Tammann Analysis of the Molecular Weight Selection of Polymorphic Crystal Nucleation in Symmetric Racemic Poly(lactic acid) Blends
Yucheng He, Dan Liu, Jiping Wang, Pengju Pan, Wenbing Hu
Abstract
High molecular weights switch the harvests of crystallinity from stereocomplex crystals to homocrystals in the symmetric racemic poly(lactic acid) (PLA) blends. We performed Flash DSC measurement of isothermal crystallization kinetics of symmetric racemic PLA blends with five molecular weights in combination with confocal Raman spectroscopy to identify their polymorphic crystal phases. The results confirmed crystallization kinetics as responsible for the molecular weight selection of polymorphic crystals. We further employed Tammann analysis on the crystal nucleation kinetics in the low-temperature region to reveal that higher molecular weights have little influence on homocrystal nucleation; rather, they suppress the nucleation rates of stereocomplex crystals, making homocrystals dominant. The observation was interpreted by the correlation hole of neighboring monomers among higher molecular weight polymers, which hinders intermolecular nucleation of alternatingly racemic polymers as favored for stereocomplex crystals.