Visible-Light Initiated Dispersion Photopolymerization of Styrene.

Polystyrene (PS) particles were synthesized in ethanol/water mixture by dispersion polymerization using visible light irradiation, with either a N-heterocyclic carbene borane-based photoinitiating system (PIS) or a disulfide. With the full PIS and poly(ethylene glycol) methyl ether methacrylate (PEGMA) as stabilizer, the size distributions were broad and the amount of PEGMA had a strong impact on the experiment reproducibility. The addition of a base solved the problem, leading to faster polymerizations, narrower size distributions and larger particles. With the disulfide as sole PIS, bigger and narrowly distributed PS particles were again formed. Quantitative conversion was achieved in each system, with particle size ranging between 100 and 350 nm. The use of poly(N-vinylpyrrolidone) as stabilizer led to significantly larger particles, up to 1.2 µm, with narrow size distributions. The production of such large latex particles by photoinitiated polymerizations is unprecedented.

Carbon Quantum Dot-Catalyzed, Highly Efficient Miniemulsion Atom Transfer Radical Polymerization Induced by Visible Light.

Owing to the benefits of using natural or artificial light sources as a stimulus, photoinduced reversible-deactivation radical polymerization (photoRDRP) techniques have been recognized to be a powerful "green" platform for the preparation of well-defined polymers. However, the development of highly efficient visible light-induced photoRDRP processes in aqueous dispersed media remains a challenge due to light scattering and refraction by monomer droplets or colloidal particles. In this work, an efficient green photocatalyst, carbon quantum dots (CQDs), was introduced to visible light-mediated miniemulsion atom transfer radical polymerization (ATRP), leading to highly efficient polymerizations with reaction rates (>80% monomer conversion within 1 h) much higher than in previous studies. This heterogeneous photocatalytic system is presumed to involve three catalytic cycles in (i) the aqueous phase, (ii) the oil-water interface, and (iii) the monomer droplets. The effect of different polymerization parameters on the polymerization reaction was investigated, including the amounts of surfactant and CQDs, CuBr2 dosage, and solid content. Excellent temporal control of the polymerization was illustrated by "ON/OFF" polymerizations, and natural sunlight was also used as an energy source. This novel CQDs-catalyzed miniemulsion photoATRP process may be easily extended to other aqueous dispersion RDRP systems. As an extension of our previous work (J. Am. Chem. Soc. 2022, 144 (22), 9817-9826) we also developed a "one-pot" method for the rapid preparation of heterogeneous hydrogels.

Development of a Borane-(Meth)acrylate Photo-Click Reaction.

In the development of 3D printing fuels, there is a need for new photoinitiating systems working under mild conditions and/or leading to polymers with new and/or enhanced properties. In this context, we introduce herein N-heterocyclic carbene-borane complexes as reagents for a new type of photo-click reaction, the borane-(meth)acrylate click reaction. Remarkably, the higher bond number of boranes relative to thiols induced an increase of the network density associated with faster polymerization kinetics. Solid-state NMR evidenced the strong participation of the boron centers on the network properties, while DMA and AFM showed that the materials exhibit improved mechanical properties, as well as reduced solvent swelling.

Styrene-Butadiene Rubber by Miniemulsion Polymerization Using In Situ Generated Surfactant.

An alternative approach for the synthesis of styrene butadiene rubber (SBR) copolymer latexes was explored in order to obtain low gel fractions and high solid contents. The ultra-turrax-assisted miniemulsion stabilized by in situ surfactant generation was adopted as the main strategy since this technique can inhibit the eventual presence of secondary nucleation producing polybutadiene particles and also control the cross-linking degree. Styrene monomer was first miniemulsified using an ultra-turrax and in situ generated surfactant using either hexadecane (HD) or octadecyl acrylate (ODA) as the hydrophobe. Dynamic light scattering (DLS) measurements of droplet size indicated faster stabilization and the production of smaller droplet diameters ca. 190 nm (PdI = 0.08) when employing in situ generated potassium oleate (K-Oleate) in comparison to SDS-based miniemulsions. High butadiene-level SBR latexes with ca. 50% solids content, a glass transition temperature (Tg) of -52 °C, and a butadiene to styrene weight ratio of 75:25, were then obtained using the miniemulsion droplets as seeds. Turbiscan and DLS measurements revealed a very stable resulting latex with SBR particle diameter of ca. 220 nm and a low polydispersity index (PdI). Secondary nucleation was prevented as indicated by the low Np/Nd value. Cryo-TEM images showed a narrow distribution of particle size as well as the absence of agglomeration. The gel content was below 10% when tert-dodecyl mercaptan (t-DM) was used as chain transfer agent (CTA).

Synthesis and Self-Assembly of Poly(N-Vinylcaprolactam)-b-Poly(ε-Caprolactone) Block Copolymers via the Combination of RAFT/MADIX and Ring-Opening Polymerizations.

Well-defined amphiphilic, biocompatible and partially biodegradable, thermo-responsive poly(N-vinylcaprolactam)-b-poly(ε-caprolactone) (PNVCL-b-PCL) block copolymers were synthesized by combining reversible addition-fragmentation chain transfer (RAFT) and ring-opening polymerizations (ROP). Poly(N-vinylcaprolactam) containing xanthate and hydroxyl end groups (X-PNVCL-OH) was first synthesized by RAFT/macromolecular design by the interchange of xanthates (RAFT/MADIX) polymerization of NVCL mediated by a chain transfer agent containing a hydroxyl function. The xanthate-end group was then removed from PNVCL by a radical-induced process. Finally, the hydroxyl end-capped PNVCL homopolymer was used as a macroinitiator in the ROP of ε-caprolactone (ε-CL) to obtain PNVCL-b-PCL block copolymers. These (co)polymers were characterized by Size Exclusion Chromatography (SEC), Fourier-Transform Infrared spectroscopy (FTIR), Proton Nuclear Magnetic Resonance spectroscopy (1H NMR), UV-vis and Differential Scanning Calorimetry (DSC) measurements. The critical micelle concentration (CMC) of the block copolymers in aqueous solution measured by the fluorescence probe technique decreased with increasing the length of the hydrophobic block. However, dynamic light scattering (DLS) demonstrated that the size of the micelles increased with increasing the proportion of hydrophobic segments. The morphology observed by cryo-TEM demonstrated that the micelles have a pointed-oval-shape. UV-vis and DLS analyses showed that these block copolymers have a temperature-responsive behavior with a lower critical solution temperature (LCST) that could be tuned by varying the block copolymer composition.

Polymer-encapsulation of iron oxide clusters using macroRAFT block copolymers as stabilizers: tuning of the particle morphology and surface functionalization.

We report the successful synthesis of superparamagnetic latex particles with a high fraction of magnetic materials and a fast magnetic response. Commercial fatty acid-modified iron oxide (IO) nanoparticles were first assembled into spherical clusters through an emulsification/solvent evaporation method. The resulting particles were stabilized with poly(2-dimethylaminoethyl methacrylate)-b-polystyrene (PDMAEMA-b-PS) amphiphilic block copolymers obtained by RAFT, and used as seeds in the emulsion copolymerization of styrene and divinylbenzene (DVB), used as cross-linking agent. The latter revealed to be key in preserving the integrity of the clusters during the emulsion polymerization reaction, and a minimum amount (i.e. 10 wt%) was necessary to obtain stable latexes composed of a core of densely packed IO nanoparticles surrounded by a thin polymer shell. DVB also had a strong influence on the particle morphology as the core-shell morphology of the composite particles could be tuned with either a smooth polymer shell or a raspberry-like surface by adjusting the DVB-to-monomer weight ratio and the feeding conditions. The amphiphilic macroRAFT not only provides colloidal stability to the magnetic latexes, but also offers a versatile platform for the design of composite particles with tailored surface properties by an appropriate choice of the hydrophilic block. Our strategy was thus successfully extended to poly(acrylic acid)-b-polystyrene (PAA-b-PS) copolymers, leading to PAA-stabilized composite particles. Both kinds of IO-encapsulated particles showed superparamagnetic properties (magnetizations at saturation of 35 and 31 emu g-1 for PDMAEMA and PAA systems, respectively), and could thus find interesting applications as magnetic carriers in the biological field due to their thermo- (for PDMAEMA) and pH- (for PDMAEMA and PAA) responsive properties.

Interaction of Cationic, Anionic, and Nonionic Macroraft Homo- and Copolymers with Laponite Clay.

The functionalization of Laponite RD platelets with different cationic, anionic, and nonionic homo- and copolymers synthesized by reversible addition-fragmentation chain transfer (RAFT) has been investigated. The effective interaction of the macromolecular RAFT agents (macroRAFTs) with the inorganic particles is known to be of crucial importance for the successful coating of minerals with polymers via RAFT-mediated emulsion polymerization to produce polymer-encapsulated inorganic particles. The macroRAFT agents synthesized in the present work contain carefully selected reinitiating R groups, which bear either ionizable tertiary amine or quaternary ammonium moieties (from 2-(dimethylamino)ethyl methacrylate, DMAEMA), negatively charged acrylic acid (AA) repeat units, or neutral polyethylene glycol (PEG) side chains, and are capable of interacting with Laponite via different adsorption mechanisms. The equilibrium adsorption of these RAFT (co)polymers was investigated by the plotting of adsorption isotherms, and either L-type or H-type curves were obtained. The hydrophobicity of the macroRAFT was shown to promote adsorption, as did the pending configuration of the PEG block. Charge repulsion between AA and the negatively charged surface of Laponite at pH 7.5, on the other hand, was prejudicial for adsorption, while the strong electrostatic interaction between the cationic DMAEMA molecules and the Laponite surface led to high-affinity-type curves.

Vitrimer Chemistry Meets Cellulose Nanofibrils: Bioinspired Nanopapers with High Water Resistance and Strong Adhesion.

Nanopapers containing cellulose nanofibrils (CNFs) are an emerging and sustainable class of high performance materials. The diversification and improvement of the mechanical and functional property space critically depend on integration of CNFs with rationally designed, tailor-made polymers following bioinspired nanocomposite designs. Here we combine for the first time CNFs with colloidal dispersions of vitrimer nanoparticles (VP) into mechanically coherent nanopaper materials. Vitrimers are permanently cross-linked polymer networks that undergo temperature-induced bond shuffling through an associative mechanism and which allow welding and reshaping on the macroscale. The choice of low glass transition, hydrophobic vitrimers derived from fatty acids and polydimethylsiloxane (PDMS), and achieving dynamic reshuffling of cross-links through transesterification reactions enables excellent compatibility and covalent attachment onto the CNF surfaces. Moreover, the resulting films are ductile, stretchable and offer high water resistance. The success of imparting the vitrimeric polymeric behavior into the nanocomposite, as well as the curing mechanism of the vitrimer, is highlighted through thorough analysis of structural and mechanical properties. The dynamic exchange chemistry of the vitrimers enables efficient welding of two nanocomposite parts as characterized by good bonding strength during single lap shear tests. In the future, we expect that the dynamic character of vitrimers becomes a promising option for the design of mechanically adaptive bioinspired nanocomposites and for shaping and reshaping such materials.

Formation of Cross-Linked Films from Immiscible Precursors through Sintering of Vitrimer Nanoparticles.

Colloidal dispersions of epoxy-acid vitrimers have been synthesized by miniemulsion polymerization. This versatile strategy enables obtaining stable cross-linked particles, even from initially incompatible precursors, while minimizing hydrolysis of the ester bonds formed during the curing. After drying of the latexes, trans-esterification exchanges occurring at high temperatures through interparticle interfaces induces an efficient sintering into homogeneous cross-linked polymer films.

Visible-Light Emulsion Photopolymerization of Styrene.

The photopolymerization of styrene in emulsion is achieved in a conventional double-wall reactor equipped with a LED ribbon coiled around the external glass wall. Styrene mixed to acridine orange is added to the water phase containing sodium dodecyl sulfate, a water-soluble N-heterocyclic carbene-borane and disulfide, and irradiated. Highly stable latexes are obtained, with particles up to a diameter of 300 nm. The ability to reach such large particle sizes via a photochemical process in a dispersed medium is due to the use of visible light: the photons in the visible range are less scattered by larger objects and thus penetrate and initiate better the polymerizations. They are also greener and cheaper to produce via LEDs, and much safer than UVs. The method presented does not require any specific glassware; it works at lower temperature and delivers larger particles compared to thermal processes at similar solids contents and surfactant concentrations.

Investigation of the Adsorption of Amphipathic macroRAFT Agents onto Montmorillonite Clay.

Recently, there has been significant interest in the use of the reversible addition-fragmentation chain-transfer (RAFT) technique to generate a variety of organic/inorganic colloidal composite particles in aqueous dispersed media using the so-called macroRAFT-assisted encapsulating emulsion polymerization (REEP) strategy. In this process, special attention should be paid to the adsorption of the macromolecular RAFT (macroRAFT) agent onto the inorganic particles, as it determines the final particle morphology and can also influence latex stability. In this work, different amphipathic macroRAFT agents were synthesized by RAFT, and their adsorption onto commercial Montmorillonite clay Cloisite Na+ (MMT) was studied by means of adsorption isotherms. Three types of macroRAFT agents were considered: a nonionic one based on poly(ethylene glycol) methyl ether acrylate (PEGA) and n-butyl acrylate (BA), anionic ones, including a block copolymer and random copolymers, based on acrylic acid (AA), BA and PEGA, and cationic ones based on 2-(dimethylamino)ethyl methacrylate (DMAEMA), BA and PEGA. Six adsorption isotherm models (Langmuir, Freundlich, Tempkin, Redlich-Peterson, Sips, and Brunauer-Emmett-Teller) were adjusted to the experimental isotherms. The nonionic macroRAFT agent formed a monolayer on the clay surface with a maximum adsorption capacity of 400 mg g-1 at pH 8, as determined from the Sips adsorption model. Adsorption of the AA-based macroRAFT agents onto MMT was moderate at alkaline pH due to electrostatic repulsions, but increased with decreasing pH. The DMAEMA-based macroRAFT agents displayed a much stronger interaction with the oppositely charged MMT surface at acidic pH due to electrostatic interactions, and the concentration of adsorbed macroRAFT agent reached values as high as 800 mg g-1. The BET model fitted the experimental data relatively well indicating multilayer adsorption promoted by the presence of the hydrophobic BA units. In addition, the cationic macroRAFT agents afforded stable MMT/macroRAFT agent complexes as evaluated by dynamic light scattering and zeta potential analyses.

Design of latex-layered double hydroxide composites by tuning the aggregation in suspensions.

Colloidal stability of polymeric latex particles was studied in the presence of oppositely charged layered double hydroxide (LDH) platelets of different interlayer anions. Adsorption of the LDH particles led to charge neutralization and to overcharging of the latex at appropriate concentrations. Mixing stable colloidal suspensions of individual particles results in rapid aggregation once the LDH adsorption neutralizes the negative charges of the polymer spheres, while stable suspensions were observed at high and low LDH doses. The governing interparticle interactions included repulsive electrical double layer forces as well as van der Waals and patch-charge attractions, whose strength depended on the amount of LDH particles adsorbed on the latex surface. The type of the LDH interlayer anions did not affect the colloidal stability of the samples. Structural investigation of the obtained latex-LDH composites revealed that the polymer spheres were completely coated with the inorganic platelets once their concentration was sufficiently high. These results are especially important for designing synthetic routes for hybrid systems in suspensions, where stable colloids are required for uniform film-formation and for the homogeneous distribution of the inorganic filler within the composite materials.

pH-Switchable Stratification of Colloidal Coatings: Surfaces "On Demand".

Stratified coatings are used to provide properties at a surface, such as hardness or refractive index, which are different from underlying layers. Although time-savings are offered by self-assembly approaches, there have been no methods yet reported to offer stratification on demand. Here, we demonstrate a strategy to create self-assembled stratified coatings, which can be switched to homogeneous structures when required. We use blends of large and small colloidal polymer particle dispersions in water that self-assemble during drying because of an osmotic pressure gradient that leads to a downward velocity of larger particles. Our confocal fluorescent microscopy images reveal a distinct surface layer created by the small particles. When the pH of the initial dispersion is raised, the hydrophilic shells of the small particles swell substantially, and the stratification is switched off. Brownian dynamics simulations explain the suppression of stratification when the small particles are swollen as a result of reduced particle mobility, a drop in the pressure gradient, and less time available before particle jamming. Our strategy paves the way for applications in antireflection films and protective coatings in which the required surface composition can be achieved on demand, simply by adjusting the pH prior to deposition.

Investigation of Four Different Laponite Clays as Stabilizers in Pickering Emulsion Polymerization.

Clay-armored polymer particles were prepared by emulsion polymerization in the presence of Laponite platelets that adsorb at the surface of latex particles and act as stabilizers during the course of the polymerization. While Laponite RDS clay platelets are most often used, the choice of the type of clay still remains an open issue that is addressed in the present article. Four different grades of Laponite were investigated as stabilizers in the emulsion polymerization of styrene. First, the adsorption isotherms of the clays, on preformed polystyrene particles, were determined by ICP-AES analysis of the residual clay in the aqueous phase. Adsorption of clay depended on the type of clay at low concentrations corresponding to adsorption as a monolayer. Adsorption of clay particles as multilayers was observed for all the grades above a certain concentration under the considered ionic strength (mainly due to the initiator ionic species). The stabilization efficiency of these clays was investigated during the polymerization reaction (free of any other stabilizer). The clays did not have the same effect on stabilization, which was related to differences in their compositions and in their adsorption isotherms. The different grades led to different polymer particles sizes and therefore to different polymerization reaction rates. Laponite RDS and S482 gave similar results, ensuring the best stabilization efficiency and the fastest reaction rate; the number of particles increased as the clay concentration increased. Stabilization with Laponite XLS gave the same particles size and number as the latter two clays at low clay concentrations, but it reached an upper limit in the number of nucleated polymer particles at higher concentrations indicating a decrease of stabilization efficiency at high concentrations. Laponite JS did not ensure a sufficient stability of the polymer particles, as the polymerization results were comparable to a stabilizer-free polymerization system.

Erratum: Dynamic Stratification in Drying Films of Colloidal Mixtures [Phys. Rev. Lett. 116, 118301 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.118301.

Publisher's Note: Dynamic Stratification in Drying Films of Colloidal Mixtures [Phys. Rev. Lett. 116, 118301 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.118301.

Dynamic Stratification in Drying Films of Colloidal Mixtures.

In simulations and experiments, we study the drying of films containing mixtures of large and small colloidal particles in water. During drying, the mixture stratifies into a layer of the larger particles at the bottom with a layer of the smaller particles on top. We developed a model to show that a gradient in osmotic pressure, which develops dynamically during drying, is responsible for the segregation mechanism behind stratification.

Temperature Response of Rhodamine B-Doped Latex Particles. From Solution to Single Particles.

Nanoparticle-based temperature imaging is an emerging field of advanced applications. Herein, the sensitivity of the fluorescence of rhodamine B-doped latex nanoparticles toward temperature is described. Submicrometer size latex particles were prepared by a surfactant-free emulsion polymerization method that allowed a simple and inexpensive way to incorporate rhodamine B into the nanoparticles. Also, rhodamine B-coated latex nanoparticles dispersed in water were prepared in order to address the effect of the dye location in the nanoparticles on their temperature dependence. A better linearity of the temperature dependence emission of the rhodamine B-embedded latex particles, as compared to that of free rhodamine B dyes or rhodamine B-coated latex particles, is observed. Temperature-dependent fluorescence measurements by fluorescent confocal microscopy on individual rhodamine B-embedded latex particles were found similar to those obtained for fluorescent latex nanoparticles in solution, indicating that these nanoparticles could be good candidates to probe thermal processes as nanothermometers.

Multipod-like silica/polystyrene clusters.

Multipod-like clusters composed of a silica core and PS satellites are prepared according to a seeded-growth emulsion polymerization of styrene in the presence of size-monodisperse silica particles previously surface-modified with methacryloxymethyltriethoxysilane. Tuning the diameter and concentration of the silica seeds affords homogeneous batches of tetrapods, hexapods, octopods, nonapods and dodecapods with morphology yields as high as 80%. Three-dimensional reconstructions by cryo-electron tomography are presented on large fields for the first time to show the high symmetry and regularity of the clusters demonstrating the good control of the synthesis process. These synthesis experiments are visited again digitally, in order to successfully refine an original simulation model and better understand the correlation between the history of the cluster growth and the final composition of the cluster mixture. Finally, using the model as a predictive tool and varying the extra experimental conditions, e.g. the composition of the surfactant mixture and the styrene concentration, result in trapping other cluster morphologies, such as tripods.

Intercalation and structural aspects of macroRAFT agents into MgAl layered double hydroxides.

Increasing attention has been devoted to the design of layered double hydroxide (LDH)-based hybrid materials. In this work, we demonstrate the intercalation by anion exchange process of poly(acrylic acid) (PAA) and three different hydrophilic random copolymers of acrylic acid (AA) and n-butyl acrylate (BA) with molar masses ranging from 2000 to 4200 g mol-1 synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, into LDH containing magnesium(II) and aluminium(III) intralayer cations and nitrates as counterions (MgAl-NO3 LDH). At basic pH, the copolymer chains (macroRAFT agents) carry negative charges which allowed the establishment of electrostatic interactions with the LDH interlayer and their intercalation. The resulting hybrid macroRAFT/LDH materials displayed an expanded interlamellar domain compared to pristine MgAl-NO3 LDH from 1.36 nm to 2.33 nm. Depending on the nature of the units involved into the macroRAFT copolymer (only AA or AA and BA), the intercalation led to monolayer or bilayer arrangements within the interlayer space. The macroRAFT intercalation and the molecular structure of the hybrid phases were further characterized by Fourier transform infrared (FTIR) and solid-state 13C, 1H and 27Al nuclear magnetic resonance (NMR) spectroscopies to get a better description of the local structure.