Small Particle Driven Chain Disentanglements in Polymer Nanocomposites

Using neutron spin-echo spectroscopy, x-ray photon correlation spectroscopy, and bulk rheology, we studied the effect of particle size on the single-chain dynamics, particle mobility, and bulk viscosity in athermal polyethylene oxide-gold nanoparticle composites. The results reveal a $\approx 25\%$ increase in the reptation tube diameter with the addition of nanoparticles smaller than the entanglement mesh size ($\approx 5\mathrm{nm}$), at a volume fraction of 20%. The tube diameter remains unchanged in the composite with larger (20 nm) nanoparticles at the same loading. In both cases, the Rouse dynamics is insensitive to particle size. These results provide a direct experimental observation of particle-size-driven disentanglements that can cause non-Einstein-like viscosity trends often observed in polymer nanocomposites.

Figure 1

Kratky plot of the contrast matched composites, ${\varphi }_{\text{part}}=0.20$, and $h/d$ ($74/26$) PEO matrices. The arrow represents the spatial range used in NSE. The inset shows the SANS profiles and the Debye fitting to the neat polymer data.

Figure 2

Dynamic structure factor of the neat PEO (filled symbols) and the PEO nanocomposites with Au nanoparticles (open symbols) at $T=400\mathrm{K}$ showing (a) identical initial Rouse decays at short times, (b) identical long-time plateau for large NP composite, and (c) reptation tube dilation of small NP composite. The symbols in (b) and (c) are for $Q=0.08{\AA }^{-1}$ (square), $Q=0.11{\AA }^{-1}$ (circle), $Q=0.15{\AA }^{-1}$ (triangle), and $Q=0.20{\AA }^{-1}$ (inverse triangle). The lines in (a) are the Rouse model predictions [21]; the lines in (b) and (c) are the global fit results from de Gennes’s equation [27] with an elementary Rouse rate, $W{l}^4=1.51{\mathrm{nm}}^4/\mathrm{ns}$.

Figure 3

(a) Relaxation time ($\tau$) and (b) stretching exponent factor ($\beta$) vs $Q$ for PEO composites with a weight fraction of 1% Au nanoparticles. The inset shows a representative autocorrelation function at $Q=0.01{\AA }^{-1}$.

Figure 4

Linear elastic ($G^{\prime }$, filled symbols) and viscous ($G^{\prime \prime }$, open symbols) moduli of the neat matrix and the composites with ${\varphi }_{\text{part}}=0.20$ NP of different sizes. Complex viscosities, $\eta *$, are plotted in the inset.