Molecular Relaxation and Rheological Behaviors of Silica/Poly(propylene oxide) Dispersions

Dispersions based on fumed silica/polyether oligomers are widely used in varying fields including adhesives, coatings, lithium ion batteries, and liquid armors. It has been found that the complicated interfacial interaction between silica and polyether and the resultant interfacial layers strongly influences the rheological responses of the dispersions while it is a long-standing issue to control the interfacial structure and rheological behavior in both the academy and industry. Investigated herein are the interfacial structure and rheological behaviors of silica/linear poly(propylene oxide) (PPG) oligomer dispersions as a function of molecular weight (0.4-4 kg/mol) of PPG and volume fraction of silica. The results show that a change of molecular weight of PPG in a narrow range strongly influences the molecular adsorption and restriction in the vicinity of silica nanoparticles and the rheology of the dispersions. Moreover, PPG with molecular weight of 0.4 kg/mol forms glassy layers of ((1.0 +/- 0.2) nm) on the surface of silica nanoparticles via hydrogen bonding between terminal hydroxyls of PPG and silanols of silica. The percolation of glassy layers at high silica loadings causes the dispersions to undergo a sol-to-gel transition at 10 rad/s. On the other hand, PPG with molecular weight of 1-2 kg/mol forms incomplete glassy layers of (0.8 +/- 0.1) nm in thickness and that of 3-4 kg/mol forms restrained layers surrounding silica nanoparticles, both yielding sol-like rheological responses up to silica volume fraction of 0.16 at 10 rad/s. Furthermore, a comparison between thresholds of percolation of glassy layers and sol-to-gel transition suggests that the degree of molecular restriction nearby silica nanoparticles and therefore the rheological behaviors of the dispersions are determined by density of terminal hydroxyls of PPG and the interaction terminal hydroxyls and silanols of silica.