Polymer-Clay Nanocomposite Particles by Inverse Emulsion Polymerization

3 Polymer-Clay Nanocomposite Particles by Inverse Emulsion Polymerization 

We have tested this hypothesis in the inverse emulsion polymerization of acrylamide (A Am). A typical recipe ineluded A Am as the monomer dissolved in water, cyclohexane as the continuous phase, sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) or sorbitan monooleate (Span-80) as the surfactant and MMT or LRD as typical nascent clays. Both oil- and water-soluble azo compounds, such as 2,2 -azobisisobutyronitrile (AIBN) and 2,2 -azobis[2-methyl-A-(2-hydroxyethyl)propionamide] (VA-086), were used as initiators. Span-80 appeared to lead to more stable inverse emulsions in the presence of clay than AOT. We first noticed a different kinetic behavior during polymerization in the presence of clay platelets. The conversion time history for the AAm inverse emulsion polymerization in the presence of MMT showed a significant decrease in the rate of polymerization in comparison with those without clay. With increasing concentration of clay particles, lower rates of polymerizations and lower final monomer conversions were found, together with increased retardation of the polymerization. The clay platelets in the inverse emulsions might act as diffusion barriers for monomer and/or initiator. 

The typical particle diameters of LRD- and MMT-containing inverse PAAm latexes were about 70 and 240 nm, respectively. We then examined the location of clays in the final latex particles. To our great surprise, cryogenic TEM 

To find out why the hydrophilic platelets were located at the interface between a hydrophilic polymer and a hydrophobic continuous phase in the inverse latexes, cryo-TEM was used to examine the locations of the platelets in the starting inverse emulsions. Conventional TEM requires dried samples and cannot be used to characterize the starting inverse emulsions containing clay platelets. We examined a model inverse emulsion comprising water, cyclohexane and clay particles, with Span-80 as the surfactant. In the absence of clay particles, the cryo-TEM image showed that water droplets of 100-200 nm in diameter were coated with surfactant molecules. The surfactant-stabilized water droplets appeared to bundle together and formed cauliflower-like structures. When either LRD or MMT was added to the inverse emulsion, the cauliflowerlike structure appeared to remain intact, but the clay platelets were located at the interface between the water droplets and cyclohexane, not inside the water 

Considering the fact that clay particles have a tendency to orient themselves at the interface between water and oil phases, we have taken advantage of this by using clay particles as the motif to stabilize inverse emulsions. As discussed earlier, for particles to be used as a Pickering-type stabilizer, they have to possess certain surface characteristics. It appeared that hydrophobized clays were better suited to stabilize inverse emulsions. The long alkyl chain of the quaternary ammonium compounds that are adsorbed on the surface of the hydrophobized clay surface enable the swelling and exfoliation of the platelets in apolar medium. 

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