Structure and molecular motion of poly(ethylene oxide) chains tethered on silica by solid-state C-13 NMR method

Solid-state C-13 cross-polarization/magic-angle spinning/dipolar Decoupling (CP/MAS/DD) nuclear magnetic resonance (NMR) spectroscopy of poly(ethylene oxide) (PEO) tethered on silica was studied to characterize the conformati on and dynamics of PEO chains. Temperature dependent NMR spectra could be i nterpreted in terms of the degree of interaction between PEO and silica mol ecules. The structures and molecular motion of the chains are strongly depe ndent on the grafting ratio (GR). Chemical shift of the methylene peak of t he 13C NMR spectrum increases with GR. 70.2 ppm in low GR indicates "train" segments strongly interacted with the silica surface, whereas of 71.2 ppm in high GR is originated from "tail" ("loop") segments separated from the s urface. The fractional amount of "train" segment decreases remarkably with increase in GR. From the temperature dependence of the line width of the NM R spectra, correlation Lime, activation energy and transition temperature o f the molecular motion were estimated. Molecular mobility of the "tail" seg ment was much higher and that of the "train" segment was lower than PEO cha ins in the amorphous region of the homopolymer bulk. "Tail" segments may th us protrude from the silica surface and have low segmental density, whereas "train" segments are trapped near the silica surface, It can be also consi dered that the fractional amount of "tail" segment increases abruptly with grafting ratio after the tethered chains of a coiled structure form one mon olayer.