Molecular dynamics of main-chain liquid crystalline polymers

Arrighi, Valeria; Higgins, J. S.; Mani, Santosh A.; Stoeber, Lutz

doi:10.1016/s0921-4526(98)01486-0

Cited by 7 publications

(4 citation statements)

References 29 publications

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“…From early 80's, neutron scattering has been widely used to investigate methyl group dynamics in the quasielastic energy range |hω| < ∼ 2 meV, or by using the FEW technique (see Section II). A huge collection of data has been obtained for several polymer systems, including standard materials as PVAc [89,103], PMMA [20,[22][23][24]34,91,104], poly(propylene oxide) (PPO) [105], poly(vinyl methyl ether) (PVME) [23,25,27,106], polyisoprene (PI) [26,92], neat poly(dimethyl siloxane) (PDMS) [93,[107][108][109], and filled with silica nanoparticles [110], head-to-tail [88,90], and head-to-head (hh) [111] polypropylene (PP), poly(ethylene propylene) (PEP) [111], and poly(methyl phenyl siloxane) (PMPS) confined in silica based nanoporous glasses [112], polymer blends as solution chlorinated polyethylene (SCPE)/PMMA [22,113], polystyrene (PS)/ PVME [114,115], hh-PP/PEP [111], or poly(ethylene oxide) (PEO)/PMMA [116,117], thin film photoresist polymers [118], and other polymer based compounds as polymer electrolites [119][120][121], proteins [122], liquid crystalline polyesters and polyethers [123,124] or propylene gl...…”

Section: Summary Of Experimental Results By Neutron Scattering On Pol...mentioning

confidence: 99%

Neutron scattering investigations on methyl group dynamics in polymers

Colmenero

Moreno

Alegría

2005

Progress in Polymer Science

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Among the different dynamical processes that take place in polymers, methyl group rotation is perhaps the simplest one, since all the relevant interactions on the methyl group can be condensed in an effective mean-field one-dimensional potential. Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time but only very recently (1998), these processes have been directly observed by high-resolution neutron scattering techniques. This paper revises and summarizes the work done on this topic over last ten years by means of neutron scattering. It is shown that the results obtained in many chemically and structurally different polymers can be consistently described in the whole temperature range -from the quantum tunnelling limit to the classical hopping regime -as well as in the librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM), which was first proposed in 1994. This model introduces a distribution of potential barriers for methyl group rotation, which is associated to the disorder present in any structural glass. The molecular and structural origin of the barrier distribution in polymers is discussed on the basis of a huge collection of investigations reported in the literature, including recent fully atomistic molecular dynamics simulations.

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Section: Summary Of Experimental Results By Neutron Scattering On Pol...mentioning

confidence: 99%

Neutron scattering investigations on methyl group dynamics in polymers

Colmenero

Moreno

Alegría

2005

Progress in Polymer Science

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Section: Molecular Simulations With Model IImentioning

confidence: 99%

“…We note that atomistic calculations of E A distributions in polymer glasses and melts have been presented by Davies and co-workers, 27,10 and Gaussian distributions have been invoked in earlier experimental works. 28,29 (2) We assume that the friction constant ζ is low enough to justify neglect of the frictional term in eq 2 in describing motion within the methyl free energy Iφ ¨+ ζφ wells, yet large enough for the dynamics of transitions between wells to be describable by an activated rate expression with transmission coefficient equal to 1. On the basis of the Kramers analysis, 30,31 the precise condition for this picture to be applicable is with E A measured in J/molecule and k B being the Boltzmann constant.…”

Section: Molecular Simulations With Model Imentioning

confidence: 99%

Segmental Dynamics of Atactic Polypropylene As Revealed by Molecular Simulations and Quasielastic Neutron Scattering

et al. 2002

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The dynamics of atactic polypropylene has been explored with quasielastic neutron scattering (QENS) measurements in the temperature range 4-460 K and momentum transfer range q) 0.2-2.25 Å-1. In parallel, molecular dynamics simulations of the same polymer have been conducted in the temperature range 260-600 K, using both a fully atomistic model and a model with united-atom methyl groups. In conjunction with the second model, a computational procedure for introducing the motion of methyl hydrogens a posteriori is proposed and tested against the fully atomistic simulation results. Simulated intermediate incoherent scattering functions I(q,t) reveal an initial exponentially decaying regime of duration ca. 1 ps, which is dominated by bond angle bending vibrations and torsional oscillations, as well as features attributable to torsional transitions of the methyl groups and to correlated conformational transitions of the backbone bonds at longer times. The time decay of I(q,t) beyond 1 ps is well-described by stretched exponential functions, the stretching exponent being around 0.5 at 600 K and decreasing with decreasing temperature. Analysis of the atomistic simulation trajectories yields distributions of relaxation times with a distinct log-Gaussian peak characteristic of methyl motion, from which a Gaussian distribution of activation energies for methyl torsional transitions with mean around 15 kJ/mol and standard deviation around 3 kJ/mol is extracted, in excellent agreement with QENS estimates. Torsional transitions of different methyls occur essentially independently of each other. QENS experiments reveal a nondecaying elastic contribution to the scattering over the time window of the measurement, which is not seen in the simulations. Apart from this, computed I(q,t) and incoherent dynamic structure factor S(q,ω) curves are in very favorable agreement with the measured QENS spectra and with earlier NMR data on atactic polypropylene.

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“…This approach , allows for a distribution of rotational times, and yields fit qualities comparable to a log-Gaussian distribution, frequently used to describe rotation of side groups. The parameters A , τ KWW , and β retain the same meanings, and the EISF [elastic incoherent structure factor] describes motion over a restricted spatial area, with the q dependence of the EISF characterizing its geometry.…”

Section: Simulation Detailsmentioning

confidence: 99%

A Molecular View of Dynamic Responses When Mixing Poly(ethylene oxide) and Poly(methyl methacrylate)

Chen

Maranas

2009

Macromolecules

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We use explicit atom molecular dynamics to investigate mobility in poly(methyl methacrylate) [PMMA] when blended with 10%, 20%, and 30% weight percent poly(ethylene oxide) [PEO]. The responses to blending of main chain motion, rotation of the two methyl groups, and rotation of the entire ester side chain, which is associated with the -relaxation, are individually assessed at temperatures well above the glass transition temperature where their characteristic times overlap. We also consider the response of main chain motion in PEO to blending with PMMA. We find three classes of behavior: methyl group rotation is not influenced by blending, ester group rotation and the main chain motion of PEO are slightly altered by the change in environment, and a substantial change in dynamics is observed for main chain motion in PMMA. The observation that the -relaxation in PMMA changes with mixing is unusual, as this is a local motion thought to be insensitive to blending. This is discussed in light of the spatial extent of localized motion and that of mixing of the two components.

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Molecular dynamics of main-chain liquid crystalline polymers

Cited by 7 publications

References 29 publications

Neutron scattering investigations on methyl group dynamics in polymers

Neutron scattering investigations on methyl group dynamics in polymers

Segmental Dynamics of Atactic Polypropylene As Revealed by Molecular Simulations and Quasielastic Neutron Scattering

A Molecular View of Dynamic Responses When Mixing Poly(ethylene oxide) and Poly(methyl methacrylate)

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