Preparation of microlatex dispersions using oil-in-water microemulsions
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The preparation of microlatex dispersions from microemulsions of a monomer (styrene, methylmethacrylate or vinyl acetate) is described. A simple method for preparing the microemulsion has been devised. This consists of forming a water-in-oil (w/o) emulsion using a low (HLB) surfactant (nonylphenol with 5, 6 or 7 moles ethylene oxide) and then titrating with an aqueous solution of a high HLB surfactant (nonylphenol with 15 or 16 moles ethylene oxide). A small amount of anionic surfactant (sodium lauryl sulphate, sodium dodecyl benzene sulphonate or dioctyl sulphosuccinate) was also incorporated to enhance the stability of the w/o emulsion and facilitate the inversion to an o/w microemulsion. The droplet-size distribution of the resulting microemulsion was determined using photon-correlation spectroscopy.
Three different methods of polymerising the microemulsion were used. These were thermally induced polymerisation using potassium persulphate, azobis-2-methyl propamidinium dichloride (AMP-water-soluble initiators) or azobisisobutyronitrile (AIBN, an oil-soluble initiator). All these initiators required heating to 60°C, i.e. above the stability temperature of the microemulsion. In this case, the microlatices produced were fairly large (37–100 nm diameter) and had a broad particle-size distribution. The second polymerisation procedure was chemically induced using a redox system of hydrogen peroxide and ascorbic acid. This produced microlatices with small sizes (18–24 nm diameter) having a narrow-size distribution. The microlatex size was roughly two to three times the size of the microemulsion droplets. This showed that collision between two or three microemulsion droplets resulted in their coalescence during the polymerisation process. The third method of polymerisation was based on UV irradiation in conjunction with K2S2O8, AMP or AIBN initiators. In this case, the microlatex size was also small (30–63 nm) with a narrow particle-size distribution.
Microlatex particles were also prepared using a mixture of monomers (styrene plus methylmethacrylate) or mixture of monomers and a macromonomer, namely methoxy (polyethylene glycol)methacrylate. The latter was used to produce “hairy” particles, i.e. with grafted polyethylene oxide (PEO) chains.
The stability of the microlatices was determined by adding electrolytes (NaCl, CaCl2, Na2SO4 or MgSO4) to determine the critical flocculation concentration (CFC). The nonionic latices were very stable giving no flocculation up to 6 mol dm−3 NaCl or CaCl2 and a CFC of 0.6 mol dm−3 for Na2SO4 or MgSO4. Charged latices were less stable than the nonionic ones. The critical flocculation temperatures (CFT) of all latices were determined as a function of electrolyte concentration. With the nonionic latices, CFC was higher than the θ-temperature for polyethylene oxide at the given electrolyte concentration. This indicated enhanced steric stabilisation as a result of the dense packing of the chains and hence an elastic contribution to the steric interaction. This was not the case with the charged latex, which showed CFT values lower than the θ-temperature. The “hairy” latices [i.e. those containing methoxy polyethylene glycol (PEG) methacrylate] were also less stable towards electrolyte (CFT was much lower than θ-temperature), indicating a low density of PEO layers.
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