Crystallization Behavior of Semicrystalline Polymers Characterized by an In Situ Fluorescence Technique and its Sensing Mechanism

In this study, fluorescence spectroscopy is used to study the bulk nonisothermal and isothermal crystallization of poly(L-lactic acid) (PLLA) by monitoring in situ, the temperature (T)- and time (t)-dependent fluorescence intensity of the fluorophores incorporated into the PLLA matrix, respectively. The crystallization behavior characterized by fluorescence resembles those measured by conventional differential scanning calorimetry, confirming the validity of this fluorescence technique for sensing crystallization. Combined in situ fluorescence results and ex situ X-ray diffraction characterizations demonstrate that this fluorescence technique shows great sensitivity not only to the degree of crystallinity but also to the crystalline microstructures formed during crystallization (e.g., α vs α' form PLLA crystals). Moreover, complementary fluorescence microscopy helps reveal the fluorescence's intrinsic crystallization sensing mechanism: As crystals form during crystallization, the bulky fluorescent probes excluded from the crystalline regions reside in the rigid/mobile amorphous (i.e., non-crystalline) regions of the resulting semicrystalline matrix, and their intramolecular motions are restricted by the neighboring crystalline domains; and thus, their fluorescence intensity is greatly enhanced upon crystallization. Owing to its high sensitivity, fluorescence is powerful for studying the crystallization behavior of ultrathin polymer films (e.g., 75 nm) and elucidating confinement and interfacial effects through future studies.