Local and global dynamics in polypropylene glycol / silica composites

TL;DR

This study investigates how silica nanoparticles affect the local and global dynamics of polypropylene glycol, revealing significant rheological changes and interfacial effects on relaxation times through spectroscopic and rheometric analysis.

Contribution

It provides new insights into the interfacial dynamics and rheological behavior of polypropylene glycol/silica nanocomposites, highlighting the impact of tethered chains and interfacial density.

Findings

Particles increase viscosity and induce non-Newtonian behavior.

No change in mean relaxation times for bulk dynamics.

Interfacial polymer chains exhibit significantly faster relaxation when isolated.

Abstract

The local segmental and global dynamics of a series of polypropylene glycol / silica nanocomposites were studied using rheometry and mechanical and dielectric spectroscopies. The particles cause substantial changes in the rheology, including higher viscosities that become non-Newtonian and the appearance of stress overshoots in the transient shear viscosity. However, no change was observed in the mean relaxation times for either the segmental or normal mode dynamics measured dielectrically. This absence of an effect of the particles is due to masking of the interfacial response by polymer chains remote from the particles. When the unattached polymer was extracted to isolate the interfacial material, very large reductions in the relaxation times were measured. This speeding up of the dynamics is due in part to the reduced density at the interface, presumably a consequence of poorer packing of tethered chains. In addition, binding of the ether oxygens of the polypropylene glycol chains truncates the normal mode, which shifts the corresponding relaxation peak to higher frequencies.