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

Abstract

We examined the agglomeration behavior of a suspension of SiO2 nanoparticles with average dimensions of ∼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) × 104. For PMMA with a critical molecular weight (Mc) of ∼3 × 104 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 (Mw: 0.3 × 104) with Mc 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 Mc or lower. The transmittance is suddenly diminished at a critical concentration C*, and the C* shifts to the higher value as the smaller molecular weight is employed and, especially in the case of Mw of ca. 0.3 × 104, C* was not found out up to ca. 50 wt%.