Mechanism of and Defect Formation in the Self-Assembly of Polymeric Polycation−Montmorillonite Ultrathin Films

Abstract

Positively charged polydiallyldimethylammonium chloride, P, was found to bind strongly to the surface of anionic montmorrillonite, M, platelets in aqueous dispersions up to a saturation (estimated to correspond to the binding of five P to one 1.0 nm × 200 nm M platelet) beyond which reversible physisorption occurred. Immersion of a substrate (glass, quartz, silica-wafer, gold, silver, and even Teflon) into an aqueous 1% solution of P and rinsing with ultrapure water for 10 min resulted in the strong adsorption of a 1.6 nm thick P on the substrate. Immersion of the P coated substrate into an aqueous dispersion of M and rinsing with ultrapure water for 10 min led to the adsorption of 2.5 nm thick M. Repeating the self-assembly steps of P and M for n number of times produced (P/M) n self-assembled films. Thickness of the M layer was found to depend on the external voltage applied during its self-assembly: applying a positive potential during the self-assembly of M increased the thickness of the M layer; application of a small negative potential decreased it slightly; however, larger negative voltages augmented it. The structure of self-assembled (P/M) n films have been characterized by a variety of techniques: X-ray diffraction, X-ray reflectivity, atomic force microscopy, transmission electron microscopic, and surface plasmon spectroscopic measurements. It was shown that clay platelets form stacks upon adsorption to the polymer layer consisting on the average two aluminosilicate sheets. The evolution of the surface roughness upon sequential deposition of P/M layers was observed by in situ AFM. Large etched pits, up to 700 nm diameter and 30 nm depth, were smoothed during P/M deposition. Small pits (188 nm diameter and 14 nm deep) were capped after one P/M deposition cycle. Surface roughness of (P/M) n films was estimated by a number of methods including surface plasmon spectroscopy. The overall roughness did not appear to correlate with the type of substrate used. On the other hand, application of an external electric field during the self-assembly of P strongly influenced the surface morphology. Application of a negative potential during the self-assembly of P improved the uniformity and regularity of the deposited layers.