Early steps in layer-by-layer construction of polyelectrolyte films: the transition from surface/polymer to...

The layer-by-layer deposition of two polyelectrolytes, quaternized poly(dimethylaminoethyl methacrylate chloride) (MADQUAT) and poly(acrylic acid) (PAA) on a silica substrate was investigated using optical reflectometry, as a function of pH (pH 4, 5.5 and 9), ionic strength (10(-3) to 10(-1) M) and type of salt. Attention was given to the successive deposited weights and to the corresponding deposited charge densities within the ten first deposited layers. Results show a change of growth regime between an early stage where the substrate had a dominating influence in the build-up and a second stage where the polymer uptake was ruled essentially by polymer-polymer interactions. The pH was seen to influence the growth via the charge densities of silica (first stage) and PAA (first and second stages). The increase of NaCl concentration induced an increase of the film weight between 10(-3) and 10(-2) M, but the trend was more sophisticated between 10(-2) and 10(-1) M where the polymer uptake increased in the first stage of the growth and decreased in subsequent layers. The film weight increased in accordance with the rank of ions in the Hofmeister series. AFM images revealed a heterogeneous film morphology with bumps and valleys, which was explained by a growth mechanism made of the successive formation and growth of polymer complexes.