Discovery of new polymer phase can lead to stronger and longer-lasting PLA

"It’s an exciting finding from the standpoint of basic science, in that we’ve found a new polymer phase and have identified a method for inducing it,” said Edith Mathiowitz, a professor of medical science and engineering at Brown. “In terms of applications, the polymer we worked with in this study has many uses, and we believe the properties we have discovered now will allow us to make it better.”

Work by previous researchers had demonstrated that treating PLA, a material with a semi-crystalline structure, with heat could increase its crystalline makeup. Researchers in Mathiowitz’s lab decided to investigate whether adding pressure to the treatment process would further influence the material’s structure.

Doctoral candidate and U.S. Navy veteran Christopher Baker, who led the project, treated PLA samples under a variety of different temperature and pressure conditions for varying amounts of time. To his surprise, he found that not only did the treatments increase the amount of crystalline area in the material, the amorphous parts of the material also became birefringent at higher temperatures and pressures. That is an indicator of a substantial structural change in the amorphous portions of the material.

Generally speaking, birefringence is a property found in crystalline materials, so seeing it in the amorphous regions of PLA was a surprise. “We didn’t expect it to have such properties,” Mathiowitz said. “So to see it in the amorphous phase was really amazing.”

Using a technique called X-ray diffraction, Baker showed that polymer strands in some of the amorphous sections had become dramatically more ordered. Further thermal analysis showed that the more ordered sections had a higher glass transition temperature. In general, amorphous materials with higher glass transition temperatures degrade at significantly slower rates.

The new amorphous phase combined with the overall increase in crystallinity in the treated samples could have significant implications for the material’s mechanical properties, the researchers said. The higher crystallinity could make it stronger, while the more ordered amorphous sections could make it last longer. That slower rate of degradation could be particularly useful in medical applications, an area in which Mathiowitz’s lab specializes.
PLA, which is biodegradable, is commonly used to make a variety of products from disposable cups to medical implants to drug delivery systems.
A past problem with using PLA for implants has been that it can degrade too fast. If this new polymer phase slows degradation, this problem may be solved. There would also be an added advantage in using PLA in drug delivery systems: if the rate at which PLA degrades can be controlled, the rate at which it delivers a drug can be altered.

“Now that we’ve shown that we can intentionally induce this phase, we think it could be very useful in many different ways,” Mathiowitz said.

The researchers plan more research aimed at quantifying changes in material properties as well as investigating whether this phase can be induced in other semi-crystalline materials.

The findings are published in the journal Polymer.

https://news.brown.edu/articles/2016/07/pla