Nanoscale Particle Motion in Attractive Polymer Nanocomposites

Using x-ray photon correlation spectroscopy, we examined the slow nanoscale motion of silica nanoparticles individually dispersed in an entangled poly (ethylene oxide) melt at particle volume fractions up to 42%. The nanoparticles, therefore, serve as both fillers for the resulting attractive polymer nanocomposites and probes for the network dynamics therein. The results show that the particle relaxation closely follows the mechanical reinforcement in the nanocomposites only at the intermediate concentrations below the critical value for the chain confinement. Quite unexpectedly, the relaxation time of the particles does not further slow down at higher volume fractions—when all chains are practically on the nanoparticle interface—and decouples from the elastic modulus of the nanocomposites that further increases orders of magnitude.

Figure 1

SAXS profiles (shifted vertically for clarity) from the PNCs at 363 K. The line on ${\varphi }_{\mathrm{NP}}=2\%$ is the fit result from spheres with radius of 24 nm and Gaussian size distribution of 0.3. The inset is the resulting structure factor peaks obtained by dividing the SAXS intensities by the fit results for the single particle form factor.

Figure 2

XPCS intensity correlation functions at $Q=0.01{\AA }^{-1}$ (top figures) at (a) $T=433\mathrm{K}$ and (b) $T=363\mathrm{K}$. The lines are the best fits to the stretched exponential forms that result in relaxation times ($\tau$) and stretching exponent ($\beta$) displayed. The lines in the $\tau (Q)$ plots show the trends for simple diffusive, $\tau \propto Q^{-2}$, and ballistic $\tau \propto Q^{-1}$ motions. Error bars represent one standard deviation.

Figure 3

(a) Temperature dependence of the NP relaxation times ($\tau$) in PNCs. The lines are the best fits to VFT equation with the known PEO parameters (see the text). (b) Linear elastic ($G^{\prime }$, filled symbols) and viscous ($G^{\prime \prime }$, open symbols) moduli as a function of deformation frequency at 363 K showing liquid-to-elastic transition in the PNCs with increasing ${\varphi }_{\mathrm{NP}}$. (c) Relaxation times obtained from the VFT fits shown in (a) and the elastic moduli of PNCs obtained determined at $\omega =0.1\mathrm{rad}/\mathrm{s}$. The shaded area indicates the region where face-to-face NP distance separation ($h$) is smaller than the chain size ($2R_g$). Error bars represent one standard deviation.