Highly filled polystyrene–laponite nanocomposites prepared by emulsion polymerization

The Impact of Polyphosphates on the Colloidal Stability of Laponite Particles

The effect of polyphosphate (polyP) adsorption on the colloidal properties of disc-shaped laponite (LRD) particles was examined in aqueous dispersions with a focus on elucidating the interparticle forces that govern the colloidal stability of the systems. The charge and aggregation rate data of bare LRD exhibited an ionic strength-dependent trend, confirming the presence of double-layer repulsion and van der Waals attraction as major surface interactions. The charge of LRD particles significantly increased in magnitude at elevated polyP concentrations as a result of polyP adsorption and subsequent overcharging of the positively charged sites on the edges of the LRD discs. A transition from stable to unstable LRD colloids was observed with increasing polyP doses indicating the formation of aggregates in the latter systems due to depletion forces and/or bridging interactions induced by dissolved or adsorbed polyP, respectively. The degree of phosphate polymerization influenced neither the charge nor the aggregation mechanism. The findings clearly confirm that polyP adsorption was the driving phenomenon to induce particle aggregation in contrast to other clay types, where phosphate derivatives act as dispersion stabilizing agents. This study provides valuable insights into the early stages of aggregation in colloidal systems involving LRD and polyPs, which have a crucial role in predicting further material properties that are important to designing LRD-polyP composites for applications such as potential phosphate sources in chemical fertilizers.

Surfactant-free synthesis of layered double hydroxide-armored latex particles

MgAl-layered double hydroxide (LDH)-armored latexes were produced by Pickering emulsion polymerization of styrene using 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) as auxiliary comonomers.

Tailored microstructure and mechanical properties of nanocomposite films made from polyacrylic/LDH hybrid latexes synthesized by RAFT-mediated emulsion polymerization

New layered double hydroxide (LDH)-filled nanocomposites with tunable microstructures and mechanical properties obtained from film-forming latexes.

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.

Epoxy Resin Catalyzed by Graphite-Based Nanofillers

Graphene stacks/epoxy nanocomposites were produced and characterized in order to analyse the effect of different graphene precursors on cure reaction of a model epoxy matrix. A kinetic analysis of the cure mechanism of the epoxy resin associated to the catalytical activity of the graphite based fillers was performed by isothermal DSC measurements. The DSC results showed that the addition of all graphite based fillers greatly increased the enthalpy of epoxy reaction and the reaction rate, confirming the presence of a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine). A kinetic modelling analysis, arising from an autocatalyzed reaction mechanism, was finally applied to isothermal DSC data, in order to predict the cure mechanism of the epoxy resin in presence of the graphite based nanofiller.

Partitioning of Laponite Clay Platelets in Pickering Emulsion Polymerization

Partitioning of laponite disklike clay platelets between polymer particles and bulk aqueous phase was investigated in Pickering surfactant-free emulsion polymerization of styrene. Adsorption of laponite clay platelets plays an important role in the stabilization of this system, influencing the particle size and the number of particles, and, hence, the reaction rate. Adsorption isotherms show that, while the laponite clay platelets are almost fully exfoliated in water, they form multilayers on the surface of the polymer particles by the end of polymerization, as confirmed by transmission electron microscopy (TEM). This observation is supported by quartz crystal microbalance, conductivity, and TEM measurements, which reveal interactions between the clay and polystyrene, as a function of the ionic strength. The strong adsorption of clay platelets leaves a low residual concentration in the aqueous phase that cannot cause further nucleation of polymer particles, as demonstrated during seeded emulsion polymerization experiments in the presence of a high excess of clay. A Brunauer-Emmett-Teller (BET)-type model for laponite adsorption on polystyrene particles matches the adsorption isotherms.

Encapsulation of clay within polymer particles in a high-solids content aqueous dispersion

By using a two-step polymerization process, it was possible to encapsulate clay platelets within polymer particles dispersed in water. First, seed polymer particles with chemically bonded clay were obtained by batch miniemulsion polymerization. Then, the clay was buried within the particles by the addition of neat monomer in a second step. The final stable dispersions can have a solids content of up to 50 wt %. Transmission electron microscopy images clearly show the presence of clay platelets inside the polymer colloids, although they are not totally exfoliated. The obtained nanocomposites showed an increase in both the storage modulus in the rubbery state and the water resistance as the clay content increases. The approach presented here might be useful for encapsulating other high-aspect ratio nanofillers.

Encapsulation of clay by ad-miniemulsion polymerization: the influence of clay size and modifier reactivity on latex morphology and physical properties

The influence of clay platelet size and type of organic modifier (reactive or nonreactive) on highly filled hybrid latex morphology and physical properties of the resultant polymer/clay nanocomposites (PCNs) were investigated. The hybrid latexes, containing clay loadings between 30 and 50 wt % clay, were prepared using ad-miniemulsion polymerization. These materials have potential use in the packaging and coating industry since clay platelets are well-known for barrier property improvements. Comparative studies on the use of montmorillonite (MMT), a large clay platelet (average size: 50-500 nm), and Laponite (Lap), small-sized clay platelets (average size: 25-40 nm), were conducted. Two different clay modifiers were used to modify the clays, i.e., a conventional nonreactive modifier (cetyltrimethylammonium bromide (CTAB)) and a reactive modifier (vinylbenzyldodecyldimethylammonium chloride (VBDAC)). Transmission electron microscopy (TEM) imaging of the hybrid latexes clearly showed strong morphological dependency on both the type of modifier and the clay platelet size. Furthermore, TEM together with small-angle X-ray scattering (SAXS) showed that the extent of clay exfoliation was strongly dependent on the reactivity of the clay modifier, irrespective of the clay platelet size. Both the type of modifier and clay platelets size were found to have an influence on different physical properties of the resultant PCNs. The influence of clay size was clearly indicated by storage modulus and thermal stability behaviors, while that of the clay modifier was indicated by the T(g). Lap-based PCNs exhibited constant or increasing storage modulus and no change in thermal stability with increasing clay content, while MMT-based PCNs showed a decreasing trend in both storage modulus and thermal stability. PCNs based on clay modified with CTAB showed a decreasing T(g) with increasing clay content, while those based on clay modified with VBDAC showed an increasing trend. It was concluded that the clay platelet size and the type of modifier play a crucial part of both the latex morphology and the physical properties of the resultant PCNs.

Preparation and Properties of PTFE-PMMA Core-Shell Nanoparticles and Nanocomposites

The preparation of polytetrafluoroethylene-poly(methyl methacrylate) (PTFE-PMMA) core-shell particles was described, featuring controlled size and narrow size distribution over a wide compositional range, through a seeded emulsion polymerization starting from a PTFE seed of 26 nanometers. Over the entire MMA/PTFE range, the particle size increases as the MMA/PTFE ratio increases. A very precise control over the particle size can be exerted by properly adjusting the ratio between the monomer and the PTFE seed. Particles in the 80–240 nm range can be prepared with uniformity indexes suited to build 2D and 3D colloidal crystals. These core-shell particles were employed to prepare nanocomposites with different compositions, through an annealing procedure at a temperature higher than the glass transition temperature of the shell forming polymer. A perfect dispersion of the PTFE particles within the PMMA matrix was obtained and optically transparent nanocomposites were prepared containing a very high PTFE amount.

Polymer Nanocomposites in Emulsion and Suspension: an Overview

Polymer nanocomposites have been a subject of intense research in the recent yeas. By nanoscale dispersion of inorganic fillers in the polymer matrices, significant enhancements in the properties of the materials have been achieved at very low filler volume fractions. Different modes of nanocomposite synthesis have been developed in the recent years which include template synthesis, in-situ polymerization, melt intercalation and polymer or prepolymer adsorption from solution. The last methodology also covers emulsion and suspension polymerization techniques for the synthesis of nanocomposites. These emulsion and suspension modes of polymer nanocomposite synthesis have the advantage that the polymerization is carried out in the presence of water which does not allow buildup of viscosity and the heat dissipation from the system is also easily achieved. The potential thermal damage to the polymer and the organic modification usually encountered in the melt intercalation is also avoided in the case of emulsion and suspension polymerization. Different polymer systems have been reported like polystyrene, polyurethanes, epoxy, poly(methyl methacrylate), poly(N-isopropylacrylamide), poly(butyl acrylate) etc. Specific synthetic methodologies like surfactant free polymerization, controlled living polymerization etc. have also been reported to successfully achieve nanocomposites with superior properties than the pure polymers. Majority of the studies bring home the conclusion that the amount of clay as well as surface modification present on clay surface significantly affect the microstructure and properties of the nanocomposite particles. Apart from clay as filler, many studies also have used the spherical inorganic particles as reinforcements.

Insight into the Broad Field of Polymer Nanocomposites: From Carbon Nanotubes to Clay Nanoplatelets, via Metal Nanoparticles

Highly ordered polymer nanocomposites are complex materials that display a rich morphological behavior owing to variations in composition, structure, and properties on a nanometer length scale. Metal-polymer nanocomposite materials are becoming more popular for applications requiring low cost, high metal surface areas. Catalytic systems seem to be the most prevalent application for a wide range of metals used in polymer nanocomposites, particularly for metals like Pt, Ni, Co, and Au, with known catalytic activities. On the other hand, among the most frequently utilized techniques to prepare polymer/CNT and/or polymer/clay nanocomposites are approaches like melt mixing, solution casting, electrospinning and solid-state shear pulverization. Additionally, some of the current and potential applications of polymer/CNT and/or polymer/clay nanocomposites include photovoltaic devices, optical switches, electromagnetic interference (EMI) shielding, aerospace and automotive materials, packaging, adhesives and coatings. This extensive review covers a broad range of articles, typically from high impact-factor journals, on most of the polymer-nanocomposites known to date: polymer/carbon nanotubes, polymer/metal nanospheres, and polymer/clay nanoplatelets composites. The various types of nanocomposites are described form the preparation stages to performance and applications. Comparisons of the various types of nanocomposites are conducted and conclusions are formulated.

Novel polymeric nanocomposites and porous materials prepared using organogels

We propose a new method for preparing polymeric nanocomposites and porous materials using self-assembled templates formed by 1,3:2,4-dibenzylidene sorbitol (DBS) organogels. DBS is capable of self-assembling into a 3D nanofibrillar network at relatively low concentrations in some organic solvents to produce organogels. In this study, we induced the formation of such physical cross-linked networks in styrene. Subsequently, we polymerized the styrene in the presence of chemical cross-linkers, divinyl benzene (DVB), with different amounts of DBS using thermal-initiated polymerization. The resulting materials were transparent, homogeneous polystyrene (PS) nanocomposites with both physical and chemical cross-links. The porous polymeric materials were obtained by solvent extraction of the DBS nanofibrils from the PS. Brunauer-Emmett-Teller (BET) measurements show that the amounts of DBS and DVB influenced the specific surface area after the removal of the DBS fibrils.