Quasielastic Neutron Scattering Study on the Dynamics of Poly(alkylene oxide)s

By means of quasielastic neutron scattering we have investigated the hydrogen dynamics in poly(alkylene oxide)s (PAOs) with different side-chain lengths at temperatures below as well as above the glass-transition. The combination of results from three different spectrometers (a time-of-flight and two backscattering instruments) has allowed covering almost 4 orders of magnitude in time - from the ps to ns range - with spatial resolution. The results evidence the simultaneous occurrence of vibrations and localized side-group motions at low temperatures and additional diffusive-like (segmental) dynamics at high temperatures. The localized processes of the side groups show (i) stretching of the scattering function, (ii) associated activation energies similar to those found for single and cooperative bond rotations of polyethylene, and (iii) spatial extents that increase with increasing temperature. Compared with poly(ethylene oxide) (PEO), the diffusive segmental process in PAOs presents (i) the same spectral shape, (ii) slower characteristic times - antiplasticization - (iii) similar deviations from Gaussian behavior. For comparison, we also report on backscattering results on the side-group dynamics of poly(n-hexyl methacrylate) in the same temperature range, that show evidence for confinement effects. We suggest that the dynamic asymmetry in systems with intrinsic dynamic heterogeneities between constituent parts is the key ingredient leading to both plasticization and confinement effects. © 2012 American Chemical Society.
This research at Oak Ridge National Laboratory’s Spallation Neutron Source was partially sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. C.G. acknowledges financial support by the International Helmholtz Research School of Biophysics and Soft Matter (IHRS BioSoft). We acknowledge support by the “Donostia International Physics Center”, the European Commission NoE SoftComp, Contract NMP3-CT-2004-502235, FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), the projects MAT2007-63681, IT-436-07 (GV), and the Spanish Ministerio de Educacion y Ciencia (Grant No. CSD2006-53).