Thermal degradation of PEO on SiO2 nanoparticles as a function of SiO2 silanol density, hydrophobicity and size

The degradation behavior of semicrystalline PEO on silica (SiO2) nanoparticles as a function of silanol density, hydrophobicity and nanoparticle size was investigated under N2 purge by derivative thermogravimetric analysis (dTGA) for adsorption amounts at or below plateau adsorption. The PEO was adsorbed onto colloidal (Stöber) onto 100 nm colloidal that had been heat-treated to vary the silanol density or hydrophobically modified, (CH3)3–SiO2, and onto 15, 40 and 100 nm SiO2 made by a water-glass process. Either one or two degradation peaks, Td, were observed for the adsorbed PEO, one at ∼180–210 °C and one at ∼250–300 °C. The degradation peak at 180–210 °C appeared either at high adsorption amounts for the 100 nm SiO2, on the 15 nm SiO2, and on hydrophobically modified SiO2, where PEO not directly hydrogen-bonded with SiOH was expected. The degradation peak at 250–300 °C correlated with SiOH density of the SiO2, and thus SiOH/Cdouble bond; length as m-dashO contacts, and the peak position increased with increase in SiOH density. With decreasing adsorption amount for the 100 nm SiO2, the 180–210 °C peak decreased with respect to the 250–300 °C peak. Thus, the freer PEO segments appeared to degrade at lower temperatures than the H-bonded segments. However, both Tds were well below those observed for neat PEO, which occurs at ca 388 °C. This may arise since PEO is oxidatively unstable, and the volatile degradation products of the relatively small number of PEO chains on the high surface area SiO2 are removed quickly, resulting in more rapid degradation.