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Elaboration of Monodisperse Spherical Hollow Particles with Ordered Mesoporous Silica Shells via Dual Latex/Surfactant Templating: Radial Orientation of Mesopore Channels

Monodisperse spherical hollow nanoparticles of mesoporous silica featuring mesopores with a radial orientation in the silica shell were synthesized via a dual-templating method. Specifically designed polystyrene latexes with anionic or cationic surface charges acted as the core templates, while cetyltrimethylammonium bromide served as a co-template to structure the mesopore formation during tetraethoxysilane hydrolysis/condensation. The particles were well-separated and presented homogeneous mesoporous silica shells. Average particle diameters were less than 200 nm, and the particles displayed high values of specific surface area and pore volume. The shell thickness and the hollow core diameter could be tuned independently while the radial pore structure was preserved. A detailed analysis of the nitrogen adsorption-desorption isotherms proved that the central cavity was completely isolated from the external medium, that is, only accessible through the radial mesopores of the shell. Consequently, our particles gather the advantages of a well-defined structure, straight penetrating channels across the silica shell, and a high accessible porous volume of the central core. These properties make them far better candidates than simple mesoporous particles for any storage and/or controlled release applications.

Controlled Radical Polymerization in Aqueous Dispersed Media

Controlled radical polymerization (CRP), sometimes also termed ‘living’ radical polymerization, offers the potential to create a wide range of polymer architectures, and its implementation in aqueous dispersed media (e.g. emulsion polymerization, used on a vast scale industrially) opens the way to large-scale manufacture of products based on this technique. Until recently, implementing CRP in aqueous dispersed media was plagued with problems such as loss of ‘living’ character and loss of colloidal stability. This review examines the basic mechanistic processes in free-radical polymerization in aqueous dispersed media (e.g. emulsion polymerization), and then examines, through this mechanistic understanding, the new techniques that have been developed over the last few years to implement CRP successfully in emulsion polymerizations and related processes. The strategies leading to these successes can thus be understood in terms of the various mechanisms which dominate CRP systems in dispersed media; these mechanisms are sometimes quite different from those in conventional free-radical polymerization in these media.

Aqueous suspension of amphiphilic diblock copolymer nanoparticles prepared in situ from a water-soluble poly(sodium acrylate) alkoxyamine macroinitiator

The simple, one step synthesis of aqueous suspensions of amphiphilic nanoparticles is presented. Those particles are prepared in the batch heterophase polymerization of styrene or -butyl acrylate, using a water-soluble poly(sodium acrylate) alkoxyamine macroinitiator. The nitroxide-mediated controlled growth of the hydrophobic block leads to the formation of poly(sodium acrylate)--polystyrene or poly(sodium acrylate)--poly(-butyl acrylate) amphiphilic diblock copolymers, able to self-assemble in water simultaneously to the growth step. When the diblock copolymers become strongly asymmetrical, with a short poly(sodium acrylate) block and a long hydrophobic one, the formed hairy nanoparticles are analogous to amphiphilic diblock copolymer crew-cut micelles.

Hierarchical structures based on self-assembled diblock copolymers within honeycomb micro-structured porous films

This article details the preparation of hierarchically ordered microporous films using the so-called breath figure approach combined with the self-assembly of well-defined poly(n-butyl acrylate)-block-polystyrene or poly(tert-butyl acrylate)-block-polystyrene copolymers synthesized by nitroxide-mediated polymerization. The first level of organization was the hexagonal pattern of pores at the micrometre length scale leading to iridescence properties and surface hydrophobicity of the honeycomb structured films. Optical microscopy with the corresponding 2-Dimensional Fast Fourier Transform highlighted the ordering of the pores over a large scale (∼1 cm2). The second level of structuring was provided by the diblock copolymers chosen for their ability to self-assemble into ordered nanophases. The nanoscale morphology of both the honeycomb films and the corresponding thermally annealed continuous films was systematically investigated by atomic force microscopy (AFM) and small angle neutron scattering (SANS). The film characterization revealed a nanostructuration of the acrylate-based coil–coil diblock copolymer within the walls of the highly ordered microporous films obtained via a simple solvent evaporation method under humid atmosphere. The four synthesized diblock copolymers exhibited different macromolecular features with respect to the Flory–Huggins interaction parameter, the glass transition temperature of each block and the weight fraction of each monomer that influenced the quality of either the micropores structuration or the nanophase segregation.

Dispersion Polymerization of Methyl Acrylate in Nonpolar Solvent Stabilized by Block Copolymers Formed In situ via the RAFT Process

The free-radical dispersion polymerization of methyl acrylate (MA) in isododecane was carried out in the presence of a poly(2-ethylhexyl acrylate) macromolecular RAFT (reversible addition-fragmentation chain transfer) agent bearing a trithiocarbonate reactive group in the middle of the chain (P2EHA-TTC). The presence of the trithiocarbonate function was crucial for the synthesis of monodisperse colloidal poly(methyl acrylate) (PMA) particles stabilized by the P2EHA segments. The hydrodynamic diameters ranged from 100 to 300 nm, using particularly low amounts of the macro(RAFT agent) (1-6 wt % vs. MA) in dispersion polymerizations carried out at 20 wt % solids content. As shown by 2D liquid chromatography, P2EHA-b-PMA or P2EHA-b-PMA-b-P2EHA block copolymers formed in situ at the early stage of the dispersion polymerization due to the reversible transfer process and played the role of particle stabilizer. The glass-transition temperature of the derived polymer films was not affected by the low amount of the chosen macromolecular stabilizer and the mechanical properties were mainly those of PMA, which makes the technique very attractive for coating applications.

pH Sensitive Hierarchically Self-Organized Bioinspired Films

In the present manuscript, we have demonstrated that hierarchically structured smart porous polymer films based on honeycomb-patterned surface can be elaborated from PS-b-P4VP pH-responsive block copolymer using the breath figure process. Despite the fast film formation by a bottom-up process, the copolymer nanostructuration was observed inside the walls of the honeycomb porous film. Atomic force microscopy (AFM), small angle X-ray and neutron scattering (SAXS and SANS) measurements were used to reveal both the hexagonal arrays formed by the pores at the micrometer length scale and the hexagonal copolymer self-assembly at the nanometer length scale. Contact angle (CA) measurements were used to point out the reversible pH-responsive wettability character of the surface. The PS-b-P4VP honeycomb film shows a contact angle variation of 20° between pH 9 and pH 3. An increase of the roughness was obtained with the pincushions hexagonal array enhancing the pH responsiveness of the polymer film with a switching CA gap of 75° when pH tuned from pH 9 to pH 3. This work presents the first report on honeycomb porous and pincushion films exhibiting a reversible pH-responsive character.

Synthesis by nitroxide-mediated aqueous dispersion polymerization, characterization, and physical core-crosslinking of pH- and thermoresponsive dynamic diblock copolymer micelles

Diblock copolymers consisting of a poly(acrylic acid) (PAA) segment and a LCST-type poly(N,N-diethylacrylamide) (PDEAAm) block were obtained by nitroxide-mediated polymerization in aqueous dispersion using a water-soluble macroalkoxyamine. The influence of several parameters on the polymerization (temperature, initial free nitroxide or macroalkoxyamine concentrations, and solids content) was evaluated in terms of kinetics, macromolecular control, and colloidal features. As determined by dynamic light scattering (DLS), stable dispersions of monodisperse particles could be obtained for solids content as high as 39 wt% without the need for any additional surfactant via a polymerization-induced self-assembly mechanism. Rendered possible by the use of a controlled/living polymerization process, the effective semi-batch incorporation of hydrophobic units (styrene) in the growing chains during the polymerization allowed the formation of physically crosslinked nanogels. The pH and temperature sensitivity were proved by means of DLS and high-sensitivity differential scanning calorimetry (HSDSC) measurements. Due to the formation of aggregates observed by size-exclusion chromatography in N,N-dimethylformamide, accurate molar masses could not be determined directly but deconvoluted hydrodynamic volume distributions suggested a good control of the polymerization.

Surfactant-free synthesis of amphiphilic diblock copolymer nanoparticles via nitroxide-mediated emulsion polymerization

Amphiphilic hairy nanoparticles are prepared in a one step, batch, heterogeneous polymerization of styrene or n-butyl acrylate, using a water-soluble poly(sodium acrylate) alkoxyamine macroinitiator based on the SG1 nitroxide.

Highly structured pH-responsive honeycomb films by a combination of a breath figure process and in situ thermolysis of a polystyrene-block-poly(ethoxy ethyl acrylate) precursor

In the present work, we show that Cu(0)-mediated controlled radical polymerization is a suitable method to synthesize high molar mass polystyrene-b-poly(ethoxy ethyl acrylate) PS-b-PEEA diblock copolymers. This method, applied at room temperature, is mandatory for complete preservation of ethoxy ethyl protecting groups during the course of polymerization. The synthesized PS-b-PEEA diblock copolymers were subsequently used for the elaboration of pH sensitive hierarchically structured honeycomb (HC) films through the Breath Figure (BF) process. The PS-b-PEEA hydrophobic honeycomb films were characterized by optical microscopy and atomic force microscopy (AFM) to reveal the hexagonal array of pores at the micrometer length scale, together with the phase segregation of the diblock copolymer. Similar to highly structured natural materials, the biomimetic honeycomb polymer films displayed intense iridescence. Moreover, the increase of surface roughness by peeling off the top layer of the PS-b-PEEA HC films produced superhydrophobic surfaces exhibiting a water contact angle of 155°. Subsequent deprotection of PEEA into pH-responsive poly(acrylic acid) (PAA) was performed in situ from the PS-b-PEEA honeycomb film by a simple thermolysis step carried out at 90 °C. The resulting PS-b-PAA honeycomb films showed a clear pH-responsive behavior with a water contact angle gap of 65° between a pH of 3 and 10.

Synthetic methodology effect on the microstructure and thermal properties of poly(n-butyl acrylate-co-methyl methacrylate) synthesized by nitroxide mediated polymerization

In this work, we report the synthesis of poly(n-butyl acrylate-co-methyl methacrylate) copolymers by the nitroxide mediated polymerization (NMP) technique, using N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide (SG1) as a control agent and 2-methylaminoxypropionic-SG1 alkoxyamine (BlocBuilder®) as the initiator. The copolymers are synthesized either by batch or semi-batch processes and the gradient profile is examined via the determination of the instantaneous fraction of monomer incorporated in the copolymer. A control of the molar mass together with low molar mass distribution (Mw/Mn < 1.4) is observed. The dependence of the copolymer glass transition temperature with conversion was followed by differential scanning calorimetry. The copolymers are investigated by carbon nuclear magnetic resonance and heteronuclear multiple bond correlation (HMBC) NMR sequences to study the effect of the monomer addition mode on the microstructure of copolymers. The thermomechanical properties of gradient copolymers are finally reported to establish the effect of the composition on the mechanical behaviour of the copolymers.