High-Resolution Incoherent Inelastic Neutron Scattering Spectra of Polyisobutylene and Polyisoprene

Adams, Mark A.; Gabrys, Barbara J.; Zając, Wojciech; Peiffer, Dennis G.

doi:10.1021/ma049651w

Cited by 11 publications

(25 citation statements)

References 17 publications

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“…Although quantum effects (not considered in the classical MDS) should certainly affect the high energy range of Z(ω), it is worthy of remark that the energies of the computed high energy maxima (see Fig. 28) roughly correspond to the reported infrarred and Raman bands [155], as well as to recent INS results by Adams et al [147].…”

Section: Molecular Dynamics Simulationssupporting

confidence: 72%

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: Molecular Dynamics Simulationssupporting

confidence: 72%

Neutron scattering investigations on methyl group dynamics in polymers

Colmenero

Moreno

Alegría

2005

Progress in Polymer Science

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“…The validation of the simulated cell has been thoroughly carried out [54,55] by direct comparison of magnitudes computed from the simulation and measured by neutron scattering on protonated and deuterated samples in earlier works [32,39,42]. The agreement found is excellent regarding both static and dynamic features.…”

Section: Molecular Dynamics Simulationsmentioning

confidence: 99%

“…From an experimental point of view, polyisobutylene (PIB, chemical formula: -[CH 2 -C(CH 3 ) 2 ]n ) is a very thoroughly investigated polymer by diverse techniques including spectroscopic techniques [28][29][30][31] as well as scattering methods [32][33][34][35][36][37][38][39][40][41][42]. It has the peculiarity of being one of the strongest polymers in Angell's terminology based on the concept of fragility [43].…”

Section: Introductionmentioning

confidence: 99%

Applicability of mode-coupling theory to polyisobutylene: A molecular dynamics simulation study

et al. 2013

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The applicability of Mode Coupling Theory (MCT) to the glass-forming polymer polyisobutylene (PIB) has been explored by using fully atomistic molecular dynamics simulations. MCT predictions for the so-called asymptotic regime have been successfully tested on the dynamic structure factor and the self-correlation function of PIB main-chain carbons calculated from the simulated cell. The factorization theorem and the time-temperature superposition principle are satisfied. A consistent fitting procedure of the simulation data to the MCT asymptotic power-laws predicted for the α-relaxation regime has delivered the dynamic exponents of the theory-in particular, the exponent parameter λ-the critical non-ergodicity parameters, and the critical temperature T(c). The obtained values of λ and T(c) agree, within the uncertainties involved in both studies, with those deduced from depolarized light scattering experiments [A. Kisliuk et al., J. Polym. Sci. Part B: Polym. Phys. 38, 2785 (2000)]. Both, λ and T(c)/T(g) values found for PIB are unusually large with respect to those commonly obtained in low molecular weight systems. Moreover, the high T(c)/T(g) value is compatible with a certain correlation of this parameter with the fragility in Angell's classification. Conversely, the value of λ is close to that reported for real polymers, simulated "realistic" polymers and simple polymer models with intramolecular barriers. In the framework of the MCT, such finding should be the signature of two different mechanisms for the glass-transition in real polymers: intermolecular packing and intramolecular barriers combined with chain connectivity.

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“…From an experimental point of view, polyisobutylene (PIB, -[CH 2 -C(CH 3 ) 2 ]n ) is a very thor-oughly investigated polymer by diverse techniques including spectroscopic techniques [8][9][10][11] as well as scattering methods. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] In particular, its collective features have been deeply investigated by neutron diffraction and the neutron spin echo (NSE) technique. 15,20 Despite having two methyl groups in its monomer, the static structure factor of PIB shows qualitatively the same appearance as that of 1,4-PB, i. e., without any particular signature of the presence of these side-groups.…”

Section: Introductionmentioning

confidence: 99%

Collective Features in Polyisobutylene. A Study of the Static and Dynamic Structure Factor by Molecular Dynamics Simulations

et al. 2013

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We present a study of the static and dynamic structure factor of polyisobutylene (PIB) by fully atomistic molecular dynamics simulations. The reliability of the simulated cell is first assured by computing the magnitudes measured by diffraction and neutron spin echo techniques on a fully deuterated sample and directly comparing the results with those previously obtained from experiments [B. Farago et al., Phys. Rev. E 2002, 65, 051803]. Taking advantage of the validated simulations, we have disentangled the contributions to the static and dynamic structure factor by using a suitable grouping of the partial correlation functions based on specific molecular groups in PIB: main-chain (MC) atoms and methyl-group (MG) atoms. Regarding the structural features, we can attribute the temperature dependence of the first structure factor peak-which is dominated by inter-chain correlations mainly from backbone atoms-predominantly to the evolution of the MC/MG cross-correlations. Paradoxically, in the momentum transfer region where the MG/MG correlations present their main peak, the total structure factor displays a minimum due to a strong negative feature of the MC/MG cross-correlations. Concerning the dynamics, the decay of the intra-molecular correlations takes place through highly correlated motions relating pairs of MGs and MG and MC atoms. At inter-molecular level, the difference between pair and self-correlations for MC atoms is enhanced as the system approaches the glass-transition, indicating a gradual increase of collectivity. This collectivity of the backbones is ultimately the responsible for the modulation of the activation energy with the structure factor found in the experiments and reproduced by the simulations. Finally, we analyze the contributions of the analytical ansatz recently proposed to describe the collective relaxation time [J. Colmenero et al., J. Chem. Phys. 2013, 139, 044906] in order to identify the key ingredient leading to the above mentioned modulation of the activation energy, which is successfully accounted for by the model.

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High-Resolution Incoherent Inelastic Neutron Scattering Spectra of Polyisobutylene and Polyisoprene

Cited by 11 publications

References 17 publications

Neutron scattering investigations on methyl group dynamics in polymers

Neutron scattering investigations on methyl group dynamics in polymers

Applicability of mode-coupling theory to polyisobutylene: A molecular dynamics simulation study

Collective Features in Polyisobutylene. A Study of the Static and Dynamic Structure Factor by Molecular Dynamics Simulations

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