Role of Substrate Type in the Process of Polyelectrolyte Multilayer Formation

Abstract

Polyelectrolyte multilayers are coatings formed by the alternate deposition of polycations and polyanions on a charged surface. In this study we examined how the type of substrate affects a multilayer prepared from poly(allylamine hydrochloride) and poly(acrylic acid). Silicon and titanium wafers were used as substrates. Their properties were systematically studied using ellipsometry, tensiometry, atomic force microscopy and streaming potential measurements. Multilayers were built up at pH = 7 with tetramethylammonium chloride as the background salt. The growth of films was monitored by ellipsometry, while the morphology and surface roughness were determined by atomic force microscopy. It was found that the thickness of multilayers containing 10 layers on silicon is 10 nm, whereas the thickness of the same film on titanium is three times higher. It was shown that multilayers formed on silicon display a grain-like structure, which was not the case for a film formed on titanium. Such morphological properties are also reflected in the surface roughness. Finally, it was shown that, in addition to the electrostatic interactions, the hydrophobicity of the substrate also plays an important role in the polyelectrolyte multilayer formation process and influences its thickness and properties.

Keywords: AFM; ellipsometry; polyelectrolyte multilayers; silicon; streaming potential; tensiometry; titanium.

Figure 1

Images (500 μm × 400 μm) of the surface of (a) Si substrate and (b) Ti substrate obtained by digital optical microscopy.

Figure 2

AFM images (5 μm × 5 μm) of (a) Si substrate and (b) Ti substrate surfaces. Higher areas are represented by lighter colours. Maximal value of the z-axis is 5 nm.

Figure 3

Results for silica (O) and titanium (X) contact angle measurements with six different test fluids interpreted by Owens–Wendt model (Equation (1)).

Figure 4

Thickness of PAH/PAA multilayers on Si (●) and Ti (■) substrates determined by ellipsometry presented as a function of the number of deposited polyelectrolyte layers.

Figure 5

The area where PAH/PAA multilayer was removed from the surface of the Si substrate, recorded by (a) digital optical microscope and (b,c) atomic force microscope. Figure (b) presents a 2D AFM image of the surface, and Figure (c) a 3D AFM image.

Figure 6

AFM images of step-edge boundary between (PAH/PAA)₅ multilayer and (a) silicon or (b) titanium surface. The corresponding height profiles are shown below the images.

Figure 7

AFM image of 5 μm × 5 μm surface of PAH/PAA multilayer made of 2 bilayers on (a) Si substrate and (b) Ti substrate. The higher areas are shown in a lighter color; z-scale value is 20 nm.

Figure 8

AFM image of 5 μm × 5 μm surface of PAH/PAA multilayer made of 5 bilayers on (a) Si substrate and (b) Ti substrate. The higher areas are shown in a lighter color; z-scale value is 20 nm.

Figure 9

Root mean square surface roughness (R_q) obtained for silicon and titanium surfaces coated with 2 or 5 PAH/PAA bilayers.