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, Tg, 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, Tm, and enthalpy, ΔHm, 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, ΔHm, Tm, and the crystallization temperature, Tc, decreased. The results suggest that the interaction of ethylene oxide segments with surface silanols inhibits the chain mobility necessary for crystallization.