Effect of substrate on the adsorption of polyelectrolyte multilayers: Study by optical fixed-angle reflectometry and AFM

Self-assembled multilayer films were formed on several chemically modified silica substrates: bare silica, aminated silica and two hydrophobically modified silicas prepared by grafting octadecyl (ODS) and propyl (PMS) trimethoxysilanes. The polyelectrolytes were poly(trimethylammonium ethyl methacrylate chloride) (MADQUAT) and poly(acrylic acid) (PAA). Analysis of the polyelectrolyte uptake was made with optical fixed-angle reflectometry and the film morphology was imaged with atomic force microscopy (AFM). All experiments were made at 20 °C and pH 5.5 in NaCl solutions 10−3 M. On all substrates the growth was not linear in the range of the first five bilayers. There was a very strong dependence of the growth on the functionality of the substrate, which was shown both by the reflectometric output (mass uptake) and by 2D AFM images (heterogeneity). With the unmodified silica substrate the growth was continued up to 20 bilayers, showing a progressive filling of holes and thus smoothing of the polyelectrolyte layer. A patterned substrate was used to demonstrate that two of the different morphologies above could be reproduced and grown on one single sample.