Polymorphic homocrystallization and phase behavior of high-molecular-weight Poly(L-lactic acid)/poly(D-lactic acid) racemic mixture with intentionally enhanced stereocomplexation ability via miscible blending

Promoting stereocomplex (SC) crystallizability is prerequisite for preparing the highly stereocomplexed polymer materials with better physical property. However, the intentionally enhanced SC crystallizability can exert severe effects on the homocrystallization and thus lead to the more complicated crystallization kinetics and polymorphic structure in stereocomplexable polymers. To elucidate the unique crystallization behavior of the stereocomplexable polymers with intentionally enhanced SC crystallizability, we choose the high-molecular-molecular (HMW) racemic poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) mixture blended with poly (ethylene glycol) (PEG) as the model system. Intriguingly, incorporation of small amount of PEG leads to the fractionated crystallization kinetics (multiple crystallization peaks) and the formation of unique beta-form homocrystals (HCs) of PLLA/PDLA component, different from the common alpha/alpha'-HCs generated in usual HMW PLLA/PDLA blends. Formation of beta-HCs is preferred in the PLLA/PDLA/PEG blends with the PEG mass fraction of 2%5% at low crystallization temperature (80-110 degrees C). We propose that the beta-HCs is formation stems from the constrained effects of precedingly-formed SCs in the early stage of crystallization; such effects become more significant in PLLA/PDLA/PEG blends due to the enhanced SC crystallizability. This study would be essential to understand the unique crystallization kinetics and polymorphic structure of HMW PLLA/PDLA mixture with intentionally enhanced SC crystallizability.