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.

Environmentally responsive particles: from superhydrophobic particle films to water-dispersible microspheres.

We describe the preparation, by precipitation copolymerization, of multifunctional divinylbenzene-co-pentafluorostyrene microspheres able to produce superhydrophobic surfaces or disperse in aqueous media upon annealing either in air or water, respectively. For that purpose, an amphiphilic block copolymer, polystyrene-b-poly(acrylic acid), was introduced in the initial feed composed of divinylbenzene and 2,3,4,5,6-pentafluorostyrene. As a result, fluorinated particles were obtained in which the diblock copolymer was encapsulated during the polymerization step. Upon annealing in dry air, the particles are completely hydrophobic and form superhydrophobic surfaces. On the contrary, annealing in water induces the reorientation of the PAA groups toward the particle interface, thus the particles can be dispersed in aqueous media. In addition, the presence of carboxylic acid groups at the particle interface permits us to switch the surface charge between negative and neutral depending on the environmental pH.

Self-organized hierarchical structures in polymer surfaces: self-assembled nanostructures within breath figures.

Herein we report the preparation of hierarchically micro- and nanostructured polymer surfaces in block copolymer/homopolymer blends. The structural order at different length scales was obtained combining two methodologies, e.g., the breath figures method to produce porous microstructures ("top-down" approach) with block copolymer self-assembly to induce microphase separation at the nanometer length scale ("bottom-up" approach). The interplay of the breath figure formation during the spin-coating and self-assembly of the triblock copolymer allowed the preparation of polymer surfaces having micrometer-sized cavities decorated with nanostructured block copolymers. The system described herein possesses unique characteristics. First, the surface chemical composition can be varied by a surface rearrangement upon annealing either to dry or humid air. Moreover, surface rearrangement is accompanied with structural changes, i.e. both topography and nanostructuration can be reversibly modified upon annealing. In terms of topograghy, a transition between holes and hills was obtained upon soft annealing to water vapor and can be recovered upon annealing to dry air. Finally, the pore nanostructure can be modulated from a micellar array to a lamellar phase when the film is exposed either to air or to tetrahydrofuran vapor.

Structured assemblies of ferromagnetic particles through covalent immobilization on functionalized polymer surfaces obtained by surface segregation.

We report a strategy to immobilize magnetic particles on polymer surfaces in an organized manner. Surface segregation of binary polymer blends provided surfaces with the desired chemical functions (carboxylic functions). These functional groups were demonstrated to be accessible and were thus able to react with magnetic particles functionalized with amine functions. The presence of a magnetic field during the covalent attachment step in direct surface patterning produced particle chains oriented parallel to the field.