Designed Multicomplexation Based on Asymmetric Polylactide Enantiomeric Blends:A Quantitative Study
To quantitatively explore the critical contributions of both molecular weights and blending ratios to the formation of stereocomplex(SC)crystallization in the poly(L-lactide)/poly(D-lactide)(PLLA/PDLA)enantiomeric blends.A series of PLLA/PDLA racemic blends were prepared by controlling the enantiomeric chain length differences and blending ratios,and it was found that in the enantiomeric blends with an enantiomeric chain length average degree of polymerization(DP)about 60 or less,when the two enantiomers had exactly the same chain lengths with the same blending ratios,SCs were formed that were more ordered with less entanglement or the amorphous regions.On the other hand,when the two enantiomeric chain lengths are unequal,the excess segments of the longer ones could remain amorphous on the surface of the SC crystallization,or complex with the other shorter enantiomers,or tightly pack between the excess segments to form homocrystals(HCs),depending on the dual effects of the difference in chain lengths and the blending ratios:when the difference in chain lengths is not significant(xL＜xD＜2xL where x is the chain segment length,and L or D refer to the poly(lactide)configuration),since xD does not exceed 2 times xL(xD＜2xL),a longer chain segment cannot accommodate the two shorter chains in the series,and the excess segments of the longer segments could stay amorphous on the SC crystallization surface.Although there is competitive crystallization between SC crystallization and HC crystallization,the former is preferred at appropriate blending ratios;when the difference in chain length(xD≥2xL)widens further,there will be enough space in the longer chain to accommodate more than one short enantiomeric chain,which means that the equal-ratio asymmetric PLLA/PDLA blends fail to form a particularly uniform single SC crystallization,and HC crystallization is found to be signs of HC crystallization.Meanwhile,the lengthy segments of the longer enantiomeric chains on the surface of the SCs could be further matched with another additional short enantiomeric chain to form multi-complexation.
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