Influence of melt-draw ratio on the crystalline behaviour of a polylactic acid cast film with a chi structure

Abstract

Herein, polylactic acid cast films were prepared with different melt-draw ratios via an extrusion casting process. The structure and properties of the prepared films were characterized. The results showed that with the increasing melt-draw ratios (MDRs), an oriented structure appeared at first, and then, a chi structure crystal was formed at higher MDRs. The vertical shish crystals showed a higher melting point than the oriented structure. The PLA molecules were extended by melt-stretching, and weak hydrogen bonds appeared between the carbonyl and methyl bond in the neighbourhood extended PLA molecular chain segments. The hydrogen bond limited the molecular mobility in the amorphous region, which increased the weight fraction of the rigid amorphous region. The stretching-induced crystallization occurred during the melt-stretching process, but the formed crystals were not strictly α form or α′ form. At lower MDRs, mainly an oriented structure was formed, and with the increase in MDR, meridional streaks in SAXS patterns appeared, accompanying the shish formation. The kebab and oriented structure signals superimposed together; this led to irregular equatorial scattering patterns. In this study, we provide a new insight into PLA crystallization via melt-extrusion cast flow.