Revealing the film-forming property and microstructure evolution of polylactide during simultaneous biaxial stretching: Toward high-performance sustainable film

The film-forming property of PLA during simultaneous biaxial stretching was investigated by the visualized contour graphic method. Meanwhile, the multiscale microstructure evolution in both amorphous and crystalline domains was studied in detail. It is revealed that the film thickness uniformity is governed by the microstructure evolution and related to the engineering stress-strain behavior. At the low stretching ratio, the excellent chain mobility and fast chain relaxation enabled by thermal activation and formation of gauche-gauche (gg) conformers make PLA show a linear elastic tensile behavior thus a good thickness uniformity. As the film subjected to yielding, the enhanced chain orientation promotes crystallization, inducing the conformational transition and the slowing down of chain relaxation. However, the localized disentanglement and disorientation of chains during yielding lead to the asynchronous structure evolution from the central to edge areas, bringing about the deterioration of film-forming property. At the high stretching ratio where PLA exhibits strain hardening, the oriented and crystalline structures of the whole film become homogeneous again, which optimizes the film thickness uniformity. Moreover, the isotropically oriented structure with tiny crystals imparts superior strengthtoughness balance, good gas barrier property, and excellent optical transparency to the highly stretched PLA films. This work provides significant guidance for developing high-quality PLA films, and can also conduct significance in production of other biaxially oriented polymer films.