Immobilized polymer fraction in hyperbranched polymer/silica nanocomposite suspensions

The glass transition behavior of hyperbranched polymer (HBP) molecules with suspensions of silica and glass particles up to the concentrated regime (25 vol %) was analyzed by modulated differential scanning calorimetry (MDSC). The reversing and nonreversing components of the MDSC signal were measured on suspensions of untreated and silylated particle of size in the nanometer and micrometer ranges. The heat capacity step (ΔCp) at the glass transition of the HBP was found to be independent of silica loading for microparticles, whereas it decreased with increasing particle amount in the case of nanoparticles. A similar behavior was observed for the enthalpy relaxation. These changes in chain dynamics and the progressive suppression of aging were attributed to immobilization effects of the HBP at the surface of the particles, which became detectable only in the case of a very high specific surface. The immobilized HBP fraction was assumed to form a shell of constant thickness around individual particles and was calculated from the ΔCp at the transition. In the case of untreated particles with a silanol surface, the immobilized shell was formed by HBP molecules H-bonded to the particles. The thickness of the shell was found to be equal to 1.9 nm, which corresponded to half the size of the HBP. In the case of methacrylsilane-treated silica, the immobilized shell thickness was found to be equal to 1.3 nm, which corresponded to a monolayer of covalently bound silane.