Preparation of spherical nanocomposites consisting of silica core and polyacrylate shell by emulsion polymerization

Nanocomposite particles consisting of silica (inorganic core) and polyacrylate (organic shell) were prepared in a form of emulsion by a new and simple method—the emulsion polymerization of acrylic monomers in the presence of silica sol. The key technique of the present emulsion polymerization, which made the formation of the nanocomposites successful, is the usage of nonionic surfactant above its cloud point. The morphology of the composite was investigated by DLS, AFM, and TEM, which clearly showed formation of the core‐shell‐type particles. A transparent film was prepared by casting the emulsion, which showed high resistibility against organic solvents. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 659–669, 2006     

Fabrication and characterization of silver core and porous silica shell nanocomposite particles

The nanocomposite particles of silver core and silica shell have been successfully prepared. Silver colloids are synthesized using polymer polyvinylpyrrolidone (PVP) as protecting agent and the silica shell is then coated by means of the Stöber process to fabricate Ag@SiO₂ core shell particles. The shell thickness can be easily controlled by the amount of tetraethyl orthosilicate (TEOS). TEM results indicate that the silica shell uniformly encapsulates the silver core particles. During the silica growth, PVP molecules will be trapped and dispersed in the silica shell. As a result, porous silica shell structure is obtained after burning off the PVP molecules and a positive correlation exists between the specific surface area of core shell particles and PVP quantity in the original silver colloids. The silica shell, even at a thickness of 25 nm, can maintain the original shape of Ag@SiO₂ particles up to 1000 °C, and prohibit silver from sintering effects.     

STUDIES ON SYNTHESIS OF SILICA SOL-ORGANIC POLYMER COMPOSITE EMULSION

An emulsion of polystyrene/poly (butylacrylate-methyl methacrylate acrylic acid) core/shell latex particles (PS/P (BA-MMA-AA)) has been prepared by use of three synthetic methods. The effects of synthetic methods on the distribution of carboxyl groups in latex particles were studied. The results show that the seed emulsion polymerization in which the pre-emulsified monomers were added by dropping method to the second stage is the best technique for obtaining the optimum distribution of carboxyl groups on the surface of the latex particles. Furthermore, by using PS/P (BA-MMA-AA), a type of novel composite emulsion of silica sol-PS/P (BA-MMA-AA) was synthesized with the above method. By observation through transmission EM, the morphology of the latex particles obtained shows that a composite structure has been formed between silica sol particles and organic polymer particles.     

Study of the preparation of silicone rubber particles with core–shell structure by seeded emulsion polymerization

Silicone rubber particles with core–shell structure were prepared by polymerization of vinyl monomers in the presence of linear or cross-linked poly(dimethyl siloxane–methyl vinyl siloxane) latexes. The monomers were added in either continuous or swelled-continuous modes. Core–shell particles with poly(butyl methacrylate), or poly(methyl methacrylate), as the shell were obtained by using either addition mode. The core–shell structure was not observed for polysiloxane–polystyrene particles. The influence of monomer addition mode, the compatibilities of the monomers and their homopolymers with silicone rubber, and the reactivity ratios of the vinyl monomers with the vinyl group of linear polysiloxane particles, on the formation of the core-shell structure is discussed.     

聚四氟乙烯改性聚丙烯酸酯乳液的合成与聚合稳定性

制备了在同一个乳胶粒中含有聚四氟乙烯和聚(甲基)丙烯酸酯链段的乳液,研究了乳胶粒的微观结构。由不同链段组成的互穿网络结构构成了乳胶粒的核,而壳则更多地由聚甲基丙烯酸酯链段组成。讨论了几种影响聚合反应稳定性的因素,包括乳化剂和引发剂的种类与用量,单体的添加方法与溶胀时间,二级核与初级核的比率,初级核的用量,温度控制等等。     

Fabrication of hollow silica particles using copolymeric spheres prepared in supercritical carbon dioxide

Amine functional polymeric spheres were prepared via the dispersion polymerization of 2-dimethylaminoethyl methacrylate and methylmethacrylate in supercritical carbon dioxide, and they were employed efficiently as templates for the fabrication of hollow silica particles. A small amount of divinyl benzene was used as a crosslinking agent to control the morphology of the copolymeric particles from clumpy solid to spherical shape. The assembly of the polymeric spheres with tetraethylorthosilicate (TEOS) through hydrothermal methods produced core-shell type hybrid particles. The whole process required neither surface treatment for the polymeric particles nor addition of any acidic or basic catalyst for the hydrolysis of silica precursor because dimethylamino groups of the copolymeric spheres were able to absorb water and catalyze the hydrolysis of TEOS for the deposition of the silica gel network. The polymeric cores were completely removed by calcination process and silica hollow particles with well-defined structure were obtained finally. The silica hollow particles were characterized by scanning electron microscopy and transmission electron microscopy, which clearly revealed that the silica spheres had the hollow structure with 151 nm wall thickness.     

Secondary particle formation in suspension polymerization using a particulate surfactant

ABSTRACT

Suspension polymerization is widely used for the preparation of microsphere and microcapsules for many applications. However, the formation of secondary particle byproducts decreases drastically the obtained microsphere yield and microcapsule shell strength. It is surprisingly finding that the secondary particles were not observed in the preparation of polymethyl methacrylate particles by suspension polymerization using particulate surfactant called Pickering emulsion. Therefore, in this work, the mechanism of secondary particle formation during suspension polymerization was investigated using various surfactants (zinc oxide, titanium dioxide, and silica nanoparticles compared to polyvinyl alcohol) and monomers (styrene, methyl methacrylate, and methyl acrylate) with different water solubilities. Results showed that submicrometer-sized secondary particles were still formed by homogeneous nucleation mainly due to radical exit from the monomer droplets. However, the formed secondary particles were unstable and then adsorbed on the main microsphere surface. The number of secondary particles increased when monomers with higher water solubility were used.     

Monodisperse polymer beads as packing material for high-performance liquid chromatography

A novel approach to monodispersed porous polymer beads allowing accurate control over a broad range of pore size distribution has been developed. It involves the use of monodispersed template particles which are used as polymeric porogens in the suspension polymerization of monomers such as styrene and divinylbenzene. The size uniformity of the template particles is retained by the final porous beads. The porous properties of the final beads are determined in large part by the characteristics of the porogenic mixture such as its composition, the molecular weight of the polymeric porogen, as well as the relative amount of monomers, polymeric and low molecular weight porogens used.     

Multiscale modeling of the effects of unlimited four‐membered ring formation on sol‐gel silica film molecular structure

A modified multiscale model of a sol‐gel silica drying process is presented that treats first‐shell substitution effects and unlimited four‐membered ring cyclization. The inclusion of four‐membered (8‐atom) rings allows the model to simulate the formation of the tetrasiloxane rings and cubic silsesquioxane cages known to be prevalent components of sol‐gel silica systems, as well as a full range of other ring‐containing structures. The polymerization process is treated using a dynamic Monte Carlo method where a discrete population of one million monomers evolves according to kinetic rules, including unimolecular‐like closure of three‐bond blocks. Compared with prior simulations with extensive cyclization that allowed only three‐membered rings, the molecular structures formed with unlimited four‐membered ring are more complex, and the occurrence of “skinning” (the rapid formation of a gel only at the surface of the film) is more pronounced and leads to more severe structure gradients. © 2012 American Institute of Chemical Engineers AIChE J, 59: 707–718, 2013     

Fabrication of anisotropic janus composite particles based on natural renewable urushiol

Urushiol praseodymium‐polystyrene (UPr‐PS) Janus composite particles were synthesized by emulsion swelling assisted protruding from UPr/PS core‐shell composite microspheres. The effects of sulfonated time and the outer electronic configuration of metal on the shell of urushiol metal/PS composite microspheres, together with the effects of monomer type, monomer dosage and swelling time on the swelling behaviour, were studied. The morphology, chemical composition and thermal performance of UPr/PS core‐shell composite microspheres and UPr‐PS Janus composite particles were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X‐ray analyzer (EDX), Thermogravimetric analysis (TG) and infrared spectrometer (IR). Results showed that the morphology could be controlled by the metal kind, the sulfonated time, the monomer type, the monomer dosage and the swelling time. Attributing to the novel anisotropic structure, the UPr‐PS Janus composite particles will be expected to be applied in oriented catalyst. POLYM. ENG. SCI., 59:411–417, 2019. © 2018 Society of Plastics Engineers     

One-stage method of preparing core–shell particles

A one-stage method of preparing code–shell particles was developed for the system containing silicone oils and glycidle methacrylate (GMA). Although the formation of core–shell particles for the systems containing silicone oils and methyl methacrylate (MMA) or styrene (ST) is possible in view of thermodynamics, the core–shell particles were not obtained. Factors such as better compatibility of silicone oils with vinyl monomers, higher swelling degree of silicone rubber in the vinyl monomers, and larger addition rate of thevinyl monomers with Si—H during the crosslinking of silicone oil containing vinyl group and Si—H (SVB and SHB) do not favor the formation of core–shell particles. X-ray photoelectron spectrometer (ESCA) was used in determining the formation of core–shell particles. The mechanism of the formation of core–shell particles is discussed. © 1995 John Wiley & Sons, Inc.     

Use of polyethyleneimine dendrimer as a novel graded-modulus interphase material in polymeric composites

The effectiveness of polyethyleneimine (PEi) dendrimer as a novel graded-modulus interphase material in polymeric composites is discussed in the context of core (polystyrene)–shell (PEi) nanoparticles affecting the mechanical properties of epoxy. The dendrimer is grafted onto the surface of polystyrene (PS) particles via a free radical polymerization reaction of styrene monomers in a non-aqueous polar solvent with t-butyl hydroperoxide (TBHP) as initiator and mild heating. The effects of both particle loading and core/shell composition are investigated. The mechanical test results in all cases show an increase in both stiffness and fracture toughness or the ability of the polymer to resist crack growth, as opposed to the commonly seen trade-off between these properties in previously studied soft particles. SEM micrographs suggest that the crosslinks with epoxy in the dendrimer network, leading to a dramatic interface stretching as the core-to-shell ratio decreases, and the capability of the dendrimer to 'harden' PS particles by diverting cracks through them are responsible for the enhancements.     

Synthesis of polymeric and hybrid nanoparticles for electroplating applications

Monodisperse polymeric particles with diameters in the range of 60-1400 nm were prepared by (emulsifier-free) emulsion polymerization and incorporated into electrolytic zinc coatings aiming to improve the corrosion resistance of electrogalvanized steel. Various types of polymeric nanoparticles were thus synthesized in order to assess the effect of the emulsifier, initiator and comonomer type on the particle morphology, stability and codeposition behavior. The polymerization experiments were carried out in laboratory-scale glass reactors and the most promising recipes were successfully scaled-up in a fully automated pilot-scale reactor. Replicates of some representative experiments, which were run both in lab and pilot-scale reactors, indicated excellent reproducibility of the polymerization process. Uniform, polymer-containing zinc coatings were produced by electrolytic codeposition of the nanoparticles from an acid zinc plating bath using a rotating disk electrode (RDE). Hybrid polystyrene/silica nanoparticles with increased silica content were also prepared via emulsifier-free emulsion polymerization, in the presence of an ultrafine aqueous silica sol, to be used in electrocoating applications. The effect of key process parameters, such as initial monomers molar ratio and pH on the size, morphology and silica content of the produced hybrid nanoparticles was investigated.     

Functionalized polystyrene latex particles as substrates for ATRP: Surface and colloidal characterization

In this work, the process for producing polystyrene particles surface functionalized with a thin shell of ATRP initiator polymerized alone or along with styrene and a crosslinker, is presented. Copolymerization of styrene and acrylic end-capped ATRP initiator to generate a thin shell on the fully polymerized core particles suffered from secondary nucleation owing to their possible incompatibility with the core particles and their own colloidal stability. One step functionalization processes, where the shell forming monomers are added directly to the 70% polymerized core particles, lead to significant changes in the resulting particle morphologies. The shot addition of these monomers led to a very uniform surface morphology without any secondary nucleation owing to quick coalescence of the secondary particles on the soft surface of the seed particles. Addition of crosslinker to the system helped in effectively eliminating the smaller particles generated due to secondary nucleation along with the chemical networking.     

In-Situ Measurement of Silica-Gel Coating on Particles of Alumina

Time-dependent growth of a silica layer on alumina particles suspended in a polymeric silica solution has been measured by monitoring the changes in the zeta potential of the alumina particles. There is an incubation period in the process, suggesting that the silica film forms by a nucleation-and-growth mechanism. The coated alumina particles are found to flocculate as their zeta potential approaches that of silica near its isoelectric point.     

Core-shell-structured silica/polyacrylate particles prepared by Pickering emulsion: influence of the nucleation model on particle interfacial organization and emulsion stability

This work reports a new evidence of the versatility of silica sol as a stabilizer for Pickering emulsions. The organization of silica particles at the oil-water interface is a function of the nucleation model. The present results show that nucleation model, together with monomer hydrophobicity, can be used as a trigger to modify the packing density of silica particles at the oil-water interface: Less hydrophobic methylmethacrylate, more wettable with silica particles, favors the formation of core-shell-structured composite when the composite particles are prepared by miniemulsion polymerization in which monomers are fed in batch (droplet nucleation). By contrast, hydrophobic butylacrylate promotes the encapsulating efficiency of silica when monomers are fed dropwise (homogeneous nucleation). The morphologies of polyacrylate-nano-SiO₂ composites prepared from different feed ratio of methylmethacrylate/butylacrylate (with different hydrophobicity) and by different feed processes are characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. The results from SEM and TEM show that the morphologies of the as-prepared polyacrylate/nano-SiO₂ composite can be a core-shell structure or a bare acrylic sphere. The stability of resulting emulsions composed of these composite particles is strongly dependent on the surface coverage of silica particles. The emulsion stability is improved by densely silica-packed composite particles.     

Formation mechanism of silicone rubber particles with core–shell structure by seeded emulsion polymerization

Silicone rubber particles with core–shell structure were prepared by polymerization of vinyl monomers in the presence of crosslinked and linear poly(dimethyl siloxane-methyl vinyl siloxane) latex. The monomers were added with either continuous or swelled-continuous addition mode. The core–shell morphology of silicone rubber/polystyrene [PST] and silicone rubber/poly(methyl methacrylate-divinyl benzene) [P(MMA-DVB)] composite particles were obtained. The effects of monomer addition mode, the compatibilities of the monomers or their homopolymer with silicone rubber, and the reactivity of polysiloxane with vinyl monomers on the formation of the core–shell structure were discussed. © 1996 John Wiley & Sons, Inc.     

Untreated Silica Nanoparticles Containing Poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid) Composite—Effect of Copolymer Composition

Poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid)/Silica [Poly(AM-co-AMPSA)/SiO₂] core-shell composite nanogels with silica as core and hydrophilic polymer with functional groups such as sulfonic acid, amido, as shell were prepared by microemulsion polymerization without surface treatment of silica and upto 5.5 mole% of AMPSA in feed to control the particle size as well as morphology. The synthesized intercalated composites were produced with controllable sizes ranging from 44–77 nm in diameter. The repared sulfonic acid based silica core-shell composite nanogels were characterized by dynamic light scattering, transmission electron microscope, Fourier transform infrared spectrophotometer, thermogravimetric analyzer, differential scanning calorimeter, scanning electron microscope and X-ray diffraction. The encapsulation of functional polymer shell onto the silica particles was driven by the hydrogen-bonding interaction between sulfonic acid and amido groups of AMPSA with Si-O linkages. The onset degradation temperature is increased from 227°C to 262°C in copolymer-silica composites which indicates improved thermal stability. The shifting of glass transition temperature from 194°C to 203°C in copolymeric composite nanogels further confirms the existence of strong interactions of silica filler with copolymer chains. Also the chemical composition of polymeric chains and the affinity of polymer chains and silica influenced the morphology.     

A simple route for preparing Au/mesoporous silica yolk/shell particles for Au-catalyzed reactions

We present a simple route to prepare mesoporous hollow silica particles containing an Au core, i.e., yolk/shell particles, by sol-gel and selective etching processes. Using tetraethoxysilane as a silica precursor, zinc acetate as a base catalyst, and cetyltrimethylammonium chloride as a soft template in the presence of Au nanoparticles, double-layered mesoporous shells were produced in one step. Elemental analysis showed that the inner shell consists of zinc silicate and the outer shell is pure silica. Au/mesoporous silica yolk/shell nanoparticles were obtained by selective etching of the zinc oxide phase with citrate buffer. The particle structure and composition were studied by transmission electron microscopy with energy disperse spectroscopy, UV-vis spectroscopy, X-ray diffraction, and nitrogen sorption experiments. Formation of double shells on the Au core in a single step was explained by a difference in the formation rates of the silica and zinc silicate phases. Au/mesoporous yolk/shell particles showed a high catalytic activity for reduction of 4-nitrophenol.     

Synthesis and characterization of poly(butyl acrylate)/silica and poly(butyl acrylate)/silica/poly(methyl methacrylate) composite particles

Poly(butyl acrylate)/poly(methyl methacrylate) (PBA/PMMA) core–shell particles embedded with nanometer‐sized silica particles were prepared by emulsion polymerization of butyl acrylate (BA) in the presence of silica particles preabsorbed with 2,2′‐azobis(2‐amidinopropane)dihydrochloride (AIBA) initiator and subsequent MMA emulsion polymerization in the presence of PBA/silica composite particles. The morphologies of the resulting PBA/silica and PBA/silica/PMMA composite particles were characterized, which showed that AIBA could be absorbed effectively onto silica particles when the pH of the dispersion medium was greater than the isoelectric potential point of silica. The critical amount of AIBA added to have stable dispersion of silica particles increased as the pH of the dispersion medium increased. PBA/silica composite particles prepared by in situ emulsion polymerization using silica preabsorbed with AIBA showed higher silica absorption efficiency than did the PBA/silica composite particles prepared by direct mixing of PBA latex and silica dispersion or by emulsion polymerization in which AIBA was added after the mixing of BA and silica. The PBA/silica composite particles exhibited a raspberrylike morphology, with silica particles “adhered” to the surfaces of the PBA particles, whereas the PBA/silica/PMMA composite latex particles exhibited a sandwich morphology, with silica particles mainly at the interface between the PBA core and the PMMA shell. Subsequently, the PBA/silica/PMMA composite latex obtained had a narrow particle size distribution and good dispersion stability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3425–3432, 2006