Greg McKenna hopes to discover how the thin film’s properties change beyond the relationship to reduced glass transition temperatures seen in ultrathin films.
One of the great things about science is research projects or experiments can be conducted with no definitive expectation of what the exact outcome will bring as long as a strong, underlying set of hypotheses exists.
For Greg McKenna, a Horn Professor and John R. Bradford Chair in the Texas Tech University Department of Chemical Engineering, ultrathin films present just such an opportunity for discovery. While extensive work has been done studying the glass transition temperatures of these films, little is known about how the mechanical properties such as modulus and yield strength change and to what extent.
Through a $525,000 grant from the National Science Foundation, McKenna hopes to push the limits of these ultrathin films to investigate how the properties change, and potentially, how that change is different than what would be expected due to the changes in their glass transition temperatures at the nanometer size scale. The effects of making films from different types of molecules, or molecular architectures, also are being investigated.
“The problem is people have seen all these really different properties in ultrathin films, and most focus on the glass transition behavior,” McKenna said. “We want to find out what are the real mechanical properties doing when you actually get that thin in addition to just seeing changes in glass transition. We're trying to work deep in the glassy state, which is where we use these materials. Hence, they are stiff and rigid, and the goal is to determine if and how the properties change differently than what people would expect if the changes were due to simply changing the glass transition.”
From watching the bubble growth we extract the viscoelastic behavior of the ultrathin film. In this case, the film was 145 nm in thickness and the temperature was 296 Kelvin (that is around room temperature).
Ultrathin films are defined as those with a thickness of less than 100 nanometers, where a nanometer equals one billionth of a meter. McKenna said there is some evidence the properties of ultrathin films change differently than does the glass transition itself, but those results are extremely limited and, consequently, cannot be considered conclusive, thus the need for further research.
Using the Texas Tech University Nanobubble Inflation Method, an experimental technique for measuring the viscoelastic properties of ultrathin polymer films that McKenna helped develop, he and group also are examining the mechanical properties of other materials in the thin-film state, starting with selenium, a gray crystalline mineral with semiconducting properties. Because it has simple molecules that can form a chain, it exhibits some polymer-like properties.
Creating ultrathin films also means having something with a large amount of surface area. McKenna said there is more going on than just observing the surface-to-volume ratios of ultrathin films, but an important aspect of the problem of ultrathin films is there is a distinct lack of strong theories to explain their behavior. Thus much of the work is driven by experimental discovery that will one day lead to improved theories. Of interest also is computer simulations possess some of the same issues as the experiments and the theories, and that is an inadequate means of treating the surfaces.
“There is a lot of government funding into nanotechnology because the future of microelectronics is actually nanoelectronics,” McKenna said. “When you use polymers in these systems, the polymers, for example, are used for dielectrics that determine the insulating properties, and that affects the clock speed of the devices. If we're going to use them, we have to know how they change, and that's where we push forward in the future.”
As another example, ultrathin films are used to coat fiber optic cables, and a large portion of those cables run under the ocean. Knowing how they will react to the harsh conditions of such use is essential to ensuring their functionality.
One challenge to enlarging the scope of experimenting on novel materials in the ultrathin film state is the fact the thin films themselves are inherently available only in extremely small quantities. Spin coating and physical vapor deposition are ways to make very thin films. Polymer films are commonly made via spin coating, while small molecule compounds and elements such as selenium can be made into thin films by the physical vapor deposition process. The Texas Tech Nanobubble Inflation Method makes possible the investigation of the properties of such extremely small amounts of materials.
Of great interest is the observation from Mark Ediger at the University of Wisconsin, who has made ultradense glasses by physical vapor deposition methods and shown some of these materials exhibit properties similar to what one would expect from a material aged for millions of years. McKenna and his coworkers at Texas Tech have previously worked with a 20 million-year-old piece of amber, and he now plans to use the Ediger process to make ultradense glasses and test them with the Texas Tech Nanobubble Inflation Method to determine if they exhibit the same behavior as does the 20 million-year-old amber tested at the macroscopic size scale.
Because molecular relaxation times near to the glass transition increase exponentially with even small changes in temperature to a point where the material becomes unsuitable and would require large amounts of time. Taking amber that is 20 million years old allows McKenna to test the material's behavior between its fictive temperature and its glass transition temperature.
“If we can measure the viscoelastic properties of ultradense glasses, we can ask the same questions that we've asked with the amber, but with a little better control,” McKenna said. “This is really fundamental work but the long-term consequences are important. We still can't describe the long-term behavior of polymers in the glassy state except empirically. This is essential to the development of advanced materials from those undersea cables to the latest high-performance composites used in modern aerospace technologies.”
And that, in essence, is what the research is about – discovery. It is fundamental research that will eventually lead to technological advances that will be used by everyone from government entities to private enterprise, who will be looking for graduates like those from Texas Tech who have extensive experience in this area.
“I hope we find some answers to the fundamental questions about glass transition,” McKenna said. “We have a hypothesis and want to test that hypothesis. I go into it and tell my students, ‘I don't want you to find what I tell you I think you should find.' They have to find what the truth is, so that's why I'm reluctant to say what the outcome will be.
“The fundamental materials research, for sure, will lead to students getting their degrees and some publications, and possibly to other important insights into the physics of glasses. More importantly, the projects should make the students learn to think.”