Miniemulsion polymerization

Synthesis of narrowly size-distributed metal salt/poly(HEMA) hybrid particles in inverse miniemulsion: versatility and mechanism

Hybrid particles containing different hydrophilic metal salts such as tetrafluoroborates of iron(II), cobalt(II), nickel(II), copper(II), and zinc(II), and nitrates of cobalt(II), nickel(II), copper(II), zinc(II), and iron(III), and cobalt(II) chloride were synthesized via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). All salts delivered narrowly size-distributed hybrid particles with the exception of iron(III), where only the nitrate salt could be successfully employed. The size and size distribution of the hybrid particles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The particle morphology and the distribution of salt in the dried particles were observed by TEM. The influences of the type of metal salts and salt content on the particle size distribution were extensively investigated.

Nano-emulsions and micro-emulsions: clarifications of the critical differences

Much research has been done over the past years on self-emulsifying drug delivery systems, their main interest being the simplicity of the formulation processes, the great stability of the systems and their high potential in pharmaceutical applications and industrial scaling-up. Self-emulsifying drug delivery systems are generally described in the literature indiscriminately as either nano-emulsions or micro-emulsions. Although this misconception appears to be common, these two systems are fundamentally different, based on very different physical and physicochemical concepts. Their differences result in very different stability behaviors, which can have significant consequences regarding their applications and administration as nanomedicines. This paper aims at clarifying the problem, first by reviewing all the physical and physicochemical fundamentals regarding these two systems, using a quantitative thermodynamic approach for micro-emulsions. Following these clarifications, we show how the confusion between nano-emulsions and micro-emulsions appears in the literature and how most of the micro-emulsion systems referred to are actually nano-emulsion systems. Finally, we illustrate how to clear up this misconception using simple experiments. Since this confusion is well established in the literature, such clarifications seem necessary in order to improve the understanding of research in this important field.

High solid content latex: preparation methods and application

One of the major challenges in emulsion polymerization over the past two decades was how to increase the solid content of latex products. In contrast to the conventional latex, high solid content (HSC) latex has a large volume fraction of dispersed phase, even larger than 70% in weight. Conventional emulsion polymerization, miniemulsion polymerization, self-emulsification polymerization and concentrated emulsion polymerization were all used to prepare HSC latexes, and many good results have been reported in recent years. Meanwhile, many applications of HSC latexes have also been developed. The present review summarized the progresses in the past few years mainly on the preparation methods and application of HSC latexes. Finally, some research directions as well as prospects were also proposed.

Stress fluctuations in drying polymer dispersions

Drying polymer dispersions usually experience tensile stress, induced by the reduction in volume and by the rigid substrate. Due to edge-in drying, the stress is usually heterogeneous over the film. Stress peaks play a decisive role in the formation of cracks. This work relies on membrane bending, a technique that provides spatially resolved stress maps. In the experiments reported here, stress fluctuations on the order of 10% on the time scale of a few seconds were found. The stress fluctuations occur coherently over the entire drying front. Fluctuations go back to slight fluctuations in humidity of the environment (as opposed to local stress relaxations due to reorganizations of the particle network). The stress fluctuations disappear when covering the sample with a lid. They can be enhanced by blowing humid or dry air across the sample surface. Modeling builds on the assumption that all stresses go back to capillary pressure created at the menisci in between different spheres at the film-air interface. The local radius of curvature changes in response to slight variations in ambient humidity according to the Kelvin equation. The fluctuations are observed under a wide variety of drying conditions and should be included in film formation models.

Narrowly size-distributed cobalt salt containing poly(2-hydroxyethyl methacrylate) particles by inverse miniemulsion

Cobalt-containing hybrid particles have been prepared through the encapsulation of cobalt tetrafluoroborate hexahydrate (CoTFB) via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). We systematically varied the amount and type of cosolvent (water, methanol, ethanol, ethylene glycol), apolar continuous phase (cyclohexane, isooctane, isopar M, hexadecane), amount of cobalt salt, and molecular weight of the polymeric surfactant. The influence of those parameters on the particle size, size distribution, and particle morphology were investigated. Narrowly size-distributed hybrid particles with good colloidal stability could be obtained in a wide range of cobalt content between 5.7 and 22.6 wt % salt relative to the monomer. The addition of a cosolvent such as water not only promotes the loading of metal salt but also has a positive influence on narrowing the particle size distribution. We assume that generally narrowly size-distributed particles can be obtained for a large variety of combinations of polar/apolar phase by adjusting the balance between osmotic and Laplace pressure via the solubility of the metal salt in the continuous phase and lowering the interfacial tension by adjusting the hydrophilic-lipophilic balance (HLB) value of the surfactant. The results show a significant advantage of the inverse miniemulsion over the direct system with respect to the variability and total amount of metal salt without losing the narrow particle size distribution and colloidal stability.

Tailored Composite Polymer-Metal Nanoparticles by Miniemulsion Polymerization and Thiol-ene Functionalization

A simple and modular synthetic approach, based on miniemulsion polymerization, has been developed for the fabrication of composite polymer-metal nanoparticle materials. The procedure produces well-defined composite structures consisting of gold, silver or MnFe(2)O(4) nanoparticles (∼10 nm in diameter) encapsulated within larger spherical nanoparticles of poly(divinylbenzene) (∼100 nm in diameter). This methodology readily permits the incorporation of multiple metal domains into a single polymeric particle, while still preserving the useful optical and magnetic properties of the metal nanoparticles. The morphology of the composite particles is retained upon increasing the inorganic content, and also upon redispersion in organic solvents. Finally, the ability to tailor the surface chemistry of the composite nanoparticles and incorporate steric stabilizing groups using simple thiol-ene chemistry is demonstrated.

Dilational lateral stress in drying latex films

Drying latex films usually experience tensile stress due to the reduction in volume. While an unconstrained film would shrink affinely in all three dimensions, a coating can only shrink along the vertical and therefore exerts tensile stress onto the substrate. Using an instrument capable of producing maps of the stress distribution, we found that dilational stress sometimes develops as well. The in-plane stress was monitored by spreading the latex dispersion on a flexible membrane. Usually, the membrane bends upward under the tensile stress exerted by the film, but it may also bend downward. Dilational stress was only found with samples showing a strong coffee stain effect, that is, samples in which there is a significant lateral flow from the center to the edge while the film dries. During drying, particles consolidate first at the edge because of the lower height in this region. Continued evaporation from the consolidated region results in a water flow toward the edge, exerting a force onto the latex particles. At the time, when the network is formed, any single sphere must be in a force-balance condition: the network must exert an elastic force onto the sphere which just compensates the viscous drag. Pictorially speaking, a spring (an elastic network) is created while an external force acts onto it. Once the flow stops, the drag force vanishes and the internal stress, which previously compensated the drag, expands the film laterally. This phenomenon can lead to buckling. Given that lateral flow of liquid while films dry is a rather common occurrence, this mode of structure formation should be widespread. It requires lateral flow in conjunction with elastic recovery of the particle network.

Waterborne, semicrystalline, pressure-sensitive adhesives with temperature-responsiveness and optimum properties

The synthesis and resulting temperature-responsive properties of semicrystalline waterborne pressure-sensitive adhesives (PSAs) were investigated. A crystalline polymer fraction was produced in situ within waterborne particles by miniemulsion polymerization of non-branched long chain acrylates. The degree of crystallinity was controlled by copolymerization with a short chain acrylate. The polymerization strategy determined the polymer architecture and film structure, which then influenced the adhesion properties. The high sensitivity of the adhesion strength of these PSAs to temperature, in the range around the crystal melting point, opens up the possibility of designing temperature-responsive adhesives. With the right distribution and concentration of crystalline polymers, a simultaneous increase in both the peel strength and the shear resistance was obtained, which is a combination that is often not found when optimizing adhesive properties.

Nitroxide-Mediated Radical Polymerization in Nanoreactors: Can Dilution or Increased Nitroxide Concentration Provide Benefits Similar to Compartmentalization?

Compartmentalization of a nitroxide-mediated radical polymerization system can lead to improved levels of both control and livingness, but at the cost of a reduced polymerization rate. Improved control is a result of the confined space effect on deactivation, whereas improved livingness stems from the segregation effect on bimolecular termination. Modelling and simulations have been carried out for the systems styrene/2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and styrene/2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy (TIPNO) using the respective nitroxide-based polystyrene macroinitaitors (alkoxyamines) at 125°C to elucidate whether similar benefits can be obtained in the corresponding bulk polymerizations by merely diluting the systems or by addition of extra free nitroxide. The results have shown that neither approach leads to simultaneous improvement in control and livingness relative to the corresponding bulk systems, thus accentuating the merits of compartmentalization in nanoreactors.