Microlatex particles

Larpent and Tandros [102] prepared microlatex particles by polymerization of PEO-MA macromonomer with MMA, styrene, and vinyl acetate. The nonionic latexes are very stable, giving no flocculation up to 6 mol dm 3 NaCl or CaCl2 and a critical flocculation concentration (CFC) of 0.6 mol dm 3 for Na2S04 or MgS04 was estimated. Charged latexes are less stable than the nonionic ones. The CFC of all latexes are determined as a function of electrolyte concentration. With the nonionic latexes, however, the critical flocculation temperature (CFT)... 

In this paper I review the salient features of polymerization in microemulsions at the present state of knowledge. I discuss the formulation of polymerizable microemulsions and show how the incorporation of monomers can modify the initial structure of the systems. The kinetic and mechanistic aspects are given and compared to those experienced in conventional emulsion polymerization. I also describe some recent results obtained on the formation of porous solid materials and functionalized microlatex particles, which seem quite promising for future applications. 

Figure 11 Hydrodynamic radius of polystyrene microlatex particles as a function of the weight ratio of surfactant to monomer. Full line, CTAB dashed line, TDEA-Cu. (From Ref. 88.)...

Another way to functionalize the surface of microlatex particles is to incorporate amphiphilic block copolymers (for example, polystyrene/polyvinylpyridine) as cosurfactants together with the classical surfactants used in the formulation [86,87]. The protruding polyvinylpyridinium chains are anchored to the glassy core through the polystyrene blocks. These blocks copolymers were shown to stabilize the oil/water interface and to effectively bind ions of transition and heavy metals via complexation. 

For medical or pharmaceutical applications, attention must be paid to the problems that can be caused by the possible toxicity of the surfactant remaining in the final product. Antonietti et al. [89] proposed the use of natural, nontoxic, and nondenaturing surfactants based on mixtures of lecithin and sodium chlolate for the formation of globular microemulsions. Pure lecithin is known to form bilayers or liposomes. The role of sodium cholate is to increase the curvature and flexibility of the interfacial layer, allowing the formation of small droplets. The final microlatex particles have a size ranging from 22 to 40 nm, depending on surfactant composition and concentration. The ability to functionalize the surface of these particles was demonstrated by the incorporation of protein molecules. 

Concerning microlatexes, particle dimensions are much smaller than those of conventional latexes. It follows that gravitational forces which tend to cause flocculation are considerably reduced (/g (f). The entropy contribution can therefore be decisive, given the large number of particles present. Furthermore, we can assume that interfacial tension between polymer droplets and the continuous medium is still very low, bearing in mind the extremely low values obtained in the initial microemulsions (7 10 dyne/cm). 

For acrylamide polymerisation in reverse AOT micelles, where A/p = 1, the comparison must be made for a polymer chain dissolved in water since this solvent is 50% present in the particles. The radius of gyration of a 6 x 10 molecular weight polyacrylamide in water is about 160 nm. This is much bigger than the size of the microlatex particle d = 30 nm), thereby confirming intramolecular collapse of the polymer. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

Core-shell nanoparticles can also be fabricated using microemulsions. This was performed using a two-stage microemulsion polymerization beginning with a polystyrene seed [62]. Butyl acrylate was then added in a second step to yield a core-shell PS/PBA morphology. The small microlatex led to better mechanical properties than those of similar products produced by emulsion polymerization. Hollow polystyrene particles have also been produced by microemulsion polymerization of MMA in the core with crosslinking of styrene on the shell. After the synthesis of core-shell particles with crosslinked PS shells, the PMMA core was dissolved with methylene chloride [63]. The direct cross-... 

Fig. 3 Changes in PMMA particle size during long term storage at 60 °C for microlatexes stabilized by different surfactants (filled triangles) TTAB (filled squares) TTAC (filled circles) CTAB (empty triangles) OTAC...

The polymerization of styrene in Winsor I-like systems by semi-continuous feeding of monomer stabilized by either DTAB, TTAB or CTAB has been systematically investigated by Gan and coworkers [69a]. Rather monodisperse polystyrene microlatexes of less than 50 nm with molecular weights of over one million were obtained at a polymer/surfactant weight ratio of 14 1. The Winsor I-like (micro)emulsion polymerization of styrene stabilized by non-ionic surfactant and initiated by oil-soluble initiators has also been reported very recently [69b]. The sizes of the large monomer-swollen particles decreased with conversion and they merged with growing particles at about 40-50% conversion. 

High polymer/surfactant weight ratios (up to about 15 1) of polystyrene microlatexes [73] have been produced in microemulsions stabihzed by polymerizable nonionic surfactant by the semi-continuous process. The copolymerization of styrene with the surfactant ensures the long-term stabihty of the latexes. Nanosized PS microlatexes with polymer content (<25 wt%) were also obtained from an emulsifier-free process [74] by the polymerization of styrene with ionic monomer (sodium styrenesulfonate, NaSS), nonionic comonomer (2-hydroxyethylmethacryalte, HEM A), or both. The surfaces of the latex particles were significantly enriched in NaSS and HEMA, providing better stabilization. 

Nanoparticles of PS (M =1.0xl0 -3.0xl0 mol ) microlatexes (10-30 nm) have also been successfully prepared from their respective commercial PS for the first time [75]. The dilute PS solutions (cyclohexane, toluene/methanol or cyclohexane/toluene) were induced to form polymer particles at their respective theta temperatures. The cationic CTAB was used to stabihze th microlatexes. The characteristics of these as-formed PS latex particles were quite similar to those obtained from the microemulsion polymerization of styrene as reported in literature. These microlatexes could also be grown to about 50 nm by seeding the polymerization of styrene with a monodisperse size distribution of D /Djj=1.08. This new physical method for preparing polymer nano-sized latexes from commercial polymers may have some potential applications, and therefore warrants further study. 

Microlatex, l.e. particles having a diameter of less than lOOnm, can be obtained, especially with polyesters MD 90 and MD 120. 

It was also shown that stable latexes of high solid content, and small particle size could be practically obtained by this emulsion polymerization technique. Such microlatexes based on acrylic polymers modified by polyesters are an interesting approach to waterborne coatings leading to high gloss paint films (9). 

The particle size of the final microlatex (rf 20-40 nm) was larger than that of the initial monomer-swollen micelle. This led to a final number of polymer particles, N, about two or three orders of magnitude smaller than that of the monomer droplets. 

As for polymerization of hydrophobic monomers in the bicontinuous phase of microemulsions, the initial structure is not preserved upon polymerization. However, a notable difference from the former systems is that the final system is a microlatex that is remarkably transparent (100% optical transmission), fluid, and stable, with a particle size remaining unchanged over years even at high volume fractions ( 60%) [20]. The microlatex consists of water-swollen spherical polymer particles with a narrow size distribution according to QELS and TEM experiments. This result is of major importance with regard to inverse emulsion polymerization, which is known to produce unstable latices with a broad particle size distribution [23]. 

To prepare stable and small microlatexes, difficult to obtain using emulsion polymerisation. In particular, the classic process in inverted emulsion leads to unstable latexes with a wide range of particle dimensions. 

Formulation is quite different for polymerisation in an oil-in-water microemulsion or an inverted microemulsion (water-in-oil). In the first case, hydrophobic monomers are liquids (e.g., styrene, methyl methacrylate) which can be dispersed in a microemulsion without addition of a solvent. Conversely, at the kind of temperatures generally used, water-soluble monomers are powders. They must first be dissolved in water (to a level of about 50%). For this reason, reverse (water-in-oil) microlatexes are formed from water-swollen polymer particles dispersed in an organic phase. 

Sedimentation measurements carried out on the final microlatexes confirm the presence of two populations of particles with very different sizes (differing by a factor of 10). Figure 6.6 illustrates the polymerisation mechanism. 

Stability of the microlatexes obtained depends critically on formulation. A good match (in terms of solubility parameters and molar volume) between oil and lipophiles can lead to very stable latexes. A poor match means unstable latexes. It has been confirmed that certain reverse microlatexes are stable for periods of several years without change in particle size. This implies a total absence of flocculation, despite the big difference in density between polymer particles 1.4) and the continuous phase ( 0.8). 

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