Brown University Researchers Induce New Polymer Phase in Polylactic Acid

The biodegradable polymer polylactic acid, also known as PLA, is widely used in manufacturing of various products, ranging from disposable cups, to drug delivery systems, to medical implants.

Treating polylactic acid, a commonly used polymer material, with heat and pressure creates crystals and causes polymer strands to become more organized. Those changes could make the material more useful in medical applications. (Mathiowitz lab / Brown University)

In a recent study reported in the Polymer journal, researchers from Brown University have demonstrated the possibility of inducing a new polymer phase in PLA by treating the polymer at various temperatures and pressures. This induced polymer phase holds the potential to reduce the degradation rate of 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. 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.

Edith Mathiowitz, Professor of Medical Science and Engineering, Brown University

PLA is semi-crystalline in nature, where parts of the molecular structure of the material are arranged into crystals while the remaining parts are disordered, or amorphous like glass.

Earlier studies had demonstrated that the crystalline structure of PLA could be increased when it is treated with heat, which could increase the strength of the material. In this study, the research group at Mathiowitz’s lab, headed by U.S. Navy veteran and doctoral candidate Christopher Baker, explored the influence of the addition of pressure on the structure of PLA.

PLA samples were treated under various temperature and pressure conditions for different durations. The pressure range used was 2,000 - 20,000 psi. Temperatures below, nearly equal, and above the glass transition temperature for PLA were used for treatments. The glass transition temperature is the point at which amorphous portions of the material transform from solid to rubbery.

Baker demonstrated that the amount of crystalline area in PLA increased during treatment. What really surprised the researchers was that the material’s amorphous parts became birefringent at higher temperatures and pressures. This implies that the amorphous parts bend light in a different fashion based on how the light is polarized, indicating that the amorphous regions of PLA undergo a substantial structural change.

As crystalline materials typically exhibit birefringence, observing this phenomenon in the amorphous portions of PLA was a real surprise. “We didn’t expect it to have such properties,” Mathiowitz said. “So to see it in the amorphous phase was really amazing.”

Using various techniques, Baker then further explored how the amorphous parts had changed. Using the X-ray diffraction technique, he demonstrated that polymer strands in some of the amorphous regions had arranged in an orderly manner.

The polymer strands are normally a tangled mess, but we found when we processed the material that the amorphous region became less entangled and much more oriented in a particular direction.

Christopher Baker, Doctoral Candidate, Brown University

Further thermal analysis revealed that the more ordered sections exhibited a higher glass transition temperature. Generally, the degradation rate is significantly lower for amorphous materials with higher glass transition temperatures.

According to the researchers, in the treated samples, the new amorphous phase in combination with the overall increase in crystallinity could have a major impact on their mechanical properties. The treated material could have better strength due to increased crystallinity and last longer due to the more ordered amorphous regions. The slower degradation rate could be particularly useful in applications in the medical field, an area that Mathiowitz’s lab specializes in.

One example is the application of PLA as a coating for implantable drug delivery systems and time-release pills. If it is possible to control the degradation rate of PLA, it is also possible to alter the rate at which the drug is delivered by the material.

Using PLA for screws and plates to stablize broken bones is another potential application of the material. As PLA implants degrade over time, a second surgery required to remove the implants can be eliminated. The rate of degradation of PLA may be higher for some of these applications. However, if this new polymer phase delays the degradation rate, PLA may become a viable option.

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

Edith Mathiowitz, Professor of Medical Science and Engineering, Brown University

Going forward, the researchers aim to measure the changes in material properties and explore the possibility of inducing this phase in other semi-crystalline materials.

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