Nanoscopic visualization of cross‐linking density in polymer networks with diarylethene photoswitches

Abstract

The in situ nanoscopic imaging of soft matter polymer structures is of paramount importance to gain a detailed knowledge of the relationship between structure, properties and functionality on the nanoscopic scale. Cross‐linking of polymer chains has tremendous effects on the viscoelastic properties of gels, and the correlation of mechanic properties on the distribution and amount of cross‐linkers is of huge relevance for applications as well as for a detailed understanding of polymers on the molecular scale. We introduce a super‐resolution fluorescence microscopy based methodology enabling the visualization and quantification of cross‐linker points in polymer systems. A novel diarylethene‐based photoswitch with a highly fluorescent closed and a non‐fluorescent open form is used as a photoswitchable cross‐linker in a polymer network. Its photophysical properties, switching behaviour, and high photostability make the dye an ideal candidate for photoactivation localization microscopy (PALM) without further modification of the structure of interest. As an example for its capability to nanoscopically visualize cross‐linking, we investigate pNIPAM microgels as system known with variations in internal cross‐linking density.