Juan Rodriguez-Hernandez

Boundary lubricant films under shear: effect of roughness and adhesion

The normal interaction and the behavior under shear of mica surfaces covered by two different triblock copolymers of polylysine-polydimethysiloxane-polylysine were studied by combining the capabilities of the surface forces apparatus and the atomic force microscopy. At low pH values these copolymers spontaneously adsorb on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties. The morphology of the adsorbed layer is determined by the molecular structure of the particular copolymer investigated. This morphology plays a fundamental role on the behavior of the adsorbed layers under shear and compression. While nonadhesive smooth layers oppose an extremely small resistance to sliding, the presence of asperities even at the nanometric scale originates a frictional resistance to the motion. The behavior of uniform nonadhesive nanorough surfaces under shear can be quantitatively understood in terms of a simple multistable thermally activated junction model. The electric charge of the adsorbed copolymer molecules and hence the adhesion energy between the coated surfaces can be modified by varying the pH of the surrounding media. In the presence of an adhesive interaction between the surfaces the behavior under shear is strongly modified. Time-dependent mechanisms of energy dissipation have to be evoked in order to explain the changes observed.

Nanogels Based on Poly(vinyl acetate) for the Preparation of Patterned Porous Films

The use of poly(vinyl acetate) (PVAc) nanogels for the fabrication of patterned porous surfaces is described. These nanogels were synthesized by controlled radical cross-linking copolymerization (CRCC) involving a xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) mechanism. This synthesis methodology allowed for the preparation of nanogels based on PVAc with a controlled constitutive chain length and average numbers of chains and cross-links. Solutions of these branched polymers were prepared in THF with a fixed amount of water and spin coated onto a surface of graphite. The surface porosity of corresponding films was observed by atomic force microscopy (AFM). Compared with linear PVAc homologues with a degree of polymerization (DP) sufficiently high to favor the formation of porous structures (DP = 50), a sharper and better defined porosity was observed with nanogels, the constitutive chains of which had the same DP. For nanogels differing only in their cross-link density, the pores were smaller and better defined in the case of the higher cross-link density, suggesting an enhanced stabilization of the water droplets during film formation. To explain these observations, it is postulated that PVAc nanogels can behave as compact particles providing steric stabilization of water droplets, which is referred to as a Pickering effect. The coalescence of water droplets would be better prevented as the cross-link density of the nanogels increases, resulting in a smaller size pore.

Triblock copolymer lubricant films under shear : Effect of molecular Cross-Linking

The normal interaction and the behavior under shear of mica surfaces covered by a triblock copolymer of poly(L-lysine)-b-polydimethysiloxane-b-poly(L-lysine) (Plys40-b-PDMS40-b-Plys40) before and after cross-linking reaction with two dicarboxylic acids were studied, combining the capabilities of the surface forces apparatus and atomic force microscopy (AFM). At low pH values, this copolymer spontaneously adsorbed on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties, forming an extremely smooth boundary layer. This smooth layer displays a very small resistance to shear under small loads, exhibiting outstanding lubrication properties. Nevertheless, the fusion of two contacting layers can be induced by compression at a certain pressure, a process that causes a marked increase in the friction forces. Cross-linking of the adsorbed polymer molecules by covalent bond formation with dicarboxylic acids increases the mechanical stability of the adsorbed layers and hinders the fusion of the boundary layers under pressure but impairs its lubrication properties to a certain extent.

Honeycomb structured porous interfaces as templates for protein adhesion

We prepared breath figure patterns decorated with a statistical glycopolymer, (styrene-co-2-{[(D-glucosamin-2-N-yl)carbonyl]oxy}ethyl methacrylate, S-HEMAGl). The preparation of the glycopolymer occurs in one single step by using styrene and S-HEMAGl. Blends of this copolymer and high molecular weight polystyrene were spin coated from THF solutions leading to the formation of surfaces with both controlled functionality and topography. AFM studies revealed that both the composition of the blend and the relative humidity play a key role on the size and distribution of the pores at the interface. The porous films shows the hydrophilic glycomonomer units are oriented towards the pore interface since upon soft annealing in water, the holes are partially swelled. The self-organization of the glycopolymer within the pores was additionally confirmed both by reaction of carbohydrate hydroxyl groups with rhodamine-isocyanate and by means of the lectin binding test using Concanavalin A (Con A).