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.

Fast assembly of bio-inspired nanocomposite films

“…The average bilayer thickness was 4.07 nm, whereas thicknesses of polymer and clay sublayers were 1.547 and 2.563 nm, respectively. Similar values were reported by Kotov et al 20 determined by surface plasmon spectroscopy. The clay/ polymer bilayer thickness was 3.9 ± 0.5 nm and for polymer and clay sublayers 1.6 ± 0.4 nm and 2.46 ± 0.5 nm, respectively.…”

“…Kleinfeld and Ferguson 7 prepared films with dripping as an analogous to dipping self-assembly method. They reported a bilayer thickness value of 3.8 nm measured by ellipsometry, which is in the same range as that given by Fan et al 28 and Kotov et al, 20 even while working with a much higher concentration of hectorite (0.2 wt%) and PDDA (5 wt%) solutions. From this sublayer thickness, a volume fraction of the inorganic sublayer in the films between 63 vol% 20,28 and 67 vol% (this work) can be calculated.…”

“…They reported a bilayer thickness value of 3.8 nm measured by ellipsometry, which is in the same range as that given by Fan et al 28 and Kotov et al, 20 even while working with a much higher concentration of hectorite (0.2 wt%) and PDDA (5 wt%) solutions. From this sublayer thickness, a volume fraction of the inorganic sublayer in the films between 63 vol% 20,28 and 67 vol% (this work) can be calculated. The mass fraction of the inorganic material was determined with 60 wt%, which is lower than that reported by Podsiadlo et al, 29 who calculated, from a TGA curve similar in shape, an inorganic loading of a dip-assembled free-standing film with about 80 wt% (∼42 vol%).…”

“…A flat rectangular substrate was sequentially dipped in PDDA solution and clay suspension. [16][17][18][19][20] The dip cycle program for each bilayer consisted of six steps: dipping into PDDA (0.05 wt%, pH 5.7) solution for 300 s, rinsing three times with water, dipping in Laponite (0.05 wt%, pH 9.7) suspension for 300 s, and finally rinsing two times with water. The preparation of a 100-bilayer film takes about 17 h.…”

From dots to doughnuts: Two-dimensionally confined deposition of polyelectrolytes on block copolymer templates

“…[10] The LbL technique has been applied to various types of substrates, compositions, [11][12][13][14] and geometries [1,[15][16][17][18][19]] using a range of PEs, including biopolymers, nanoparticles, and clays. [20][21][22][23][24][25][26] PEM coatings are widely applicable in medicine, sensing, molecular electronics, and surface modifications. [27][28][29][30][31][32][33][34][35][36][37][38][39][40] Constructing PEMs on patterned substrates opens another dimension in LbL construction, which may be useful for applications that require lateral structuring or high surface area (e.g., photonics, catalysis, controlled release, and membranes).…”