Rotation of Methyl Side Groups in Polymers: A Fourier Transform Approach to Quasielastic Neutron Scattering. 1. Homopolymers

Arrighi, Valeria; Higgins, J. S.; Burgess, A. N.; Howells, W.S.

doi:10.1021/ma00112a021

Cited by 56 publications

(92 citation statements)

References 8 publications

(13 reference statements)

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“…Hence, first quasielastic neutron scattering data (QENS, see Section II) on classical hopping of methyl groups in polymers were analyzed, as for the case of crystalline systems, by assuming a unique value of the rotational potential. However, this procedure provided, in contrast to the result observed in crystalline systems, a non-Arrhenius temperature dependence of the hopping rate, and an apparent temperature dependence of the geometry of the motion, with large deviations from threefold rotation [20][21][22][23][24][25][26].…”

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confidence: 62%

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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confidence: 62%

Neutron scattering investigations on methyl group dynamics in polymers

Colmenero

Moreno

Alegría

2005

Progress in Polymer Science

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“…A full characterization of this motion is beyond the scope of this paper and will be addressed in a future work involving BS measurements on the sample PMMA-d 5 (hydrogens only at the ester methyl group). Nevertheless, it is important to emphasize that the spatial scale of the rotational motion we have identified well above T g is well beyond what would be expected for a simple π-flip (N ) 2, R ≈ 2 Å) which was observed in the neighborhood of T g .…”

Section: S S H Sg (Qt) By S I (Qt) ) S H Mc (Qt)/a H Mc (Q) the Rmentioning

confidence: 99%

“…Among the different dynamical processes taking place in PMMA, we first mention those related to rotational motions of both methyl groups: (i) The low values of the energy barriers in the ester methyl group have allowed resolution of rotational tunneling by quasielastic neutron scattering; 1,2 accordingly, its classical rotation is very fast. [2][3][4][5][6] (ii) Because of stronger hindrance, the rotation of the R-methyl group occurs only at higher temperatures. 7,8 Furthermore, other subglass and glass relaxation processes in PMMA have been studied for a long time mainly by dynamic mechanical analysis, dielectric spectroscopy, and advanced NMR spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

Self- and Collective Dynamics of Syndiotactic Poly(methyl methacrylate). A Combined Study by Quasielastic Neutron Scattering and Atomistic Molecular Dynamics Simulations

et al. 2006

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We have investigated the molecular dynamics of syndiotactic poly(methyl methacrylate) well above the glass transition by combining quasielastic neutron scattering and fully atomistic computer simulations. The incoherent scattering measured by backscattering on a sample with deuterated ester methyl groups has revealed the single-particle motions of hydrogens in the main chain and in the R-methyl groups. Moreover, with neutron spin-echo experiments on the fully deuterated sample we have accessed the collective motions at the two first maxima of the structure factor. The simulated cell, which has been previously validated regarding the structural properties et al. Macromolecules 2006, 39, 3947], shows a dynamical behavior that, allowing a shift in temperature, reproduces very accurately all the experimental results. The combined analysis of both sets of data has shown that: (i) The segmental relaxation involving backbone atoms deviates from Gaussian behavior.(ii) The dynamics is extremely heterogeneous: in addition to the subdiffusion associated with the R-process and the methyl group rotations, we have found indications of a rotational motion of the ester side group around the main chain. (iii) At a given momentum transfer and depending on the molecular groups considered, the time scales for collective motion are spread over about 1 order of magnitude, the correlations involving the main chain decaying much more slowly than those relating side groups. (iv) At the length scale characteristic for the overall periodicity of the system (that corresponding to the first structure factor peak), the experimentally observed collective dynamics relates to the backbone motions and is of interchain character; there, coherency effects are observed for all correlations, though side groups display weaker collectivity. (v) At the second structure factor peak, coherency remains only for correlations involving the main chains.

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“…The ester relaxation in particular has been the subject of a series of studies from several groups. [6][7][8][9][10] Neutron scattering differentiates between hydrogen and deuterium so that deuterium labeling is a convenient way of highlighting the motion of the part of the molecule of interest. In fact, the deuterium cross section is much less than that of hydrogen so that the labeling leaves the hydrogenous segments showing and suppresses the signal from those parts containing deuterium.…”

Section: Introductionmentioning

confidence: 99%

“…6 This situation was eventually clarified by the realization that there is a distribution of activation energies in the system caused either by intramolecular or intermolecular interactions, which vary for each group perhaps due to conformational or packing heterogeneities frozen in at the glass temperature. 8,10 A further question arose about the apparent persistence to very low temperatures of quite rapid reorientation-where the frequency extrapolated from higher temperatures would fall far below the resolution limit of the spectrometer. To explain this observation it is necessary to look more closely at the motion of the methyl group.…”

Section: Introductionmentioning

confidence: 99%

Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): A deuteron magnetic resonance study

Cereghetti

Kind

Higgins

2004

The Journal of Chemical Physics

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In isotactic poly͑methyl methacrylate͒ ͑PMMA͒, we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance ͑DMR͒ in a deuterated sample. We find that the motion of the CD 3 -group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T ϭ291 K and Tϭ70 K reveal a broad temperature dependent probability distribution of autocorrelation times c for the 2 /3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies (E a ) with variance E a ϭ13.8Ϯ0.5 meV ͑1.33 kJ/mol͒. The line shape transition between Tϭ70 K and Tϭ23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width E a /E a of the E a distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between Tϭ70 K and Tϭ23 K.

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Rotation of Methyl Side Groups in Polymers: A Fourier Transform Approach to Quasielastic Neutron Scattering. 1. Homopolymers

Cited by 56 publications

References 8 publications

Neutron scattering investigations on methyl group dynamics in polymers

Neutron scattering investigations on methyl group dynamics in polymers

Self- and Collective Dynamics of Syndiotactic Poly(methyl methacrylate). A Combined Study by Quasielastic Neutron Scattering and Atomistic Molecular Dynamics Simulations

Tacticity effects on the barriers to rotation of the ester methyl group in poly (methyl methacrylate): A deuteron magnetic resonance study

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