An optical microscopy study on the phase structure of poly(L-lactide acid)/poly(propylene carbonate) blends

The dependence of phase structure of PLLA/PPC blends on the blend ratio, the heat-treatment temperature and time was investigated by optical microscopy. It is found that, at lower PPC content, e.g., less than 30%, the PLLA crystalline spherulites fill the whole space with the PPC dispersed in the amorphous region of PLLA. No evident phase separation has been observed under optical microscope. When the content of PPC reaches 40%, phase separation takes place. The phase separation of the PLLA/PPC blend happens prior to the crystallization of PLLA. Therefore, the heat-treatment temperature and time are the two most important factors that control the phase structure of the blend. At low heat-treatment temperatures, e.g., lower than 190 °C, the PPC and the amorphous PLLA part compose a continuous phase with the crystalline PLLA domains dispersed in it. When the sample was heat-treated at 200 °C for 5 min, a bicontinuous phase structure was observed. With further increase of the heat-treatment temperature, the crystalline PLLA composes the continuous phase with PPC domains randomly dispersed in it. Similar phase reversal phenomenon has also been observed by heat-treating the samples at 200 °C for different times. It is further confirmed that the crystallization of PLLA in the blends is influenced by the different phase structures. For example, the crystallinity of PLLA in the blend increases with increasing heat-treatment temperature.