Initial Solvent-Driven Nonequilibrium Effect on Structure, Properties, and Dynamics of Polymer Nanocomposites

Unusual structures and dynamic properties found in polymer nanocomposites (PNCs) are often attributed to immobilized (adsorbed) polymers at nanoparticle-polymer interfaces, which are responsible for reducing the intrinsic incompatibility between nanoparticles and polymers in PNCs. Although tremendous effort has been made to characterize the presence of immobilized polymers, a systematic understanding of the structure and dynamics under different processing conditions is still lacking. Here, we report that the initial dispersing solvent, which is not present after producing PNCs, drives these nonequilibrium effects on polymer chain dynamics at interfaces. Employing extensive small-angle scattering, proton NMR spectroscopy, and rheometry experiments, we found that the thickness of the immobilized layer can be dependent on the initial solvent, changing the structure and the properties of the PNC significantly. In addition, we show that the outcome of the initial solvent effect becomes more effective at particle volume fractions where the immobile layers begin to interact.

Figure 1

The structure factors $S(qD)$ of silica nanoparticles in PNCs are plotted with $qD$ where $D$ is particle diameter, for PEG MWs of (a) $0.4\mathrm{kg}/\mathrm{mol}$ and (b) $20\mathrm{kg}/\mathrm{mol}$. The solid curves and open symbols represent EtOH-- and water-started PNCs, respectively.

Figure 2

(a) Normalized FID, magic-sandwich echo (MSE), MAPE-, and DQ-filtered intensities of EtOH-PEG0.4k-0.5 and water-PEG0.4k-0.5. The black dashed dot lines are MSE-filtered FID. (b) The calculated strongly immobilized polymer fractions as a function of ${\varphi }_c$ in ethanol- and water-started PNCs with closed and open symbols, respectively. (Inset) Calculated layer thickness $\delta$. (c) nDQ curve. (d) Schematic illustrations of the nanoparticle surface in PNCs. Red and green chains are adsorbed and bulk polymers distinguished by the yellow dashed line.

Figure 3

Storage and loss modulus, $G^{\prime }$ and $G^{\prime \prime }$, of PNCs as a function of complex shear strain $\gamma$ at 1 Hz, for PEG $0.4\mathrm{kg}/\mathrm{mol}$, (a) EtOH- and (b) water-started PNCs. The results of PEG $0.4\text{k}$ PNCs at ${\varphi }_c$ lower than 0.3 are given in Fig. S10. For PEG $20\mathrm{kg}/\mathrm{mol}$, (c) EtOH- and (d) water-started PNCs. The complex shear modulus $G^{*}$ of PNCs are compared for different initial solvents with (e) PEG $0.4\mathrm{kg}/\mathrm{mol}$ at $\gamma =0.6\%$ and (f) PEG $20\mathrm{kg}/\mathrm{mol}$ at $\gamma =0.1\%$. The normalized layer-to-layer distances with $R_g$ are drawn with ${\varphi }_c$ assuming random close packing, based on the EtOH-started samples. A complementary frequency sweep experiment is provided in Fig. S9.

Figure 4

Schematic illustration of nanoparticle dispersions in (a) EtOH-started (thick layer) and (b) water-started (thin layer) PNCs at low and high ${\varphi }_c$. The strongly immobilized polymers are colored in solid red and blue for low and high MW, respectively, and the faded blue is used to represent loops and tails of the immobilized layers.