Physical Networks of Metal-Ion-Containing Polymer Brushes Show Fully Tunable Swelling, Nanomechanical and Nanotribological Properties

The interaction between weak polyacid brushes and metal ions can lead to the formation of a wide variety of complex structures across the polymer grafts. In the case of poly(hydroxyethyl methacrylate) brushes derivatized with succinate side groups; (PHEMA-SA), the coordination with Zn2+ or Ca2+ species can be tuned by varying the solution pH, below and above the pK(a) of the polyacid, brush. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) indicates, that Zn2+ ions generate relatively weak, localized monodentate bridges along PHEMA-SA grafts at basic pH. These Zn2+ brush conjugates swell profusely in water, are compliant and very lubricious, as observed by combining variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM) methods. In contrast, incubation of PHEMA-SA brushes with Ca2+ or Zn2+ at acidic pH leads to the formation of more extended, bidentate linkages forming a physical network between the metal centers and the surrounding grafts. This type of coordination causes brush dehydration and the stiffening of the films, as well as high friction, due to the energy dissipation required to perturb the dense, brush physical network by the shearing AFM probe. Regulating the interaction between metal ions and ionizable polymer brushes emerges as a versatile and easily accessible tool to control the interfacial properties of grafted polymer films. The achieved modulation of nanomechanical and nanotribological characteristics is technologically relevant, in that it allows both function and performance to be tuned for widely applicable polymer coatings.