AFM Study of Polymer Brush Grafted to Deformable Surfaces: Quantitative Properties of the Brush and Substrate Mechanics

Polymers brushes (polymer chains end-tethered to a substrate) have been extensively studied with atomic force microscopy (AFM). Force indentation curves are collected while squeezing the sample surface with the AFM probe. The analysis of these curves allows obtaining the equilibrium brush thickness and grafting density by using an appropriate model for mechanical deformation of the brush. However, this approach becomes inaccurate when the substrate is deformable, which is frequently the case. In this situation, the collected force distance curves include information about both the brush and substrate deformations. Here we describe a method which takes into account both the brush and substrate deformations. Quantitative accuracy of the presented method is demonstrated by applying the method to measuring a poly(ethylene oxide) brush grafted to the cross-linked poly(2-vinylpyridine) substrate swollen in aqueous media of different acidity. By analyzing the AFM force curves, we simultaneously obtain the grafting density, equilibrium brush thickness, and the Young's modulus of the substrate. The method is verified by independent measurements of the substrate Young's modulus and direct measurements of the brush thickness using ellipsometry. The method demonstrates a very good agreement between the estimated and directly measured Young's modulus of the substrate and molecular characteristics of the brush.