Episode 2: Glasses found denser than the supercooled liquid

In this podcast episode, MRS Bulletin's Sophia Chen interviews Zahra Fakhraai of the University of Pennsylvania on her group's research to better understand how a substance condenses into glass. They studied the liquid–liquid phase transition in vapor-deposited thin films of N,N0-bis(3-methylphenyl)-N,N0-diphenylbenzidine (TPD). They discovered a new high-density supercooled liquid phase in glasses deposited in the thickness range of 25-55 nm. Their findings could lead to more precise theoretical descriptions of glasses. 

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics of biomaterials, quantum materials, sustainability, artificial intelligence, perovskites, and robotics. My name is Sophia Chen.  Zahra Fakhraai makes glass in her lab. But Fakhraai, a University of Pennsylvania chemist, doesn’t do it in the way you might imagine, with red-hot molten bulbs on the end of a blowpipe. She makes extremely thin films of glass, a thousand times thinner than a human hair. This glass forms as organic molecules in a vapor condense on a substrate. By creating and testing this thin glass, Fakhraai is working to better understand how a substance condenses into glass. Glass is this strange phase of matter because its molecules are completely disordered like in a liquid, but it’s dense and hard like a solid. 

ZAHRA FAKHRAAI: People don't quite understand why the molecules move as slowly as they do, and then why they fall out of equilibrium to form a glass. That has been one of the most longstanding questions in condensed matter physics.

SC: Fakhraai’s team studies thin films because compared to large samples, films stabilize more rapidly, causing them to become denser and harder. This is because molecules close to a substance’s surface can move much more quickly than molecules in the bulk. In contrast, stabilization can take a long time in large chunks of glass. For example, tree resin, which is a natural glass, hardens into amber over millions of years. But researchers can create a thin film glass as dense and as hard as amber, in a matter of an hour or so. Fakhraai’s team makes glass out of an organic molecule known as TPD. TPD is used to make organic LEDs, although Fakhraai chose to study the material as a proof of concept. 

ZF: It's a molecule that has been studied a lot, so we understand a lot of its properties.

SC: When making this thin glass, Fakhraai’s team stumbled across something strange. The glass was much denser than they expected. 

ZF: Under certain conditions, we can actually make them to be very, very dense, actually not only denser than this supercooled liquid, but actually denser than the crystal even.

SC: It’s quite remarkable that the glass was denser than the crystalline phase. Typically, the crystalline state of a material is denser than its glass state. That’s because you can usually fit more molecules in the same space when molecules are arranged in an orderly crystal than if they are disordered. Fakhraai says you can think of in analogy with passengers on a crowded subway.

ZF: If people came in one by one, they were just forming rows and rows of people, you can actually probably put in more people in that station, than if people come in randomly and they have to be mindful of their neighbors. It's sort of similar that the molecules are stuck. So they can't like, move around and make a better arrangement. And so the density suffers.

SC: The glass was also denser than another phase of matter known as its supercooled liquid phase, which is also unusual. Fakhraai says that this dense glass phase implies the existence of an even denser supercooled liquid phase that they have not directly observed. This liquid phase could be even denser than the crystalline phase. To understand the phase further, she plans to measure the heat capacity, conductivity, and mechanical properties of the material. In addition, Fakhraai hypothesizes this super dense phase could exist in many other types of organic molecules in thin films. Their findings could also lead to more precise theoretical descriptions of glasses.

ZF: Our data, while consistent with a lot of the models of glass transition that are out there, it doesn't exactly fit. And so there needs to be some modifications of these glass transition theories, which is exciting.

SC: This work was published in a recent issue of the Proceedings of the National Academy of Sciences (https://doi.org/10.1073/pnas.2100738118). My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.