Unveiling the Layered Structure of Sulfobetaine Polymer Brushes through Bimodal Atomic Force Microscopy

Many zwitterionic polymer brushes exhibit highly stimuli-responsiveproperties stemming from the strong dipole and electrostatic interactionof their building blocks. Here, we showed how a combination of twoatomic force microscopy (AFM) modes can reveal the layered structureof poly(sulfobetaine methacrylate) brushes synthesized by surface-initiatedatom-transfer radical polymerization. Due to polydispersity and anti-polyelectrolyteeffect, a diffused layer emerges on top of a condensed layer of thebrush as a function of salt concentration. The amplitude-modulationmode of the AFM, owing to the tip's dynamic motion, can onlyprobe the more stable condensed layer near the substrate, whereasthe force-spectroscopic mode with its high sensitivity can accuratelydetect the diffused layer and hence determine the total brush thickness.Infrared spectroscopy and quartz crystal microbalance monitoring revealedthe strong ion-screening effect and higher brush hydration propensityof multivalent ions. Different cation valencies also showed subtleeffects on the dimensionality of the layered structure. Our resultshighlight the usefulness of AFM in revealing various contextual phenomenathat arise from the unique properties of zwitterionic polymers.

Automated summary

In this study, two atomic force microscopy (AFM) modes were used to reveal the layered structure of poly(sulfobetaine methacrylate) brushes synthesized by surface-initiated atom-transfer radical polymerization. The AFM amplitude-modulation mode probed the stable condensed layer near the substrate, while the force-spectroscopic mode accurately detected the diffused layer and determined the total brush thickness. The results showed the ion-screening effect and higher brush hydration propensity of multivalent ions, and how different cation valencies affected the dimensionality of the layered structure. This study highlights the utility of AFM in revealing contextual phenomena arising from the unique properties of zwitterionic polymers.