MECHANISM OF AND DEFECT FORMATION IN THE SELF-ASSEMBLY OF POLYMERIC POLYCATION-MONTMORILLONITE ULTRATHIN FILMS

Positively charged polydiallyldimethylammonium chloride, P, was found to bind strongly to the surface of anionic montmorrillonite, M, platel ets in aqueous dispersions up to a saturation (estimated to correspond to the binding of five P to one 1.0 nn x 200 nm M platelet) beyond wh ich 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. Immersi on of the P coated substrate into an aqueous dispersion of M and rinsi ng with ultrapure water for 10 min led to the adsorption of 2.5 nm thi ck M. Repeating the self-assembly steps of P and M for n number of tim es produced (P/M)(n) self-assembled films. Thickness of the M layer wa s found to depend on the external voltage applied during its self-asse mbly: applying a positive potential during the self-assembly of M incr eased the thickness of the M layer; application of a small negative po tential decreased it slightly; however, larger negative voltages augme nted it. The structure of self-assembled (P/M)(n), films have been cha racterized by a variety of techniques: X-ray diffraction, X-ray reflec tivity, atomic force microscopy, transmission electron microscopic, an d surface plasmon spectroscopic measurements. It was shown that clay p latelets form stacks upon adsorption to the polymer layer consisting o n 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, w ere smoothed during P/M deposition. Small pits (188 nm diameter and 14 nm deep) were capped after one P/M deposition cycle. Surface roughnes s of (P/M)(n) films was estimated by a number of methods including sur face plasmon spectroscopy. The overall roughness did not appear to cor relate 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.