Scientists Conduct Simulation of Glass Transition in Polymer Film

Glasses are not technically solid in a crystallized form, however, they are substances frozen in a liquid-like structure.

There are many fundamental questions regarding how exactly glasses form from flowing liquid to solid glass. A main factor, materials scientists study when examining phenomena about glass, like its formation, is the glass-transition temperature where this occurs.

An international team of computational physicists and chemists have revealed how the polymer structure bears on the glass-transition temperature in the forming of glass in atactic polystyrene (PS), a frequently used glass substance. The team’s research finding is reported this week in the Journal of Chemical Physics, from AIP Publishing.

Alexey Lyulin, a physicist at Technische Universiteit Eindhoven in the Netherlands and visiting faculty member at Stanford University, led the research conducting supercomputer-processed simulations.

Glass transition is actually a mysterious phenomenon. It is still not understood completely, even for very simple liquids.

Alexey Lyulin, Physicist, Technische Universiteit Eindhoven

Lyulin also said that polymers are not simple liquids. These polymers have very long chainlike molecules, and, in general, they do not crystallize but create amorphous, glassy solids. This glassy state is vital for many applications, such as nanolithography. The polymer interface is important as well, because this is where important mechanics and heat transfer occurs between different molecules.

Although Lyulin and his collaborators examined the polymer glass transition in bulk samples, they were particularly interested in thin films of polystyrene. These polystyrene films often have one molecule thickness at the nanolevel. While exploring the glass transition for polymers at this level, Lyulin revealed that scientists want to study more about the relevant dynamics and the structure.

In our paper, we studied only polystyrene structure and what happens to this structure at glass transition when you go to very thin, nanometer-thick films.

Alexey Lyulin, Physicist, Technische Universiteit Eindhoven

Lyulin also noted that the authors of this paper were familiar with experimental research that revealed in the thin, free-standing film without a substrate, the transition temperature of polystyrene is very low, compared to bulk polystyrene, with a difference of about 60 °C.

"It's a huge effect -- the biggest effect that is observed in polymer films," Lyulin said. "And then we tried to understand why, what is so specific about polystyrene."

The authors theorized that many benzene rings in the polystyrene film are pressed to the periphery of the film and reveal an interesting behavior of these ring interactions, also know as aromatic or pi-pi interactions.

It means that the very strong interactions between benzene rings are somehow weakened inside the film. And because of this weakening, the glass transition occurs lower in temperature.

Alexey Lyulin, Physicist, Technische Universiteit Eindhoven

Various groups within the research team tested their theory with a multipronged method. One group arranged the initial film samples, one operated computer simulations and another group assisted in analyzing the results.

According to Lyulin, the team also noticed that the polymer cooling rates influenced the transition temperature. They tested more than 100 polystyrene films of different thickness, structure and at different temperatures, which took more than six months, and the computer simulation cooling rates were many orders faster than in experiments.

Lyulin stated that the strong confirmation of their surprising theory emphasizes that the findings provide fundamental insights about the molecular structure of the polystyrene film as the glassy substance approaches the transition.

These pi-pi aromatic interactions play a very, very important role in this specific polymer and in any polymer that contains aromatic rings. The pi-pi interactions lead to specific orientation, ordering of these aromatic groups and then to specific structure that has very important consequences for this glassy material.

Alexey Lyulin, Physicist, Technische Universiteit Eindhoven

Lyulin also added that this appears to occur with other non-polymeric materials, such as graphene, which has pi-pi interactions between carbon rings. He believes that he and his collaborators will continue this research and math the results with other experimentalists and theorists.

"It would be very interesting to study and compare this effect dynamically, what happens to the mobility of these rings, how they relax and what happens to the mobility of other polymer segments upon cooling in the system," Lyulin said. "It would be very interesting to compare both static and dynamic Tg (transition temperature) values"

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