Conformational Changes of Polymer Chains in a Shear Field Studied by Fluorescence Resonance Energy Transfer

The in situ detection of the microconformation of polymers in a shear field from the molecular level not only plays an important role for demonstrating the molecular mechanism of polymer chains' dynamic behavior such as deformation and diffusion, but also serves as a crucial link in developing advanced polymer processing technology. However, due to the conformational change scale of polymer chains falling within the range from 0.1 nm to 10 nm, conventional characterization methods, such as dynamic light scattering and fluorescence microscopy techniques, are difficult to meet the requirements of spatial resolution needed for characterization. To address this problem, the fluorometer was modified by adding a Couette rheofluorescence cell on the detection optical path. Since the Couette fluorescence cell can apply different shear rates to the sample, the modified fluorometer is able to detect in situ fluorescence signals of samples in a shear field. In order to characterize the conformational changes of the polymer chain, fluorescent donor and acceptor groups were labeled to a single polymer chain through covalent bonds Based on the positive correlation between the fluorescence resonance energy transfer (FRET) efficiency and the distance between the fluorescent donor and acceptor groups, a FRET spectroscopy has been used to achieve in situ characterization of the solution shear conformation of polymer chains by analyzing the change of the FRET efficiency between fluorescent donor and acceptor groups labeled on the same polymer chain as a function of the shear rate. For the first time, it was observed in situ at the molecular level that polymer chains under shear showed the conformational change modes of " whole driven by local" in dilute solutions and " local driven by whole" in semi-dilute solutions, respectively. This study provides new methods and ideas for experimentally carrying out in situ studies on the conformational evolution of polymer chains, and has an advancing effect on the development of molecular rheology.