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, sigma similar to 2 molecules/nm(2) (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 degreesC for the unattached PEO(375)-silane to -54 degreesC for the attached PEO(375)-SiO2 chains. In the case of PEO(5000)-SiO2, a similar maximum weight percent coverage (similar to38%), corresponding to sigma similar to 0.2 molecules/nm(2), 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, DeltaH(m), that were decreased with respect to the neat PEO(5000)-silane, but no Tg, was observed. With decreasing sigma, as the surface area occupied by the grafted chains increased with respect to the calculated chain dimensions in solution, DeltaH(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.