Qualitative Effect of the Polymerization Rate on the Nanoparticle Dispersion in Poly(methyl methacrylate)/Silica Nanocomposite Films

In this study, we investigate the effect of the polymerization rate, mainly mediated by the initiator concentration, on the nanoparticle dispersion in the nanocomposite films formed by poly­(methyl methacrylate) (PMMA) and [3-(methacryloyloxy)­propyl]­trimethoxysilane-modified silica nanoparticles (M-SiO₂) via free radical polymerization of the precursory solution, that is, 15.5 wt % M-SiO₂ nanoparticles dispersed in the methyl methacrylate (MMA) monomer, in which the tethered silanes at the particle surface bearing the reactive vinyl groups were capable of polymerizing with the MMA monomer. At slower polymerization, the nanoparticles self-organized to form a large-length scale network structure with the mass fractal of average dimension of 2.7; contrarily, the nanoparticles exhibited better dispersion when the polymerization proceeded at a faster rate. According to the scenario of perturbed polymerization kinetics in the presence of nanoparticles, we propose a possible mechanism through which the MMA monomers at a lower polymerization rate may constitute longer “multiple grafted PMMA chains” along particle surfaces by an optimal balance between the polymerization of the monomer and the grafting reaction of the monomer onto the particle surface, forming the M-SiO₂-rich clusters by interparticle bridging; the multiple grafting reaction also occurred in between any two neighboring M-SiO₂ nanoparticles situated respectively at the two adjoining clusters to induce the intercluster bridging, thereby leading to a hierarchical fractal network. However, both the polymerization and the grafting reaction of MMA monomers at a higher polymerization rate formed the shorter free and grafted PMMA chains, providing steric stabilization to retain better dispersity of nanoparticles.