The stereocomplex formation and phase separation of PLLA/PDLA blends with different optical purities and molecular weights

In this study, the poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blends with different optical purities of PLLA and various molecular weights of PDLA are prepared by solution mixing, and the stereocomplex formation and phase separation behaviors of these blends are investigated. Results reveal that optical purity and molecular weight do not vary the crystal structure of PLA stereocomplex (sc) and homochiral crystallites (hc). As the optical purity increasing in the blends, the melting temperature of sc ( T sc ) and the content of sc (Δ H sc ) increased, while the melting temperature of hc ( T hm ) hardly changes, although the content of hc (Δ H hm ) decreased gradually. The T sc and Δ H sc are also enhanced as the molecular weight of PDLA reduces, and the Δ H hm reduces rapidly even though the T hm does not vary apparently. With lower optical purities of PLLA and higher molecular weights of PDLA, three types of crystals form in the blends, i.e. , PLA sc, PLLA hc and PDLA hc. As molecular weight decreases and optical purity enhances, the crystal phase decreases to two (sc and PDLA hc), and one (sc) finally. This investigation indicates that the phase separation behavior between PLLA and PDLA in the PLLA/PDLA blends not only depends on molecular weights, but also relies on the optical purities of polymers.