Surface Enrichment of Polymer-Grafted Nanoparticles in a Miscible Polymer Nanocomposite

We show that the polymer-grafted nanoparticles (NPs) initially well-dispersed in a polymer matrix segregate to the free surface of a film upon thermal annealing in the one-phase region of the phase diagram because the grafted polymer has a lower surface energy than the matrix polymer. Using a combination of atomic force microscopy, transmission electron microscopy, and Rutherford backscattering spectrometry, the evolution of the poly(methyl methacrylate)-grafted silica NP (PMMA NP) surface excess in 25/75 wt % PMMA NP/poly(styrene-ran-acrylonitrile) films is observed as a function of annealing time at 150 °C (T < TLCST). The temporal growth of the surface excess is interpreted as a competition between entropic contributions, surface energy differences of the constituents, and the Flory–Huggins interaction parameter, χ. For the first time in a miscible polymer nanocomposite mixture, quantitative comparisons of NP surface segregation are made with the predictions of theory derived for analogous polymer blends. These studies provide insight for designing polymer nanocomposite films with advantageous surface properties such as wettability and hardness and motivate the need for developing rigorous models that capture complex polymer nanocomposite phase behaviors.