Scanning Drop Friction Force Microscopy

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick–slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick–slip–stick–slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip–stick–slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces

Soft NanocompositesFrom Interface Control to Interphase Formation

We report measurements of structure, mechanical properties, glass transition temperature, and contact angle of a novel nanocomposite material consisting of swellable silsesquioxane nanoparticles with grafted poly­(ethyl methacrylate) (PEMA) brushes and PEMA matrices with varying molecular weight. We measured the interparticle distance at the surface of the composites using scanning probe microscopy (SPM) and in the bulk of ∼0.5 μm thick films by grazing incidence small angle X-ray scattering (GISAXS). For a given molecular weight of the brush unstable dispersions at high molecular weight of the matrix indicate an intrinsic incompatibility between polymer-grafted-nanoparticles and homopolymer matrices. This incompatibility is affirmed by a high contact angle between the polymer-grafted-nanoparticles and the high molecular weight matrix as measured by SPM. For unstable dispersions, we measured a decreased glass transition temperature along with a decreased plateau modulus by dynamic mechanical thermal analysis (DMTA) which indicates the formation of a liquid-like layer at the brush–matrix interface. This proves the ability to decouple the structural and mechanical properties from the potential to be swollen with small molecules. It opens a new area of use of these soft nanocomposites as slow release materials with tailored mechanical properties

Thin Polyelectrolyte Multilayers Made by Inkjet Printing and Their Characterization by Nanomechanical Cantilever Sensors

Measurements with nanomechanical cantilever (NMC) sensors often reveal only qualitative results. Here we overcome this issue by inkjet printing well-defined polyelectrolyte multilayers (PEMs). We present a method that allows fabricating a 40 bilayer (BL) thick and 5 mm long line made of poly­(allylamine hydrochloride) (PAH) and polystyrene sulfonate (PSS). NMC sensors were used to quantify the uptake of water in thin PEMs. We measured and analyzed the mass loading and the swelling response of the PEMs upon exposure to relative humidity between 5% and 80%. For a film made of 5 BLs we determined a Young’s module of ∼390 MPa for low humidity (<5%). Thicker PEM films made by 10 BLs exhibited a higher Young’s module of ∼560 MPa. The Young’s module decreased in both cases to 2–3 MPa at 80% relative humidity. Furthermore, the NMC measurements of mass and swelling upon exposure to humidity indicated a thickness-dependent swelling of the PEMs

Phototunable Response in Caged Polymer Brushes

A light-responsive brush was obtained by surface-initiated ATRP of a methacrylate monomer containing ionizable −COOH side groups caged with the photoremovable group 4,5-dimethoxy-2-nitrobenzyl (NVOC). In the caged form, the polymer brush (PNVOCMA) is neutral and hydrophobic due to the presence of the aromatic chromophore. Upon irradiation the NVOC group is removed and a polyanion (polymethacrylic acid, PMAA) chain is generated. The charged brush can swell and collapse depending on the pH and the exposure dose (i.e., uncaging degree). The behavior and properties of the brush layer for different photoconversion degrees were studied. On the basis of quartz crystal microbalance measurements, a threshold of 50% uncaging was identified in order to achieve significant swelling and pH response of the brush. Between 50 and 80% the photoconversion the response of the brush could be light-modulated. For photoconversions >80% only small changes in the response were detectable. X-ray reflectivity (XRR) and scanning force microscopy allowed us to measure thickness, roughness and swelling of the brushes at intermediate photoconversions. Combined XRR and grazing-incidence small-angle scattering experiments evidenced a change in the internal structure of the brush upon exposure and indicated the occurrence of domain segregation as a consequence of the coexistence of hydrophobic and charged groups in the brush structure

Thiadizoloquinoxaline-Based Low-Bandgap Conjugated Polymers as Ambipolar Semiconductors for Organic Field Effect Transistors

Two novel conjugated polymers with high molecular weight, <b>PBDTTQ-3</b> and <b>PAPhTQ</b>, were synthesized by tuning alkyl chains and alternating the electron-donating ability of the thiadiazoloquinoxaline (TQ) moiety. Both polymers have excellent solubility in common organic solvents. UV–vis–NIR absorption and cyclic voltammetry indicate a bandgap of (0.76 eV) and high electron affinity level (−4.08 eV) for <b>PBDTTQ-3</b>. Two dimensional wide-angle X-ray scattering shows that both polymers are only poorly ordered in the bulk but possess a close π-stacking distance of 0.36 nm. Despite the disorder in thin film observed by grazing incidence wide-angle X-ray scattering, <b>PBDTTQ-3</b> exhibits good ambipolar transport, with a maximum hole mobility of 0.22 cm² V^–1 s^–1 and comparable electron mobility of 0.21 cm² V^–1 s^–1

Scanning Drop Friction Force Microscopy

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick–slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick–slip–stick–slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip–stick–slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces

Redox Active Polymer Brushes with Phenothiazine Moieties

We have investigated two different concepts to synthesize redox active polymer brushes using surface initiated atomic transfer radical polymerization (SI-ATRP). This polymerization technique allows the synthesis of well-defined grafted polymer brushes. In the initial step the surface was functionalized with a self-assembling monolayer of the SI-ATRP starter. Then, polymer brushes carrying phenothiazine moieties were grafted from the surface via SI-ATRP. The first concept consists of polymerizing monomers with phenothiazine pendant moieties to directly incorporate the redox functionality as side group in the growing polymer brush. The second concept consists of using grafted activated ester brushes which are functionalized with phenothiazine redox moieties in a successive reaction step. The electrochemical properties of the grafted redox active brushes were examined by cyclic voltammetry. Furthermore, the surface morphology and the chemical composition of the polymer brushes were characterized using scanning force microscopy (SFM), X-ray techniques, and UV/vis spectroscopy. Apart from their redox behavior, the synthesized brushes revealed increased mechanical stability on the nanoscale

Semifluorinated Alkanes at the Air–Water Interface: Tailoring Structure and Rheology at the Molecular Scale

Semifluorinated alkanes form monolayers with interesting properties at the air–water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression–expansion cycles. Starting from a reference system perfluor­(dodecyl)­dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air–water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression–expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure

Stability of a Split Streptomycin Binding Aptamer

Here we investigated the stability of an aptamer, which is formed by two RNA strands and binds the antibiotic streptomycin. Molecular dynamics simulations in aqueous solution confirmed the geometry and the pattern of hydrogen bond interactions that was derived from the crystal structure (1NTB). The result of umbrella sampling simulations indicated a favored streptomycin binding with a free energy of Δ<i>G</i>_bind^° = −101.7 kJ mol^–1. Experimentally, the increase in oligonucleotide stability upon binding of streptomycin was probed by single-molecule force spectroscopy. Rate dependent force spectroscopy measurements revealed a decrease in the natural off-rate (<i>k</i>_{off‑COMPLEX} = 0.22 ± 0.16 s^–1) for the aptamer–streptomycin complex compared to the aptamer having an empty binding pocket (<i>k</i>_{off‑APTAMER} = 0.49 ± 0.11 s^–1). This decrease in the natural off-rate corresponds to a decrease in the Gibbs free energy of ΔΔ<i>G</i>^sheer ≈ −3.4 kJ mol^–1. The simulated binding pattern and the experimental results led to the conclusion that hydrogen bonds between both RNA strands mainly contribute to the decrease in natural off-rate of the aptamer system studied

Stability of a Split Streptomycin Binding Aptamer

Here we investigated the stability of an aptamer, which is formed by two RNA strands and binds the antibiotic streptomycin. Molecular dynamics simulations in aqueous solution confirmed the geometry and the pattern of hydrogen bond interactions that was derived from the crystal structure (1NTB). The result of umbrella sampling simulations indicated a favored streptomycin binding with a free energy of Δ<i>G</i>_bind^° = −101.7 kJ mol^–1. Experimentally, the increase in oligonucleotide stability upon binding of streptomycin was probed by single-molecule force spectroscopy. Rate dependent force spectroscopy measurements revealed a decrease in the natural off-rate (<i>k</i>_{off‑COMPLEX} = 0.22 ± 0.16 s^–1) for the aptamer–streptomycin complex compared to the aptamer having an empty binding pocket (<i>k</i>_{off‑APTAMER} = 0.49 ± 0.11 s^–1). This decrease in the natural off-rate corresponds to a decrease in the Gibbs free energy of ΔΔ<i>G</i>^sheer ≈ −3.4 kJ mol^–1. The simulated binding pattern and the experimental results led to the conclusion that hydrogen bonds between both RNA strands mainly contribute to the decrease in natural off-rate of the aptamer system studied