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.