Fractionated Crystallization Kinetics and Polymorphic Homocrystalline Structure of Poly(L-lactic acid)/Poly(D-lactic acid) Blends: Effect of Blend Ratio

Stereocomplex (SC) crystallization has been an effective way to improve the physical performances of stereoregular polymers. However, the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers, which influences the physical properties of obtained materials. Herein, we select the medium-molecular-weight (MMW) poly((L)-lactic acid)/poly((D)-lactic acid) (PLLA/PDLA) asymmetric blends with different PDLA fractions (f(D)=0.01-0.5) as the model system and investigate the effects of f(D) and crystallization temperature (T-c) on the crystallization kinetics and polymorphic crystalline structure. We observe the fractionated (i.e., multistep) crystallization kinetics and the formation of peculiar beta-form homocrystals (HCs) in the asymmetric blends under quiescent conditions, which are strongly influenced by both f(D) and T-c. Precisely, crystallization of beta-form HCs is favorable in the MMW PLLA/PDLA blends with high f(D) (>= 0.2) at a low T-c (80-100 degrees C). It is proposed that the formation of metastable beta-form HCs is attributed to the conformational matching between beta-form HCs and SCs, and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization. Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.