Three-Dimensional Visualization of the Inhomogeneous Cold Crystallization of Polymers

Crystallinity determines the performance of polymers, and inhomogeneity in crystallinity occurs in both planar and depth dimensions of polymers during the manufacturing process. However, it is challenging to provide a multidimensional, visualized, and quantified evaluation of the polymer crystallization process. In this contribution, we proposed a three-dimensional strategy to visualize the isothermal cold crystallization behaviors of poly(lactic acid) (PLA) based on fluorescence labeling targeting carboxyl groups. The uniform fluorescence distribution at different depths was observed for the amorphous state of PLA, while decayed fluorescence sites along depths were identified for PLA with an oriented polymer chain and ordered crystalline structure. Such a phenomenon could be ascribed to the diffusion affinity differences between the amorphous and crystalline regions for the fluorescence probe. The temperature and time dependencies of the fluorescence volume were quantitatively studied, conforming to the classical isothermal crystallization Avrami kinetic equation. Accordingly, the progressive cold crystallization process, interpreted as the decayed fluorescence volumes, resulted in the preferred crystalline structure and enhancement of Young's modulus for PLA. Therefore, we have realized the three-dimensional visualization and quantification of PLA cold crystallization, guiding the structural evaluation and performance prediction for polymers. It is anticipated that such a strategy could broaden the evaluation scope for the physical and chemical properties of various polymers.