Nonisothermal Melt Crystallization Behavior of Semicrystalline Polymers Monitored Using an In Situ Fluorescence Technique

Upon cooling a semicrystalline polymer from its amorphous melt state, it undergoes melt crystallization where organized microstructures develop through a process of nucleation and crystal growth. Understanding the crystallization kinetics of a semicrystalline thermoplastic is key to tuning crystallinity and microstructure, which play integral roles in the material’s final properties such as toughness, gas permeability, and degradation rate. Nonisothermal crystallization, in particular, has great technological relevance to polymer engineering processes such as injection molding, film blowing, and fiber spinning, all of which rely on fast cooling rates. Spectroscopic, scattering, calorimetric, and rheological techniques have been conventionally used for studying nonisothermal crystallization, but can be limited in their sensitivity, tunability, and availability. Our group has recently developed a fluorescence technique for sensing the melting transitions of semicrystalline thermoplastics by incorporating fluorescent probes into polymer matrices. Herein, this methodology has been extended to an in-situ study of nonisothermal melt crystallization by monitoring the T-dependent fluorescence intensity of the fluorophores incorporated into a polymer matrix. As crystals form upon cooling from the amorphous melt state, the intramolecular motions of fluorophores are restricted and thus their T-dependent fluorescence intensity data exhibit a stepwise increase during nonisothermal crystallization. The first derivative of the T-dependent fluorescence intensity data can provide insight into the onset, peak, and endset crystallization temperatures, all of which align reasonably well with conventional differential scanning calorimetry measurements. This facile, sensitive, and contact-free fluorescence technique can access faster cooling rates (up to ~100 oC min-1) than many other existing methods for nonisothermal crystallization studies, which is more relevant to industrial polymer processing conditions. Additionally, the fluorescence detection mechanism shows great sensitivity not only to the degree of crystallinity but also to the crystalline microstructure formed during nonisothermal crystallization. Furthermore, unique fluorescent labeling is expected to foster novel studies on the local crystallization within heterogeneous polymeric systems including blends, composites, and multilayer films. Such local crystallization studies are out of reach for most conventional techniques that measure spatially averaged properties. Overall, this nonisothermal crystallization study expands the capabilities of this novel fluorescence technique for advancing the field of semicrystalline thermoplastic design and processing.