2018 Volume 54 Issue 12 Pages 444-450
Poly（lactic acid）（PLA） has been widely used as an alternative to petroleum-based polymers. Commercially,PLA is prepared by the fermentation of lactic acid produced from natural renewable sources such as cornin a typical compost environment. In fact, the synthesis method is environmentally benign with no emissionof carbon dioxide. Although PLA was initially intended for use in biomedical applications, these interestingtechnological features such as ease of polymer processing, good clarity, and acceptable mechanical propertieshave widened its use as a material in packaging, electronic, and automobile applications. However, PLA has aproblem of low thermal and mechanical properties due to low crystallinity. In order to increase the crystallinityof PLA sample, many researchers focused on stereo-complex crystals（Sc） from poly（L-lactic acid） and poly（D-lactic acid） blend. The melting point of Sc is approximately 50℃ higher than α-crystals of PLA. In thisreview, I discussed crystallization of Sc from the glassy state and the molten state and crystallization undershear flow. Time-resolved polarized optical microscopy, DSC, synchrotron radiation X-ray scattering werecarried out. During cooling down from the molten state, several tens micron crystals and 20 nm long spacingperiod were observed, while crystallization from the glassy state, we could observe very small crystals belowthe melting point of α-crystals. Furthermore, bimodal long spacing period, 60 and 15 nm because of αlamellarcrystal growth and melting during heating up were observed. Furthermore, this review discussed about theeffect of shear flow imposed prior to crystallization on higher-order structure formation and accelerationof Sc crystal growth. The density fluctuations of 100-nm scale were observed prior to nucleation by in situsimultaneous wide- and small-angle X-ray scattering measurements. These density fluctuations grew with timeand the intensity increased with increasing shear rate. Furthermore, the results revealed that the poly（L-lacticacid） and poly（D-lactic acid） chains were only partially interpenetrated. The strength of density fluctuationprior to nucleation was strongly dependent on the mixed phases.
