Deep Insight into the Effect of Microstructure Evolution on Film-Forming Properties of Polylactide during Uniaxial Hot-Stretching Process

Polylactide films were prepared by uniaxial hot-stretching to gain deep insight into the effect of microstructure evolution on film-forming properties. The film thickness uniformity visualized by the 3D contour graphic method was found to be deeply affected by the microstructural evolution and likely to be predicted by analyzing the stress–strain curves. A linear stress–strain behavior under the low stretching ratio endows PLA with a good film-forming property, which is ascribed to the great chain mobility enabled by thermal activation and the formation of gg conformers, although a little strain-induced crystallization synchronously occurred. Meanwhile, the yielding accompanied by chain slippage and crystal fragmentation led to the asynchronous structural evolution and the different chain relaxation behaviors from the middle to the side area of film, bringing about the deterioration of thickness uniformity. At the high stretching ratio, PLA showed a strain hardening behavior and regained good film-forming property due to the high chain orientation, inducing the structure consistency of the whole film. And the highly stretched film with excellent thickness uniformity exhibited outstanding optical transparency and mechanical performances. This work provides some guide for producing high-quality PLA films via uniaxial hot-stretching, and the related conclusion can be applicable to the preparation of other polymer films.