The role of solid particles in emulsion polymerization – synthesis and kinetic studies

Control of particle morphology and chemical functionality in polymer dispersions have been of growing interest in the scientific community for the past decades. The possibility of regulating asymmetry, in both shape and chemical composition, has been sought as a way of creating complex advanced materials. In these materials the mechanical and physico-chemical properties of different phases are combined, in a synergistic way, in an attempt of mimicking nature’s behaviour. This thesis particularly deals with nanocomposite materials: materials where at least one of the different phases has two or three dimensions of less than 100 nanometres. Among the plethora of synthetic pathways developed for the controlled synthesis of nanocomposite colloids, this work focuses on a process called Pickering emulsion polymerization; a seeded emulsion polymerization reaction where a polymer phase is formed in-situ in the presence of a stabilizing nano-sized colloid formed ex-situ. The product of the reaction typically is that of a polymeric particle surrounded by a dense shell of stabilizing agent. The main advantages are the ease of operation, absence of high shear homogenization steps and of molecular surfactants. The latter is of key importance for instance in coating applications where surfactant migration during and after film formation can be detrimental for the final film properties. In this work, initially Pickering emulsion polymerization is thoroughly explored from a kinetic and mechanistic viewpoint using a model system consisting of SiO2 nanoparticles and styrene or methyl methacrylate as monomers. These relatively well-known systems are used to draw more conclusive theories on the mechanism governing particle formation and specifically the mode of stabilizer adsorption at the polymer interface. Once assessed the main processes influencing the fate of the reaction, a first step towards the implementation of added complexity in the system is taken by moving towards polymeric block copolymer stabilizers, where the chemical composition can be tailored by changing the type of monomer used. Both dispersion and emulsion polymerization approaches are discussed, with a particular focus on the development of protocols which do not contain added coloration or malodorous compounds. This increases the complexity of the system as the adopted chain-transfer agents require to operate in monomer starve fed conditions in order to allow control over chain-growth. This was found not to be compatible with dispersion polymerization, or polymerization induced selfassembly, reaction conditions. Nevertheless, a solution to the problem is proposed which yielded polymer self-assemblies of various morphologies. Finally, nanometric polymeric stabilizers (i.e. crosslinked block copolymer micelles, or nanogels) produced by the more successful emulsion polymerization approaches are adopted in Pickering emulsion polymerization reactions as sole stabilizers. The controlled destabilization of the nanogels by pH adjustment and background electrolyte addition led to polymer colloids of Janus, patchy or armoured morphology. Such particles are characterized by a given number of surface protrusions, with the same chemical composition as the nanogels adopted in the protocol

A mechanistic investigation of Pickering emulsion polymerization

Pickering emulsion polymerization offers a versatile way of synthetising hybrid core–shell latexes where a polymer core is surrounded by an armour of inorganic nanoparticles. A mechanistic understanding of the polymerization process is limited which restricts the use of the technique in the fabrication of more complex, multilayered colloids. In this paper clarity is provided through an in-depth investigation into the Pickering emulsion polymerization of methyl methacrylate (MMA) in the presence of nano-sized colloidal silica (Ludox TM-40). Mechanistic insights are discussed by studying both the adsorption of the stabiliser to the surface of the latex particles and polymerization kinetics. The adhesion of the Pickering nanoparticles was found not to be spontaneous, as confirmed by cryo-TEM analysis of MMA droplets in water and monomer-swollen PMMA latexes. This supports the theory that the inorganic particles are driven towards the interface as a result of a heterocoagulation event in the water phase with a growing oligoradical. The emulsion polymerizations were monitored by reaction calorimetry in order to establish accurate values for monomer conversion and the overall rate of polymerizations (Rp). Rp increased for higher initial silica concentrations and the polymerizations were found to follow pseudo-bulk kinetics

Toward sulfur-free RAFT polymerization induced self-assembly

Polymerization induced self-assembly (PISA) using methacrylate-based macromonomers as RAFT agents is an unexplored, attractive route to make self-assembled colloidal objects. The use of this class of RAFT-agents in heterogeneous polymerizations is however not trivial, because of their inherent low reactivity. In this work we demonstrate that two obstacles need to be overcome, one being control of chain-growth (propagation), the other monomer partitioning. Batch dispersion polymerizations of hydroxypropyl methacrylate in the presence of poly(glycerol methacrylate) macromonomers in water showed limited control of chain-growth. Semicontinuous experiments whereby monomer was fed improved results only to some extent. Control of propagation is essential for PISA to allow for dynamic rearrangement of colloidal structures. We tackled the problem of monomer partitioning (caused by uncontrolled particle nucleation) by starting the polymerization with an amphiphilic thermoresponsive diblock copolymer, already “phase-separated” from solution. TEM analysis showed that PISA was successful and that different particle morphologies were obtained throughout the polymerization

Effect of the addition of salt to Pickering emulsion polymerizations using polymeric nanogels as stabilizers

Nanogels made from crosslinked block copolymer micelles are used as stabilizers in the Pickering emulsion polymerization of styrene. The effect of the addition of salt, i.e. NaCl, on the emulsion polymerization is studied. It is shown that an increase in ionic strength of the dispersing medium in these polymerizations led to the formation of latexes of larger diameters. Along with an increase in size, the morphology of these polymer colloids changed from Janus to patchy with an increase in number of nanogels adsorbed on the polymer surface, as a function of the salt concentration in water. In particular, at the highest tested ionic strength, ca. 25 mM, fully armored polymeric particles surrounded by a dense layer of adsorbed stabilizing nanogels were formed. Kinetic studies carried out at varying NaCl concentrations suggested that particle formation in the reaction followed a combination of a coagulative nucleation mechanism, characterized by a clustering process of Janus precursors to form bigger aggregates, and droplet nucleation. Preliminary film formation studies on latexes made with n-butyl acrylate as a comonomer indicated the potential of this technique for the production of coherent polymer films which included a substructure of functional nanogels

Dwellings, jabuticabas, and affections — trajectories with Sylvia Caiuby Novaes

Sylvia Caiuby Novaes é professora do Departamento de Antropologia da Universidade de São Paulo (USP) e dedica-se há cerca de 50 anos à pesquisa e ao ensino em antropologia. Entre outras realizações, ela é uma das pioneiras da antropologia visual no Brasil, é fundadora do Laboratório de Imagem e Som da Antropologia (LISA) e editora responsável pela revista Gesto, Imagem e Som. Revista de Antropologia (GIS). Nesta entrevista, realizada por mais de 30 orientandos de diferentes gerações, Sylvia fala sobre sua trajetória, projetos, visão de mundo, suas diversas viagens, o fascínio pelas pesquisas de campo e a universidade. Ao contar sobre sua trajetória acadêmica e pessoal, Sylvia traz reflexões sobre sua relação com a fotografia e a produção de imagens.  Sylvia Caiuby Novaes is a Professor in the Department of Anthropology at the University of São Paulo (USP) and has been dedicated to research and teaching in anthropology for nearly 50 years. Among other accomplishments, she is one of the pioneers of visual anthropology in Brazil, is the founder of the Laboratory of Image and Sound of Anthropology (LISA) and the editor in charge of the Gesture, Image and Sound.  Journal of Anthropology (GIS). In this interview, conducted by more than 30 advisees from different generations, Sylvia talks about her trajectory, projects, worldview, her various travels, her fascination with field research and the university. When telling about her academic and personal trajectory, Sylvia reflects on her relationship with photography and the production of images

Robust open cellular porous polymer monoliths made from cured colloidal gels of latex particles

The coagulation of oppositely charged latexes, prepared from the soap-free emulsion polymerisation of styrene using water as the reaction medium, resulted in the obtainment of colloidal gels that were porous in nature and held together by electrostatic interactions. Chemical crosslinking, involving the introduction of a water-soluble crosslinker, resulted in the obtainment of stronger chemical bonds between particles affording a rigid porous material known as a monolith. It was found that, in a simpler approach, these materials could be prepared using a single latex where the addition of ammonium persulfate both resulted in the formation of the colloidal gel and initiated the crosslinking process. The pore size of the resulting monoliths was predictable as this was observed to directly correlate to the particle diameter, with larger pores achieved using particles of increased size. All gels obtained in this work were highly mouldable and retained their shape, which allowed for a range of formats to be easily prepared without the requirement of a mould

Synthesis of Janus and patchy particles using nanogels as stabilizers in emulsion polymerization

Polymer nanogels are used as colloidal stabilisers in emulsion polymerization. The nanogels were made by the covalent crosslinking of block copolymer micelles, the macromolecular building blocks of which were synthesized using a combination of catalytic chain transfer emulsion polymerization and reversible addition fragmentation chain-transfer (RAFT) of methacrylate monomers. The use of the nanogels in an emulsion polymerization led to anisotropic Janus and patchy colloids, where a latex particle was decorated by a number of patches on its surface. Control on the particle size and patch density was achieved by tailoring of the reaction conditions, such as varying the amount of nanogels, pH and salt concentration. Overall, the emulsion polymerization process in the presence of nanogels as stabilizers is shown to be a versatile and easily scalable route towards the fabrication of Janus and patchy polymer colloids

New developments in PFPE lubrication: application to tinned electrical contacts

International audienceElectrical contacts performances and reliability can be improved by the use of a well-suited lubricant. Much work has been done and is still performed in this field. Different types of lubricants having different compositions and properties have been described. Perfluorinated polyethers (PFPE) are the lubricant class of choice when seeking high thermal and chemical stability, wide operating temperature ranges, and ultimately long lifetime and reliability. It has been shown in previous studies that linear and branched PFPE performed differently according to the contact surface finishes and that depending on the type of wear occurring in the contact different viscosities were beneficial. Indeed a universal lubricant improving the properties of all the various metallic finishes is still something to look forward to. In this paper, we show the first results on the friction and electrical properties of tinned electrical contacts lubricated with a new type of PFPE lubricant, Fomblin® HV25. This new PFPE oil has inherent high-viscosity, ca. 32000cSt (at 20°C), without any thickening agent (e.g. PTFE powder). Reciprocating friction tests were performed with a dedicated instrumented device. Improvements in friction reduction and in electrical behavior as compared PFPE oils with standard viscosities are described

Investigation of an Intensified Thermo-Chemical Experimental Set-Up for Hydrogen Production from Biomass: Gasification Process Integrated to a Portable Purification System—Part II

Biomass gasification is a versatile thermochemical process that can be used for direct energy applications and the production of advanced liquid and gaseous energy carriers. In the present work, the results are presented concerning the H2 production at a high purity grade from biomass feedstocks via steam/oxygen gasification. The data demonstrating such a process chain were collected at an innovative gasification prototype plant coupled to a portable purification system (PPS). The overall integration was designed for gas conditioning and purification to hydrogen. By using almond shells as the biomass feedstock, from a product gas with an average and stable composition of 40%-v H2, 21%-v CO, 35%-v CO2, 2.5%-v CH4, the PPS unit provided a hydrogen stream, with a final concentration of 99.99%-v and a gas yield of 66.4%