Thermodynamics and Kinetics of Polymer Grafted Nanoparticle Composites

In this work, model binary and ternary polymer nanocomposites (PNCs) of poly(methyl methacrylate) grafted silica nanoparticles (PMMA-NP), poly(styrene-ran-acrylonitrile) (SAN), and PMMA homopolymers are examined to probe thermodynamic and kinetic parameters underlying phase behavior and nanoparticle assembly. Using complimentary techniques, the addition of PMMA to PMMA-NP/SAN is observed to increase the miscibility in off-critical compositions. This compatibilization is attributed to interfacial segregation of PMMA and supported by molecular dynamics simulations. Knowing the phase diagram, the interplay between phase separation and wetting is then studied by quenching binary PNC films into the two phase region. The morphology exhibits wetting layers of PMMA-NPs at the surface and substrate separated by “pillars” of PMMA-NPs that span the thickness. These “pillars” enhance the film modulus and hardness as measured by nanoindentation. PMMA-NP surface excess increases rapidly with time and reaches a plateau that is dependent on quench depth. The diffusion coefficients of PMMA-NPs are measured and compared with prevailing models. Overall, these results highlight the complex parameter space in PNCs and provide new insights and control over bulk and surface morphologies of PNCs.