Cationic surfactant blocks radical-inhibiting sites on silica

Surfactant-catalyzed room-temperature radical polymerization of methyl methacrylate (MMA) was conducted in the presence of fumed silica nanoparticles and water. Three types of surfactants, cationic (CTAB), nonionic (Triton X-100) or anionic (SDS), were used to catalyze the decomposition of the initiator, 2,2'-azobisisobutyronitrile (AIBN). The surfactant-catalyzed decomposition rate constant for AIBN at room temperature was found to be independent of the surfactant type. However, the rates of polymerization of the MMA emulsion gels at room temperature were found to depend on the types of surfactant with: cationic>nonionic>anionic. An inhibition period was observed for the polymerizations with nonionic and anionic surfactants. The radical-inhibition was likely due to the reactions between the radicals and the silanol groups on fumed silica. This inhibition can be reduced by using cationic surfactants to block these surface silanols.