The structure of chain end-grafted nanoparticle/homopolymer nanocomposites

The morphological structure of mixtures of homopolymers with inorganic nanoparticles, onto which polymer chain ends are grafted, is described herein. In the bulk, at very low grafting densities, Sigma, the morphology of these polymer nanocomposites (PNCs) is largely determined by the attractive enthalpic interactions between the nanoparticle cores, mediated by entropic interactions with the grafted polymer chains. These competing interactions have been shown to be responsible for the anisotropic organization of the nanoparticles within polymer hosts. At high grafting densities, however, even in cases where the host chains and the grafted chains are of identical chemical structure, the entropic brush layer/free host chain interactions are dominant, leading to miscibility (isotropic dispersion of the nanoparticles within the polymer host) or to microscopic and macroscopic phase separation. In the limit of very small nanoparticles, radii R approximate to 1 nm, the brush/melt interactions are not important yet the nanoparticles exhibit a high tendency toward miscibility within homopolymer hosts. This paper begins with a discussion of the bulk morphology of PNCs in order to provide a context for the subsequent description of the morphological structure of thin film brush coated nanoparticle-homopolymer mixtures. A fundamental difference between these bulk and thin film PNCs is that a preferential attraction of grafted nanoparticles to interfaces renders thin film PNCs thermodynamically less stable than their bulk analogs. Morphological phase diagrams, which delineate regimes where the PNCs are miscible, incompatible or partially miscible, are parameterized in terms of the nanoparticle curvature, the chain grafting density, the grafting chain degree of polymerization, N, that of the free host chains, P, and the thermodynamic (Flory-Huggins) interaction parameter, chi, between the host and grafted chains.