Do Very Small POSS Nanoparticles Perturb s-PMMA Chain Conformations?

Small-angle neutron scattering (SANS) measurements of syndiotactic s-PMMA polymers mixed with weakly attractive 1.0 nm diameter polyhedral oligomeric silsesquioxane (POSS) nanoparticles (NPs) show no observable changes in the chain radius of gyration R_g, regardless of the polymer molecular weights, the amount of residual solvent, or the POSS NP loading and dispersion (from 0 to 20 vol %). In retrospect, these results are not surprising since scaling arguments imply that chain size in the concentrated region of the phase diagram of a polymer solution is ideal and independent of the polymer volume fraction ø, and only as the semidilute region is entered with decreasing concentration does the chain size for a good solvent begin to increase due to polymer excluded volume and then scales with concentration as ø^-1/8. For typical polymer nanocomposites the NP concentrations are less than 50% v/v, so the polymers are still generally within the concentrated regions of their phase diagrams, where ideal chain conformations are observed for small molecule solvents. By combining the present results with previous results from the literature, we conclude that spherical NPs apparently have little effect on the conformations of polymer chains, especially in typical polymer nanocomposites that only incorporate moderate amounts of NPs.