Non-fouling polymer films on hard-anodized aluminum substrates: Nanomechanical properties and modelling

In this paper, hard-anodized aluminum is used as a substrate to support a polymeric anti-biofouling coating based on a non-toxic, environmentally benign zwitterionic polymer (sulfobetaine methacrylate (pSBMA)). The coating is processed from an aqueous solution of the monomer via a photografting technique. Nanostructured coatings are obtained with a morphology that suggests tethered, cross-linked brushes akin to nanoropes. The antifouling properties are demonstrated using protein and bacteria adsorption on structured surfaces. Using the nanoindentation technology with a variety of testing methods, in wet and dry coating conditions, the elastic modulus and hardness of the polymer film and the substrate are obtained. The static elastic modulus and hardness are in the range of 2.0 GPa and of 220 MPa, respectively, with little effects of whether the films are tested in wet or dry condition. Moreover, the apparent modulus of the polymer coating is measured with different testing frequencies and loading rates using a cylindrical flat-punch and spherical indenters. The values obtained are in broad agreement with those obtained with the Berkovich indenter, but reflect the visco-elastic nature of the film, depending on the testing frequency and loading rate. The resistance of the coating to scratching is studied by applying fixed loads on the top surface, where the load and scratch profiles during the scratch are recorded and analyzed. Finite element models of the nanoindentation and scratch process are proposed and compared to the experimental results. The numerically obtained indentation depth-load curves and scratch profiles show a strong agreement with the experimentally obtained ones. Furthermore, stress and strain distributions on the coating surface are analyzed and the coefficient of friction is calculated for two different loads.