Layer-by-layer deposition on a heterogeneous surface: Effect of sorption kinetics on the growth of polyelectrolyte multilayers.

Surface functionalization by means of controlled deposition of charged polymers or nanoparticles using the layer-by-layer (LbL) approach has been used to modify mostly engineered materials with well-defined surface chemistry and morphology. In this regard, natural and inhomogeneous interfaces have gained very little attention. Furthermore, natural substrates are susceptible to alterations by factors commonly used to control the growth of multilayers, such as pH, temperature and ionic strength. Here, we study the impact of sorption kinetics of a bilayer system (Poly(diallyldimethylammonium chloride) (PDDA) and Poly(sodium 4-styrenesulfonate) (PSS)) on a natural heterogeneous wood surface at neutral pH, without salt addition, on the multilayer buildup. To overcome analytical limitations we introduce a complementary approach based on UV reflectance spectroscopy, atomic force microscopy (AFM) and zeta potential measurements. Compared to immersion times used for ideal substrates, we found that a high surface coverage requires relatively long immersion, approximately 30min, into polyelectrolyte solutions, while a sufficient removal of polyelectrolyte excess during the washing step, requires even longer, about 100min. Based on these findings, we show that film growth can be controlled kinetically. Long immersion times provide well-defined and regular multilayers. The obtained data points to specific requirements to be considered when LbL treatments are applied to rough, porous and heterogeneous surfaces, and thereby sets a basis for a successful transfer of various surface functionalization approaches already shown on ideal surfaces.

High-Resolution Adhesion Mapping of the Odd-Even Effect on a Layer-by-Layer Coated Biomaterial by Atomic-Force-Microscopy.

The adhesion behavior of polyelectrolyte multilayers consisting of poly(diallyldimethylammonium chloride), PDDA, and poly(styrenesulfonate), PSS, toward a silicon AFM tip was studied during their build-up on wood, a chemically heterogeneous, micrometer rough biomaterial and compared with a nanometer rough substrate, namely quartz. The atomic force microscopy-based force mapping approach generated high-resolution topography-, and adhesion maps within the first bilayers, which point toward a homogeneous layer-by-layer build-up on the biomaterial surface, and therefore indicate an even charge distribution. By analyzing the force-distance curves in every pixel of the mapping, new insights into the specific interactions of the polyelectrolyte multilayers at the surface were achieved. The characteristic odd-even effect of polyelectrolyte multilayers cannot only be determined on quartz, but also on the biomaterial wood, however, only after an offset of two bilayers. This is potentially due to the specific roughness and charge of wood in comparison to commonly used quartz.

Superoleophobic Meshes with Relatively Low Hysteresis and Sliding Angles by Electropolymerization: Importance of Polymer-Growth Control.

To study the effects of polymer growth and polymer nanostructure formation on stainless-steel meshes (opening size of 100 µm), we report the behavior of three electrodeposited polymers with varied deposition charge to achieve superoleophobic meshes with low adhesion (low hysteresis (H) and sliding angles (α)). We show the importance of controlling the polymer growth around the mesh wires (two-dimensional growth) to avoid the re-covering of the mesh openings. Indeed, both the polymer micro-/nanostructures as well as the presence of the holes are necessary to highly reduce the solid-liquid contact area and to achieve superoleophobic properties with low adhesion. Herein, with the best polymer, superoleophobic properties are obtained with apparent contact angles of 160° for sunflower oil (H=13°, α=10°), 155° for hexadecane (H=29°, α=30°), 143° for dodecane (sticking behavior), and 134° for decane (sticking behavior).

Superoleophobic Meshes with Relatively Low Hysteresis and Sliding Angles by Electropolymerization: Importance of Polymer-Growth Control.

Invited for this month's cover is the group of Prof. Frederic Guittard from Université Nice Sophia-Antipolis. The cover picture shows a superoleophobic stainless-steel mesh obtained by electrodeposition of fluorinated conducting polymers. Read the full text of the article at 10.1002/cplu.201300315.

Surface structuration (micro and/or nano) governed by the fluorinated tail lengths toward superoleophobic surfaces.

As compared to superhydrophobic surfaces, the challenge to obtain superoleophobic properties, surfaces against low-surface-tension probe liquids such as hexadecane, is very important because of their high tendency to wet. From the molecular design of the monomer, it is possible to obtain in one step superoleophobic surfaces by electrodeposition. Hence, we report the synthesis and the characterization of an original series of fluorinated 3,4-ethylenedioxypyrrole (EDOP) derivatives. The electrodeposited polymer films are characterized by contact angle measurements (static and dynamic with various probe liquids), optical profilometry, and scanning electron microscopy. In the view toward reaching superoleophobic properties, a common approach is to increase the number of fluoromethylene units of the surface post-treatment agent. Here, surprisingly, it is possible, in one step, to reach more efficient antioil surface properties by decreasing the length of the fluorinated tail (F-octyl to F-hexyl). This fact can be explained by a double scale of structuration (micro and nano) induced using only F-hexyl tails.