Dr. Hao Shen

"Single-Molecule Soft Matter Chemistry"

Worldwide polymer production surpassed 300 megatons in 2015, and is still rapidly expanding. The large demand for polymeric materials stems from the fact that polymeric materials can be tailored to suit a variety of needs in real world applications. Polymers can achieve extraordinary mechanical strength, flexibility, and conductivity, yet polymers are lighter and exhibit higher corrosion resistances than classic materials such as steel. Despite the rapid expansion of polymers in everyday applications, the fundamental understandings of structure-function relationships of polymeric materials is lacking. Polymers, being soft matters, makes it challenging to study these materials in which repeating–units can form mesoscopic structures that are distinctly different at microscopic and macroscopic scales.

Traditional characterization techniques, although powerful, provide ensemble averaged results, where crucial performance-dictating rare subpopulations are not quantified. Microscopy techniques (electron microscopy, scanning near-field microscopy and atomic force microscopy) can provide real-time observation with high spatial resolutions. However, they are usually destructive to polymer samples, and only characterize their outer-most atomic layers. Advancements in photon detectors now allow a single fluorophore to be readily detected under ambient conditions. These advancements have enabled the application of noninvasive optical microscopy in the field of polymer science. With the integration of super-resolution approaches, single-molecule optical microscopy can provide a nanometer spatial resolution at millisecond time scales.

The Shen research group focuses on utilizing single-molecule optical microscopy to study polymer chemistry. Specifically, we would like to address two fundamental questions: (1) how do anomalous processes in a polymerization reaction, such as auto-acceleration, radical trapping, gelation, and diffusion induced termination, lead to the polydispersity of polymer chains? (2) What is the most probable orientation and conformation for a single polymer chain when many chains are assembled into a bulk product? Knowing the answers to these questions will provide guidance toward preparing more uniform polymeric materials exhibiting enhanced performances. The state-of-the-art super-resolution imaging approaches are used to tackle the aforementioned questions. In particular, we track the 3D motion of fluorescent reporters in real-time during a polymerization reaction to probe the reaction kinetics at various locations of the monomer solution. Moreover, we develop novel imaging techniques to simultaneously detect multiple emitting sites within the diffraction limit of light. These techniques will enable the direct probe of chain orientation and conformation with multiple fluorescent labels attached.