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

Abstract

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.

Study of the structure and dynamics of poly(vinyl pyrrolidone) by molecular dynamics simulations validated by quasielastic neutron scattering and x-ray diffraction experiments

“…We note that shifts in temperature and/or timescale are usually required for a perfect matching of results from MD simulations and real samples. We can mention the cases of orthoterphenyl, 42 polyisoprene, 43 poly(methyl methacrylate), 5,44 poly(vinyl acetate), 45 poly(ethylene propylene), 31 and poly(vinyl methyl ether). 46 Apart from slight differences in the microstructure or in the density, a possible source for this discrepancy could be that the force field used in the simulations was not perfectly calibrated.…”

“…[1][2][3] Moreover, it has been shown that increasing the size of side groups has a deep impact on the structure and dynamics of polymers. Of particular interest is the family of poly(nalkyl-methacrylates), 2,[4][5][6][7][8][9][10][11][12] where x-ray and neutron diffraction analysis show that nanosegregation occurs between backbone and side groups. The effects of these nanosegregated domains on the local dynamics remain however unclear.…”

Study of the dynamics of poly(ethylene oxide) by combining molecular dynamic simulations and neutron scattering experiments

“…Similar behavior has also been found in other polymer systems. 37,70,71 Obviously, the de-Gennes approach proposed for monoatomic simple liquids cannot be applied rigorously to more complex systems such as polymers. As a general conclusion, from this thorough comparison, we can state that not only the self-motions of H-atoms but also the collective dynamics of PEO are well reproduced by the simulated cell.…”

“…The complementary use of these techniques has proven to be an extremely successful route to address a number of similar problems in the field of soft matter. 7,[27][28][29][30][31][32][33][34][35][36][37][38][39] Regarding MD simulations on PEO, a relatively large number of publications can be found in literature. 25,26,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]60 Some of them deal mainly with structural properties, 48,50,51,[53][54][55][56]60 but the local dynamics is also addressed in others; 42,52,57 sometimes the works have been motivated by its use as polyelectrolyte 40,41,…”

Collective dynamics of glass-forming polymers at intermediate length scales

“…dielectric or mechanical spectroscopy also allow covering such a low-Q limit. On the other extreme, neutron scattering has already been used to investigate the dynamic structure factor at the first structure factor peak Q max , reveling thereby the "genuine" α-relaxation, in a relatively large number of glassforming systems, and in particular in polymers [1][2][3][4][5][6][7][8][9][10][11][12][13]. In fact, the application of neutron scattering techniques to protonated and deuterated samples (addressing thereby the self-motions of hydrogens and the collective response, respectively, see Appendix A) has been key to characterize the dynamics of the structural relaxation at inter-and intra-molecular length scales [8,14,15].…”