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

Abstract

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.

Recent advances in enhancing stereocomplexation between poly(lactide) enantiomeric chains

“…Additionally, the spatial heterogeneity and arrangement of PLLA and PDLA chains which has been displayed under the ''Effect of mixture mass ratios or molar ratios of PLA enantiomeric blends'' section in less-entangled blends also affect the formation of SC crystallization. 73 Besides the conventional melt blending, solution blending techniques, and supercritical fluid (SCF), newer techniques such as annealing, 20,21,45,46,62 electrospinning, 33,35,50,112 printing, 105 and templating 113 have been reported to prepare SCs between enantiomeric PLLA and PDLA chains. In another study, S. H. Im et al 111 introduced a new technique for SC crystallization using an oil-in-water (O/W) emulsion blending method.…”

“…3. Guo et al 73 synthesized a wide molecular weight range of PLA enantiomers (6.7–210 kDa) and prepared different blending ratios of PLLA/PDLA racemic blends to confirm the existence of multi-complexation and attain exclusive SC crystallization (L: 6.7 kDa, D: 95 kDa) between the L/M-MW PDLA and H-MW PLLA chain. Notably, the PLA chains still have unavoidable entanglement due to the absence of ECC.…”

Role of the branched PEG-b-PLLA chain in morphological structures and thermodynamics for PEG-b-PLLA-g-glucose copolymers with different architectures

“…4,10 It is well known that copolymers and blends are two common but effective approaches to enhancing the properties of the polymer matrix. 11,12 Therefore, PLA block copolymers have been widely concerned in recent years. 13 Such PLA block copolymers have been investigated, including PS/PLLA block copolymers, 14 PCL/PLLA block copolymers, [15][16][17][18][19] PEG/PLLA block copolymers, 17,[20][21][22][23] etc.…”

“…Scheme 3 Synthesis of 5A-MPEG-b-PLLA-g-Glu (5A-MPELAG)(11), and 5A-Glu-g-PLLA-b-PEG-b-PLLA-g-Glu (5A-PELAG)(12).…”

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

“…Since Y. Ikada proposed the stereocomplex (SC) crystallization of PLLA and PDLA in 1987, a large number of studies on the formation of the SC have been reported. The chemical and physical properties of SC PLA strongly exceed those of its parent enantiomeric pure, homocrystalline polymers, such as higher mechanical strength, better thermal stability, lower thermodegradation, etc. , For example, the blending of PLLA and PDLA can result in the formation of SCs with a melting point of about 50 °C higher than that of homocrystals (HCs). − …”