Local, Global, and Particle Dynamics in Polypropylene Glycol / Silica Nanocomposites

The local segmental and global dynamics of polypropylene glycol / silica nanocomposites were studied using rheometry, SANS, and mechanical and dielectric spectroscopies. The particles cause substantial changes in the rheology, including higher and non-Newtonian viscosities. 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 both the local and global 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 truncates the normal mode, which contributes an additional shift of the corresponding relaxation peak to higher frequencies.

At particle concentrations sufficient to cause the viscosity to be shear-rate dependent, the transient viscosity exhibits stress overshoots. If the shear flow is interrupted and then resumed, the magnitude of the overshoot is governed by the quiescent time. Both the time scale of this structural recovery and its temperature-dependence are markedly different from those determined for the chain dynamics. That is, the recovery of the startup transient involves interparticle interactions that are largely decoupled from the local viscosity. However, these interactions do not correspond to aggregation of the particles over any significant length scale, as evidenced by the absence of any upturn in the scattering at low q (60 – 80 nm) and by the invariance of the scattering during flow-induced changes in the transient viscosity.