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 nonisothermal crystallization through nucleation and crystal growth. Understanding the nonisothermal crystallization kinetics of a semicrystalline thermoplastic is key to tuning its crystallinity, microstructure, and material properties. Conventional spectroscopic, calorimetric, and rheological techniques have been used for studying nonisothermal crystallization, but they are limited in sensitivity, tunability, and availability. Our group has developed a new fluorescence technique for sensing semicrystalline polymers’ melting transitions by incorporating fluorescent probes into polymer matrices. Herein, this methodology has been extended to study nonisothermal crystallization by monitoring the T-dependent fluorescence intensity of a dye-labeled polymer. As crystals form upon cooling from the melt state, the intramolecular motions of fluorophores are restricted and thus their T-dependent fluorescence intensity data exhibit a stepwise increase during crystallization. The crystallization processes monitored by fluorescence agree well with those measured by conventional differential scanning calorimetry. Moreover, fluorescent labeling could enable novel location-specific studies on the local crystallization within heterogeneous polymeric systems (e.g., multilayer films), which are out of reach for most conventional techniques that measure spatially averaged properties.