Researchers are developing a polymer that can be adapted to both high and low temperatures

The modern world is full of synthetic polymers, long-chain molecules designed by scientists to fill all sorts of applications.

Researchers at the FAMU-FSU College of Engineering have developed two closely related polymers that respond differently to high and low temperature thresholds, despite their similar design. The polymer pair can be used in applications in medicine, protein synthesis, protective coatings and other fields. Their work is published in macromolecules.

“Normally, in order to obtain one extreme behavior, we have to prepare a polymer for that specific application, and if you want to obtain another extreme behavior of the polymer, you have to prepare a completely different polymer,” said co-author Hueyong. Chung, associate professor at FAMU-FSU College of Engineering. “But now, through this work, we have one type of polymer that can be rapidly adapted with minimal interference with both functions.”

The researchers’ polymer is made of a sulfoxide, a compound made of sulfur, oxygen and carbon molecules. One version contains an additional ingredient, a pair of hydrogen atoms known as a methylene group. This small structural difference is enough for each polymer to respond differently to changes in temperature.

Every mixture has critical temperatures above or below, at which the components are completely dissolved in the solution, regardless of the concentration of the different components in the mixture.

One version of the researchers’ polymer is soluble in water at lower temperatures but becomes insoluble at higher temperatures. The other version displays the opposite behaviour. It is insoluble at low temperatures but melts when temperatures rise above a critical point.

“This paradoxical behavior, which appeared with only one minor change, was a surprising finding,” said postdoctoral researcher Biswajit Saha, lead author of the study. “It’s an exciting avenue for future research.”

Besides developing this new temperature-controlled polymer, the research team made other discoveries.

A new mechanism governs the critical temperature threshold

Previous research has shown that hydrogen atom bonds determine the temperature above which temperature-sensitive polymers dissolve in solution, which is called the upper critical solution threshold.

But Chung’s group found that the attraction between the poles of different positively and negatively charged molecules — a process known as dipole-dipole interaction — also predicted the temperature at which the polymer would mix in the water. Notably, his group has experimentally demonstrated the existence of this interaction as a driving force for thermal behaviour.

Two-phase thermal behavior

Most solutions experience a one-phase change when they pass their temperature threshold. But the polymer developed by Chung’s team undergoes phase changes in two stages. This feature could open up potential new applications in medicine, such as a single drug capsule that dissolves in the heat of a patient’s stomach in two phases, allowing for precise drug delivery.

“We were lucky to get all these diverse ideas in one design,” Chung said. “Being the only polymer that can be programmed to achieve different behaviors means that this molecule can easily adapt to different applications.”