Thickening of the Immobilized Polymer Layer Using Trace Amount of Amine and Its Role in Promoting Gelation of Colloidal Nanocomposites

Immobilized polymer layers surrounding nanoparticles are proposed to be of essentially vital importance for the reinforcement of nanofiller to polymer matrices, but there is still a need to clarify its contribution to diverse rheological performance like colloidal stability and gelation. In this study, we find for the first time that introducing a trace amount of secondary/tertiary amine efficiently thickens the immobilized glassy layer in hydrophilic fumed silica (FS) filled polypropylene glycol (PPG) from 1.5 to 4.5 nm, which simultaneously promotes gelation of the liquid-like dispersion even containing extremely low contents of FS (<2 vol %). By coordinately using modulated differential scanning calorimetry and rheology methods, we find strong evidence that (1) the amine-promoted gelation is due to thickening and easy-percolation of the inner glassy layer converted from an outer uncrystallizable layer, and (2) the dispersion rheology could be well normalized within the framework of a two-phase model incorporating effective volume fraction of nanoparticles plus the glassy layers. We also highlight the importance of the surface chemistry of FS for adjusting the polymer immobilization and dispersion rheology.