Poly(vinyl alcohol) stabilization of acrylic emulsion polymers using the miniemulsion approach

To enhance the grafting between poly(vinyl alcohol) (PVA) and acrylic monomers at the water/droplet interface, the miniemulsion polymerization approach was investigated. Miniemulsions of n-butyl acrylate and methyl methacrylate (50/50 wt %) employing PVA as the stabilizer and hexadecane (HD) as the costabilizer were studied. HD played a critical role in determining the stability of the droplets. Miniemulsions prepared using cetyl alcohol as costabilizer were evaluated in terms of the concentration of cetyl alcohol and the time dependence of the droplet size. These miniemulsions showed a higher rate of degradation due to Ostwald ripening compared to miniemulsions prepared using hexadecane. The droplet size decreased exponentially with increasing PVA, with a lower limit being reached beyond which more PVA had no effect. The PVA partitioning on the surface of droplets was determined by serum extraction using a serum replacement cell. The amount of adsorbed PVA chains on the droplets increased from similar to1.0 to similar to1.95 mg/m(2) in the range 3-10 wt % PVA and then became constant, as indicated by an increase in the serum concentration of PVA. The trends of the partitioning of PVA on the droplets and the droplet size as a function of PVA concentration were consistent. The sera of miniemulsions prepared with varying MMA/BA compositions were extracted using the serum replacement cell. The amount of serum PVA greatly increased above 75 wt % MMA composition even though the number of droplets increased, indicating that the cross-sectional area of adsorbed PVA molecules on the droplets greatly increased above 75 wt % MMA. The concentration dependence of the grafted PVA on the final particles was similar to that of the number of PVA molecules adsorbed on the monomer droplets, strongly supporting the hypothesis that the water/monomer interface is the main grafting site in the miniemulsion polymerization. The effect of monomer composition on the grafting of PVA was studied. The addition of a small amount of MMA (10-15 wt %) greatly changed the surface characteristics of the final particles and affected the grafting of PVA.