Glass Is Just a Frozen Liquid That Ages in Time

Plexiglas, in fact an acrylic glass, is the commercial name of Polymethyl methacrylate (PMMA) or poly (methyl 2-methylpropenoate) is the synthetic polymer of methyl methacrylate. It is thermoplastic - deformable, melts to a liquid when heated and freezes to a brittle, glassy state when cooled sufficiently - transparent as glass and often used as an alternative to conventional glass.

What is less known is the fact that "Glasses, including polymer glasses, are essentially frozen liquids," according to Kenneth S. Schweizer, Professor of Materials Science at the University of Illinois. "They appear solid, but because they are frozen liquids, the molecules continually undergo small motions that lead to a time dependence of properties."

So, they're essentially always moving, at imperceptible speeds, but constantly changing.

Unlike window glass, which melts at roughly 1,200 degrees above room temperature, polymer glasses have melting points much closer to room temperature. So close, in fact, that many polymer glasses retain some liquid-like properties at room temperature, including motion at the molecular level.

"The movements are so small and so slow, we can't see them without the aid of sophisticated measuring tools," Schweizer said. "Nevertheless, this residual motion can significantly change the material's mechanical and thermal properties over time."

As the material gradually reconfigures and approaches equilibrium at room temperature, the movements become slower and slower. Under sufficiently cold conditions, this "relaxation" time can become astronomically large, even longer than the age of the universe for some materials.

"Among other possible effects, the aging process causes polymer glasses to become stiffer and often more brittle," said Schweizer, who also is a professor of chemistry, of chemical and biomolecular engineering, and a researcher at the university's Frederick Seitz Materials Research Laboratory.

Over time, the molecules crowd closer together, increasing the density and changing the mechanical properties of the material.

This is the first theory that developed a way to relate the physical properties of a polymer glass to the time scale of molecular movement, this information being "[...]especially important in engineering applications where small changes in dimensions, stiffness or other properties can affect long-term performance or reliability," as Schweizer said.

Among the many applications of polymer glass, there are residential and commercial aquariums, lenses of exterior lights of automobiles, the spectator protection in ice hockey stadiums, and as implants like intraocular lenses in the eye when the original lens has been removed in the treatment of cataracts.

The material is used to produce laserdiscs, and sometimes also for DVDs, and even artificial fingernails.