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.
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.
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.
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.
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.
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
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.