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

Abstract

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.

Nanophase Separation and Exotic Dynamic Behavior in Comb-Like Polymers

“…Quasielastic spectra recorded on fully protonated PAOs samples revealed the motions of hydrogens located both in the main chain and along the side group [8]. The low-temperature results show clear indications for the occurrence of localized motions (elastic and quasielastic components), while at the highest temperature investigated the spectra look quasielastic-like with the features of a diffusive-like motion.…”

“…Diffraction experiments were carried out on D16 and D7 at the ILL and on DNS at the JCNS in Garching, Germany. Further details on the experiments can be found elsewhere [5][6][7][8].…”

Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials

“…33 These polymers were subject of a thorough QENS investigation combining three different spectrometers to cover the widest possible dynamic range. 34 the side groups and simultaneous anomalous diffusion associated to the segmental motions involved in the -relaxation. The spatial extent of the local motions was found to increase with temperature due to the softening of the structure and no special signature of confinement was identified in the side group dynamics of PAOs.…”

“…For example, for comb-like polymers (polymers with long alkyl side-groups) nano-phase separation has been identified and, under certain conditions, also confinement effects. [32][33][34] The chain dynamics of polymer rings 35 and the branch-point motions of star polymers 36 have also been investigated by NSE during the recent past years, providing direct microscopic insight into their intriguing rheological properties.…”

“…In addition, the self-motions of hydrogens in the alkyl side-groups were investigated on a labeled PHMA sample (deuterated main chain and protonated side group) by NSE 32 [empty squares in Fig. 3(b)] and also by BS-ToF 34 [ Fig. 4(ac)].…”