Poly(ethylene oxide) Silananted Nanosize Fumed Silica:  DSC and TGA Characterization of the Surface

Low-molecular-weight poly(ethylene oxide) (PEO) was grafted onto high-surface-area (A380) fumed silica (SiO2) by the use of a silane coupling agent, (CH3O)3−Si−(CH2)3−(OCH2CH2) x −OCH3, with x = 6−9, corresponding to an average molecular weight for the PEO segment of 375 g/mol, PEO(375)−silane. The PEO-grafted silica was characterized by thermogravimetric analysis and differential scanning calorimetry. In the case of PEO(375)−SiO2, the maximum grafting ratio, σ ∼ 2 molecules/nm2 (approximately 33 wt %), corresponded to less coverage than that expected for a monolayer of crystalline upright chains. No crystallization was observed, and the glass transition temperature, T g, increased from −90 °C for the unattached PEO(375)−silane to −54 °C for the attached PEO(375)−SiO2 chains. In the case of PEO(5000)−SiO2, a similar maximum weight percent coverage (∼38%), corresponding to σ ∼ 0.2 molecules/nm2, was obtained. At the highest grafting ratio, the calculated unperturbed chain dimensions in solution were approximately the same as those of the area occupied by the grafted PEO(5000) chains. A melt endotherm, with a melt temperature, T m, and enthalpy, ΔH m, that were decreased with respect to the neat PEO(5000)−silane, but no Tg, was observed. With decreasing σ, as the surface area occupied by the grafted chains increased with respect to the calculated chain dimensions in solution, ΔH m, T m, and the crystallization temperature, T c, decreased. The results suggest that the interaction of ethylene oxide segments with surface silanols inhibits the chain mobility necessary for crystallization.