Erik Wischerhoff

ToF-SIMS study of alternate polyelectrolyte thin films: Chemical surface characterization and molecular secondary ions sampling depth

Multilayered assemblies of alternate polyelectrolytes have been synthesized by dipping charged silicon wafers successively into solutions of polyelectrolytes of opposite charge. In this study, three types of assemblies and several thicknesses are investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), in combination with other characterization techniques (X-Ray Photoelectron Spectroscopy (XPS), X-Ray Reflectivity (XRR) and Atomic Force Microscopy (AFM)). The sensitivity of ToF-SIMS to the extreme surface provides a powerful tool to verify the chemical structure, as well as the spatial homogeneity of the topmost layers. Monolayers of complex polyelectrolytes differing only by the end of the pendant group or by the monomer chain length can be distinguished easily, notwithstanding the interference with the information coming from the underlying layers. The chemical imaging capability of ToF-SIMS allows the identification of the defects and contaminants in the surface layer, as well as the verification of the thickness uniformity at a local scale (similar to 1 mu m). In addition, the proof of a regular build-up is given by the disappearance of the substrate signal (Sif) when the number of layers increases. On the other hand, the question of the information depth in ToF-SIMS, which constitutes an important issue for the characterization of very thin films, is addressed. The attenuation depth in the organic film is determined for atomic and molecular secondary ions (Si+, SiOH+, SiO3H-), mainly by the correlation with XPS and XRR data. The decay of the mean emission depth when the ion size increases makes the largest molecular ions the most surface sensitive.

Polyelectrolyte adsorption onto planar surfaces: a study by streaming potential and ellipsometry measurements ☆

Possible applications of polyelectrolyte monolayers and multilayers obtained via layer-by-layer deposition ask for well-defined growth as a function of the number of the adsorption cycles. Essential parameters for the regular adsorption of subsequent polymer layers by electrostatic interactions are the charge of the outermost surface region, the surface of the substrate, and the nature of the polyelectrolyte. Therefore such adsorbed polyelectrolyte layers were characterized by streaming potential measurements in combination with ellipsometric studies. After the adsorption of highly charged polycations such as poly(ethylene imine) on negatively charged surfaces, the surface charge changes: A marked increase of the zeta potential as well as a shift of the isoelectric point is observed for standard polyelectrolytes. These changes depend on the storage of the adsorbed layers under water, or on shear applied in the measuring cell. Only one polyelectrolyte disposing of a large hydrophobic fragments formed stable layers. In this case, no significant changes are observed in dependence on the storage conditions dr on shear. By varying the number of polyelectrolytes layers, the dependence of the zeta potential on the pH is found to be identical. Accordingly, the surface charge is nearly independent of the number of layers. The layer thickness in a dried state is determined by variable angle spectroscopic ellipsometry, demonstrating the step-by-step growth of the multilayers

Thermoresponsive PEG-based polymer layers: surface characterization with AFM force measurements.

Thermoresponsive polymer-coated surfaces based on poly(2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) [P(MEO(2)MA-co-OEGMA)] allow switching between cell attachment and detachment. Here, we investigate the temperature-dependent surface interactions between the polymer coating and a colloidal probe in an aqueous medium by means of atomic force microscopy (AFM) force-distance measurements. The analysis of the adhesion forces from AFM retraction curves identifies two kinds of regimes for the copolymer at temperatures below and above the lower critical solution temperature (LCST). Whereas at 25 degrees C the surface interactions with the polymer in the swollen state are dominated by repulsive forces, at 37 degrees C the surface interactions switch to attractive forces and a stronger adhesion is detected by AFM. Running several heating/cooling cycles repeatedly shows that switching the surface properties provides reproducible adhesion force values. Time-dependent measurements give insight into the switching kinetics, demonstrating that the cell response is coupled to the polymer kinetics but probably limited by the cellular rearrangements.

Tailoring of stimuli-responsive water soluble acrylamide and methacrylamide polymers

The tailoring of the lower critical solution temperature (LCST) polymers of acrylamide and methacrylamide in water is achieved by chemical modification of freely water soluble precursor polymers poly[N-2-hydroxypropylmethacrylamide], poly[N,N-bis(hydroxyethyl)acrylamide] and poly[N-(tris(hydroxymethyl)-methyl)acrylamide]. Two principal reactions, namely acetylation and cinnamoylation, are applied. By varying the acylating agent as well as the extend of acylation, the LCST can be tailored easily. The cloud points observed for the different polymer series do not correlate with the apparent hydrophilicity of the parent polymers according to the content of hydroxyl groups. The results thus exemplify the difficulties to predict the behavior of modified thermosensitive polymers by simply analysing the balance of hydrophilic to hydrophobic molecular fragments. Chemical modification by cinnamoylation provides photoreactive copolymers e.g susceptible to photocrosslinking. When the polymers are prepared by polymerization using a disulfide-functionalized azo-initiator efficient grafting of the modified copolymers on gold surfaces is possible to prepare ultrathin hydrogel films, as demonstrated by Surface Plasmon Resonance.

Ferroelectric and electroclinic characterisation of a new organic siloxane bimesogen

Abstract A new siloxane based twin dimer has been synthesised which exhibits both a broad ferroelectric phase (from sub ambient temperatures to 37.5°C), and a Sa phase extending over 12.5°C (from 37.5°C to 50°C). The electroclinic effect within the Sa phase is very pronounced, occurs throughout the Sa phase and exhibits electroclinic coefficients as large as 2°/Vrms/μm at a temperature of Tsc*/Sa+5°C. The response times are correspondingly low (as little as 25μs at the end of the Sa range) and the maximum induced tilt angles very large, up to 17° at TSc*/sa+6°C and even higher closer to the phase transition. The effect of the soft mode contribution to the measured tilt angle can be seen far back into the ferroelectric phase. Characterisation has been performed by electro optic analysis, X ray studies, textural observations and measurement of spontaneous polarisation.

Structural studies on thin organic coatings built by repeated adsorption of polyelectrolytes

Polyelectrolyte multilayers are obtained by repeated physisorption of oppositely charged polyions. We report on the effect of the linear charge density of a series of aromatic ionenes on the growth of stable ionene/poly(vinyl sulfate) multilayers. Stable multilayers could not be grown in water below a critical linear charge density, corresponding to a charge parameter of about 0.7 (in water at 20 degrees C). We then present two potential ways to overcome the critical charge density limitation. The first one relies on the existence of a specific matching between a polycation and a polyanion, partly based on non-electrostatic intramolecular interactions, giving rise to a surface-constrained complexation leading to multilayers of extremely high supramolecular order. The growth mechanism of the multilayers is different from what is the usual case for stronger polyelectrolytes, since it occurs in a loose unstructured boundary layer at the sample surface, and involves organization processes similar to those occurring upon formation of insoluble structured polyelectrolyte complexes. The relationship between thickness and number of deposition cycles (d vs, n) could be described by an n(1.36) scaling law. The second way is to use a chemical reactivation of the outermost polyelectrolyte layer after its deposition by electrostatic adsorption, in order to regenerate a net charge at the interface allowing further adsorption. This technique is larger in scope, since it may lead to interesting properties such as non-centrosymmetry of the multilayers

On the influence of the architecture of poly(ethylene glycol)-based thermoresponsive polymers on cell adhesion.

Thermoresponsive polymer surface coatings are a promising tool for cell culture applications. They allow for a mild way of cell detachment that preserves the activity of membrane proteins-a prerequisite for reliable cell analysis. To enlarge the application range of these coatings to cells with different adhesion properties, we synthesized various novel poly(ethylene glycol)-based thermoresponsive polymers and describe how (i) their chemical structure and (ii) their surface density affect their efficiency. In order to quantify the influence of both factors, the time for cell spreading and rounding efficiency were observed. As a result, efficiency of cell rounding, which is closely correlated to cell detachment, is less affected by both factors than the time needed for cell spreading. This time can effectively be adjusted by the molecular architecture which includes the length of the polymer backbone and the side chains. Based on this work, recommendations are given for future optimization of functionality of thermoresponsive polymer coatings for cell culture applications.

Engineering Adhesion to Thermoresponsive Substrates: Effect of Polymer Composition on Liquid–Liquid–Solid Wetting

Adhesion control in liquid-liquid-solid systems represents a challenge for applications ranging from self-cleaning to biocompatibility of engineered materials. By using responsive polymer chemistry and molecular self-assembly, adhesion at solid/liquid interfaces can be achieved and modulated by external stimuli. Here, we utilize thermosensitive polymeric materials based on random copolymers of di(ethylene glycol) methyl ether methacrylate (x = MEO2MA) and oligo(ethylene glycol) methyl ether methacrylate (y = OEGMA), that is, P(MEO2MAx-co-OEGMAy), to investigate the role of hydrophobicity on the phenomenon of adhesion. The copolymer ratio (x/y) dictates macromolecular changes enabling control of the hydrophilic-to-lipophilic balance (HBL) of the polymer brushes through external triggers such as ionic strength and temperature. We discuss the HBL of the thermobrushes in terms of the surface energy of the substrate by measuring the contact angle at water-decane-P(MEO2MAx-co-OEGMAy) brush contact line as a function of polymer composition and temperature. Solid supported polyelectrolyte layers grafted with P(MEO2MAx-co-OEGMAy) display a transition in the wettability that is related to the lower critical solution temperature of the polymer brushes. Using experimental observation of the hydrophilic to hydrophobic transition by the contact angle, we extract the underlying energetics associated with liquid-liquid-solid adhesion as a function of the copolymer ratio. The change in cellular attachment on P(MEO2MAx-co-OEGMAy) substrates of variable (x/y) composition demonstrates the subtle role of compositional tuning on the ability to control liquid-liquid-solid adhesion in biological applications.

Ultrathin multilayers made by alternate deposition of ionenes and polyvinylsulfate: from unstable to stable growth

We report on the influence of ionic strength of polyelectrolyte solutions on the growth of multilayers made of polyvinylsulfate and a series of aromatic ionenes of decreasing charge density. Stable multilayers growth was only possible for polyelectrolytes of sufficiently high charge density, confirming the existence of a critical charge density for the formation of multilayers. We also show that, for systems near the critical charge density, the addition of salt to the solution may result in a transition from unstable to stable multilayer growth

Surface modification with thermoresponsive polymer brushes for a switchable electrochemical sensor

Elaboration of switchable surfaces represents an interesting way for the development of a new generation of electrochemical sensors. In this paper, a method for growing thermoresponsive polymer brushes from a gold surface pre-modified with polyethyleneimine (PEI), subsequent layer-by-layer polyelectrolyte assembly and adsorption of a charged macroinitiator is described. We propose an easy method for monitoring the coil-to-globule phase transition of the polymer brush using an electrochemical quartz crystal microbalance with dissipation (E-QCM-D). The surface of these polymer modified electrodes shows reversible switching from the swollen to the collapsed state with temperature. As demonstrated from E-QCM-D measurements using an original signal processing method, the switch is operating in three reversible steps related to different interfacial viscosities. Moreover, it is shown that the one electron oxidation of ferrocene carboxylic acid is dramatically affected by the change from the swollen to the collapsed state of the polymer brush, showing a spectacular 86% decrease of the charge transfer resistance between the two states.