Structure and Molecular Motion of Poly(ethylene oxide) Chains Tethered on Silica by Solid-State 13C NMR Method

Abstract

Solid-state 13C 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 conformation and dynamics of PEO chains. Temperature dependent NMR spectra could be interpreted in terms of the degree of interaction between PEO and silica molecules. The structures and molecular motion of the chains are strongly dependent on the grafting ratio (GR). Chemical shift of the methylene peak of the 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 surface. The fractional amount of “train” segment decreases remarkably with increase in GR. From the temperature dependence of the line width of the NMR spectra, correlation time, activation energy and transition temperature of the molecular motion were estimated. Molecular mobility of the “tail” segment was much higher and that of the “train” segment was lower than PEO chains in the amorphous region of the homopolymer bulk. “Tail” segments may thus protrude from the silica surface and have low segmental density, whereas “train” segments are trapped near the silica surface. It can be also considered that the fractional amount of “tail” segment increases abruptly with grafting ratio after the tethered chains of a coiled structure form one monolayer.