Static and Dynamic Control of Fingerprint Landscapes of Liquid Crystal Network Coatings.

Surface topography variations of liquid crystal networks in their functional coatings provide unique properties in these systems. Chiral-nematic polymer coatings self-organize in a fingerprint texture with the molecular helices parallel to the substrate with alternating domains of molecular units with parallel and perpendicular director orientation as controlled by the concentration of a reactive chiral additive. Driven by surface-tension differences and altered by anisotropic polymerization shrinkage, the coating may form hills and valleys hundreds of nanometers in size with different molecular alignment. The director orientation in the corrugations could be controlled by monomer diffusion during polymerization. Polymerization in the presence of a dichroic dye gives topographic elevations in which the molecules are oriented along the normal. Polymerization by means of a dichroic photoinitiator gives topographic elevations in which the molecules align parallel to the surface. By balancing the monomer diffusion and anisotropic polymerization shrinkage, relatively flat surfaces are also achieved. The different surfaces exhibit distinct topographical deformations when subjected to external stimuli, such as an AC electric field. This method can be universally extended to LC polymers with other alignment configurations.

Angle-Dependent Atomic Force Microscopy Single-Chain Pulling of Adsorbed Macromolecules from Planar Surfaces Unveils the Signature of an Adsorption-Desorption Transition.

The adsorption-desorption behavior of polymer chains is at the heart of macromolecular surface science and technology. With the current developments in atomic force microscopy (AFM), it has now become possible to address the desorption problem from the perspective of a single macromolecule. Here, we report on desorption of single polymer chains on planar surfaces by AFM-based single molecule force spectroscopy (SMFS) as a function of the pulling angle with respect to the surface-normal direction. SMFS experiments were performed in water with various substrates using different polymers covalently attached to the AFM probe tip. End-grafting at the AFM tip was achieved by surface-initiated polymerization using initiator functionalized tips. We found that the desorption force increases with a decreasing pulling angle, i.e., an enhanced adhesion of the polymer chain was observed. The magnitude of the desorption force shows a weak angular dependence at pulling angles close to the surface normal. A significant increase of the force is observed at shallower pulling from a certain pulling angle. This behavior carries the signature of an adsorption-desorption transition. The angular dependence of the normalized desorption force exhibits a universal behavior. We compared and interpreted our results using theoretical predictions for single-chain adsorption-desorption transitions.

Correction: The persistence length of adsorbed dendronized polymers.

Correction for 'The persistence length of adsorbed dendronized polymers' by Lucie Grebikova, et al., Nanoscale, 2016, 8, 13498-13506.

The persistence length of adsorbed dendronized polymers.

The persistence length of cationic dendronized polymers adsorbed onto oppositely charged substrates was studied by atomic force microscopy (AFM) and quantitative image analysis. One can find that a decrease in the ionic strength leads to an increase of the persistence length, but the nature of the substrate and of the generation of the side dendrons influence the persistence length substantially. The strongest effects as the ionic strength is being changed are observed for the fourth generation polymer adsorbed on mica, which is a hydrophilic and highly charged substrate. However, the observed dependence on the ionic strength is much weaker than the one predicted by the Odijk, Skolnik, and Fixman (OSF) theory for semi-flexible chains. Low-generation polymers show a variation with the ionic strength that resembles the one observed for simple and flexible polyelectrolytes in solution. For high-generation polymers, this dependence is weaker. Similar dependencies are found for silica and gold substrates. The observed behavior is probably caused by different extents of screening of the charged groups, which is modified by the polymer generation, and to a lesser extent, the nature of the substrate. For highly ordered pyrolytic graphite (HOPG), which is a hydrophobic and weakly charged substrate, the electrostatic contribution to the persistence length is much smaller. In the latter case, we suspect that specific interactions between the polymer and the substrate also play an important role.

Single-molecule force measurements by nano-handling of individual dendronized polymers.

A nano-handling technique based on atomic force microscopy (AFM) is presented that allows reliable measuring of force-extension profiles of single-polymer molecules. The basis of the method is a properly functionalized solid substrate, to which a small amount of the polymers in question is adsorbed. The sample is first imaged in amplitude modulation mode in solution with a functionalized AFM cantilever, and a polymer chain is picked up with the AFM tip at one of the ends of the polymer molecule. Force curves are recorded by stretching the attached polymer molecule many times. After the force experiments, the molecule is imaged again. In this fashion, one can ascertain that the force experiments are truly carried out with one individual molecule. With this technique, the force response of amino-functionalized dendronized polymers was studied and it could be accurately described by the freely jointed chain model with chain elasticity. A monotonic dependence of the mechanical properties of these polymers up to generation 4 was found. The elastic constant was independent of the generation and solution composition. On the other hand, the effective Kuhn length increased with the generation at higher salt concentrations. The mechanical response of dendronized polymers can be tuned with solution composition.

Dispersion Characteristics and Aggregation in Titanate Nanowire Colloids.

Titanate nanowires (TiONWs) are synthesized through the hydrothermal method and characterized in acidic aqueous dispersions by using electrophoresis, dynamic light scattering, and atomic force microscopy. The TiONWs have a rodlike shape with an average length of about 600 nm and a thickness of 35 nm. They are positively charged under the conditions used. Their surface charge properties and aggregation are investigated in the presence of oppositely charged poly(styrene sulfonate) (PSS) polyelectrolyte. Charge neutralization followed by a subsequent charge reversal process is observed, which is attributed to the adsorption of PSS. The colloids are unstable near the charge neutralization point and stable at lower and higher PSS doses, in good qualitative agreement with the theory developed by Derjaguin, Landau, Verwey, and Overbeek (DLVO). The nanowires prefer to align along the walls, leading to "spaghetti-like" oriented aggregates. The aggregation processes of bare and PSS-coated TiONWs are monitored at different concentrations of an inert electrolyte; slow aggregation is found at low salt levels, whereas aggregation is rapid beyond the critical coagulation concentration, as predicted by the DLVO theory, which describes the colloid stability of the TiONWs adequately in all the systems investigated. Coating of the nanowires with the polyelectrolyte leads to a critical coagulation concentration 75 times higher than that of the bare titanates, indicating the enormous stabilizing effect of PSS.