Isotropically small crystalline lamellae induced by high biaxial-stretching rate as a key microstructure for super-tough polylactide film

Abstract

At present, polylactide (PLA) film with high toughness is on expectation, but in general, it is brittle with 45 MPa tensile strength and only 3% elongation at break. The present work clarifies that isotropically small crystalline lamellae is the key microstructure to obtain super-tough PLA (180 MPa tensile strength and 80% elongation at break). The systematic study on two parameters of biaxial-stretching technique, i.e., stretching rate and draw ratio, allows us understanding the development of microstructure, especially the crystalline phase, the regular chain packing in the crystal lattice, and the evolution of the higher-order structure. The integrated structural analyses based on Fourier transform infrared spectroscopy (FTIR), 2D-wide angle X-ray diffraction (2D-WAXD) and 2D-small angle X-ray scattering (2D-SAXS) declare that the structurally-irregular amorphous PLA starts to develop the mesophase and δ-crystal (including α-crystal) with the crystallite size of several tens of nanometers by slow biaxial-stretching (3 mm/s). At this stage, the biaxially oriented PLA (BOPLA) still shows relatively poor mechanical properties. When the stretching rate and the draw ratio increased above a certain level, i.e., 75 mm/s and 5 × 5, respectively, the small δ-crystallites of about 10 nm size with isotropic orientation are mainly formed. This structural evolution has been found to result in the drastic increase of the toughness of BOPLA film, which is about 4 times higher than the PLA film produced by conventional stretching. In this way, the present work shows for the first time that the simple way to toughen the PLA film is to induce a well-dispersed higher-order structure consisting of many but small δ-crystallites as seen in the model case of BOPLA.