Dennis Engberg

Brillouin scattering and dielectric relaxation in PPG of different chain lengths and end groups

Abstract We have investigated the structural relaxational behavior of poly(propylene glycol), PPG, for polymer chains of increasing number of monomer units, n(=3, 7, 70), and for different end groups (OH and CH3) using Brillouin scattering and dielectric relaxation. In the case of the OH capped systems the relaxation dynamics are rather insensitive to chain length variations, whereas large effects are demonstrated when changing to CH3 end groups. For chains with n≲20, there is a radical decrease in the sound velocity and the average relaxation time, when the hydroxyl terminations are replaced by methyl terminations. By using the nonpolar CH3 endcapping the linking of adjacent chains, as is the case in the hydrogen bonded OH capped systems, is avoided.

The segmental and rotational dynamics of PPO investigated by neutron scattering and molecular dynamics simulations

The relaxation properties of poly(propylene oxide) at temperatures above the glass‐transition temperature are studied by quasi‐elastic neutron scattering and molecular dynamics (MD) simulations. The contributions to the dynamic structure factor from the segmental motion and the methyl group jump rotation are separated using a threefold jump‐rotation model for the methyl side group motion, with a delta function for the distribution of relaxation times, and a stretched exponential function for the segmental motion. This simple model describes momentum transfer and time dependence of the data over a wide temperature range. The results show that with this separation, both neutron and MD data can be well described, and the analysis of both contributions is necessary in order to obtain reliable results for the segmental relaxation and rotational relaxation times at temperature above T g .

Diffusive solvent dynamics in a polymer gel electrolyte studied by quasielastic neutron scattering

A quasielastic neutron scattering study has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). The dynamics of the solvent, which is crucial for the ion conduction in this system, was probed using the hydrogen/deuterium contrast variation method with nondeuterated solvent and a deuterated polymer matrix. Two relaxation processes of the solvent were studied in the 10-400 microeV range at different temperatures. From analysis of the momentum transfer dependence of the processes we conclude that the faster process ( approximately 100 microeV) is related to rotational diffusion of the solvent and the slower process ( approximately 10 microeV) to translational diffusion of the solvent. The translational diffusion is found to be similar to the diffusion in the corresponding liquid electrolyte at short distances, but geometrically constrained by the polymer matrix at distances beyond approximately 5 A. The study indicates that the hindered diffusion of the solvent on a length scale of the polymer network interchain distance ( approximately 5-20 A) is sufficient to explain the reduced macroscopic diffusivity and ion conductivity of the gel electrolyte compared to the liquid electrolyte.

Brillouin scattering study of structural relaxation in Mg(ClO4)2-PPG

Abstract Acoustic properties and short-time (10 −11 s) structural relaxation behaviour in poly(propylene glycol), PPG-4000, complexed with different concentrations of Mg(ClO 4 ) 2 , have been examined using the Brillouin scattering technique. Below T g the Mg 2+ coordinates preferably to the OH − groups of the polymeric chain ends. Above T g the Mg 2+ coordinations to ether oxygens become increasingly more important.