Spatial dependence of ordering process in bulk materials of polylactide and its multiple system during hygrothermal aging

Abstract

With a growing availability of polylactide (PLA) intended to three-dimensional molding applications, its spatial dependence of structural evolution during hygrothermal aging needs to be further investigated, in order to evaluate the service and degradation of PLA. This work mainly focuses on the spatial dependence of ordering process in the bulk materials of PLA and PLA/poly(butylene adipate-co-terephthalate) (PBAT) multiple system during hygrothermal aging. It is the first time that the microbeam wide angle X-ray diffraction (μWAXD) combined with differential scanning calorimetry and scanning electron microscopy are utilized to quantitatively analyze the spatial dependence of ordering process in the three-dimensional samples. Interestingly, owing to the presence of substantial mesophase and the preferential penetration of water molecules, stronger diffraction intensity in the skin layer can be identified in PLA/PBAT blends at the initial stage of hygrothermal aging, meanwhile neat PLA remains a negligible crystallinity. We ascribe the accelerated crystallization in PLA/PBAT blends to the higher Xmeso (6.3–9.0%) for unaged PLA/PBAT blends. Moreover, at the initial stage of hygrothermal aging, the mesophase can act as active nuclei to promote the formation of crystal, and the PBAT could act as heterogeneous nucleation to facilitate the transformation of mobile amorphous (MAF) to mesophase. However, at the late stage the original mesophase consumes up and the increasing ordered phase could act as physical cross-linking point to hinder the transforming into local ordering, leading to the slower transformation rate in PLA/PBAT blends than that in neat PLA. Notably, the obvious change of phase morphology can be observed at the late stage, and a blurred interface between PLA and PBAT may infer an improved compatibility. The reason for improved compatibility between PLA and PBAT can range a wide combination of factors including lower molecular weight PLA produced by hydrolysis and transesterification.