The phase transition behaviors of melt‐quenched asymmetric poly(l‐lactide acid)/poly(d‐lactide acid) (PLLA/PDLA) blends are investigated by the in‐situ wide angle X‐ray diffraction. Results indicate that the crystal structures and their transition of PLLA/PDLA specimens depend on the composition, molecular weight, and temperature. In the specimens with lower molecular weights (∼9 kg/mol), the poly(lactic acid) homocrystallites (HC), mesophase, and stereocomplex crystallites (SC) develop in specific blends. As the content of PDLA increases, the formation temperature of HC increases to a fixed value, while that temperature for SC reduces to a constant value, and the formation temperature of mesophase does not vary with composition. For the specimens with moderate molecular weights (∼30 kg/mol), the HC, modified HC, and SC develop in specific composition, the formation temperatures of both the HC are constant, and the formation of SC reduces to a fixed temperature. In the blends with molecular weight of ∼65 kg/mol, HC and SC develop, and the formation of HC does not vary, but that temperature for SC reduces slightly as more PDLA incorporated. After melting of HC, another enhancement of SC is observed in all the PLLA/PDLA specimens. The unique crystallization phenomena for these PLLA/PDLA specimens are analyzed.

Blending of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) would produce polylactide stereocomplex crystallites (PLA SC). In the PLLA/PDLA specimens prepared by solution mixing or melt blending, the isomeric polymers were mixed in the molecular level. When the distance of these isomers increased, the crystallization behaviors of PLLA/PDLA were not clear yet. In this study, PLLA/PDLA blends were prepared by different electrospinning methods, i.e., spinning by mixed solution (DL-A), side-by-side mode (DL-B), and double spinnerets (DL-C). Results revealed that the distances between isomeric molecules increased in the order of DL-A, DL-B, and DL-C, and this distance influences the phase transition and subsequently crystallization behaviors. In the DL-A, the homochiral crystallites (HC) developed at first, and then SC formed in the specimen with lower content of PDLA. The formation of SC was observed at first, and less or no content of HC was developed with higher content of PDLA. For the DL-B group, the formation of HC was observed at ∼100 °C, and the SC were produced around 140 °C irrespective of composition. More content of SC was produced during or after melting of HC. When referring to the DL-C group, the HC developed at the first stage, and the SC were produced during or after melting of PLA HC. In addition, the SC that developed in the three groups have different nucleation efficiencies for the subsequent formation of HC. The nucleation efficiency increased in the order of DL-A < DL-B < DL-C, and these results hinted that not only the content but also the distribution of SC affects the formation of HC.

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A series of poly(ethylene glycol)‐poly(l‐lactide) (PEG‐PLLA) di‐, and tri‐block copolymers that consisted of different lengths of PEG and PLLA blocks were synthesized, and the crystallization behavior of PEG and PLLA blocks were investigated. Results demonstrate that PEG and PLLA blocks interact with each other, and the crystallization and melting behaviors of copolymers are dependent on the composition of PEG and PLLA. The crystallization of PEG block is confined by the existence of PLLA block, the more fraction of PLLA, the greater restriction on PEG. However, this confinement is reduced after PEG with the larger molecular weight is incorporated into the copolymers. The PEG acts as a solvent, which reduces the viscosity and enhances the chain mobility of the PLLA. The melting temperature of PLLA block decreases when the PEG length is increasing or when the molecular weight of PLLA is decreasing. With the increase of the molecular weight of the PLLA block, the crystallinity of PLLA firstly increases and then decreases. For the copolymers with different structures and molecular weights of PEG blocks, the crystallinity and melting point of PLLA block exhibit different unique component dependence.