasn’t been a successful ideal glass made from polymers until Forrest’s study.
“Until about 10 years ago, there was no way experimentally to access ideal glass materials. Before then, we had seen advances in the theory, but vapor deposition is by far the biggest experimental advance in glass research. Now, we’ve opened the experimental potential up to polymer materials as well!”
Polymers are an attractive material for glasses because there are a wide variety of natural and synthetic polymers available; many polymers do not crystallize well, making them ideal for glass formation; and by changing the length of the polymers, researchers have control over the temperature at which they solidify into glass. In principle, the vapour deposition technique can be used with any polymer — including those with more technological significance.
Forrest and his colleagues discovered that the key to creating an ultra-stable glassy material from polymers was starting from very pure sample of polymers with a uniform length. Since polymer molecules are chains of smaller units (known as monomers), the molecules can be a variety of different lengths. This parameter is very important in determining the properties of the overall polymer material. By distilling the polymers, the researchers were able to get the pure samples needed to successfully create stable, vapor deposition polymer glasses.
In order to determine the comparative age of the glass to be over three million years old, the researchers took a stable glass and timed how long it took to melt back into a liquid. By looking at the temperature dependence of these times and following the trend back to the much lower temperature where the glass was deposited, researchers can get a better idea of how long it would take to make these glasses under normal processes.
While the samples mimicking ancient glasses are unique and noteworthy in their own right, they may also eventually allow researchers to design and manufacture stable glassy materials, with the potential for a wide variety of future applications using designer polymer glasses.