Formation of shish-kebabs in injection-molded poly(L-lactic acid) by application of an intense flow field

Unlike polyolefins (e.g., isotactic polypropylene), it is still a great challenge to form rich shish-kebabs in biodegradable poly(l-lactic acid) (PLLA) because of its short chain length and semirigid chain backbone. In the present work, a modified injection molding technology, named oscillation shear injection molding, was applied to provide an intense shear flow on PLLA melt in mold cavity, in order to promote shear-induced crystallization of PLLA. Additionally, a small amount of poly(ethylene glycol) (PEG) with flexible chains was introduced for improving the crystallization kinetics. Numerous shish-kebabs of PLLA were achieved in injection-molded PLLA for the first time. High-resolution scanning electronic microscopy and small-angle X-ray scattering showed a structure feature of shish-kebabs with a diameter of around 0.7 μm and a long period of ∼20 nm. The wide-angle X-ray diffraction results showed that shish-kebabs had more ordered crystalline structure of α-form. A significant improvement of the mechanical properties was obtained; the tensile strength and modulus increased to 73.7 and 1888 MPa from the initial values of 64.9 and 1684 MPa, respectively, meanwhile the ductility is not deteriorated. Interestingly, when shish-kebabs form in the PLLA/PEG system, a bamboo-like bionic structure comprising a hard skin layer and a soft core develops in injection-molded specimen. This unique structure leads to a great balance of mechanical properties, including substantial increments of 26, 20, and 112% in the tensile strength, modulus, and impact toughness, compared to the control sample. Further exploration will give a rich fundamental understanding in the shear-induced crystallization and morphology manipulation of PLLA, aiming to achieve superior PLLA products.