Microscopic Dynamics of Polyethylene Glycol Chains Interacting with Silica Nanoparticles

We present high resolution neutron spectroscopic investigations of polyethylene glycol matrices interacting attractively with neat ${\mathrm{SiO}}_2$ nanoparticles. We observe a very rich dynamical picture that significantly contradicts earlier conclusions on such systems. Investigating a short chain matrix we realized that a fraction of chains is attached at the nanoparticle surface suppressing completely its translational diffusion. Nevertheless these attached chains undergo an unchanged segmental dynamics seemingly forming a micellelike corona of chains attached with their OH end groups. Changing to methyl-terminated chains the picture changes drastically, now showing a tightly adsorbed layer that however is not glassy as often assumed but undergoes fast picosecond local dynamics. With the singular importance of end groups, mean field approaches are not applicable and future simulations should be redirected to model such unexpected phenomena.

Figure 1

The intermediate scattering function of the pure PEG (OH) sample measured at 413 K. The solid lines show the full Rouse model fits, the dashed lines indicate the sole contribution from the internal chain relaxation modes.

Figure 2

Comparison of the intermediate scattering functions of the two PEG-silica samples [OH (circles) and ${\mathrm{CH}}_3$ (squares) end groups]. The pointed line represents the pure melt. The solid and dashed lines are respective model fits (see text). The inset compares the initial decay at short times for the nanocomposites and the pure melt.

Figure 3

The time-of-flight spectra of the matrix [$\mathrm{PEG}\mathrm{\text{−}}(\mathrm{OH}{)}_2$] and both nanocomposites at 413 K compared to the resolution (thin line). The $Q$ dependence of the intensity of the small elastic peak on top of the PEG-silica spectra is shown in the inset for three temperatures. The dashed line represents the ${\mathrm{SiO}}_2$ structure factor.

Figure 4

For the case of $\mathrm{PEG}\mathrm{\text{−}}(\mathrm{OH}{)}_2$ at 413 K the robustness of the fit results (solid lines) is demonstrated by an explicit variation of the parameters (dashed lines).