Turning up the heat on the search for better plastics

By Scott Lyon

If you have ever poured hot coffee into a cheap plastic cup, you may recall that sinking feeling as the cup seems to wilt. Not quite solid, not quite liquid, the cup totters between two states of matter.

This shift from a hard, glass-like state to  a soft and rubbery state is called the glass transition, and understanding it could lead  to applications such as self-repairing plastic, high-efficiency solar cells and better batteries.

A research team at Princeton has turned up the heat on the search for a more detailed understanding of plastics by studying the temperature at which materials undergo the glass transition.

The team performed highly precise measurements of the glass transition temperature in plastics known as block copolymers, which are chains of at least two repeating polymer units. What they found could help scientists create new polymers that, like window glass, have an underlying disordered structure that lends strength and durability.

Block copolymers are an active area of research because they can be made to have highly desirable properties, such as healing after mechanical injury and being simultaneously stiff and tough.

“When you think about anything from telescopes probing space to fiber optic cables under the oceans, all of it relies on glass,” said Dane Christie, a graduate student in  the Department of Chemical and Biological Engineering (CBE) and the first author of a recent paper published in the journal ACS Central Science.

“For most polymers, the glass transition temperature is probably the single most important material parameter,” said

Richard Register, the Eugene Higgins Professor of Chemical and Biological Engineering, who co-led the study with Rodney Priestley, an associate professor of CBE. “It’s what makes a tire rubber rubber and a soda bottle a relatively stiff plastic.”

But measuring the exact glass transition temperature of these materials, which self- assemble into shapes that bring various sections of the polymer chains into contact with each other, is not easy. To do so, Christie attached fluorescent labels that allowed him to track the precise movement of the parts of the chain as he raised the temperature. This allowed him to observe how the materials change during the transition from a liquid to a glass state.

The results showed a dramatic and asymmetric change in the intrinsic values of the glass transition temperature within each piece of the chain, especially near the interfaces, facts missed by previous studies.

The study was funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation through the Princeton Center for Complex Materials.