P. Carlsson

Structural properties of poly(propylene oxide) from diffraction experiments and reverse Monte Carlo simulation

Structural characteristics of an amorphous polymer melt, poly(propylene oxide) (PPO), have been studied by combining neutron and x-ray diffraction experiments and computer modeling using the reverse Monte Carlo (RMC) technique. The neutron diffraction experiments were performed on hydrogenous as well as deuterated samples. The experimentally determined nearest-neighbor distances were found to be in good agreement with literature data. The RMC modeling was applied for interpretation of the diffraction data to obtain more detailed structural information on bond angles, intermediate and long range correlations. For the intermediate range structure, the experimental structure factors demonstrate a first diffraction peak at about 1.45 A−1, which from the RMC produced model can be related to the interchain distance of an almost random packing of the polymer chains. To investigate the chain conformation, partial atomic pair correlation functions have been calculated for atoms belonging to monomers close in seque...

Neutron-scattering studies of a polymer electrolyte, PPO–LiClO4

The structure and dynamics of a prototype polymer electrolyte, PPO–LiClO4, have been investigated using neutron diffraction (ND) and quasi-elastic neutron scattering (QENS). For comparison, corresponding studies of pure PPO have also been performed. The diffraction data reveal large structural changes which are induced by the dopant salt. The phenomena can be explained by local ordering of the chain segments around the solvated cations and by contraction of neighbouring chains via cationic cross links. The QENS results indicate that the segmental motions of the polymer chains, which have been considered to be of vital importance for the ionic mobility, slow down considerably when coordinated to cations. The results are discussed in relation to proposed models for the structure and the conductivity mechanism.

Modelling of segmental dynamics in polymer electrolyte PPO-LiClO4, by surface fitting of quasi-elastic neutron scattering data

Abstract The structural dynamics of an amorphous model polymer electrolyte, poly(propylene oxide) (PPO) complexed with lithium perchlorate (LiClO 4 ), has been investigated using quasi-elastic neutron scattering (QENS). Pure PPO was investigated for reference. A combination of data from several analyser reflections, has enabled an investigation of a (Fourier) time range, wide enough to encompass parts of the segmental relaxation as well as the methyl group motion. The two processes have been identified and investigated by a simultaneous surface fit of the data from all the detectors and resolutions to a model where the segmental motion is represented by a stretched exponential relaxation function with relaxation times proportional to Q − ν , and the methyl group motion is represented by the three-fold jump rotation model. The dynamics of the pure polymer is well described by the model, using fitting parameters which are in good agreement with literature data, and we conclude that the surface fitting approach is a successful method for analysing the data. The results for the salt-containing polymer show that the segmental relaxation is strongly slowed down and more stretched, whereas no significant change was seen for the methyl group motion. The adequacy of the model in this case cannot be fully evaluated due to the still limited data on the segmental relaxation.

Molecular-dynamics simulation of structural and conformational properties of poly(propylene oxide)

Molecular-dynamics simulations of poly(propylene oxide) (PPO) have been performed in the temperature range 300 K⩽T⩽450 K using a newly developed atomistic force field. Chains with 1, 11 and 45 repeat units have been considered. We find excellent agreement for the short and intermediate range order as deduced from x-ray and neutron diffraction measurements with deviations (excluding quantum mechanical effects for the nuclear motion) lying within experimental uncertainties. The static structure factor S(q) shows a first sharp diffraction peak due to interchain correlations which is shifted to smaller wave vectors with increasing temperature. The molecular weight dependence of the short and intermediate range order is found to be weak and the obtained density and thermal expansivity compare favorably with experimental data. The backbone dihedral angle distributions are essentially identical for PO11 and PO45, but deviate slightly from those for PO1, and the probability for gauche conformations is quite large...

Structural and dynamical properties of polymer electrolytes PPOLiClO4

Abstract The structure and dynamics of a prototype of polymer electrolytes, PPOLiClO 4 , have been investigated using neutron diffraction and quasi-elastic neutron scattering (QENS). The aim is to gain insights into microscopic structures and processes that may be of importance for the ionic conductivity mechanism. The diffraction experiments reveal large structural changes on an intermediate length scale, 4–10 A, as the polymer is doped with salt. The effects may partly be explained by local segmental ordering of the polymer ether-oxygens around the solvated cations. The QENS results indicate that the segmental motions, which have been discussed to be of vital importance for the ionic mobility, slow down considerably when coordinated to cations. The results are discussed in relation to proposed models for the structure and the conductivity mechanism.

Neutron diffraction investigations of the cation coordination in an amorphous polymer electrolyte, PPO-LiClO4

The structure of an amorphous polymer electrolyte model material, PPOLiClO4, has been investigated on the short-range length scale using neutron diffraction H:D isotope substitution experiments. We report experimental results which may be attributed to a lithium ether‐oxygen distance of about 2.1 A, The findings indicate, within the experimental accuracy, that the cation environment of the Li ion in PPOLiClO4 O:M 16:1 is approximately the same as reported for crystalline and amorphous complexes of PEO and lithium salts.

Structural investigations of polymer electrolyte poly(propylene oxide)-LiClO4 using diffraction experiments and reverse Monte Carlo simulation.

The structure of an amorphous polymer electrolyte, poly(propylene oxide) (PPO) complexed with LiClO4, has been studied using reverse Monte Carlo (RMC) simulations. The simulations require no force field but are based on experimental data only, in this case from x-ray and neutron diffraction experiments. Excellent agreement between the experimental data and the structures resulting from the RMC simulation is obtained. Samples with ether-oxygen to lithium concentrations (molar ratios) O:Li=16:1 and 5:1 were studied and compared to results of pure PPO from a previous study. We focus on the effects of the solvated salt on the structure of the polymer matrix, the spatial distribution of ions, and the correlations between the anions and the polymer chains. Analyzing the structures produced in the simulations, we find that for a concentration 16:1, the interchain distance is approximately the same as in pure PPO but more well defined. For a concentration 5:1, we find a larger and less well-defined interchain distance compared to the 16:1 concentration. This signifies that at the 16:1 salt concentration, there is enough free volume in the polymer host to accommodate the ions, and that the solvation of salt induces ordering of the polymer matrix. At the higher salt concentration 5:1, the polymer network must expand and become less ordered to host the ions. We also note, in accordance with previous studies, that the solvation of salt changes the conformation of the polymer chain towards more gauche states. The simulations furthermore reveal marked correlations between the polymer chains and the anions, which we suggest arise predominantly from an interaction mediated via cations, which can simultaneously coordinate both ether oxygens in the polymer chains and anions. Interanionic distances at 5 A, which are consistent with two or more anions being coordinated around the same cation, are also observed. On a larger scale, the RMC structure of PPO-LiClO4 16:1 clearly indicates the presence of salt-rich and salt-depleted domains having a length scale of <20 A. In view of such a heterogeneous structure of PPO-LiClO4 16:1, it is plausible that the increased ordering of the polymer matrix is due to rather well-defined structural arrangements within the salt-rich domains, and that the characteristic interchain distance in the salt-rich domains is similar to that of the pure polymer.

Low-frequency vibrations in monomers, dimers and polymers of propylene glycol

Abstract We have studied the low-frequency dynamics of oligomers (n = 1 and n = 2) and polymers (n = 45) of propylene glycol using molecular dynamics (MD) simulations. The polymer structure was built from a reverse Monte Carlo (RMC) simulation of the static structure factors S(Q) obtained for ordinary and deuterated poly(prolylene oxide) from neutron diffraction. The fraction of the different stereo-isomers obtained in this way was checked by performing 13C nuclear magnetic resonance studies on the polymers. The RMC polymer structure was used as a starting structure for MD simulations of the polymer, whereas the shorter-chain oligomers were simulated using random starting structures. The vibrational density of states was calculated from the Fourier transform of the velocity autocorrelation function. Our results indicate that the low-frequency peak below 100 cm−1, generally referred to as the boson peak, is to a large extent due to intermolecular degrees of freedom, the peak position and shape being rather...

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 .

The segmental dynamics of a polymer electrolyte investigated by neutron spin echo technique

The structural dynamics of a polymer electrolyte model material, PPO-LiClO4, has for the first time been studied using the Neutron Spin Echo technique. The dynamics was studied at a Q-value close to the first sharp diffraction peak, which has been shown originate from inter-chain correlations. Pure PPO was investigated for reference and we find for both materials, that a stretched exponential function can describe the relaxation observed, and that the relaxation times are, within the compared temperature range, proportional to reported relaxation times from viscosity measurements. We therefore attribute the observed dynamics to α-relaxation and ascribe an inter-chain character to it. The relaxation of PPO-LiClO4 is found to be slower and more stretched compared to that of pure PPO, which, with the inter-chain character of the relaxation in mind, may be attributed to that the salt induced transient cationic cross links between chains. We furthermore find that the shape of the α-relaxation in PPO (stretchin...