Direct Neutron Scattering Measurements of Grafted Polymer Chain Conformations from Functionalized Nanoparticles

The conformations of grafted polymers play an important role in determining the physical properties of polymer nanocomposites. Small-angle neutron scattering (SANS) is performed to quantify the conformation of poly(methyl methacrylate)($M_{w} > $ 27,000 g/mol) and polystyrene chains ($M_{w} >$ 57,000 g/mol) which are attached to iron oxide nanoparticles ($R_{np} = 2.5$ nm, $\sigma = 0.73$ chains/$\mathrm{nm^{2}}$) and small fractal aggregates ($R \approx 11$ nm, $\sigma = 0.2$ chains/$\mathrm{nm^{2}}$), respectively. Unlike light scattering or microscopy, SANS can directly measure the grafted polymer chain conformations. In a homopolymer melt, we find the grafted chains adopt stretched conformations near the nanoparticle surface, and transition to ideal, random coils past a cutoff distance $r_{c}$, in agreement with scaling arguments in the literature. We find the conformation of the polymer chains is largely unaffected by the ratio of the degree of polymerization of the matrix ($P$) to that of the brush ($N$). Finally, we extend this work to measure grafted polymer conformation in solution as a function of solvent quality, and find the grafted chains behave as swollen coils with an excluded volume parameter $\nu$ that decreases as the solvent cools to the $\Theta$ temperature.