Microlatex Characteristics

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). 

The arguments put forward to explain thermodynamic stability of microemulsions can also explain the stability of these microlatexes [6.22]. Concerning the former, it has been shown that the dispersion entropy of the droplets becomes larger than the extremely unfavourable contribution from interfacial energy, when the droplet radius goes below a critical radius. Moreover, the formation of microemulsions requires very low interfacial tensions. 

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). 

The combined effect of these factors leads in certain conditions to transparent and highly stable microlatexes. The question of thermodynamic stability remains, however. 

Microlatexes have been characterised by means of various techniques, including light scattering, viscosity, and electron microscopy. Unlike alcohol-based microemulsions, which raise many problems of dilution, microlatexes are easy to dilute, behaving like suspensions of hard spheres, without strong interaction between particles. After polymerisation, attractive forces due to the monomer-cosurfactant at the interface are considerably reduced (see above). 

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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. 

Many parameters govern formation and stability of colloidal dispersions. Among these are temperature, type and concentration of the various components, the hydrophile-lipophile balance (HLB) of the emulsifier (s), and the presence or absence of a cosurfactant, all of which can greatly infiuence their characteristics. Choosing a system is thus a very complicated task and often undertaken empirically. However, a procedure can be established as a guide to formulation, on the basis of a few simple ideas, sufficient to obtain some remarkably stable microlatexes. 

As flocculants for a wide range of substrates (such as cellulose fibres) and in water treatment. Reverse latexes are destabilised and reversed by adding excess water or possibly another surface active agent. Microlatexes prepared in microemulsion polymerisation are self-reversing and therefore do not require addition of another surfactant to favour this transformation. Furthermore, the corresponding polymers, confined within such small particles, with such low polydispersity, should exhibit better characteristics in this respect. 

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