Effect of the different architectures and molecular weights on stereocomplex in enantiomeric polylactides-b-MPEG block copolymers

Role of the Branched PEG-b-PLLA Block Chain in Stereocomplex Crystallization and Crystallization Kinetics for PDLA/MPEG-b-PLLA-g-glucose Blends with Different Architectures

The isothermal crystallization behavior and corresponding morphology evolution of poly(d-lactic acid) (PDLA) blends with PLLA_6.7k or MPEG-b-PLLA_6.7k-g-glucose with different architectures and different PLLA-grafted copolymer contents were investigated. The formation of stereocomplexes (SCs) in between the chain branched structure of MPEG-b-PLLA_6.7k-g-glucose and PDLA chains acting as the physical crosslinking points slows down the motion of PDLA chains, but the SCs could act as a heterogeneous nucleating agent for the late formation of homocrystals (HCs) in the blend system, accelerating the crystallization kinetics of HCs through enhancing the nucleation density. For PDLA/MPEG-b-PLLA_6.7k-g-glucose blends, the mobility of SCs in the blend system and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a T_c of 130 °C. The evolution of the crystal growth and structure during the isothermal crystallization process by rheometry has revealed the interesting role of the branched chains of MPEG-b-PLLA_6.7k-g-glucose in the mechanism of the crystallization in PDLA blends.

Competitive Mechanism of Stereocomplexes and Homocrystals in High-Performance Symmetric and Asymmetric Poly(lactic acid) Enantiomers: Qualitative Methods

To systematically explore the critical contributions of both molecular weights and crystallization temperature and chain length and molar ratios to the formation of stereocomplexes (SCs), our group quantitatively prepared a wide MW range of symmetric and asymmetric poly(lactic acid) (PLA) racemic blends, which contains L-MW PLLA with M _n > 6k g/mol. The crystallinity and relative fraction of SCs increase with T _c, and the SCs are exclusively formed at T _c > 180 °C in M/H-MW racemic blends. When MWs of one of the enantiomers are over 6k and less than 41k, multiple stereocomplexation is clear in the asymmetric racemic blends and more ordered SCs form with less entanglement or the amorphous region compared to those for the MW of the enantiomers over 41k in the symmetric/asymmetric enantiomers. When the MW of the blends is more than 41k, SCs and homocrystals (HCs) coexist in the symmetric enantiomers and the multicomplexation can restrict the asymmetric enantiomers. This study provides a deep comprehensive insight into the stereocomplex crystallization mechanism of polymers and provides a reference value for future research attempting to prepare stereocomplex materials.

Substantially Enhanced Stereocomplex Crystallization of Poly(L-Lactide)/Poly(D-Lactide) Blends by the Formation of Multi-Arm Stereo-Block Copolymers

Stereocomplex-type polylactide (SC-PLA) created by alternate packing of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains in a crystalline state has emerged as a growingly popular engineering bioplastic that possesses excellent hydrolytic stability and thermomechanical properties. However, it is extremely difficult to acquire high-performance SC-PLA products via melt-processing of high-molecular-weight PLLA/PDLA blends because both SC crystallites and homocrystallites (HCs) are competitively formed in the melt-crystallization. Herein, a facile yet powerful way was employed to boost SC formation by introducing trace amounts of some epoxy-functionalized small-molecule modifiers into the enantiomeric blends during reactive melt-blending. The results show that the SC formation is considerably enhanced with the in situ generation of multi-arm stereo-block PLA copolymers, based on the reaction between epoxy groups of the modifiers and hydroxyl end groups of PLAs. More impressively, it is intriguing to find that the introduction of only 0.5 wt% modifiers can induce exclusive SC formation in the blends upon isothermal and non-isothermal melt-crystallizations. The outstanding SC crystallizability might be attributed to the suppressing effect of such unique copolymers on the separation of the alternately arranged PLLA/PDLA chain segments in molten state as a compatibilizer. Furthermore, the generation of these copolymers does not result in a significant increase in melt viscosity of the blends. These findings suggest new opportunities for the high-throughput processing of SC-PLA materials into useful products.