Quasielastic Neutron-Scattering Study of the Local Dynamics of Poly(ethylene glycol) Dimethyl Ether in Aqueous Solution

Abstract

Neutron quasielastic scattering experiments were carried out on aqueous solutions of 500 u molecular weight poly(ethylene glycol) dimethyl ether (PEG-DME) in deuterated water. The intermediate scattering functions extracted from the measured neutron scattering can be fitted by a combination of a fast second-order exponential decay (t < 2 ps) and a slower first-order exponential decay. The analysis of the momentum transfer (Q) dependence of the decay constant for the slower component shows that it has an approximately constant value at Q's less than 13 nm-1 and then increases linearly up to the highest momentum transfer (25 nm-1). Both the slope of the higher-Q linear region and the individual Q-dependent decay constants show a minimum in the PEG-DME weight fraction range of 0.6−0.9. Further analysis of the neutron-scattering data to check the effect of multiple scattering in the sample shows that only the shape of the fast decay (t < 2 ps) is affected by this correction. A direct quantitative comparison is made between experiment and molecular dynamics simulations. Fourier transforming the experimental data from the frequency domain into the time domain to yield the intermediate scattering function allows for a quantitative comparison with the equivalent function calculated from the simulations. Furthermore, a Monte Carlo simulation of the experiment based on simulation results is used to account for the effect of multiple scattering quantitatively, which represents a novel approach to dealing with the complications arising from multiple scattering. This correction is significant, resulting in excellent agreement between experiment and the simulations. Both simulation and experiment give rise to a maximum in the relaxation time for PEG-DME proton motion in the PEG-DME weight fraction range of 0.6−0.9. On the basis of the simulations, this maximum arises from competition between the slowing down of the torsional transitions due to hydrogen bonding between the water and the PEG-DME ether oxygens, and the addition of a sufficient quantity of water results in an increasing fraction of large water clusters and more mobile water (i.e., a low-viscosity solvent). The former dominates at low dilution, and the latter dominates at higher dilution, leading to the enhanced backbone motion of the PEG-DME and the observed maximum in the residence time of the PEG-DME protons as a function of composition.