Thermal Properties and Structural Evolution of Poly(l-lactide)/Poly(d-lactide) Blends

In order to obtain the forming regulation and structural evolution of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) stereocomplexes (sc-PLA), a commercial high molecular weight PLLA was blended with a series of PDLAs with different weight-average molecular weights (Mw) in the range of 7.5–290 kg·mol–1 by solution blending. The thermal properties, morphology, and thermal stability of the PLLA/PDLA blends were investigated by differential scanning calorimetry (DSC), field emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TGA). The crystallization of sc-PLA and homochiral PLA (homo-PLA) was competitive and controlled by the Mw of PDLA. The phase structure of the PLLA/PDLA blends depended on the melting temperature. By quenching sc-PLA from 280 to 290 °C, a bicontinuous phase structure was observed for PLLA and PDLA. When quenching sc-PLA from 230 to 270 °C, a stereoamorphous mesophase of PLA (sam-PLA) was obtained, which originated from the residual strong hydrogen bonding between PLLA and PDLA. sam-PLA was a new discovery different from sc-PLA and homo-PLA. The PLLA/PDLA blends are not fully compatible among sam-PLA, PLLA, and PDLA. During melting, sc-PLA changed into sam-PLA, and the hydrogen bonding in sam-PLA was destroyed and weakened with increasing temperature at <∼270 °C. At higher than ∼270 °C, the hydrogen bonding became extremely weak and even disappeared.