Ahmed Mourran

Supramolecular Assemblies of a Bis(terpyridine) Ligand and of its [2×2] Grid-type ZnII and CoII Complexes on Highly Ordered Pyrolytic Graphite

The synthesis and characterisation of bis(terpyridine)-derived ligands which are capable of forming [2x2] grid-like complexes is presented. Additional pyridine substituents on these ligands do not interfere with the complexation process. Adsorbing one of the pure ligands on highly ordered pyrolytic graphite (HOPG) shows a highly ordered structure stabilised by additional weak intermolecular C-H...N hydrogen bonds partially through the extra-pyridines as could be shown by scanning tunneling microscopy (STM) investigations. Similar adsorption experiments with one of the corresponding [2x2] Co(II) grid-type complexes on graphite, led also to a well-organised structure with interdigitation of the extra-pyridine moieties.

When Colloidal Particles Become Polymer Coils

This work concerns interfacial adsorption and attachment of swollen microgel with low- to medium-level cross-linking density. Compared to colloids that form a second, dispersed phase, the suspended swollen microgel particles are ultrahigh molecular weight molecules, which are dissolved like a linear polymer, so that solvent and solute constitute only one phase. In contrast to recent literature in which microgels are treated as particles with a distinct surface, we consider solvent-solute interaction as well as interfacial adsorption based on the chain segments that can form trains of adsorbed segments and loops protruding from the surface into the solvent. We point out experimental results that support this discrimination between particles and microgels. The time needed for swollen microgels to adsorb at the air/water interface can be 3 orders of magnitude shorter than that for dispersed particles and decreases with decreasing cross-linking density. Detailed analysis of the microgels deformation, in the dry state, at a solid surface enabled discrimination particle like microgel in which case spreading was controlled predominantly by the elasticity and molecule like adsorption characterized by a significant overstreching, ultimately leading to chain scission of microgel strands. Dissipative particle dynamics simulations confirms the experimental findings on the interfacial activity and spreading of microgel at liquid/air interface.

Contact angle hysteresis on superhydrophobic stripes

We study experimentally and discuss quantitatively the contact angle hysteresis on striped superhydrophobic surfaces as a function of a solid fraction, ϕS. It is shown that the receding regime is determined by a longitudinal sliding motion of the deformed contact line. Despite an anisotropy of the texture the receding contact angle remains isotropic, i.e., is practically the same in the longitudinal and transverse directions. The cosine of the receding angle grows nonlinearly with ϕS. To interpret this we develop a theoretical model, which shows that the value of the receding angle depends both on weak defects at smooth solid areas and on the strong defects due to the elastic energy of the deformed contact line, which scales as ϕS(2)lnϕS. The advancing contact angle was found to be anisotropic, except in a dilute regime, and its value is shown to be determined by the rolling motion of the drop. The cosine of the longitudinal advancing angle depends linearly on ϕS, but a satisfactory fit to the data can only be provided if we generalize the Cassie equation to account for weak defects. The cosine of the transverse advancing angle is much smaller and is maximized at ϕS ≃ 0.5. An explanation of its value can be obtained if we invoke an additional energy due to strong defects in this direction, which is shown to be caused by the adhesion of the drop on solid sectors and is proportional to ϕS(2). Finally, the contact angle hysteresis is found to be quite large and generally anisotropic, but it becomes isotropic when ϕS ≤ 0.2.

Cubosomes from hierarchical self-assembly of poly(ionic liquid) block copolymers.

Cubosomes are micro- and nanoparticles with a bicontinuous cubic two-phase structure, reported for the self-assembly of low molecular weight surfactants, for example, lipids, but rarely formed by polymers. These objects are characterized by a maximum continuous interface and high interface to volume ratio, which makes them promising candidates for efficient adsorbents and host-guest applications. Here we demonstrate self-assembly to nanoscale cuboidal particles with a bicontinuous cubic structure by amphiphilic poly(ionic liquid) diblock copolymers, poly(acrylic acid)-block-poly(4-vinylbenzyl)-3-butyl imidazolium bis(trifluoromethylsulfonyl)imide, in a mixture of tetrahydrofuran and water under optimized conditions. Structure determining parameters include polymer composition and concentration, temperature, and the variation of the solvent mixture. The formation of the cubosomes can be explained by the hierarchical interactions of the constituent components. The lattice structure of the block copolymers can be transferred to the shape of the particle as it is common for atomic and molecular faceted crystals.

Waterborne physically crosslinked antimicrobial nanogels

Supramolecular nanomaterials are formed by reversible connection of different building blocks; commonly non-covalent interactions lead to the formation of these materials. In this report, we present the preparation of very stable physically crosslinked nanogels (PCNGs) via a simple one pot reaction in water as solvent. Branched poly(ethylene imine) (PEI) is functionalized with C-10 alkyl chains and azetidinium groups yielding an amphiphilic polymer, which due to the hydrophobic interaction of the alkyl chains and the ionic repulsion of the azetidinium groups forms PCNGs with high colloidal stability. As the dynamic hydrophobic interactions are the main driving force in the formation of these nanogels, the PCNG show a temperature responsive behavior with respect to the zeta potential, particle size (hydrodynamic diameter), and polydispersity index. The potential of the PCNGs to form protective coatings is shown by the formation of ultrathin films on mica and highly oriented pyrolytic graphite. Finally the antimicrobial efficacy of the PCNGs was proven against a wide range of bacteria.

Regimes of wetting transitions on superhydrophobic textures conditioned by energy of receding contact lines

We discuss an evaporation-induced wetting transition on superhydrophobic stripes and show that depending on the elastic energy of the deformed contact line, which determines the value of an instantaneous apparent contact angle, two different scenarios occur. For relatively dilute stripes, the receding angle is above 90°, and the sudden impalement transition happens due to an increase of a curvature of an evaporating drop. For dense stripes, the slow impregnation transition commences when the apparent angle reaches 90° and represents the impregnation of the grooves from the triple contact line towards the drop center.

Dynamic Switching of Helical Microgel Ribbons

We report on a microscopic poly(N-isopropylacrylamide) hydrogel ribbon, coated by a thin gold layer, that shows helical coiling. Confined swelling and shrinkage of the hydrogel below and above its characteristic volume phase transition leads to a temperature actuated reversal of the sense of the helix. The extent and the shape of the winding are controlled by the dimensions and mechanical properties of the bilayer ribbon. We focus on a cylindrical helix geometry and monitor the morphing under equilibrium and nonequilibrium conditions, that is, when the temperature changes faster than the volume (millisecond range). For slow temperature variations, the water release and uptake follow the equilibrium transition trajectory determined by the time needed for the diffusion of water into and out of the microscopic gel. Much faster variations of the temperature are accomplished by internal heating of embedded gold nanorods by IR-light irradiation. This causes elastic stresses that strongly affect the motions. This method enables well-reproducible deviations from the equilibrium transition path and allows us to control rather precisely the spatiotemporal transformation in a cyclic repetitive process. Actuation and response are sensitive to small variations of temperature and composition of the aqueous sol in which the gel is immersed. The principle as described may be used to detect specific analytes that bind either to the surface of the gold layer or within the gel and can modify the interaction between the water and the gel. The reported nonequilibrium morphing implies that the system dissipates energy and may also be able to perform work as required for a microscopic motor.

Monolayer properties of asymmetrically substituted sexithiophene.

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain. It displays thermotropic liquid crystalline (LC) behavior. Both compounds form readily single molecular layers from solution. Remarkably, full monolayer coverage can be achieved before multilayer growth starts. LC properties promote preordering near the interface as well as exchange of molecules between the growing domains, thus regulating the domain sizes. As a result, the LC compound 2 forms single-molecule islands with larger domain sizes compared to compound 1. Surface X-ray investigations indicate that the 6T cores are tilted relative to the layer normal. The tilt angle is as large as 54° for compound 2 compared to 28° for compound 1. For molecule 2, interfacial constraints and packing requirements because of the asymmetric substitution cause a rather loosely organized core structure.

Substituted Septithiophenes with End Groups of Different Size : Packing and Frustration in Bulk and Thin Films

We report on three different liquid crystalline compounds with a central septithiophene core and alkylated end groups of strongly increasing bulkiness. In principle, the thiophene cores prefer to pack parallel to optimize their π-π interactions, which becomes sterically impossible for the bulkier end groups. Using X-ray diffraction, we find that the way out of this packing dilemma is toward liquid-crystal phases of higher dimensionality in the order smectic → columnar ↔ bicontinuous cubic. For the smectic phase, packing in a monolayer is no problem; for the other ones packing considerations become more stringent in films due to the boundaries. Surface X-ray techniques and atomic force microscopy indicate an appreciable difference between monolayer and three-layer films, in which the monolayers appear to escape from packing frustration by generating superstructures. We propose a basic structure of columns parallel to the substrate that provides a compromise between preserving some π-π interactions and packing the bulky alkyl groups.

Preparation and application of poly(alkoxytitanate) as a TiO2 precursor with high storage stability

Titanium dioxide is a basic material of our daily life. Because of its favourable properties, such as harmlessness, chemical stability, photocatalytic activity, or whiteness it is increasingly applied in both micro and nano particles and thin films and coating. One of the available procedures for film forming is the sol–gel technology, an inexpensive low temperature process with wide possibilities to vary film properties by changing the composition of the precursor solution or other parameters. In the paper a new precursor polymer for TiO2 film-preparation with high storage and processing stabilities is introduced and applied in thin film forming. The new precursor poly(alkoxytitanate) is prepared by a one step, water-free sol–gel method. A smooth TiO2 film can be prepared using this precursor by spin-coating followed by H2-plasma curing. Comparing to a common precursor such as Ti(O–iPr)4, this precursor has a good solubility in different solvents and a much higher storage stability. The easy to modify precursor end groups enable the tailoring of properties regarding to hydrolysis to both TiO2 particles and films.