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

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• 作者：A.-C. Genix ; A. Arbe ; F. Alvarez ; J. Colmenero ; B. Farago ; A. Wischnewski ; D. Richter
• 刊名：Macromolecules
• 出版年：2006
• 出版时间：September 5, 2006
• 年：2006
• 卷：39
• 期：18
• 页码：6260 - 6272
• 全文大小：517K
• 年卷期：v.39,no.18(September 5, 2006)
• ISSN：1520-5835

文摘

We have investigated the molecular dynamics of syndiotactic poly(methyl methacrylate) well abovethe glass transition by combining quasielastic neutron scattering and fully atomistic computer simulations. Theincoherent scattering measured by backscattering on a sample with deuterated ester methyl groups has revealedthe single-particle motions of hydrogens in the main chain and in the ages/gifchars/alpha.gif" BORDER=0>-methyl groups. Moreover, with neutronspin-echo experiments on the fully deuterated sample we have accessed the collective motions at the two firstmaxima of the structure factor. The simulated cell, which has been previously validated regarding the structuralproperties [Genix, A.-C.; et al. Macromolecules 2006, 39, 3947], shows a dynamical behavior that, allowing ashift in temperature, reproduces very accurately all the experimental results. The combined analysis of both setsof 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 ages/gifchars/alpha.gif" BORDER=0>-process andthe methyl group rotations, we have found indications of a rotational motion of the ester side group around themain chain. (iii) At a given momentum transfer and depending on the molecular groups considered, the timescales for collective motion are spread over about 1 order of magnitude, the correlations involving the mainchain decaying much more slowly than those relating side groups. (iv) At the length scale characteristic for theoverall periodicity of the system (that corresponding to the first structure factor peak), the experimentally observedcollective dynamics relates to the backbone motions and is of interchain character; there, coherency effects areobserved for all correlations, though side groups display weaker collectivity. (v) At the second structure factorpeak, coherency remains only for correlations involving the main chains.