Melt extension-induced shish-kebabs with heterogeneous spatial distribution of crystalline polymorphs in lightly crosslinked poly(lactic acid)

Abstract

Applying flow to stimulate the shish-kebab formation in semi-rigid poly(lactic acid) (PLA) is far from mature compared to that in flexible polyolefins, which waits to be solved for both academia and industry. Herein, we conduct a systematic investigation on the flow-induced crystallization of lightly crosslinked PLA melt by a combination of extensional rheology and in situ synchrotron wide and small-angle X-ray scattering. With Hencky strain above a critical value of 1.5–2.0, abundant shish-kebabs with high orientation and high regularity are identified to form, which leads to not only tremendous acceleration of PLA crystallization kinetics but also superb heat tolerance of finally solidified sample. With an affine deformation assumption, the generation of shish-kebabs is recognized to require the entanglement network deformation in crosslinked component chains reaching 67% of its maximum elongation ratio, much higher than that reported in polyethylene. In addition, we have confirmed a heterogeneous spatial distribution of crystalline polymorphs in shish-kebab body due to its morphological duality. The β-form contributes mainly to the central shish originating from strongly stretched crosslinked component chains, while the α/α′-form to the lamellar kebab from moderately orientated free chains. Then a close relationship between chains deformation, crystalline polymorphs, crystal morphology and material property is well established. This study has disclosed that imposing melt extension to lightly crosslinked PLA can result in extraordinary crystallization capacity of PLA material comparable to that of flexible polyolefins, showing potential significance for guiding the industrial processing.