Sensing the melting transition of semicrystalline polymers via a novel fluorescence technique

The melting point (T_m) is an important parameter that dictates the physical properties and thereby applications of semicrystalline thermoplastics. Conventional melting transition characterization techniques, e.g., differential scanning calorimetry, ellipsometry, optical microscopy, and X-ray based methods, probe T_m by monitoring temperature (T)-dependent thermal properties, morphologies, and microstructures. Although these techniques have been well developed, they tend to have their own limitations, e.g., X-ray based techniques require X-ray beams and most previous techniques can only measure spatially averaged properties throughout a film cross section or an entire bulk sample. Here, we have developed a new, simple, and versatile fluorescence technique for probing the melting transitions of semicrystalline thermoplastics. With this approach, fluorescent probes are incorporated into a semicrystalline polymer, either by physical doping or covalent labeling, and their T-dependent fluorescence intensity data exhibit a stepwise decrease nearby T_m because of the reduced restriction of intramolecular motion when crystals start to melt. Interestingly, the first derivative of the obtained T-dependent fluorescence intensity data can reveal more details of the underlying melting transition, e.g., the onset and endset of the melting transition as well as the peak melting temperature. The melting point values determined by fluorescence agree well with those characterized by conventional differential scanning calorimetry, confirming the validity of our fluorescence technique for probing melting transitions. In addition, this fluorescence technique can be applied with various types of fluorescent probes (e.g., fluorophores exhibiting either aggregation-induced emission or aggregation-caused quenching effects) and generalized to many semicrystalline thermoplastics (e.g., poly(L-lactic acid), poly(caprolactone), and poly(ethylene oxide)), while maintaining excellent sensitivity to melting transitions. Overall, our fluorescence technique represents an easy and contact-free melting point characterization approach that may allow for novel location-specific T_m investigations within heterogeneous polymeric systems (e.g., multilayer films, blends, and composites). Such a novel fluorescence technique can expand the toolbox for T_m determination and potentially lead to unprecedented insights into the design principles for semicrystalline thermoplastics.