Surface graft layers, stability

We now consider a system that involves bare clay sheets and a polymer mixture that contains both functionalized and nonfunctionalized species. In particular, some fraction of the polymers contain "stickers" that are highly attracted to the surface. Aside from the sticker sites, the functionalized and nonfunctionalized chains are chemically identical. The functionalized polymers will bind to the clay and form a grafted layer. The remainder of the polymers will interact witii the anchored species. Since the free and grafted chains are comparable in length, the free polymer can readily penetrate the grafted layer and form a broad interphase, which promotes the formation of exfoliated hybrids While the experimental result will depend on the kinetics of the process, the SCF calculations can indicate, for example, how the fraction of functionalized chains in the mixture affects the thermodynamic stability of the product To test the feasibility of this approach, we varied the fraction of functionalized polymers within a melt of chains of comparable length. 

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

---