A new mechanism for the silica nanoparticle dispersion-agglomeration transition in a poly(methyl methacrylate)/silica hybrid suspension

We examined the agglomeration behavior of a suspension of SiO2 nanoparticles with average dimensions of similar to 15 nm in a solution of tetrahydrofuran and polydisperse poly(methyl methacrylate) (PMMA) with a weight-average molecular weight in the range of (0.3-31) x 10(4). For PMMA with a critical molecular weight (M-c) of similar to 3 x 10(4) or larger, at which PMMA chains show effective entanglement, a critical polymer concentration (C*) was clearly observed. At C*, the dispersed SiO2 nanoparticles came into contact with one another and rapidly agglomerated. C* increased with decreasing molecular weight. However, no clear C* was observed for PMMA (M-w: 0.3 x 10(4)) with M-c or lower molecular weights. The molecular weight dependence of the observed C* can be explained by the depletion effect, but the lack of a clear C* for low molecular weight PMMA cannot be explained with this theory. Because C* occurs in the vicinity of the critical concentration, at which the random coils in the solution come into contact with one another and begin to overlap, the entanglement of random coils is considered to be the driving force behind nanoparticle agglomeration. However, no C* was observed because effective entanglement does not occur for PMMA with M-c or lower.