Comparison of melting and crystallization behaviors of polylactide under high-pressure CO₂, N₂, and He

This study investigated the melting and crystallization behaviors of polylactide (PLA) under high-pressure CO₂, N₂, and helium (He) using a high-pressure differential scanning calorimeter and a wide-angle X-ray diffractometer. The results showed that the PLA's melting temperature was depressed only when contacted with pressurized CO₂ where at high CO₂ pressures the lubricating gas molecules induced more imperfect melt and cold crystals during the cooling and heating cycle. PLA's melt crystallization was analyzed during both isothermal and nonisothermal processes. In contrast to the effect of dissolved CO₂ that expedited the PLA's crystallization rate, N₂ showed almost a neutral impact on the PLA's crystallization kinetics. Because of the lower solubility, N₂ gas dissolved in the PLA had a diminutive plasticization effect, and thereby it could only counterbalance its negative hydraulic pressure effect. Moreover, as the helium pressure increased, the PLA's final crystallinity was reduced due to the dominant effect of helium's hydraulic pressure.